Tumors in the Adenohypophysis

Abstract

The human pituitary gland is located in the sella turcica and consits of the adenohypophysis and the neurohypophysis. The adenohypophysis, comprising approximately 80% of the entire pituitary gland, produces six hormones, including growth hormone (GH), prolactin (PRL), and adrenocorticotropin (ACTH) – as well as the three glycoprotein hormones – thyrotropin or thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Tumors of the pituitary gland are classified based on the five-tier cassification scheme that is now embodied in the World Health Organization classification of tumors. Clinical, endocrine, imaging, and operative as well as histological findings all contribute to the current classification of these tumors. Ultrastructural study of pituitary tumors is also important in their classification. In this chapter, we discuss morphological characteristics of pituitary tumor subtypes including GH-, PRL-, ACTH-, TSH-, FSH/LH-producing adenomas, as well as silent and null cell adenomas and pituitary carcinomas.

3.1 Introduction

The human pituitary is an oval, bean-shaped, and bilaterally symmetric organ located in the sella turcica, near the hypothalamus and optic chiasm, surrounded by the sphenoid bone and covered with the sellar diaphragm. It is a composite endocrine organ divided in two parts: the adenohypophysis, which derives from an evagination of stomodeal ectoderm (Rathke pouch), and the neurohypophysis, which arises from the neuroectoderm of the floor of the forebrain. The adult pituitary weighs about 0.6 g and measures about 13 mm transversely, 9 mm anteroposteriorly, and 6 mm vertically. A reduction in weight is evident in old age, and an increase occurs during pregnancy and lactation. Although the pituitary size regresses after cessation of lactation, the reversion is not complete, the gland weighing 1 g or more in multiparous women.

The adenohypophysis (anterior lobe) comprises approximately 80% of the entire pituitary and includes the pars distalis (PD), the pars intermedia (PI), and the pars tuberalis (PT). It produces six distinct hormones, including the three amino acid hormones – growth hormone (GH), prolactin (PRL), and adrenocorticotropin (ACTH) – as well as the three glycoprotein hormones – thyrotropin or thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Collectively, these affect the function of virtually all cells in the body [1]. Our perceptions of the functional anatomy and cytology of the human pituitary have undergone considerable change in the last three decades of the twentieth century. The enduring concept of the human adenohypophysis comprising five cell types irreversibly committed to produce six hormones gradually gave way to a new paradigm. In the course of work aimed at developing a morphological classification of pituitary adenomas, three distinct tumor types not related to any of the five known cell types were recognized. This finding suggested the existence of three previously unrecognized cell types. Two of these appeared to produce pro-opiomelanocortin (POMC), the prohormone shared by anterior lobe corticotrophs and cells of the PI, thus challenging the assumption that the human PI is vestigial and lacks functional significance. Subsequently, it was demonstrated that, following normal embryonic growth, the PI migrates into the anterior lobe during the late fetal period. In later life, the PI-derived POMC-producing cells may proliferate, giving rise to either type of silent “corticotroph” adenoma [67]. To date, the parent cell of this third, newly recognized tumor type has eluded detection. A minor component of the adenohypophysis, the PT, surrounds the anterolateral aspect of the pituitary stalk and is assumed to play only a minor role in adenohypophysial function. Hormone secretion by the adenohypophysis is regulated primarily by hypothalamic stimulating and inhibiting hormones, which, synthesized in various nuclei of the hypothalamus, are transported via the portal vessels to adenohypophysial cells. Recent evidence indicates that the regulation of adenohypophysial hormone secretion is more complicated than previously thought. Peripheral target organ gland hormones exert a powerful feedback effect, not only on the hypothalamus but also directly on adenohypophysial cells. In addition, several growth factors and cytokines affect hormone secretion. One novel finding is that several growth factors as well as hypothalamic hormones are produced by adenohypophysial cells as well. Via paracrine/autocrine effects, these substances can modulate adenohypophysial hormone release.

As a downward extension of the hypothalamus, the neurohypophysis or pars nervosa consists of three portions, including the median eminence, the hypophysial stalk or “infundibulum,” and the pars posterior or posterior lobe. The latter plays an important role in the secretion of vassopressin and oxytocin and consists of terminations of nerve fibers arising in the supraoptic land paraventricular nuclei of the hypothalamus. These nerve endings contain neurosecretory granules and are surrounded by specialized glial cells termed pituicytes. The posterior pituitary hormones, synthesized in the supraoptic and paraventricular nuclei of the hypothalamus, are bound to carrier proteins (neurophysins) and are ­transported via the unmyelinated nerve fibers to the posterior lobe, where they are stored in the neurosecretory granules until subsequently released [2].

Given the complex anatomy of the sellar region, as well as the crucial role of the pituitary in regulating the body’s hormonal balance, the clinical manifestations of its diseases are highly variable. It is assumed that some lesions exhibit primarily endocrine effects, whereas others produce mechanical, compressive effects on critical structures surrounding the land. In numerical and clinical terms, pituitary tumors are the most significant lesions affecting the sellar region. Although these may affect either the adenohypophysis or neurohypophysis, nearly all originate in the former. Neurohypophysial tumors are not only rare but also show far less diversity. The neurohypophysis is, however, a favored recipient site of various metastatic tumors.

Pituitary tumors consisting of adenohypophysial cells represent a unique form of neoplasia. In concept and practice, they differ from virtually all other tumors affecting the sellar region, for example, meningeal, neural, glial, vascular, osseous, and embryonal neoplasms. Of these, some clinically and radiographically mimic pituitary adenoma, thus making a firm preoperative distinction impossible. Also entering into the differential diagnosis of adenoma are various non-neoplastic, “tumor-like” lesions.

The primary focus of this chapter is a review of our current knowledge of adenohypophysial tumors and a discussion of their differential diagnosis.

3.2 Tumors of the Adenohypophysis

Tumors of the adenohypophysis are not only the principal tumors of the sellar region, but with the possible exception of meningiomas, also the most frequent primary intracranial neoplasms seen in clinical practice. They represent approximately 10–15% of all operated intracranial tumors and are encountered in 20–25% of autopsy-obtained pituitaries. Thus, neoplastic transformation in the pituitary is a relatively common event but one not always manifesting clinically (Fig. 3.1).

Fig. 3.1

Incidental microadenomas are a frequent finding in the elderly. They are benign, well-demarcated tumors, which either lack immunoreactivity for pituitary hormones or stain mainly for prolactin as is shown in the picture. Original magnification ×40

Although no age group is exempt from the development of adenomas, there is a clear tendency for their frequency to increase with age the highest incidence being between the third and the sixth decades. They are only rarely diagnosed in prepubertal patients. On the basis of surgical series, pituitary tumors occur more often among women, particularly prolactin cell adenomas in premenopausal women. The basis of their prevalence in women is unclear, especially given the fact that in autopsy series incidental adenomas are equally distributed between the two sexes. The expression of estrogen and other sex steroid receptors in the normal pituitary may in part account for the female preponderance. Other factors may also be involved in that clinical manifestation is more conspicuous and easily recognized in women [3].

