The skeletal system refers to all structures, both rigid and semirigid, which provide structural foundation, support the body’s soft tissues, protect internal organs, and provide necessary leverage for muscle function.
Skeletal systems may vary widely between species, but all serve to provide both functional structure and protection from external injury. For most insects, crustaceans, and other invertebrates, this structure is the outermost part of the body and is called an exoskeleton. In vertebrates, this structure is internal and is called an endoskeleton. Vertebrate skeletons are a group of relatively rigid structures composed of a combination of connectives tissues generally classified as bone and cartilage (Encyclopedia Britannica 2018). This chapter will focus on the development, function, and anatomical specifics of the vertebrate skeletal system.
Skeletal System Development
The skeletal system is comprised of mainly two types of connective tissue: bone and cartilage. Bones are a specialized form of connective tissue with a mineralized matrix and are generally considered the organs of the skeletal system. Their mineralized matrix, composed of hydroxyapatite crystals, results in the extremely hard structure of bones that allow them to provide support and protection (Ross and Pawlina 2006, p. 202).
Because bones tend to vary widely in terms of size, shape, structure, and even function, there are a number of ways in which bones can be classified. One classification system involves the structural arrangement of the bone tissue. Cancellous, or “spongy bone,” refers to the latticelike trabeculae that form the innermost part of bones (Ross and Pawlina 2006, p. 203). In contrast, “compact bone” refers to the solid, dense layer that forms on the outside of bones. The periosteum is a sheet of fibrous connective tissue that surrounds the outside of the bone (Zalisko and Kardong 2001, p. 40). Bones are also often characterized according to their shape. Long bones are elongated in one direction when compared to the other and are generally composed of a shaft and two ends. An example of a long bone would be a femur. Short bones are generally described as having a diameter that is equal in all dimensions, like the carpal bones of many species. Flat bones, like the sternum, are generally thin with a small layer of spongy bone sandwiched between two outer layers of thick compact bone. The final shape category is referred to as irregular bones. These bones have shapes that do not fit with the other three categories, like vertebrae (Ross and Pawlina 2006, p. 204).
Bones can also be classified according to embryologic origin. Different portions of the skeleton are generated by different developmental processes. The axial skeleton is derived from somites, the appendicular skeleton is a product of the lateral plate mesoderm, and the craniofacial bones are generated by the cranial neural crest (Gilbert 2000). The formation of the bone is generally classified by one of two processes.
Bones can form through either (a) endochondral ossification or through (b) intramembranous ossification. Endochondral ossification relies on a cartilaginous template as a bone precursor, where intramembranous ossification is the direct conversion of mesenchymal tissue into bone tissue (Gilbert 2000). Generally, bones that are weight bearing, like bones of the appendicular skeleton or the vertebral bones, are formed by endochondral ossification. In contrast, intramembranous bones form directly from the mesenchyme without a cartilage precursor. Sesamoid bones, periosteal bones, and dermal bones all undergo intramembranous ossification. Bones like the patella, portions of the skull, the clavicle, and even the turtle shells are formed by the direct bone formation method of intramembranous ossification (Gilbert 2000). Though the two methods may differ in their initial stages, the preliminary bone laid down by both methods is quickly replaced by identical new bone (Ross and Pawlina 2006, p. 216).
In endochondral (endo, within; and chondral, cartilage) bone formation, cartilage is a specialized connective tissue that is composed of chondrocytes and a significant extracellular matrix. Chondrocytes are specialized cells within cartilage that both produce and maintain the extracellular matrix (Ross and Pawlina 2006, p. 184). The avascular character of cartilage means that there is no blood flow through any cartilaginous tissues. This lack of vascular channels, vital for the cartilage cells survival, accounts for the dependence of the chondrocytes on the extensive extracellular matrix (Ross and Pawlina 2006, p. 182). Essential nutrients reach the chondrocytes via extensive diffusion from the blood vessels in the perichondrium through the matrix (Zalisko and Kardong 2001, p. 39).
