Embryology of Skeletal System PDF

Üsküdar University School of Medicine Skeletal system and Muscle Development Asım Savlu M.D. Histology and Embryology  Mesenchymal embryonic cells can differentiate in many ways : Fibroblasts, chondroblasts, osteoblasts Not only sclerotome but...

Üsküdar University School of Medicine Skeletal system and Muscle Development Asım Savlu M.D. Histology and Embryology  Mesenchymal embryonic cells can differentiate in many ways : Fibroblasts, chondroblasts, osteoblasts Not only sclerotome but parietal layer of the lateral plate mesoderm also has the bone tissue formation capacity Parietal layer of the lateral plate mesoderm forms bones of the pelvic and shoulder girdle, limbs, sternum  Neural crest cells in the head region participate in formation of bones on the face and skull In flat bones of the skull mesenchyme in the dermis directly differentiates to bone which is called intramembranous ossification For the bones of the base of the skull and the limb mesenchymal cells first give rise to a hyaline cartilage model, then this template differentiates to bone which called endochondral ossification SKULL Neurocranium: forms the protective case around the brain: is divided into two parts 1) The membranous part (flat bones) derived from paraxial mesoderm and neural crest cells. Mesenchyme of these two sources undergoes to intramembranous ossification. Resulting needle like bone spicules radiate from primary ossification 2) Cartilagenous part ( base of skull) Viscerocranium: forms the skeleton of the face Newborn Skull At birth, the flat bones of the skull are separated from each other by a narrow seams of connective tissue: sutures At points where more than two points meet, the sutures are wide and are called fontanelles The most prominent of these is the anterior fontanel Sutures and fontanels allow the bones of the skull to overlap during the birth  Several sutures and fontanels remain membranous for a considerable time after birth The bones of the vault continue to grow after birth, mainly because the brain grows In the first few years after birth palpation of the anterior fontanel may give valuable information as to whether ossification of the skull is proceeding normally and whether intracranial pressure is normal In most cases anterior fontanelles closes at 18th month  Cartilaginous neurocranium or the chondrocranium of the skull initially consists of a number of separate cartilages Those that lie in front of the rostral limit of the notochord which ends at the level of the pituitary gland in the center of the cella turcica, are derived from the neural crest cells They form the prechordal chondrocranium Those that lie posterior to this limit arise from paraxial mesoderm and form the chordal chondrocranium The base of the skull is formed when these cartilages fuse and ossify by endochondral ossification Viscerocranium Consists of the bones of the face is formed mainly from the first two Phryngeal arches The dorsal portion of the first arch maxillary process give rise to maxilla, zygomatic bone and part of the temporal bone Ventral portion mandibular process contains the Meckel cartilage, Mesenchyme around the meckel cartilage condenses and forms mandible by intramembranous ossification, Meckel cartilage disappears except sphenomandibular ligament Dorsal tip of the mandibular process along with that of the second pharyngeal arch gives rise to incus, malleus and the stapes Viscerocranium Mesenchyme formation of the bones of the face is derived from the neural crest cells At first the face is small comparing the neurocranium This appearence is due to 1) Virtual abcence of the paranasal air sinuses 2) The small size of the bones, particularly the jaws With the appearence of the teeth and the development of the sinuses, the face looses its babyish aharacteristics Vertebra and the Vertebral Column Vertebrae is derived from the sclerotome portions of paraxial mesoderm A typical vertebra consists of a vertebral arch and foramen, a body, transverse processes and a spinous process During the fourth week sclerotome cells migrate around the spinal notochord to merge with the cells from the opposing somite on the other side of the neural tube  As development continues the sclerotome portion of each somite goes under a resegmentation Resegmentation occurs when the caudal half of each scleretome grows into and fuses with the cephalic half of each subsequent sclerotome Thus each vertebra is formed by the combination of caudal half of one somite and the cranial half of its neighbor Patterning of the shapes of the different vertebrae is regulated by HOX genes  Mesenchymal cells between cephalic and caudal parts of the original sclerotome segment do not proliferate but fill the space between the two precartilaginous vertebral bodies In this way they contribute to the formation of the intervertebral disc Although notochord regreses entirely in the region of the vertebral bodies, it persists and enlarges in the region of the intervertebral disc and contributes to nucleus pulposus which is later surrounded by the circular fibers of annulus fibrosus, These two structures form the intervertebral disc  Resegmentation of the sclerotomes into definitive vertebra causes myotomes to bridge the intervertebral discs and