The overwhelming majority of neoplastic lesions arising in the adenohypophysis are adenomas. Nearly all are histologically benign, slow-growing, well-demarcated, and confined to the sella turcica. In other cases, however, they exhibit rapid proliferation and are invasive of dura, bone, and vascular adventitia. Invasion of these structures is indicative of malignancy. Pituitary carcinoma is exceedingly rare and is defined as a metastasizing tumor giving rise to cerebrospinal and/or distant systemic metastases [4–7]. Brain invasion, although less well understood, is also considered a sign of malignancy.

Based on their remarkable variation in biological ­behavior, numerous attempts have been made to classify pituitary adenomas into distinct categories. In the present chapter, we present the five-tier classification scheme of pituitary tumors now embodied in the World Health Organization International Histological Classification of Tumors [8, 9]. This approach takes into consideration the clinical and laboratory findings, neuroimaging findings as well as histologic, immunocytochemical, imaging, and ultrastructural features.

3.3 Classification of Pituitary Tumors Based on Clinical Findings and Endocrine Data

Although these parameters are clinical and biochemical, in most cases, they correlate with tumor morphology and immunohistochemistry. As such, they are valuable to diagnostic pathologists [1, 10]. Ordinarily, the history and physical examination provide important indications as to the endocrine status of the patients. Suspicions of hormone excess and/or deficiency must then be validated by careful endocrine testing. An endocrine diagnosis is reached by measuring pituitary and target land hormone levels in both basal and dynamic states. Such measurements are sensitive diagnostic indicators in the approximately 70% of pituitary tumors that are hormonally active. The remainder are functionally “silent” and present as expanding sellar masses that cause panhypopituitarism or nonendocrine symptoms due to compression of other anatomic structures in the sellar region. Thus, a variety of clinical features, in either isolation or combination, can be associated with pituitary tumors.

Endocrinologically functioning adenomas cause pituitary hormone excess and a variety of distinctive hypersecretory states. These include hypesecretion of GH, PRL, ACTH, and, rarely, TSH. Corresponding clinical phenotypes include acromegaly or gigantism, the amenorrhea-galactorrhea syndrome, Cushing’s disease/Nelson’s syndrome, and hyperthyroidism. Clinically nonfunctioning pituitary adenomas, mainly gonadotrophic or null cell adenomas, present as expanding sellar masses. Owing to compression or injury to the nontumorous pituitary, its stalk or the hypothalamus, they are often associated with various degrees of hypopituitarism.

The clinical presentation of both functioning and nonfunctioning pituitary adenomas may include a constellation of neurologic symptoms. Suprasellar extension with compression of the optic chiasm results in a characteristic bitemporal hemianoptic pattern of visual loss. Encroachment on hypothalamic structures causes alterations in the sleep cycle, alertness, and behavior. Occasional transgression of the lamina terminalis brings pituitary adenomas into region of the third ventricle with resultant obstructive hydrocephalus. Lateral extension of pituitary adenomas with entry into one or both cavernous sinuses occurs quite commonly and produces cranial neuropathies (cavernous sinus syndrome). Some tumors extend in other directions and, if sufficiently large, can involve the anterior, the middle, and occasionally the posterior fossae, wherein they can produce a full spectrum of neurologic deficits. Common symptoms of large pituitary tumors include headache and increased intracranial pressure.

As noted above, an important effect of a large pituitary tumor is the development of hypopituitarism. Although its causes are varied, it is usually the result of compression or destruction of the hypothalamus and/or pituitary stalk. As the hypothalamus plays a major role in regulating pituitary secretory activity, hypopituitarism may be of hypothalamic origin, that is, the result of decreased or absent secretion of regulatory hypothalamic hormones. Another important presentation is the so-called “stalk-section effect” wherein anterior lobe dysfunction (hyperprolactinemia due to cessation of dopamine delivery to lactotrophs) and diabetes insipidus are principal effects.

Stimulatory hormones such as growth hormone-releasing hormone (GH-RH), corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), and gonadotropin-releasing hormone (GnRH) as well as inhibitory hormones, such as dopamine (DA) or somatostatin (SST), are synthesized by parvocellular neurons of the hypothalamus. They are often released from axon terminals in the external zone of the median eminence into hypophysial portal vessels. The pituitary gland has a unique blood supply originating in the interior and superior hypophysial arteries, both branches of the internal carotid arteries [11]. The inferior hypophysial arteries transport arterial blood directly to the pituitary capsule, a few rows of adenohypophysial cells under the capsule, and the neurohypophysis. The superior hypophysial arteries are divided into two main branches; of these, one penetrates the infundibulum and terminates in the surrounding capillary network, whereas the other, a descending branch known as the loral artery, provides direct arterial blood supply to the anterior lobe without passing through the infandibulum. From the functional point of view, the capillary network surrounding the infundibulum plays a crucial role in the regulation of adenohypophysial endocrine activity. From hypothalamic nerve endings, the releasing and inhibitory hormones pass into the capillary network. Deriving from this network, the long portal vessels extend down the hypophysial stalk to terminate in adenohypophysial capillaries where the hormones gain ready access to the various secretory cells. Short portal vessels originating in the distal stalk and posterior lobe also enter the adenohypophysis.

Compression of the pituitary stalk not only results in disruption of the blood flow to the hypophysial portal system but also impairs transit of vasopressin and oxytocin via nerve fibers to the posterior lobe. Since the normal functional ­activity of the posterior lobe depends upon the integrity of its nerve fiber tracts, disruption of this pathway results in diabetes insipidus (see below).

Although the clinical manifestations of hypopituitarism are influenced by its etiology, severity, and rate of development, a characteristic evolution of pituitary failure is ­apparent. Secretion of LH and FSH are usually affected first, followed sequentially by TSH and GH. The ACTH axis is most resilient and is generally the last to be affected. Prolactin deficiency is rare except as a component of Sheehan’s syndrome (postpartum pituitary necrosis). In contrast, hyperprolactinemia is far more common, and due to loss of production or effective transport and release dopamine, the hypothalamic prolactin-inhibiting hormone. Moderate hyperprolactinemia (±100 ng/mL) can occur in association with any structural lesion of the sellar region. Thus, its presence should not prompt a reflex diagnosis of PRL-producing adenoma. This is actually related to the fact that PRL secretion is under the inhibitory control of various hypothalamic “prolactin inhibitory factors,” of which dopamine is the most important. Processes that impair the hypothalamic release of dopamine (compressive or destructive hypothalamic lesions), or impair its adenohypophysial transfer (compressive or destructive lesions of the pituitary stalk), disinhibit PRL cells with resultant hyperprolactinemia. Also an important symptom associated with hypopituitarism is diabetes insipidus, a result of the diminished functional activity of the posterior lobe. It is intriguing that diabetes insipidus practically never occurs in patients with pituitary adenomas.