Based on the type and amount of protein fibers that are present in the extracellular matrix, cartilage is further divided into three subcategories: hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is the most common type of cartilage and is found in many places throughout the skeletal system (Zalisko and Kardong 2001, p. 40). The matrix of hyaline cartilage is produced by chondrocytes and contains type II collagen molecules, proteoglycans, and multi-adhesive glycoproteins. Hyaline cartilage serves a variety of functions and is found in both fetal and adult skeletons. During fetal development, the hyaline cartilage serves as the model for the developing skeleton. In initial stages of development, most long bones are represented by hyaline cartilage templates that resemble the shape of the eventual mature bone (Hill 2018). As development progresses, it is replaced by bone until cartilage remains only on articular surfaces of joints and on the ends of ribs. These cartilaginous structures persist in adulthood and provide a smooth, well-lubricated surface for the ends of bones within a joint. Hyaline cartilage is also found in the larynx, trachea, and bronchi of adults (Ross and Pawlina 2006, p. 194).
Elastic cartilage differs from the other types of cartilage due to the presence of elastin in the extracellular matrix. While elastic cartilage maintains the resilience and pliability of hyaline cartilage, the structure of its extracellular matrix gives it additional properties of elasticity and flexibility. This type of cartilage is commonly found in the external ear, the auditory tube, and the epiglottis (Ross and Pawlina 2006, p. 190).
The last cartilage subtype, fibrocartilage, is hyaline cartilage that has been combined with dense connective tissue. It is histologically similar to hyaline cartilage, except for an increased number of collagen fibers present in fibrocartilage (Zalisko and Kardong 2001, p. 40). This type of cartilage is analogous to a shock absorber. It is often present in locations that require significant resistance to shearing and compressive forces. Fibrocartilage is present in intervertebral disks, the pubic symphysis, menisci, and occasionally at a given point of tendon insertion. It is composed of both type I and type II collagen and functions to resist distortion under stressors (Ross, p. 191).
General Skeletal System Structure
The bodies skeletal system can be subdivided into the axial skeleton and the appendicular skeleton (Pasquini et al. 2007). The term “axial skeleton” refers to the bones and cartilage that protect the soft tissue structures of the head, neck, thorax, and abdomen. These structures include the skull, the hyoid apparatus, the vertebral column, the ribs, and the sternum (Pasquini, p. 26).
The Axial Skeleton
Cranial Skeleton (Skull)
The size and shapes of vertebrate skulls may differ drastically between different animals, but a common morphological pattern to the bones and their locations exist between most vertebrate species. The dorsal portion of the skull incorporates the frontal bones and the parietal bones, which together form most of the dorsal covering of the cranial cavity. The occipital bones occupy a large portion of the back of the skull and include the paired occipital condyles and the foramen magnum. A lateral view of the skull shows the upper portion of the jaw, or the maxilla, articulating with the lower jaw bone, the mandible. The temporal process of the zygomatic bone and the zygomatic process of the temporal bone join together to form the zygomatic arch. Caudal to the maxilla and just below the zygomatic arch lay the palatine bone, the presphenoid, and the basisphenoid. The hard palate of the skull is formed by the palatine processes of the premaxillae, the maxillae, and the palatine bones.
The vertebral column, or the backbone, is the main phylogenetic trait of all vertebrate species. While basic structural aspects of the vertebral column can differ between species, general trends in development, structure, and function can be seen across all vertebrates. The mammalian vertebral column can be broken down into five different areas or subsections: cervical vertebrae, thoracic vertebrae, lumbar vertebrae, sacral vertebrae, and caudal vertebrae. Segments of fibrous cartilage are located between the bony vertebrae and are known as intervertebral disks (Zalisko and Kardong 2001, p. 53).
In mammals, the first two cervical vertebrae are referred to as the atlas and the axis. The atlas is a ring-shaped vertebra with paired, winglike transverse processes. The axis sits just caudal to the atlas and has a broad spinous process that projects over the atlas. In most mammals, the axis would be followed by five additional vertebrae with articular processes, vertebral foramen, a neural arch, a spinous process, centrum, and transverse processes (Zalisko and Kardong 2001, p. 52).
The thoracic vertebrae of mammals are generally tall with short transverse processes and spinous processes angled caudally. There is a small depression where the head of the rib articulates with the centrum known as the facet.
Lumbar vertebrae can be differentiated from thoracic vertebrae based on the cranial orientation of the transverse and spinous processes (Zalisko and Kardong 2001, p. 52).
In mammals, the individual sacral vertebrae are fused together to form a single sacrum. The number of fused sacral vertebrae differs between species. There are three fused vertebrae in dogs and cats, four in a rabbit, and five in humans and horses (Zalisko and Kardong 2001, p. 52).