this alteration gives them the capacity to move the spine For the same reason, intersegmental arteries at first lying between the sclerotomes pass midway over the vertebral bodies Spinal nerves come to lie near the intervertebral discs and leave the vertebral column through the intervertebral foramina As the vertebrae form, Two primary curves; Thorasic and sacral curvatura are established by the child learns to hold his head, Lumbar curvature is established by the time the child learns walking Ribs and Sternum Bony portion of each rib is derived from the sclerotome cells that remain in the paraxial mesoderm and that grow out from the costal proceses of thorasic vertebrae Costal cartilages are formed by scelerotome cells that migrates into the adjacent lateral plate mesoderm The sternum develops independetly in the parietal layer of lateral plate mesoderm in the ventral body wall Two sternal bands are formed in the parietal layer of the lateral plate mesoderm on either side of the midline and fuse to form the cartilaginous models MUSCULAR SYSTEM STRIATED SKELETAL MUSCULATURE Head musculature is derived from seven somitomeres which are partially segmented whorls of mesenchymal cells derived from the paraxial mesoderm Musculature of the axial skeleton, body wall, and the limbs is derived from somites initially form as somitomeres and extend from the occipital region to the tail bud After segmentation these somitomeres undergo a process of epithelization and form a ‘ball’ of epithelial cells with a cavity at the center  Ventral region of each somite then become mesenchymal again and forms sclerotome Cells in the upper region of the somite form the dermatome and the two muscle forming areas at the ventrolateral and dorsomedial lips or edges respectively Cells from these two areas migrate and proliferate to form progenitor muscle cells ventral to the dermotome therby forming dermomyotome  Some cells from the ventrolateral region also migrate into adjacent parietal layer of the lateral plate mesoderm Here they form infrahyoid, abdominal wall ( rectus abdominus, internal and external oblique, and transversus abdominus) and the limb muscles The remaining cells in the myotome form the muscles of the back , shoulder girdle, and the intercostal muscles  Initially, there is a well defined border between each somite and the parietal layer of the lateral plate mesoderm called the lateral somitic frontier. This frontier separates two mesodermal domains in the embryo 1.The primaxial domain that comprises the region around the neural tube and contains only somite derived (paraxial mesoderm) cells 2.The abaxial domain that consists of the parietal layer of the lateral plate mesoderm together with the somite cells that have migrated across the lateral somitic frontier  Muscle cells that cross this frontier (those from the ventrolateral edge of the myotome) enter the lateral plate mesoderm comprise the abaxail muscle cell precursors and receive many of their signals of differentiation from lateral plate mesoderm Those that remain in paraxial mesoderm and do not cross the frontier comprise the primaxial muscle cell precursors and receive many of their developmental signals from the neural tube and notochord Regardless of their domain each myotome receives its inervation from spinal nerves derived from the same segment as the muscle cells  The lateral somitic frontier also defines the border between the dermis derived from dermatomes in the back and the dermis derived from lateral plate mesoderm in the body wall It also defines a border for the rib development, such that the bony components of each rib are derived from the primaxial sclerotome cells and the cartilaginous parts of those ribs that attach the sternum are derived from the sclerotome cells that migrate across the the lateral somitic frontier (abaxial cells) Skeletal Muscle and Tendons During differentiation, precursor cells, the myoblasts fuse and form long multinucleated muscle fibers Myofibrils soon appear in the cytoplasm and by the end of the third month cross striations of the typical skelatal muscle appear Tendons are derived from sclerotome cells lying adjacent to myotomes Head Musculature All voluntary muscles of the head region are derived from paraxial mesoderm including the muscles of the tongue, eye and that associated with the pharyngeal arches Limb musculature The first indication of the limb musculature is observed in the 7th wk of development as a condensation of mesenchyme near the base of the limb buds The mesenchyme is derived from the dorsolateral cells of the somites that migrate into the limb bud to form the muscles Cardiac Muscle Cardiac Muscle develops from splanchnic mesoderm surrounding the endothelial heart tube Myoblasts adhere to one another by special attachements that later develop into intercalated discs Myofibrils develop as in the skeletal muscle but myoblasts do not fuse During later development a few special bundles of muscle cells with irregularly distributed myofibrils become visible and differentiate in Purkinje cells Smooth Muscle Cells Smooth muscle for the dorsal aorta and the large arteries is derived from the lateral plate mesoderm and neural crest cells In the coronary arteries smooth muscle originates from proepicardial cells and neural crest cells Smooth muscle in the wall of the gut and gut derivatives is derived from the splanchnic layer of the lateral plate mesoderm that surrounds the structures Only the sphinctor and dilator muscles of the pupil and the muscles tissue in the mammary gland and sweat glands are derived from ectoderm LIMB GROWTH AND DEVELOPMENT The limbs, including the shoulder and pelvic girdles, comprise the appendicular skeleton. At the end of the fourth week of development, limb buds become visible as outpocketings from the ventrolateral body wall The forelimb appears first followed by the hindlimb 1 to 2 days later.  