Although the most common cause of hypopituitarism is pituitary adenoma, other potential causes should be considered. These include: nonpituitary neoplasms of the sellar region, for example, craniopharyngiomas; metastases to the pituitary from carcinomas of the breast, lung, colon and prostate; vascular disorders, for example, pituitary apoplexy; inflammatory lesions, for example, lymphocytic hypophysitis, giant cell granuloma. sarcoidosis, (Erdheim–Chester disease); infectious diseases due to bacteria, mycobacteria fungi and rarely parasites, as well as idiopathic lesions, for example, Langerhans’ cell histiocytosis. In addition, posttraumatic dysfunction of the hypothalamic-pituitary axis, prior pituitary surgery or radiotherapy, and even genetic or familial abnormalities should be taken into consideration in the differential diagnosis of hypopituitarism [12, 13].

3.4 Classification of Pituitary Tumors Based on Imaging and Operative Findings

Given its strategic location at the skull base, information regarding pituitary tumor size, location, extension, and invasion is necessary when one wishes to draw conclusions impacting treatment and prognosis. Classification of pituitary adenomas on the basis of size and invasiveness is determined largely by imaging studies (magnetic resonance imaging, computed tomography, conventional radiography) as well as by intraoperative findings. By convention, pituitary tumors <1 cm in their greatest diameter are considered microadenomas whereas larger examples are termed microadenomas. In this regard, the radiologic classification of Hardy is easily applied and clinically useful [14]. It takes into consideration not only tumor size, but extension, configuration, and invasiveness as well. Microadenomas are designated grade 0 or grade I tumors, depending on whether the sellar configuration is normal or altered in a minor way. Macroadenomas causing diffuse sellar enlargement, focal destruction, or extensive erosion of the skull base are referred to as grade 11, 111, and IV, respectively. Macroadenomas are further subclassified on the basis of their extrasellar extensions, whether suprasellar, parasellar, inferior, or a combination of these.

3.5 Classification of Pituitary Tumors Based on Routine Hematoxylin&Eosin (H&E) Stain

Once a diagnosis of pituitary adenoma has been established on the basis of clinical, laboratory, and/or imaging findings, therapeutic decision-making begins. Options include surgical resection, receptor-mediated pharmacotherapy, and radiation treatment. Although some latitude exists for the initial use of medical treatment in selected endocrine-active pituitary adenomas (PRL, GH, and TSH-producing adenomas), given its rapid and consistent beneficial effects, surgery remains the initial therapy of choice in most instances. One important indication for surgery is the need of tissue for pathologic characterization. In the majority of cases the distinction of pituitary adenoma from nontumorous land can readily be made. This is of obvious importance, but can be challenging, given the small and fragmented nature of many specimens obtained transphenoidally. After exclusion of normal compressed or hyperplastic adenohypophysial tissue the nature of the adenoma has to be assessed [15–18].

Grossly, pituitary tumors are tan-gray to purple in color and creamy in texture, contrasting with the relative firmness of the normal gland. The latter accounts for the reluctance with which normal tissue can be smeared. The histologic growth pattern of pituitary adenomas varies, ranging from diffuse to sinusoidal or papillary due to a tendency to perivascular pseudorosette formation. The recognition of these histologic details is of importance only in view of the spectrum of sellar lesions that enter into the differential diagnosis of pituitary adenoma. Given the diversity of pathologic processes that clinically and radiologically masquerade as a primary tumor, an optimal specimen devoid of artifacts is essential. We avoid frozen sections and far prefer touch or smear preparations in which the distinctive cytologic monomorphism of adenoma usually permits a ready diagnosis.

The most important routine method, the gold standard for diagnosis of pituitary adenomas, is the H&E stain. On histologic sections as well, the most important characteristics of pituitary adenoma are cellular monomorphism and lack of acinar organization. In contrast, the cells of the normal adenohypophysis are organized in a delicate acinar pattern, each acinus consisting of an admixture of different cell types surrounded by well-defined reticulin-rich network of anastamosing capillaries with fenestrated endothelium. According to their staining properties, adenohypophysial cells have been traditionally classified in three categories corresponding to acidophilic, basophilic and chromophobic types [16, 18].

Pituitary adenomas are usually well demarcated and consist of compressed nontumorous adenohypophysis (by a pseudo-capsule) with condensed stroma. Unlike many benign tumors of other locations, pituitary adenomas have no “true” or fibrous capsule. Some histologically benign tumors have an indistinct border wherein clusters of adenoma cells extend into the adjacent nontumorous adenohypophysis.

The pale-staining posterior pituitary lobe is composed of nerve fibers, their expansion (Herring bodies) and terminations filled with neurosecretory material, and delicate, functionally specialized astrocytes (pituicytes).

In the H&E-stained sections, minor variations in normal pituitary anatomy can be seen. None are of clinical significance. Common among these variables is the so-called basophil invasion which consists of migration of basophilic adenohypophysial cells of PI origin into the posterior lobe. Accumulation of such cells increases with age and appears to begin at the interface of the anterior and posterior lobes and may be impressive in extent. Occasional examples mimic pituitary adenoma. Given the origin of the anterior pituitary from stomatodeum, the findings of remnants, usually on the upper surface of the posterior lobe, is not surprising. Microscopically, they resemble serous acini. Such rests may be the basis of rare salivary gland tumors of the sellar region. Rathke’s cleft remnants are frequently encountered as glands and cleft-like spaces at the interface of the anterior and posterior lobes. Cells comprising the wall of such structures may be cuboidal columnar, mucin-producing, or ciliated, or sometimes of adenohypophysial type. Progressive accumulation of secretions within such cysts gives rise to Rathke’s cleft cysts, either sizable and symptomatic or small and incidental autopsy findings. Intravascular hyaline bodies are on occasion seen in the capillaries of pituitary stalk wherein they appear as eosinophilic, cylindrical, hyaline bodies resembling intravascular thrombi. Lymphocytic foci are seen in somewhat over 10% of normal pituitaries usually between the anterior and posterior lobes (interlobar groove). Histologically, their extent pales in comparison with the often destructive infiltrate seen in lymphocytic hypophysitis.

On the basis of cytoplasmic staining affinity using the H&E method, pituitary tumors were once classified in three categories: acidophilic, basophilic, and chromophobic adenomas. Perhaps as a result of its simplicity and convenience, this approach to classification endured for decades. It is now obsolete, as much clinical and pathological overlap in functional tumor types occurs within these elementary categories. The scheme assumed that acidophilic adenomas were GH secreting and that basophilic adenomas producing ACTH-chromophobic lesions were hormonally inactive. With the emergence of new methodology, however, it became all too clear that the tinctorial characteristics of the cell cytoplasm correlate poorly with reliable cell type recognition, secretory activity, or cytogenesis. Thus, not all acidophilic tumors produce GH, nor are all GH-producing tumors acidophilic; some basophilic tumors do not cause Cushing’s s disease and more than half of chromophobic tumors are endocrinologically active, variously secreting GH, PRL, ACTH, TSH, LH/FSH, and/or α-subunit.

