Bones and Skeletal Tissues
forms the cranial bones of the
skull (frontal, parietal, occipital, and temporal bones) and the
clavicles. Most bones formed by this process are ﬂat bones. At
about week 8 of development, ossiﬁcation begins within ﬁbrous
connective tissue membranes formed by
Tis process involves four major steps, depicted in
Postnatal Bone Growth
During infancy and youth, long bones lengthen entirely by in-
terstitial growth of the epiphyseal plate cartilage and its replace-
ment by bone, and all bones grow in thickness by appositional
growth. Most bones stop growing during adolescence. How-
ever, some facial bones, such as those of the nose and lower jaw,
continue to grow almost imperceptibly throughout life.
Growth in Length of Long Bones
Longitudinal bone growth mimics many of the events of endo-
chondral ossiﬁcation and depends on the presence of epiphyseal
cartilage. Te cartilage is relatively inactive on the side of the epi-
physeal plate facing the epiphysis, a region called the
. But the epiphyseal plate cartilage abutting the diaphysis
organizes into a pattern that allows fast, eﬃcient growth. Te carti-
lage cells here form tall columns, like coins in a stack. Te cells at the
“top” (epiphysis-facing) side of the stack abutting the resting zone
. Tese cells
divide quickly, pushing the epiphysis away from the diaphysis and
lengthening the entire long bone.
Meanwhile, the older chondrocytes in the stack, which are
closer to the diaphysis (
in Figure 6.10), hyper-
trophy, and their lacunae erode and enlarge, leaving large in-
terconnecting spaces. Subsequently, the surrounding cartilage
matrix calciﬁes and these chondrocytes die and deteriorate,
Tis leaves long slender spicules of calciﬁed cartilage at the
epiphysis-diaphysis junction, which look like stalactites hang-
ing from the roof of a cave. Tese calciﬁed spicules ultimately
become part of the
, and are in-
vaded by marrow elements from the medullary cavity. Os-
teoclasts partly erode the cartilage spicules, then osteoblasts
quickly cover them with new bone, and ultimately spongy bone
replaces them. Eventually as osteoclasts digest the spicule tips,
the medullary cavity also lengthens. During growth, the epi-
physeal plate maintains a constant thickness because the rate of
cartilage growth on its epiphysis-facing side is balanced by its
replacement with bony tissue on its diaphysis-facing side.
Longitudinal growth is accompanied by almost continuous
remodeling of the epiphyseal ends to maintain the proportion
between the diaphysis and epiphyses. Bone remodeling involves
both new bone formation and bone resorption
to form the
Ossiﬁcation centers appear in the ﬁbrous connective tissue
Selected centrally located mesenchymal cells cluster and
differentiate into osteoblasts, forming an ossiﬁcation center
that produces the ﬁrst trabeculae of spongy bone.
Osteoid is secreted within the ﬁbrous membrane and calciﬁes.
• Osteoblasts begin to secrete osteoid, which calcifes in a
rapped osteoblasts become osteocytes.
Woven bone and periosteum form.
• Accumulating osteoid is laid down between embryonic blood
vessels in a manner that results in a network (instead oF
concentric lamellae) oF trabeculae called woven bone.
ascularized mesenchyme condenses on the external face of the
woven bone and becomes the periosteum.
Lamellar bone replaces woven bone, just deep to the
periosteum. Red marrow appears.
• Trabeculae just deep to the periosteum thicken. Mature lamellar
bone replaces them, forming compact bone plates.
• Spongy bone (diploë), consisting oF distinct trabeculae, persists
internally and its vascular tissue becomes red marrow
represent much lower magniﬁcation than diagrams