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Bone can be formed in two ways:
Intramembranous ossification
Direct mineralization of matrix
secreted by osteoblasts .
Endochondral ossification
Deposition of bone matrix on
a preexisting cartilage
matrix .
• 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
•
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.
• 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.
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).
•
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).
• 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.
Cartilaginous stage of vertebral development
During the 6th week, chondrification centers appear in each
mesenchymal vertebra .
• 5 secondary ossification centers appear in the vertebrae after
puberty:
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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.
•
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.
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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.
• . 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.
• 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.
• 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.
• . 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.
• 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.
• 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.
• . 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.
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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.
•
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.
• . 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.
• Poland anomaly. The
left pectoralis major
muscle is absent.
• Figure 10.7 Prune
belly syndrome: a
distended abdomen
from atrophy of
abdominal wall
musculature.