In addition to the H&E stain, silver stain for reticulin fibers and periodic acid-Schiff (PAS) technique aid in the identification of pituitary tumors, whereas the latter is perhaps most useful, as it shows not only positivity in ACTH adenomas and some glycoprotein hormone-producing tumors but also highlights basement membranes of the capillary network. On the other hand, silver stains show only lack of reticulin fibers in adenomas equated with lack of the acinar pattern, a classic diagnostic feature of adenomas. Silver stain is also preferred to the demonstration of pituitary hyperplasia, as its main morphological feature is the expansion of acini (Fig. 3.2).

Fig. 3.2

The Gordon-Sweet silver technique for reticulin staining is useful in distinguishing hyperplasia (a) from normal adenohypophysial tissue (b) or from adenomas (c). While normal adenohypophysis (b) displays a regular network of delicate reticulin fibers, hyperplasia (a) causes enlargement, distortion, and confluence of acini. In contrast, loss of acinar structure is evident in pituitary tumors (c). Original magnification ×100

3.6 Adenohypophysial Cell Hyperplasia

By definition, hyperplasia is a numerical, quantifiable increase of one or occasionally two cell types in response to physiologic demands. Attendant cytologic changes may also be seen. Only occasionally does neoplastic transformation supervene upon the hyperplastic process. Physiologic hyperplasia regularly affects the pituitary, the best example being hyperplasia of PRL cells in pregnancy and lactation. Several disease states are also accompanied by pituitary hyperplasia.

Pituitary hyperplasia is infrequent, not readily recognized, and often undiagnosed. The diagnostic difficulty is compounded by regional variation in the distribution of several pituitary cell types, inadequate or poor surgical specimens, and lack of precise diagnostic criteria for some forms of hyperplasia. From the morphologic point of view, three types of pituitary hyperplasia can be distinguished [19, 20]

Diffuse pituitary hyperplasia consists of a numerical increase of secretory cells without major alterations in cell morphology and acinar architecture on silver stain. When diffuse pituitary hyperplasia is marked, the acini maybe slightly but rather evenly enlarged without nodularity. When not pronounced, this morphologic type may be difficult or even impossible to recognize in fragmented specimens. Only tedious cell counts in large specimens or autopsy glands can confirm the presence of diffuse hyperplasia.

Focal pituitary hyperplasia represents a small, circumscribed accumulation of a single pituitary cell type. Such minute nodules are usually incidental findings in intact autopsy specimens. They have no apparent clinical basis and are of no significance in surgical pathology.

Nodular pituitary hyperplasia is a more advanced, widespread form of focal pituitary hyperplasia. Depending on the degree of cell proliferation, participating acini are variably enlarged and populated by an increased number of the affected cell type. If the changes are marked, focal disruption of the reticulin network and confluence of the acini take place. It is important to note that the hyperplastic mass is almost never monomorphous, other cell types being intermingled.

In general, pituitary cell hyperplasia involves cells of a single type. Only on occasion is more than one cell type affected simultaneously. The most common form of hyperplasia involves the PRL-producing cells. Not only is PRL cell hyperplasia seen in physiologic situations such as pregnancy and lactation, but it may also be associated with various pathologic processes, for example, as a component of stalk section effect, adjacent to occasional ACTH-producing adenomas, and in long- standing primary hypothyroidism where it results from the trophic effects of TRH. In contrast, GH cell hyperplasia is rare, occurring mainly as the result of an extrapituitary GH-RH-producing neuroendocrine tumor, for example, pancreatic islet cell tumor, pheochromocytoma, bronchial carcinoid, and so forth. Hyperplasia of TSH cells occurs exclusively in the context of long-standing primary hypothyroidism. LH/FSH cell hyperplasia is rare and difficult to recognize. It is well seen in patients with various forms of long-standing primary hypogonadism for example, Klinefelter’s and Turner’s syndromes. ACTH cell hyperplasia does occur but its importance as a cause of Cushing’s disease is still controversial. ACTH cell hyperplasia is a regular feature of untreated Addison disease and of CRH-producing extrapituitary tumors.

3.7 Classification of Pituitary Tumors Based on Their Immunohistochemical Assessment

The development of immunohistochemistry permits the conclusive identification of the various cell types in the adenohypophysis [21]. As a result, it was pivotal in the establishment of a functional classification of pituitary adenomas and in their ultra-structural characterization. Correlation with clinical features and endocrine activity also became possible. The standard immunohistochemical battery includes the use of antibodies to GH, PRL, ACTH, TSH, FSH, LH, and the α-subunit of the glycoprotein hormones. Based on immunohistochemistry, five different cell types producing six adenohypophysial hormones became recognized. Of the five known anterior lobe cell types, two – somatotrophs (GH cells) and lactotrophs (PRL cells) – belong to the “acidophilic series,” whereas the three other cell types, corticotrophs (ACTH cells) and other derivatives of the POMC-producing cell line, thyrotrophs (TSH cells), and gonadotrophs (FSH and/or LH cells), belong to the “basophilic series.” The anatomical regional distribution of the various cell types varies within the gland, making it difficult to quantitate cell numbers based on the examination of small tissue fragments. Somatotrophs comprise approximately 50% of adenohypophysial cells and are located mainly in the “lateral wings” of the PD. Somatotroph adenomas generally arise at this site. Lactotroph represent 10–25% of adenohypophysial cells and are maximally concentrated in the posterior aspect of the lateral wing just anterior to the neural lobe. Most lactotroph adenomas originate in this area. Corticotrophs represent adenomas hypophysial cells, the majority of which reside within the central or “mucoid wedge.” This is the usual site for functioning, corticotroph adenomas. Corticotrophs in the region of Rathke’s cleft and in the posterior lobe (see basophil invasion above) presumably give rise to nonfunctioning or “silent” corticotroph cell adenomas. Thyrotrophs, accounting for fewer than 5% of all adenohypophysial cells, occupy a small zone in the anteromedial region of the central wedge. Although thyrotroph adenomas are seldom discovered while still microadenomas, most originate at this site. Gonadotrophs are widely distributed throughout the PD, having no favored site of accumulation. As such gonadotroph adenomas do not have a predictable site of origin

Hormone immunohistochemistry aside, great efforts have been made to determine whether pituitary adenomas could be ascribed to a generic immunophenotype that would reliably distinguish specific adenoma types. It is now clear that the demonstration of immunoreactivity for pituitary hormones is the simplest diagnostic method of doing so, particularly in clinically nonfunctioning adenomas. For the basic diagnosis of pituitary adenoma, histology and immunohistochemistry at the light microscopic level correlate optimally with clinical imaging, and operative findings. However, the use of transmission electron microscopy is essential to classify pituitary tumors precisely, and to determine their cytogenesis, degree of differentiation, and cellular makeup (Fig. 3.3).

