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Chapter 6: The Skeletal System by John F. NeasThe articular and skeletal systems develop primarily from mesoderm. Bone begins to form about the fourth week of prenatal development by either intramembranous ossification or endochondral ossification. Both processes begin with migration of mesenchymal (mesodermal) cells into the area where bone formation will occur. Most bones develop from mesenchymal cells that develop into chondroblasts that condense into a hyaline cartilage precursor; the hyaline cartilage is later replaced by bone in through endochondral ossification. Some bones (e.g., facial bones and certain flat bones of the cranium) develop by intramembranous ossification whereby mesenchymal cells develop into osteoblasts that directly form bone without first passing though a cartilaginous stage. Sesamoid bones are specialized intramembranous bones that develop within tendons. Bone formation, or ossification, begins about the fourth week of embryonic development, but ossification centers are not observed until about the eighth week. Ossification is not completed in certain bones until about twenty-five or thirty years of age. Histology Epiphyseal Growth Plate This micrograph of the epiphyseal growth plate shows the dynamic process of endochondral ossification. The transition between epiphyseal cartilage and new bone occurs in six functional and morphological stages: Zone of reserve cartilage (R): typical hyaline cartilage with chondrocytes arranged in small clusters surrounded by a large amount of moderately stained matrix. Zone of proliferation (P): clusters of cartilage cells undergo successive mitotic divisions to form columns of chondrocytes. Zone of maturation (M): cell division ends and chondrocytes increase in size. Zone of hypertrophy and calcification (H): chondrocytes become greatly enlarged and vacuolated and the matrix becomes calcified. Zone of cartilage degeneration (D): chondrocytes degenerate and lacunae of the calcified matrix becomes invade by osteogenic cells and capillaries from the marrow cavity of the diaphysis. Osteogenic zone (O): osteogenic cells differentiate into osteoblasts that congregate on the surface of the spicules of calcified cartilage matrix where they begin bone formation. Membranous Bone This micrograph shows a full thickness view of the skull vault of a fetal human being. This bone forms by the process of intramembranous ossification. The external surface of the skull is invested by periosteum (Px) that merges with the deep layers of the overlying skin. The internal surface of the skull is also lined by periosteum (Pi) that constitutes the outermost membranous covering of the brain (dura mater). C = cancellous bone. Endochondral Ossification This micrograph shows endochondral ossification in the bones of a human fetal digit in two specimens prepared with different stains. This micrograph also illustrates a typical synovial joint, in this case an interphalangeal joint (IP). The articular surfaces of the two phalanges are covered by hyaline cartilage (C). A fibrous capsule (Cp), inserted into the articulating bones at some distance from the articular cartilages, maintains the articular surfaces in close contact. The synovial cavity (S) is lubricated by synovial fluid. Note the extensor tendon (E) that inserts into the terminal phalanx. B = bone. Histology-Epiphyseal growth plate Histology-Endochondral ossification Development of the Skull The skull consists of a neurocranium that forms a protective case, which surrounds the brain and special sensory organs (optic, auditory, and olfactory), and a viscerocranium (facial skeleton, or splanchnocranium) that forms the ear ossicles, hyoid bone, laryngeal and tracheal cartilages, and certain processes of the skull. Unlike most bone and cartilage of the skull that develops from mesoderm, most of the pharyngeal arch cartilages and their derivatives probably develop from the neural crest. The basal part of the skull has a cartilaginous model. The roof and sides of the skull develop directly in membrane. Neurocranium The head mesenchyme that produces the neurocranium initially forms a membrane around the neural tube. The outer portion of this membrane develops from the sclerotome (myotome) and produces cartilage and bone of the neurocranium. Two distinct populations of mesodermal cells contribute to the formation of the skull around the developing brain—the chondrocranium and the dermatocranium. Cartilaginous Neurocranium (Chondrocranium) After 5 weeks of development, the central nervous system is a hollow tube that extends the length of the body. A series of cartilages appear in the mesenchyme of the head beneath and alongside the expanding brain and around the developing eyes, ears, and nose. Five additional pairs of cartilages develop in the walls of the pharynx within the pharyngeal or branchial arches. The largest is the first, or mandibular, arch. The cartilages