1. Bone can be formed in two ways: Direct mineralization of matrix secreted by osteoblasts. Deposition of bone matrix on a preexisting cartilage matrix. Intramembranous ossificationEndochondral ossification

2. The earliest formation is the establishment of cartilagenous model of the future bone e.g long bones will appear as straight homogeneous model of hyaline cartilage

3. Endochondral bone formation. A. Mesenchyme cells begin to condense and differentiate into chondrocytes. B. Chondrocytes form a cartilaginous model of the prospective bone. C,D. Blood vessels invade the center of the cartilaginous model, bringing osteoblasts (black cells) and restricting proliferating chondrocytic cells to the ends (epiphyses) of the bones.

7. Bones of the skull of a 3-month-old fetus showing the spread of bone spicules from primary ossification centers in the flat bones of the skull.

8. Skeletal structures of the head and face. Mesenchyme for these structures is derived from neural crest (blue), paraxial mesoderm (somites and somitomeres) (red), and lateral plate mesoderm (yellow).

9. Dorsal view of the chondrocranium, or base of the skull, in the adult showing bones formed by endochondral ossification. Bones that form rostral to the rostral half of the sella turcica arise from neural crest (blue). Those forming posterior to this landmark arise from paraxial mesoderm (chordal chondrocranium) (red).

10. Lateral view of the head and neck region of an older fetus, showing derivatives of the arch cartilages participating in formation of bones of the face.

12. Cartilaginous stage of vertebral development During the 6 th week, chondrification centers appear in each mesenchymal vertebra.

13. 5 secondary ossification centers appear in the vertebrae after puberty:

14. Formation of the vertebral column at various stages of development. A. At the fourth week of development, sclerotomic segments are separated by less dense intersegmental tissue. Note the position of the myotomes, intersegmental arteries, and segmental nerves. B. Proliferation of the caudal half of one sclerotome proceeds into the intersegmental mesenchyme and cranial half of the subjacent sclerotome.. Note the appearance of the intervertebral discs. C. Vertebrae are formed by the upper and lower halves of two successive sclerotomes and the intersegmental tissue. Myotomes bridge the intervertebral discs and, therefore, can move the vertebral column.

21. A. Child with anencephaly. Cranial neural folds fail to elevate and fuse, leaving the cranial neuropore open. The skull never forms, and brain tissue degenerates. B. Patient with meningocele. This rather common abnormality may be successfully repaired.

22. Examples of children with craniosynostosis. A. Child with scaphocephaly caused by early closure of the sagittal suture. Note the frontal and occipital bossing. B. Child with brachycephaly caused by early closure of the coronal and lambdoidal sutures.

23. . Three-month-old infant with achondroplasia. Note the large head, short extremities, and protruding abdomen. B,C. Achondroplasia in a 15-year-old girl. Note dwarfism of the short-limb type, the limbs being disproportionately shorter than the trunk. The limbs are bowed; there is an increase in lumbar lordosis; and the face is small relative to the head.

24. Development of the limb buds in human embryos. A. At 5 weeks. B. At 6 weeks. C. At 8 weeks. Hindlimb development lags behind forelimb development by 1–2 days.

25. Figure 9.14 Schematic of human hands. A. At 48 days. Cell death in the apical ectodermal ridge creates a separate ridge for each digit. B. At 51 days. Cell death in the interdigital spaces produces separation of the digits. C. At 56 days. Digit separation is complete.

26. . Lower extremity of an early 6-week embryo, illustrating the first hyaline cartilage models. B,C. Complete set of cartilage models at the end of the sixth week and the beginning of the eighth week, respectively.

28. A. Child with unilateral amelia. B. Patient with a form of meromelia called phocomelia. The hands and feet are attached to the trunk by irregularly shaped bones.

29. Digital defects. A. Brachydactyly, short digits. B. Syndactyly, fused digits. C. Polydactyly, extra digits D. Cleft foot, lobster claw deformity. Any of these defects may involve either the hands or feet or both.

30. . Cross section showing the developing regions of a somite. Sclerotome cells are dispersing to migrate around the neural tube and notochord to contribute to vertebral formation. B. Example of a typical vertebra showing its various components.

31. Development of the somite. A. Paraxial mesoderm cells are arranged around a small cavity. B. As a result of further differentiation, cells in the ventromedial wall lose their epithelial arrangement and become mesenchymal. Collectively, they are called the sclerotome. Cells in the dorsolateral wall of the somite form limb and body wall musculature, while cells at the dorsomedial portion migrate beneath the remaining dorsal epithelium (the dermatome) to form the myotome.

32. Stages in the development of a somite. A. Mesoderm cells are arranged around a small cavity. B. Cells of the ventral and medial walls of the somite lose their epithelial arrangement and migrate in the direction of the notochord. These cells collectively constitute the sclerotome. Cells at the dorsolateral portion of the somite migrate as precursors to limb and body wall musculature. Dorsomedial cells migrate beneath the remaining dorsal epithelium of the somite to form the myotome. C. Cells forming the myotome continue to extend beneath the dorsal epithelium. D. After ventral extension of the myotome, dermatome cells lose their epithelial configuration and spread out under the overlying ectoderm to form dermis.

33. . Transverse section through the thoracic region of a 5-week embryo. The dorsal portion of the body wall musculature (epimere) and the ventral portion (hypomere) are innervated by a dorsal primary ramus and a ventral primary ramus, respectively. B. Similar to Figure 10.2A later in development. The hypomere has formed three muscle layers and a ventral longitudinal muscle column.

34. Poland anomaly. The left pectoralis major muscle is absent.

35. Figure 10.7 Prune belly syndrome: a distended abdomen from atrophy of abdominal wall musculature.