Fig. 3.3

In some adenoma types immunostaining for specific adenohypophysial hormones may not be sufficient for conclusive diagnosis; it may even be misleading. In such cases electron microscopy is mandatory for correct diagnosis. (a) Immunoreactivity for PRL in a prolactin cell adenoma showing the specific Golgi immunostaining pattern. (b) Silent subtype 3 adenoma may exhibit immunostaining for prolactin in scattered tumor cells. However, the positivity is noted over the entire cytoplasm. (c) Corticotroph adenoma (see also Fig. 3.4) showing intense ACTH immunostaining in the cytoplasm of adenoma cells. (d) In gonadtroph adenomas of the female type (see also Fig. 3.5) a light, scattered immunostaining for ACTH may be apparent. Original magnification ×400

3.8 Classification of Pituitary Tumors Based on Ultrastructure

Although this approach is time consuming, expensive, and requires considerable expertise, electron microscopy provides valuable information regarding the cellular composition cytogenesis and secretory activity of a tumor. Using transmission electron microscopy, pituitary adenomas can be distinguished from non-neoplastic lesions and from tumors of nonadenohypophysial origin [16, 18, 20, 22, 23]. A shortcoming of ultrastructural investigation relates to small sample size, which introduces the possibility of “sampling error.”

3.9 GH-Producing Adenoma

GH excess manifests in two clinically related phenotypes. The first and more common of the two is acromegaly the result of sustained GH excess that begins or persists after puberty [1, 16, 18, 24]. When GH excess manifests prior to epiphysial closure, the result is excessive linear growth or gigantism. Despite the multisystem nature of GH excess and the often dramatic physical transformation is produced, this disorder is seldom diagnosed at an early stage. Thus, pituitary adenomas underlying acromegaly or gigantism have generally progressed to the macroadenoma stage at diagnosis. Pituitary tumors associated with hypersecretion of GH are heterogeneous and can be separated into five distinct adenoma types showing differences in incidence, immunohistochemical profile, ultrastructural morphology, and biologic behavior. Of the five types, two are monomorphous GH cell adenomas composed of either densely or sparsely granulated GH cells. The remainder are plurihormonal tumors , that include mammosomatotroph adenoma, mixed GH-PRL cell adenoma, and acidophil stem cell adenoma. The latter are discussed in a separate section below.

3.10 Densely Granulated GH Cell Adenoma

These tumors comprise what has been termed the “classic acidophilic adenoma of acromegaly.” It accounts for approximately 8% of all pituitary adenomas and is characterized by a relatively slow growth rate, limited invasiveness, and an overall indolent biologic. Strong uniform cytoplasmic immunoreactivity for GH is evident in most adenoma cells. Reactivity may also be seen for PRL, (α-subunit, and/or TSH). Ultrastructural analysis shows this tumor to consist of uniform, polyhedral, or elongate cells a predominantly spherical or ovoid nuclei. The adenoma cells contain a full complement of cytoplasmic organelles including well-developed Golgi and rough endoplasmic reticulum (RER). The most prominent ultrastructural feature of this tumor is abundance of mature, GH-containing cytoplasmic secretory granules measuring 150–600 nm (mainly 400–500 nm) in diameter.

3.11 Sparsely Granulated GH-Cell Adenoma

This tumor corresponds to the chromophobic variant of somatotroph adenoma. Slightly more common than the acidophilic form, it is more prevalent in women and is known to be more aggressive, more rapidly growing and less responsive to somatostatin analog treatment [25]. Immunoreactivity for GH is often limited to the Golgi zone, whereas positivity in the rest of the cytoplasm is but moderate to weak. Ultrastructural features of this tumor include scant secretory granules measuring 100–200 nm. The most distinctive feature of its cells is the presence of a so-called fibrous body, which, composed of ail admixture of intermediate (cytokeratin) filaments and smooth endoplasmic reticulum (SER), is located in the Golgi region, and often indents the nucleus [26].

In GH cell adenomas treated with long-acting somatostatin analogs, mild cell shrinkage, accumulation of lysosomes, and interstitial as well as perivascular fibrosis can often be seen. These alterations are inconsistently present and are usually not marked.

3.12 PRL Cell Adenomas

This most frequent form of pituitary adenomas is also the most common primary tumor affecting the pituitary. Its clinical presentation relates either to the hormonal consequences of hyperprolactinemia or, particularly in postmenopausal women and in males, to neurological symptoms due to significant tumor size. The principal endocrine features of hyperprolactinemia include amenorrhea, galactorrhea, and infertility in women and decreased libido and impotence in men [16, 18, 27].

PRL cell I adenomas are either chromophobic or amphophilic with a sizable pale Golgi zone. Distinctive psammomatous calcification is seen in a minority of tumors. Production of “endocrine amyloid” may also be encountered. Immunohistochemistry shows prolactinomas to be monohormonal tumors containing only immunoreactive PRL. Most show a characteristic paranuclear pattern of PRL immunopositivity corresponding to the conspicuous Golgi region. Diffuse cytoplasmic immunostaining for PRL is a feature only in a small minority of cases. Thus, two ultrastructural types of PRL cell adenoma are recognized.

3.13 Sparsely Granulated PRL Cell Adenoma

This is the most frequent tumor type (Fig. 3.4). Its cells have the same striking appearance of hormonal activity as nontumorous PRL cells, abundant RER often in large concentric whorls, and prominence of the Golgi apparatus. The latter often contains pleomorphic, immature secretory granules. Secterory granules are generally sparse, measuring 120–300 nm. The ultrastructural hallmark of sparsely granulated PRL cell adenoma is the presence of granule exocytosis, the extrusion of secretory granules. These are often “misplaced exocytosis,” taking place at the lateral cell surfaces, far from the vascular pole of the cell.

Fig. 3.4

Corticotroph adenoma. The cytoplasm contains numerous secretory granules displaying characteristic morphology. Original magnification ×11,760

3.14 Densely Granulated PRL Cell Adenomas

These are rare and may be associated with short-term dopamine agonist therapy. Otherwise, they share identical clinical, biochemical, and prognostic profiles with sparsely granulated variant. Densely granulated PR cell adenomas contain abundant cytoplasmic secretory granules, thus their acidophilic appearance on H&E stain and diffuse use cytoplasmic PRL immunopositivty. Secterory granule are spherical oval to irregular in configuration and both larger (600 nm) and more numerous than those of the sparsely granulated variant.

The decreasing prevalence of PRL cell adenomas in surgical material is attributed to a major shift in management of these tumors from surgical toward medical therapy with dopamine agonists, such as bromocriptine or pergolide. Such medical treatment results in a striking morphologic change. In contrast to the uniform morphology of untreated tumors, PRL cell adenomas exposed to dopaminergic agonists display smaller cells in which PRL immunopositivity is scant or barely detectable. With protracted treatment, a decrease in cytoplasmic volume results in a “small cell” appearance, and marked perivascular and interstitial fibrosis. By electron microscopy, the tumor consists of small cells with markedly heterochromatic multiply indented nuclei, and a narrow rim of cytoplasm possessing few membranous organelles, scattered lysosomes, and only few randomly distributed secretory granules. Some rumors contain a mixed population of suppressed cells and cells displaying varying degrees of endocrine activity, a feature of nonuniform involution. Cessation of treatment brings about a reversal of these changes.

