The development, topographical relations and innervation of the epiphysis cerebri in the albino rat
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- Kappers, J.A. Zeitschrift für Zellforschung (1960) 52: 163. doi:10.1007/BF00338980
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The anlage of the epiphysis first appears on the 14th day of embryonic development.
The epiphysis develops by follicular proliferation of the wall of the epiphyseal evagination. In the rat, especially the rostro-dorsal part of this wall is involved in the process.
Leptomeningeal mesenchyme containing vessels grows in between the pineal follicles. The adult organ shows a dense vascular network. It is a compact structure consisting of cellnests the follicular structure of which can be still distinguished at many places.
In embryos, aged 14 days, fibres could be clearly demonstrated in the caudal commissure. The first habenular commissural fibres were shown in embryos of the 16th day.
During development the true pineal recess of the third ventricle disappears by fusing of the epiphyseal peduncles. In the adult, a small recess bordered by the intercalary lamina is sometimes present between the habenular and the caudal commissure. This recess of the third ventricle has been termed intercommissural recess.
During early postnatal development the pineal body gradually shifts in a caudal and somewhat dorsal direction. In the adult brain it shows a superficial position just in front of the cerebellum between the caudal collicles. Its lateral parts are sometimes covered by the caudal poles of the cerebral hemispheres.
The epiphysis is enveloped by a sheath of leptomeningeal origin. The floor of the confluens sinuum covers the dorsal surface of the organ.
The pineal body keeps its connexion with the commissural region by means of a thin and slender avascular epiphyseal stalk consisting of pinealocytes, pinealoblasts and fibrocytes. This stalk can be divided topographically in three parts. Dorsal to it either two veins, vena prosencephali mediana and vena cerebri magna, are present or a single large vena cerebri magna. These veins discharge into the confluens sinuum. The vena prosencephali mediana may fuse either phylogenetically or ontogenetically with the vena cerebri magna originally present. This, then, results in the formation of a single large vena cerebri magna draining, a. o., the choroid plexuses of the third as well as those of both lateral ventricles and the basal veins.
In the epiphyseal stalk nerve fibres could be constantly demonstrated. Most of the fibres derive from the habenular commissure the contribution of fibres from the caudal commissure being only small. The nerve bundle in the epiphyseal stalk has been termed commissuro-epiphyseal tract.
The fibres of the commissuro-epiphyseal tract are aberrant commissural fibres. They are of no functional significance for the innervation of the pinealocytes. They may leave the stalk or the rostral part of the epiphysis passing into the surrounding leptomeningeal tissue. Other fibres, after having entered the pineal organ, perform loops of 180° returning to the commissural region in the epiphyseal stalk. In one case an ending of a commissuro-epiphyseal fibre was seen. This, however, was most probably not a functional terminal. Very rarely, single commissuro-epiphyseal fibres were seen running as far as the caudal part of the organ.
The epiphysis shows an extensive autonomic innervation which is principally supplied by two nervi conarii, bilaterally present. These nerve fascicles course in the tentorium cerebelli penetrating into the epiphysis underneath the floor of the confluens sinuum.
After removal of both superior cervical ganglia the fibres of the nervi conarii degenerate evidently being postganglionic fibres originating in these ganglia.
The terminal autonomic pineal innervation happens by means of interfollicular strands of fibres from which thin single fibres or very small fibre bundles were seen branching and penetrating the follicles or cellnests of pinealocytes, thus running intrafollicularly.
Structures being most probably autonomic motor terminals have been observed. They are exclusively related to the pinealocytes, not to the vascular walls.
Among the interfollicular fibre strands interstitial cells have been observed. The present author is inclined to share the opinion of investigators holding that these cells are not of a nervous nature but belong to the same category of cells as the lemnocytes do.
After removal of both superior cervical ganglia the terminal intrapineal autonomic innervation was seen to disappear completely. In very few preparations only some rare fibres were left. Evidently, pineal innervation is mainly, if not exclusively, orthosympathetic.
On the ground of the observations mentioned as well as of the results of recent electronmicroscopical investigations it has been concluded that the interas well as the intrafollicular fibres and fibre strands consist of the thin terminal ramifications of postganglionic nerve fibres originating in the superior cervical ganglia. Proof of the existence of a sensory innervation of the epiphysis has not been found so far. The opinion of some authors, holding that, in general, the terminal autonomic “neurofibrillar network” would be formed by the interstitial cells presumably being synaptically connected with postganglionic fibres is discussed and considered improbable.
In the rat, so far, neither nervous nor vascular relations have been observed pointing to the existence of an „epithalamo-epiphyseal-“ or „habenulo-epiphyseal complex“ that, in any way, could be compared with the hypothalamo-hypophyseal complex.