associated with the brain enlarge and fuse to form a chondrocranium that cradles the brain and sense organs. The chondrocranium consists of a basal plate, trabecular region, and cartilaginous sensory capsules that surround the organs of special sense (acoustic and vestibular, olfactory, and possibly optic). The walls and floor of the chondrocranium are incomplete at 8 weeks, and there is no cranial roof. During the ninth week, numerous centers of endochondral ossification appear within the chondrocranium to form the bones that support the brain. Membranous Neurocranium (Dermatocranium) The dermatocranium develops through membranous ossification to form the large flat bones that cover the brain and facial region, including the frontals, parietals, part of the occipital, and the squamous temporal bones. Fibrous sutures separate the large bones of the cranial vault that cover the brain (i.e., dermatocranium) during the fetal period and at birth. Where two or more such bones meet, the sutures are wide and form large membranous areas of the skull called "soft spots," or fontanelles, that provide spaces between the developing bones. The fontanelles permit the skull to undergo changes of shape, called molding, during parturition, and they allow for rapid growth of the brain during infancy. Complete ossification of the fontanels normally occurs by twenty to twenty-four months of age. Six fontanelles form: (1) anterior (frontal) fontanel, the most prominent, a diamond-shaped area on the anteromedian portion of the skull in the coronal suture; (2) posterior (occipital) fontanel, at the back of the skull in the median line in the lambdoidal suture; (3) paired anterolateral (sphenoidal) fontanels, one on either side of the skull directly below the anterior fontanel; and (4) the paired posterolateral (mastoid) fontanels on the posterolateral sides of the skull between the squamosal, parietal and occipital bones. The cranial sutures include (1) the coronal that extends from the anterior fontanel to the anterolateral fontanel (between the frontal and parietal bones); (2) the lambdoidal from the posterior fontanel to the posterolateral fontanel (between the parietal and occipital bones); (3) the sagittal that extends the anteroposterior median length of the skull between the anterior and posterior fontanels (between the two parietals); and (4) a squamosal suture that connects the posterolateral and anterolateral fontanels. Viscerocranium The viscerocranium is the portion of the skull that develops from the pharyngeal (branchial) arches that are apparent in the neck region during the fourth week of development. The branchial arches in fish and amphibians participate in formation of gills. In mammals, the pharyngeal arches produce the jaws, ear ossicles, hyoid bone, and larynx. The viscerocranium consists of six pairs of cartilaginous rods, one in each of the pharyngeal arches. These rods first appear as condensations of mesenchyme in the pharyngeal arch mesoderm surrounding the foregut. The cartilaginous bars probably develop, at least in part, from neural crest cells. The viscerocranium, like the neurocranium, develops through endochondral and intramembranous ossification. Dermal bone partly replaces portions of the pharyngeal cartilages, but others persist as cartilage throughout life. Cartilaginous Viscerocranium The cartilaginous viscerocranium consists of the first and second pharyngeal cartilages. The mesenchyme of the first pharyngeal (mandibular) arch divides into a short maxillary process and a long mandibular process. The maxillary process ossifies to form the malleus and incus. The mesenchymal core of the mandibular process chondrifies to form Meckel’s cartilage, the ventral end of which participates in forming the mandible. The body of the mandible develops by intramembranous ossification lateral to Meckel’s cartilage. The mesenchymal core of the second pharyngeal (hyoid) arch forms the stapes, styloid process, and lesser horn and upper body of the hyoid bone. The ventral portion of the third pharyngeal cartilage ossifies to become the greater horn and lower body of the hyoid. The mesenchymal core of the fourth, fifth, and sixth pharyngeal arches transforms and fuses into the laryngeal cartilages. The fourth (and possibly the fifth) pharyngeal arches produce the epiglottic, thyroid and cuneiform cartilages. The sixth arch probably contributes to the cricoid, arytenoid and corniculate cartilages. The tracheal cartilages possibly develop from supposed post-sixth arch cartilages. Membranous Viscerocranium Most of the facial bones develop by intramembranous ossification and these supplement the cartilaginous viscerocranium. The membrane bones form lateral to the cartilages of the first pharyngeal arch and in its maxillary and mandibular processes. Four dermal ossifications appear in the maxillary process of the first pharyngeal arch: premaxilla, maxilla, zygoma, and squamous temporal bones. The palatine, vomer, and