3.15 Acth Cell Adenomas

The majority of corticotroph adenomas are basophilic and display strong positivity with the PAS method. Immunohistochemistry demonstrates the presence of ACTH and other POMC-related peptides in the cytoplasm of adenoma cells is [16, 18, 28]. Corticotroph adenomas are most often monomorphous and monohormonal. Rarely, however, they exhibit immunopositivity for α-subunit, LH, or PRL. Typical corticotroph adenomas are associated with signs and symptoms of corticosteroid excess (Cushing’s disease), that is, moonlike facies, acne, hirsutism, truncal obesity, abdominal striae, easy bruising, mood changes, hypertension, osteoporosis, insulin resistance, diabetes mellitus, and muscle weakness. Such tumors exhibit a marked female preponderance. Only about half of the adenomas in Cushing’s disease are detectable by imaging procedures: the remainder are very small. Macroadenomas are uncommon in Cushing’s disease and are usually invasive and difficult to cure. The same is true of a subset of Cushing’s adenomas that were treated by adrenalectomy (Nelson’s syndrome). Such tumors may have been radiographically undetectable at presentation or aggressive sizable adenomas from the start. In any event, unlike the tumors of Cushing’s disease, those of Nelson’s syndrome are usually invasive macroadenomas associated with hyperpigmentation (melanocyte-stimulating hormone effect), visual field defects, and headaches. A large proportion of pituitary carcinomas have their origin in Nelson’s syndrome.

Although the histologic and immunohistochemical appearance of tumors associated with Nelson’s syndrome is similar to the previously described adenomas of Cushing’s disease, they do show slightly different ultrastructural features. In corticotroph adenomas associated with Cushing’s disease the adenoma cells are elongated or angular with ovoid nuclei showing occasionally indentations. The cytoplasm is abundant and contains prominent RER, free ribosomes, and polysomes, as well as a conspicuous Golgi complex. Secretory granules measuring 150–450 nm are numerous and exhibit highly characteristic morphology, being spherical, teardrop, or heart shaped and showing variable electron density (Fig. 3.5). The other characteristic ultrastructural marker is bundles of keratin immunoreactive intermediate filaments disposed around the nucleus [2, 29, 30]. Excessive accumulation of these filaments, a phenomenon referred to as Crooke’s hyalinization. Usually occurs in surrounding nontumorous corticotroph cells. On occasion, the adenoma cells may show Crooke’s change, filaments occupying large areas of the cytoplasm displacing organelles and secretory granules to the cell periphery. Tumors composed of Crooke cells are termed Crooke’s cell adenomas. They are typically Cushing’s disease-associated, often invasive and recur more frequently than other ACTH cell adenomas. In Nelson’s syndrome, the electron microscopic features of the tumor cells are similar to those seen in Cushing’s disease, but with one important exception – that the high levels of cortisol are not a feature of Nelson’s syndrome, and because Crooke’s hyaline change is the negative feedback effect of elevated glucocorticoid levels, intermediate filaments lack in Nelson’s adenomas.

Fig. 3.5

Gonadotroph adenomas of the female type comprise polar cells with long processes containing most of the small secretory granules. The Golgi complex shows vacuolar transformation (honeycomb Golgi). Original magnification ×13,640

3.16 TSH Cell Adenoma

This tumor is rare, representing only about I % of all pituitary adenomas [31–33]. Clinically, most TSH cell adenomas present with the signs and symptoms of hyperthyroidism. The thyroid gland is diffusely enlarged. The diagnostic hallmark is the presence of an in appropriately high TSH level in the presence of elevated peripheral thyroid hormone concentrations. A minority of tumors occur in the setting of hypothyroidism. From the histopathologic standpoint, the diagnosis of thyrotroph adenomas is often difficult due to their variable morphology. Generally, they are composed of chromophobic angular-shaped cells disposed in a sinusoidal or diffuse pattern. Interstitial and perivascular fibrosis may be conspicuous in some cases. By immunohistochemistry, the cells are positive for TSH and often α-subunit. A minority of thyrotroph adenomas also show variable reactivity for GH and/or PRL. At the ultrastructural level, the TSH cells are spindle-shaped and possess long cytoplasmic processes as well as spherical to ovoid nuclei, often with prominent nucleoli. The cytoplasm contains moderately developed organelles and small (50–200 nm) secretory granules peripherally situated beneath the cell membrane.

3.17 FSH/LH Cell Adenoma

Gonadotroph adenomas may be associated with increased serum levels of FSH, LH, and/or u-subunit, but the majority of patients have gonadotropin levels within normal limits for their age. Most tumors occur in middle and older age, men being more often affected. Even if a tumor is hormonally active, gonadotropin excess does not result in clinical hyperfunction. Thus, patients with this adenoma typically present with hypogonadism and symptoms of mass effect, mainly visual disturbance and hypopituitarism [16, 18, 34, 35].

Histologically, gonadotroph adenomas are chromophobic tumors featuring pseudorosettes and papillae. Microcysts may also be evident. PAS stains may highlight the presence of secretory granules beneath the cell membrane. Differences in the pattern of immunostaining may be seen in men and women. Adenomas of men are more likely to demonstrate immunoreactivity for FSH and/or LH, staining being variable and often unevenly distributed. In contrast, tumors in women are often poorly immunoreactive, some showing scant if any staining for gonadotropins. Among pituitary adenomas, gonadotropic tumors are the only ones exhibiting sex-linked differences in ultrastructural appearance. The so-called “male type” possesses slightly dilated RER a prominent Golgi complex with sparse, small (200 nm) secretory granules, and, in 50% of cases, vary in degrees of oncocytic changes. In contrast, gonadotroph adenomas of the “female type” feature a unique morphological marker, the so-called “honeycomb Golgi complex” in which the sacculi transform into clusters of spheres containing a low-density proteinaceous substance.

3.18 Silent Adenomas

The term “silent adenoma” has been applied to three clinically nonfunctioning tumors, each morphologically distinct from null cell adenomas. Unlike the latter, silent adenomas consist of cells often showing well-defined immunoreactivity for hormones, most frequently ACTH. In contrast, null cell adenomas are immunonegative or contain only few cells that are immunopositive for FSH/LH and/or (a-subunit. Two of the three silent adenomas show morphologic resemblance to the corticotroph adenomas of Cushing’s disease, whereas null cell adenomas reflect endocrine differentiation, but show no markers of any specific pituitary cell type. At present, aside from immunohistochemistry, electron microscopy is required for the conclusive identification of the silent adenomas, particularly those of subtype 3 [16, 18, 36, 37].

3.18.1 Silent “Corticotroph” Adenoma Subtype I

Morphologically these tumors are indistinguishable from the adenomas of Cushing’s disease. The amphophilic, PAS-positive tumor cells are immunoreactive for ACTH and other POMC-related peptides. Ultrastructurally, there are similarly no differences between the two lesions. However, an unusual and unexplained characteristic of silent corticotroph adenoma subtype I is the frequent occurrence of hemorrhage and infarction. Recent findings point to an origin of this tumor from PI-derived POMC cells, the function of which is still unknown.