The transition to the caudal vertebrae is accompanied by a significant loss in prominence of the arches and processes. The number of caudal vertebrae is dependent on tail length, and there is significant difference between species and even individuals. In humans, all caudal vertebrae fuse together into a single piece called the coccyx (Zalisko and Kardong 2001, p. 52).
Ribs are comprised of a bony vertebral rib and costal cartilage. Ribs are bicipital, with the capitulum, or head of the rib, articulating with the centrum and the tubercle articulating with the transverse process. The bony portion between the capitulum and the tubercle is called the neck, and the rest of the rib is referred to as the body or shaft (Zalisko and Kardong 2001, p. 52).
The sternum, also sometimes referred to as the breastplate, is a flat bone composed of usually eight sternebrae that attach to the ventral aspects of the ribs. The first and most cranial sternebra is called the manubrium, and the most distal sternebra is called the xiphoid. Six additional sternebrae lie in between the manubrium and the xiphoid making up the body of the sternum.
The Appendicular Skeleton
The term “appendicular skeleton” includes limb bones as well as the bones that connect the limbs to the axial skeleton (Pasquini et al. 2007, p. 26).
For the majority of mammalian species, the pectoral girdle is characterized by significant mobility as well as structural support. Because of its intrinsic mobility, the thoracic girdle has an increased reliance on surrounding musculature for proper support and appropriate transmission of forces. The mammalian pectoral girdle generally only includes a clavicle and scapula. The paired clavicles serve to anchor the thoracic limbs to the axial skeleton. The scapula is a flat bone located on the dorsal portion of the cranial thorax that serves as an attachment point for many of the muscles and tendons of the arm, chest, and back. Each scapula has a spine that terminates in a point called the acromion. The acromion process articulates with its respective clavicle serving as the attachment point for muscles of the arm and chest (Pasquini, p. 26).
The humerus is the largest forelimb bone. The proximal head of the humerus articulates with the glenoid cavity of the scapula to create the shoulder joint. Two processes, known as the greater and lesser tubercles, are present distal to the humeral head and neck. These serve as attachment points for important forelimb musculature. The body of the humerus terminates into the medial articular surface called the trochlea and the lateral articular surface called the capitulum. The distal portion of the humerus articulates with the proximal portion of the forearm bones to form the elbow joint. The trochlear notch of the ulna articulates with the trochlear surface of the humerus. The head of the radius articulates with the capitulum of the humerus. The distal ends of the radius and ulna articulate with multiple carpal bones to form the wrist joint. The five metacarpals and phalanges make up each hand and its digits (Pasquini, p. 72).
When compared to the pectoral girdle, the bones of the pelvic girdle tend to be solid and fixed weight bearing structures. Its structure provides strong foundational stability and support for movement. The fused bones of the ilium, acetabulum, ischium, and pubis make up the hemipelvis (Evans and de Lahunta 2010, p.81). The ilium is comprised of a narrow body and a concave wing. The acetabulum is a socket that serves as the attachment point for the femur to the pelvic girdle (Newton 1985). The ischium forms the caudal floor of the pelvis, and the pubis forms the cranial floor of the pelvis. Each hemipelvis attaches to the other on the opposite side to form the ischial and pubic symphyses (Pasquini, p. 26).
The femur, fibula, tibia, metatarsal bones, tarsal bones, and phalanges make up each pelvic limb. The femur is the long bone of the hind limb and is comprised of a proximal head, neck, and greater trochanter, a shaft, and distal lateral and medial condyles (Carrier et al. 2006, p.2225). The head of the femur attaches to the pelvic girdle at the acetabular fossa via the round ligament forming the hip joint. The tibia and fibula are the hind limb bones just distal to the femur. The distal femur, proximal tibia, and patella form the knee joint. The fibula is long and slender and lies lateral to the thicker tibia bone. The ankle joint is formed by the tarsal bones. The five metatarsals and phalanges make up each foot and its digits (Pasquini, p. 26).
Despite differing appearances and functions, the skeletal systems of most vertebrates have many similarities both developmentally and anatomically. The basic building blocks of the skeletal system, bone and cartilage, remain relatively consistent throughout vertebrate skeletons with few anatomical and developmental variations. The assemblage of these basic building blocks also retains a consistent theme when compared across different branches of the vertebrate phylogenic tree. The skeletal differences that exist between species can be attributed to differing anatomical physics needed for different environmental and lifestyle needs. Despite these differences, all species’ skeletal systems are an essential basic component of life providing structural foundation, support of the body’s soft tissues, protection of internal organs, and necessary leverage for muscle function.
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