pterygoid lamina develop from dermal ossification around the maxillary process. (For development of the palate, see Chapter 24—Development of the Respiratory System.) The mesenchyme of the mandibular process condenses around the outer side of the first arch and undergoes intramembranous ossification to form two membrane bones: the mandible and tympanic plate (that will later fuse with the squamous temporal bone and otic capsule). The vomer develops from two ossification centers that flank the lower border of the perpendicular ethmoidal plate. The nasal, lacrimal and inferior nasal concha bones develop from single centers of intramembranous ossification in close association with the nasal capsule. Development of the Vertebral Column, Rib Cage, and Sternum The postcranial axial skeleton develops from the sclerotome (vertebral centra, neural arches, intervertebral discs and ligaments associated with the vertebral column, true ribs, costal cartilages, sternum) and dermatome (vertebral neural spines). The notochord, a flexible rod of mesodermal tissue that lies in a position of the future vertebral column anterior to the developing spinal cord, develops from specialized embryonic cells called chordamesoblast. The notochord forms the mesenchymal axial skeleton in the human embryo and acts to organize tissues during the development of the vertebral column. The development of the vertebral column is a sequential process that involves the notochord and paired block-like condensations of mesenchymal called somites that form on both sides of the neural tube. Somites develop toward the end of the third week of embryonic development. During the fourth week of development, the somites differentiate into (1) the dorsolateral dermatome that produces the dermis of the skin, (2) the medial myotome that forms most of the skeletal muscle of the body, and (3) the ventromedial sclerotome that produces the vertebral column and contributes to the floor of the cranium. At about twenty-eight days of development, sclerotomal cells begin to migrate away from each somite pair in three main directions and cluster around the notochord where they differentiate into chondrocytes. 1. Mesenchymal cells migrate ventromedially, meet in the midline, and form a series of membranous or precartilaginous blocks that surround the notochord. Patches of mesenchyme separate these cartilages, which will develop into the vertebral bodies (centra). The centra develop by union of the caudal half of one sclerotome and the cranial half of the next lower sclerotome. The caudal portions also form the intervertebral discs and ligaments. A pair of chondrification centers fuse across the midline to form the cartilaginous vertebral centra around the spinal cord, creating a model of the complete vertebra. Articulations in the cervical, thoracic, and lumbar regions, develop where adjacent cartilaginous blocks come into contact. The cartilages fuse together in the sacrum and coccyx. Most of the notochord undergoes resorption and eventually disappears as the vertebral bodies expand and ossify around it. Only small portions of the notochord persist in adult humans as gelatinous centers of the intervertebral discs called the nucleus pulposus between adjacent vertebrae. Surrounding mesenchymal cells later differentiate into chondrocytes and produce the fibrocartilage of the annulus fibrosus. 2. Mesenchymal cells migrate dorsally to form the vertebral (neural) arches and spines that cover the neural tube. The precartilaginous neural arches and transverse processes develop from the cranial half of the next lower sclerotome. Chondrification centers in each half of the neural arches fuse dorsally with their counterparts of the opposite side to complete the neural arches and spines, and they extend laterally to form the transverse processes. 3. A group of mesenchymal cells pass ventrolaterally as costal processes into the developing body wall to form the ribs and costal cartilages of the rib cage. These structures are initially continuous but, by the eighth week, the ribs have separated from the vertebrae. Ribs develop at every vertebra, but they remain small and later fuse with the growing vertebrae in the cervical, lumbar, sacral, and coccygeal regions. The ribs of the thoracic vertebrae enlarge, following the curvature of the body wall. They fuse with the cartilages of the sternum when they reach the ventral midline. The sternum develops independent of the ribs and is first apparent about thirty-five days as a pair of mesenchymal bars lateral to the ventral midline in the thoracic region. The bars connect the ends of the rib cartilages on each side of the thorax. The bars gradually convert into precartilage, converge toward the midline as the body wall develops, and fuse at their cranial ends. The sternum secondarily becomes segmented into sternebrae as several ossification centers develop along the cartilaginous sternum, but fusion gradually reduces the number. The