3.18.2 Silent “Corticotroph” Adenoma Subtype 2

This primarily affects men. Histologically, most are chromo-phobic and, in contrast to subtype I “corticotroph” adenomas, show only mild, patchy PAS-staining and ACTH immunoreactivity. Positivity for β-endorphin is often stronger. Ultrastructurally, the tumor is less obviously corticotropic. Its cells are polyhedral without polarity, contain secretory granules smaller (200–350 nm) than those of ACTH-secreting and silent sub-type I adenomas, and lack intermediate filaments. On the other hand, the secretory granules are similar to those of Cushing’s and silent subtype 1 adenomas.

3.18.3 Silent Adenoma Subtype 3

This intriguing tumor type is a nosologic enigma. Its clinical presentation and morphologic features have been well characterized; yet the issue of histogenesis remains to be settled. It was originally thought to be related to the two previously discussed silent adenomas based on variable but usually scant immunoreactivity for ACTH and other POMC-related peptides in some examples. Most tumors are immunonegative for ACTH. More often, one sees scattered immunoreactivity for GH, PRL, and α-subunit. Lastly, some tumors are entirely immunoreactive. The ultrastructure of silent adenoma subtype 3 is complex. It is composed of large, polar cells, the cytoplasm of which contains RER, often copious amounts of SER, and a very well developed Golgi apparatus. Secretory granules vary in number. Measuring about 200 nm, they often collect at one pole of the cytoplasm, as is the case with well-differentiated glycoprotein hormone-producing cells. Based on these ultrastructural features, the tumors seem to be actively secreting, but what is being produced remains to be determined. In view of their variable, confusing immunophenotype, the diagnosis requires ultrastructural confirmation.

3.19 Null Cell Adenomas

Null cell adenomas are mainly found in adults, particularly the oncocytic variant. The term “null” signifies the lack or paucity of morphological, especially ultrastructural markers that would indicate either a cell of origin or a direction of differentiation [1, 16, 18]. Histologically, these tumors vary from chromophobic to eosinophilic and granular (oncocytic) and exhibit either a diffuse pattern or pseudorosette formation. Immunostains are often negative or show only scattered positivity for one or more hormones, usually combinations of FSH, LH, or α-subunit. On occasion, scattered cells even show immunoreactivity for GH, PRL, or ACTH. Although null cell adenomas may be immunonegative for hormones and lack function, endocrine differentiation is evident as reactivity for neuron-specific enolase, chromogranin, and/or synaptophysin. At the ultrastructural level, null cell adenomas vary. Chromophobic tumors are composed of cells with small quantities of cytoplasm containing poorly developed RER and Golgi as well as scant, small (100–250 nm) secretory granules. The cells of the oncocytic variant are larger. Their sole ultrastructural characteristic is the excessive mitochondrial accumulation. Despite marked mitochondrial abundance, the same RER and Golgi as well as secretory granules are always evident. In those tumors containing somewhat more differentiated cells, these usually show features of glycoprotein hormone-producing cells. This is not surprising because from the histologic, immunohistochemical and ultrastructural aspects, an apparent overlap exists between null cell adenomas – oncocytomas and gonadotroph adenoma, being difficult to draw the line between the two entities in many cases.

3.20 The Contribution of Molecular and Genetic Techniques to the Study of Pituitary Tumors

Several novel techniques have recently been introduced to analyze the molecular and genetic aspects of pituitary tumors [38–40]. These have advanced our understanding of molecular pathogenesis of these lesions. The development of pituitary adenomas appears to be a multistep, multicausal process involving tumor initiation followed by tumor promotion. Only the most relevant findings are reviewed in the following paragraphs. These aspects of pituitary development and tumors are covered in Chaps. 4 and 5.

3.21 Clonal Origin of Pituitary Tumors

A fundamental and still controversial issue related to pituitary tumorigenesis is the question of whether neoplastic transformation of adenohypophysial cells is due to hypothalamic dysfunction or simply the result of an acquired mutation of a single cell. Using the allelic X-chromosome inactivation analysis, several laboratories have confirmed the monoclonal composition of virtually all pituitary adenomas. Thus pituitary adenomas are considered monoclonal expansions of a single somatically mutated and transformed cell [41, 42].

3.22 Hypothalamic Factors and Pituitary Tumors

Despite having demonstrated the clonality of most if not all pituitary tumors, a contribution of hypothalamic hormones to pituitary tumorigenesis has been considered [43]. For good reason there is renewed interest in integrating their role in the current multi-step monoclonal model. For example, it has been demonstrated that the abnormal activity of ­hypothalamic hypophysiotrophic hormones, in either the form of excess stimulation or deficient inhibition, may contribute to the genesis and/or progression of pituitary tumors. It has also been shown that somatotroph hyperplasia of long-standing duration can undergo adenomatous transformation. High-level ectopic GH-RH production in patients with GH-RH-producing extrapituitary tumors also results in somatotroph hyperplasia followed in some cases by adenoma formation [44, 45]. Animal models also provide support for the notion. For example, rats transgenic for GH-RH develop somatotroph hyperplasia and subsequently pituitary adenoma. It has also been shown that the dopamine receptor (D2) knockout rodents develop PRL-producing pituitary adenomas [46, 47, 68].

3.23 Endocrine Factors

Both experimental studies and clinical investigations have provided evidence that endocrine abnormalities may predispose, promote, or even induce the development of pituitary adenomas [1, 48]. For example, thyrotroph adenomas are known to develop in patients with long-standing primary hypothyroidism as are corticotroph adenomas in untreated Addison’s disease. It is also known that the protracted estrogen stimulation contributes to transformation and/or neoplastic progression of PRL cell adenomas in the rodent and human pituitaries.

3.24 Genomic Alterations in Pituitary Adenomas

Since it became clear that somatic mutation(s) in a single adenohypophysial cells is the event requisite to pituitary tumorigenesis, vigorous attempts have been made to identify and characterize the responsible mutations [49–51]. Activating mutations of two oncogenes, GSPT1 and H-ras, have been found in human pituitary adenomas. In addition, H-ras and c-myc oncogenes as well as mutations of p53, nm23 and Rb genes, have been identified disproportionately more often in aggressive tumors. For example, mutaion of the Rb gene has been seen in pituitary carcinomas. These observations provide evidence that amplification of oncogenes (H-ras and c-myc) and inactivation of tumor suppressor genes (p53, nm23, and Rb) may play a role in initiation and/or tumor progression. The recent application of microarray technology has shown large number of genes in pituitary tumors to be abnormal.

