ossification). The splanch nic meso—derm gives rise to cardiac and smooth muscle.
The skeletal system develops from paraxial mesoderm. By the end of the fourth week, the sclerotome cells form embryonic connective tissue, known as mesenchyme. Mesenchyme cells migrate and differentiate to form fibro—blasts, chondroblasts, or osteoblasts.
Bone organs are formed by two methods.
Flat bones are formed by a process known as intra—membinous ossification, in which bones develop directly within mesenchyme.
Long bones are formed by a process known as en—dochondral ossification, in which mesenchymal cells give rise hyaline cartilage models that subsequently become ossified.
Skull formation.
Neurocranium is divided into two portions: The membranous neurocranium consists of flat bones that surround the brain as a vault. The bones appose one another at sutures and fontanelles, which allow overlap of bones during birth and remain membranous until adulthood.
The cartilaginous neurocranium (chondro—cranium) of the base of the skull is formed by fusion and ossification of number of separate cartilages along the median plate.
Viscerocranium arises primarily from the first two pharynge arches.
Appendicular system: The pectoral and pelvic girdles and the limbs comprise the appendicular system.
Except for the clavicle, most bones of the system are end chondral. The limbs begin as mesenchymal buds with an apical ectodermal ridge covering, which exerts an inductive influence over the mesenchyme.
Bone formation occurs by ossification of hyaline cartilage models.
The cartilage that remains between the diaphysis and the epiphyses of a long bone is known as the epiphysial plate. It is the site of growth of long bones until they attain their final size and the epiphysial plate disappears.
Vertebral column.
During the fourth week, sclerotome cells migrate medially to surround the spinal cord and notochord. After proliferation of the caudal portion of the sclerotomes, the vertebrae are formed, each consisting of the caudal part of one sclerotome and cephalic part of the next.
While the notochord persists in the areas of the vertebral bod ies, it degenerates between them, forming the nucleus pulposus. The latter, together with surrounding circular fibers of the annulus fibrosis, forms the intervertebral disc.
New words
skeletal – скелетный
mesoderm – мезодерма
cartilage – хрящ
fibroblasts – фибробласты
chondroblasts – хондробласты
osteoblasts – остеобласты
paraxial – параксиальный
flat – плоский
bone – кость
9. Muskular system
Skeletal (voluntary) system.
The dermomyotome further differentiates into the myo—tome and the dermatome.
Cells of the myotome migrate ventrally to surround the in—traembryonic coelom and the somatic mesoderm of the ventrolateral body wall. These myoblasts elongate, become spindle—shaped, and fuse to form multinucleated muscle fibers.
Myofibrils appear in the cytoplasm, and, by the third month, cross—striations appear. Individual muscle fibers increase in diameter as myofibrils multiply and become arranged in groups surrounded by mesenchyme.
Individual muscles form, as well as tendons that connect muscle to bone.
Trunk musculature: By the end of the fifth week, body—wall musculature divides into a dorsal epimere, supplied by the dorsal primary ramus of the spinal nerve, and a ventral hypomere, supplied by the ventral primary ramus.
Epimere muscles form the extensor muscles of the vertebral column, and hypomere muscles give rise to lateral and ven tral flexor musculature.
The hypomere splits into three layers. In the thorax, the three layers form the external costal, internal intercostal, and transverse thoracic muscle.
In the abdomen, the three layers form the external oblique, internal oblique, and transverse abdomii muscles.
Head musculature.
The extrinsic and intrinsic muscles of the tongue are thought to be derived from occipital myotomes that migrate forward.
The extrinsic muscles of the eye may derive from preop—tic myotomes that originally surround the prochordal plate.
The muscles of mastication, facial expression, the pharynx, and the larynx are derived from different pharyngeal arches and maintain their innervation by the nerve of the arch of origin.
Limb musculature originates in the seventh week from soma mesoderm that migrates into the limb bud. With time, the limb musculature splits into ventral flexor and dorsal extern groups.
The limb is innervated by spinal nerves, which penetrate the limb bud mesodermal condensations. Segmental branches of the spinal nerves fuse to form large dorsal a ventral nerves.
The cutaneous innervation of the limbs is also derived from spinal nerves and reflects the level at which the limbs arise.
Smooth muscle: the smooth muscle coats of the gut, trachtea, bronchi, and blood vessels of the associated mesenteries are derived from splanchnic mesoderm surrounding the gastrointestinal tract. Vessels elsewhere in the body obtain their coat from local mesenchyme.
Cardiac muscle, like smooth muscle, is derived from splanchnic mesoderm.
New words
ventral – брюшной
somatic – соматический
cytoplasm – цитоплазма
cross—striations – поперечные бороздчатости
extensor – разгибающая мышца
dorsal – спинной
ivertebral – позвоночный
arche – дуга
abdomen – живот
facial – лицевой
branch – ветвь
10. Skeleton
The bones of our body make up a skeleton. The skeleton forms about 18 % of the weight of the human body.
The skeleton of the trunk mainly