Initially, the limb buds consist of a mesenchymal core derived from the parietal (somatic) layer of lateral plate mesoderm that will form the bones and connective tissues of the limb, covered by a layer of cuboidal ectoderm. Ectoderm at the distal border of the limb thickens and forms the apical ectodermal ridge (AER) This ridge exerts an inductive influence on adjacent mesenchyme, causing it to remain as a population of undifferentiated, rapidly proliferating cells, the progress zone. As the limb grows, cells farther from the influence of the AER begin to differentiate into cartilage and muscle. In this manner, development of the limb proceeds proximodistally.  In 6-week-old embryos, the terminal portion of the limb buds becomes flattened to form the hand and footplates and is separated from the proximal segment by a circular constriction Later, a second constriction divides the proximal portion into two segments, and the main parts of the extremities can be recognized  Fingers and toes are formed when cell death in the AER separates this ridge into five parts. Further formation of the digits depends on their continued outgrowth under the influence of the five segments of ridge ectoderm Condensation of the mesenchyme to form cartilaginous digital rays, and the death of intervening tissue between the rays completes the development  While the external shape is being established, mesenchyme in the buds begins to condense, and these cells differentiate into chondrocytes By the sixth week of development, the first hyaline cartilage models, foreshadowing the bones of the extremities, are formed by these chondrocytes  Ossification of the bones of the extremities, endochondral ossification, begins by the end of the embryonic period. Primary ossification centers are present in all long bones of the limbs by the 12th week of development. From the primary center in the shaft or diaphysis of the bone, endochondral ossification gradually progresses toward the ends of the cartilaginous model  At birth, the diaphysis of the bone is usually completely ossified, but the two ends, the epiphyses, are still cartilaginous. Shortly thereafter, ossification centers arise in the epiphyses. Temporarily, a cartilage plate remains between the diaphyseal and epiphyseal ossification centers. This epiphyseal plate, plays an important role in growth in the length of the bones. Endochondral ossification proceeds on both sides of the plate When the bone has acquired its full length, the epiphyseal plates disappear, and the epiphyses unite with the shaft of the bone.  Synovial joints between bones begin to form at the same time that mesenchymal condensations initiate the process of forming cartilage. Thus, in the region between two chondrifying bone primordia, called the interzone the condensed mesenchyme differentiates into dense fibrous tissue. This fibrous tissue then forms articular cartilage, covering the ends of the two adjacent bones; the synovial membranes; and the menisci and ligaments within the joint capsule The joint capsule itself is derived from mesenchyme cells surrounding the interzone region.  LIMB MUSCULATURE Limb musculature is derived from dorsolateral cells of the somites that migrate into the limb to form muscles These muscle components are segmented according to the somites from which they are derived  With elongation of the limb buds, the muscle tissue first splits into flexor and extensor components and then additional splittings and fusions occur, such that a single muscle may be formed from more than one original segment. The resulting complex pattern of muscles is determined by connective tissue derived from lateral plate mesoderm  As soon as the buds form, ventral primary rami from the appropriate spinal nerves penetrate into the mesenchyme. At first, each ventral ramus enters with dorsal and ventral branches derived from its specific spinal segment, but soon branches in their respective divisions begin to unite to form large dorsal and ventral nerves Thus, the radial nerve, which supplies the extensor musculature, is formed by a combination of the dorsal segmental branches, whereas the ulnar and median nerves, which supply the flexor musculature, are formed by a combination of the ventral branches.  Immediately after the nerves have entered the limb buds, they establish an intimate contact with the differentiating mesodermal condensations, and the early contact between the nerve and muscle cells is a prerequisite for their complete functional differentiation. Spinal nerves not only play an important role in differentiation and motor innervation of the limb musculature, but also provide sensory innervation for the dermatomes.