manubrium forms first, followed by the sternal body and then the xiphoid process. Ossification of the vertebral column and ribs begins in the eighth week, continues throughout childhood, and becomes complete in young adulthood. Only the shortest ribs completely ossify. In the others, the distal portions remain unossified, forming the costal cartilages. Growth continues for many years; in vertebrae, the bases of the neural arches enlarge until 3–6 years of age; the spinous processes and vertebral bodies grow until 18–25 years of age. Cartilaginous Neurocranium (Chondrocranium) Membranous Neurocranium (Dermatocranium) Development of the Vertebral Column, Rib Cage, and Sternum The appendicular skeleton probably develops from myotomes and in situ from mesenchyme derived from somatic mesoderm. This includes the limbs, limb girdles, and their associated ligaments and synovial membranes. The clavicles are dermal bones of the trunk that develop from dermatomes. Unlike other bones of the appendicular skeleton, the clavicle first shows membranous and then atypical endochondral ossification. Its classification is usually as a membrane bone. The development of the appendicular skeleton begins at the end of the fourth week and progresses through the embryonic period. The first condensations of mesenchyme in the appendicular skeleton are in the region of the future girdles with the pectoral (shoulder) girdle appearing a little before the pelvic (hip) girdle. The cartilaginous precursors of the upper and lower extremities make their appearance toward the end of the fourth week as small elevations at the sides of the trunk called limb buds. Each limb bud consists of a mass of undifferentiated mesoderm, or mesenchyme, partially covered with a layer of ectoderm, called the apical ectodermal ridge (AER). The AER promotes bone and muscle development. The anterior (upper) pair of extremities, the arm buds, develop ahead of the posterior (lower) pair, or leg buds, by a few days. The upper larger parts of each limb develop before the lower smaller parts (the same is true for chondrification and ossification). A mesenchymal skeleton first develops in the limbs; some of the masses of mesoderm surrounding the developing bones will develop into the skeletal muscles of the extremities. By the sixth week, the limb buds constrict around the middle portion, producing paddle-shaped distal segments called hand and foot plates. Digital rays that will form the digits of the hands and feet appear by about the fifth week. The death of cells between the phalangeal cartilages produces individual fingers by about the end of the sixth week. As the limb buds elongate, migrating mesenchymal tissues differentiate into specific cartilaginous bones. After 5 weeks of development, the pectoral limb buds are cartilaginous and scapular cartilages are developing in the mesenchyme of the trunk. By the seventh week, cartilaginous models of all the major skeletal components are well formed, the arm, forearm, and hand are evident in the upper limb bud, and the thigh, leg, and feet appear in the lower limb bud. By the eighth week, the shoulder, elbow, and wrist areas become apparent, and the upper limb bud is appropriately called the upper extremity; the appearance of the knee and ankle heralds the time when the lower limb bud is appropriately referred to as the lower extremity. The developing limbs are first directed caudally. At about the middle of the fifth week, bends develop at the future locations of the shoulder and elbow joints and a lateral rotation of the apical ridge changes the orientation of the elbows. Similarly, at about the eighth week, the apical ridge of the pelvic limb rotates medially, changing the orientation of the knee joint. Thus, the elbows become directed backward and the knees forward. The long bones are of endochondral origin and their cartilaginous precursors are miniatures of the adult bone. Endochondral ossification begins in the future limb bones by the eighth week. The primary centers of chondrification develop in the shafts (diaphyses) of the long bones. The centers of chondrification correspond to the primary centers of ossification that appear a few days later. Gradually, the hyaline cartilage tissue is replaced by bony tissue. Ossification starts at about the middle of the shaft and proceeds peripherally until only the ends (epiphyses) remain as cartilage. The coxal bones ossify at three separate centers (ilium, ischium, and pubis). After approximately 10 weeks of development, the shafts of the limb bones are undergoing rapid ossification, but the distal bones of the carpus and tarsus remain cartilaginous. At birth, primary centers of ossification occur in all the limb bones except the patella, and carpal and certain tarsal bones. Ossification continues throughout the fetal period. Secondary centers of ossification appear in the epiphyses near the end of the fetal period. The number and location of these epiphyseal centers vary