3.25 Plurihornomanility

The development of light microscopic immunohistochemistry and its ancillary techniques, such as double immunostaining and immunoelectron microscopy, challenged and negated the long-accepted “one cell-one hormone” theory, as they showed plurihormonality to be a common occurrence in both normal and neoplastic pituitary cells (Fig. 3.6) [52, 53]. Although the presence of more than one hormone in the same cell was initially hard to explain, modern studies showed that precursor cells can differentiate toward the spectrum of cell types that populate the adult adenohypophysis. Current evidence suggests that corticotrophs arise as a lineage distinct from that of the other pituitary cell types. The cells belonging to other lines, for example, somatotrophs, lactotrophs, thyrotrophs, and gonadotrophs, appear to be related in that they utilize common transcription factors. This is especially true for somatotrophs and lactotrophs, because, in contrast to other cell types that function independently, lactotrophs have a strong dependence on somatotrophs. Several different transcription factors regulating the transformation of primordial pituitary cells to mature secretory cells have been identified. These include Rpx/Hes1, Pitx1, Ptx2, Lhx3/LIM3/P-lim, Prop-1 and Pit-1/GH factor 1 [54, 55].

Fig. 3.6

Double-immunogold method demonstrates colocalization of GH (immunolabeling with 10-nm gold particles) and PRL (immunolabeling with 20-nm gold particles) in the same secretory granules of mammosomatotroph in a bihormonal tumor. Original magnification ×31,160

It is not clear whether plurihormonal cells occur more ­frequently in adenomatous or in normal, nontumorous pituitaries [56]. Under physiological conditions the presence of plurihormonal cells can be related to the phenomenon of “transdifferentiation,” which involves reversible transformation of one cell type to another. In neoplasms, mutation or gene deletion may lead to the development of new immunohistochemical or ultrastructural phenotypes, thus accounting for cell heterogeneity [57–59].

Plurihormonal pituitary adenomas may be monomorphous, that is, composed of a distinct morphologic cell type, which nonetheless secretes more than a single hormone. Yet other tumors consists of two or more morphologically different cell types. For example, several plurihormonal adenomas associated with acromegaly produce GH and one or more glycoprotein hormones, primarily α-subunit [60, 61]. These patients have acromegaly, elevated serum GH and insulin-like growth factor (IGF)-I serum levels. Immunohistochemistry demonstrates cells producing GH and α-subunit, less often TSH FSH, and/or LH. By electron microscopy, the appearance of tumors is chiefly monomorphous, similar to that of densely granulated somatotroph adenomas [1, 16, 18].

3.26 Mixed Somatotroph-Lactotroph Adenomas

This tumor is most commonly composed of densely granulated somatotrophs and sparsely granulated lactotrophs. On hematoxylin and eosin stained sections, it consists of acidophilic cells interspersed with chromophobic cells. By immunohistochemistry GH and PRL are demonstrated in different cell populations. Electron microscopy documents the bimorphous nature of the tumors.

3.27 Acidophil Stem Cell Adenoma

These rare, hyperprolactinemia-associated tumors tend to grow rapidly in young individuals. They are chromophobic or slightly acidophilic, immunohistochemically reactive for PRL and to a lesser extent GH, but monomorphous. In some cases, GH immunoreactivity may not be apparent. Ultrastructurally, acidophil stem cell adenomas are monomorphous but demonstrate both lactotroph and somatotroph markers, that is, granule extrusions and fibrous bodies. The tumors may be oncocytic, even in young patients, and display a unique and diagnostic from of giant mitochondria. The sparse, randomly distributed secretory granules are small (50–200 nm) [62].

3.28 Mammosomatotroph Adenoma

Morphologically similar to densely granulated somatotroph adenomas, the tumor is monomorphous in cellular makeup and strongly acidophilic. Immunohistochemistry shows reactivity for both GH and PRL within the same cells. Staining for PRL is variable and many tumors alsocontain α-subunit. The diagnosis is confirmed by electron microscopy.

3.29 Cell Proliferation Markers

Several cell proliferation markers including proliferative cell nuclear antigen (PCNA) MIB-1 I (Ki-67), p-27, cyclins, topoisomerase II-α, AGNOR (argyropilic nuclear organization region), and BrdUrd (bromodeoxyuridine) can be used to document kinetic abnormalities that play a role in tumor progression (63–65). As detected by the MIB-1 I antibody, Ki-67 expression is a useful marker of proliferative activity, invasiveness, and prognosis in a variety of tumor systems. Although many pituitary tumors show a slow rate of growth, others enlarge more rapidly and invade the neighboring tissue. Only rare examples give rise to distant cerebrospinal and/or systemic metastases (pituitary carcinomas). The prognostic value of cell proliferation markers in pituitary tumors has been confirmed in several studies show in a correlation between high labeling indices and aggressive behavior. Particularly high MIB-1 and PCNA labeling is seen in metastases (pituitary carcinoma) as well as in their respective primary tumors (Fig. 3.7). Measurements of microvessel density show increased angiogenesis in various types of malignant tumors. Although microvessel density is lower in pituitary adenomas than in the nontumorous gland, pituitary carcinomas have increased the microvessel density. An emerging marker is MGMT (O6-methylguanine-methyltransferase) whose lack of staining in pituitary tumors predicts responsiveness to treatment with temozolomide [66]

Fig. 3.7

Although pituitary tumors generally exhibit a slow growth rate previous studies have shown a relationsionship between the expression of cell-proliferation markers and aggressive tumor behavior. (a) Pituitay tumors showing a low (<1%) MIB-1 labeling index. (b) Pituitary tumor showing high (>7%) MIB-1 labeling index. Original magnification ×250

3.30 Atypical Pituitary Neoplasms and Pituitary Carcinomas

The diagnosis of pituitary carcinoma presents a challenge. Based on the current WHO classification of pituitary neoplasms, documentation of metastasis is required for diagnosis of pituitary carcinoma. However, tumors demonstrating invasive growth, increased mitotic index, Ki-67 labeling index ≥ 3% and extensive nuclear reactivity for p53 are termed “atypical pituitary neoplasms” [8, 9]. Pituitary carcinomas are very rare and are defined as primary neoplasms of the adenohypophysis that undergo craniospinal and/or systemic spread [4–7]. Brain invasion, a feature evident only at autopsy, is also an indicator of malignancy. The pathogenesis of pituitary carcinoma is controversial. For example, it is unclear whether carcinomas develop from adenomas or arise de novo from the endocrinologic standpoint; pituitary carcinomas are more often hormone-secreting than nonfunctioning tumors. Among functioning carcinomas, the most common types are PRL- or ACTH- producing. GH-, TSH-, and FSH/LH-producing tumors are very rare. Metastatic involvement of the central nervous system is more often craniospinal leptomeningeal than parenchymal. Favored sites of systemic spread include liver, lung, bone, and lymph nodes.

Morphologically the histopathology of pituitary carcinomas varies. In some cases, the histology is indistinguishable from that of benign adenomas. Most, however, show increased numbers of mitotic figures, nuclear atypia, hyperchromasia, pleomorphism, nucleolar prominence, and necrosis (Fig. 3.8). Cellular atypia is usually more conspicuous in the metastases than in the primary tumors. Immunohistochemistry shows the same degree of reactivity for hormones as in adenomas. In most cases, significantly increased MIB-1 labeling and microvessel density, as well as increased p53 protooncogene expression, are noted. As noted earlier, ras mutations can be seen in PRL cell carcinomas.

Fig. 3.8

Pituitary carcinoma showing nuclear and cellular atypical. Original magnification ×250