in different long bones, but generally, there is at least one center in each epiphysis. A mass of cartilage called the epiphyseal plate persists between the bone that forms in the epiphyses and diaphysis. The epiphyseal plate permits an increase in length of the shaft by the addition of bone at the cartilage plate by endochondral ossification. The only secondary centers normally present at full term are in the distal epiphysis of the femur and occasionally in the proximal epiphysis of the tibia. More epiphyseal centers continue to develop during childhood and early adolescence. At birth, cartilaginous areas are present in the humeral head, in the wrist, between the bones of the palm, and fingers, and in the coxae. Growth ceases in late adolescence when the skeleton acquires its adult size. At that time, the epiphyseal plates finally resorb and become replaced by bone and the epiphyses permanently fuse to the diaphysis.
Joints form where two cartilages are in contact, or articulation. The surfaces within the joint cavity remain cartilaginous, but the rest of the bones undergo ossification. The classification of articulations is by degree of movement permitted and nature of the material that joins the articulating bones. Diarthroses (synovial joints) are freely movable joints with a joint cavity between the bones and a fibrous capsule at their periphery. Diarthroses develop by the third month as the epiphyses of adjacent endochondral bones develop distinct shapes and as the muscles contract and move the joints. The sites of developing diarthroses are obvious at six weeks as mesenchyme concentrates in the areas where precartilage cells differentiate. The future joints at this stage appear where mesenchymal cells are less concentrated. As cartilage cells develop within a developing bone, a thin flattened sheet of cells grows around the cartilaginous model to become the perichondrium. These cells are continuous across the space between the adjacent developing bones. The flattened mesenchymal cells that surround the gap differentiate into the joint capsule. Early during the third month, the mesenchymal cells remaining within the joint capsule begin to migrate toward the epiphyses of the adjacent developing bones. The cleft eventually enlarges into a joint cavity. The surfaces of the epiphyses that contact the joint cavity develop thin pads of hyaline cartilage that become the articular cartilages of the functional joint. As the joint continues to develop, mesothelial cells combine with connective tissue on the inside of the joint capsule to form a highly vascular synovial membrane that secretes a watery synovial fluid to lubricate the joint. Ligaments arise from the mesenchyme developing in joints. In certain developing diarthroses, the mesenchymal cells do not migrate from the center of the joint cavity. Instead, they produce fibrocartilaginous wedges, called menisci, as in the knee joint, or to complete cartilaginous pads, called articular discs, as in the sternoclavicular joint. Most diarthroses have developed by the end of the third month. Shortly thereafter, fetal muscle contractions called quickening produce movement at these joints. Movement at the joint enhances the nutrition of the articular cartilage and prevents fusion of connective tissues within the joint. The classification of diarthrotic joints is according to the type of connection between the bones and the type of movement allowed. Types of diarthrotic joints include ball and socket, condyloid, gliding, hinge, pivot and saddle. Examples of synovial joints include the knee, elbow, and wrist. Synarthroses are joints where little or no movement occurs. Unlike diarthrosis, the mesenchyme between articulating bones of a synarthrosis does not produce a joint cavity. Instead, the fibrous tissue persists and holds the bones together more or less rigidly. Syndesmoses are mostly immovable joints where connective tissue joins bones. The connective tissue may be ligamentous (stylohyoid ligament, tibiofibular ligaments, ligaments of vertebral laminae) or a dense fibrous connective tissue (sutures of the cranium). A gomphosis is a form of syndesmosis in which a conical process fits into a socket (as with the joint between the root of a tooth and the alveolus in the jawbone). Synchondroses are slightly movable joints in which hyaline cartilage separates the bones, as occurs between the ribs and sternum. This cartilage may become ossified in later development, as between the epiphyses and shaft of a long bone. The interzonal mesenchyme may chondrify into fibrocartilage, as at the pubic symphysis and spinal articulations separated by intervertebral discs. Synostoses are synarthrotic joints originally formed by connective tissue or cartilage that grow together by the replacement of intervening tissues by bone. Synostoses include the epiphyseal union of long bones and the sutures between bones of the skull.
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