Page 268 - Atlas of Histology with Functional Correlations

lacunae and become osteocytes. As in endochondral ossification, trapped

osteocytes in the lacunae establish the cell-to-cell connection through the
canaliculi.

The mandible, maxilla, clavicles, and most of the flat bones of the skull are
formed by the intramembranous ossification. In the developing skull, the
ossification centers grow radially, replace the connective tissue, and then fuse. In

newborns, the fontanelles (soft spots) in the skull represent the membranous
regions where intramembranous ossification of skull bones is still in the process
of ossification. The surrounding mesenchymal tissue that does not ossify
becomes the periosteum and endosteum of the new bones.

Supplemental micrographic images are available at

www.thePoint.com/Eroschenko13e under Bone Development.

FIGURE 7.19 | Endochondral Ossification:

Development of Long Bone (Panoramic View,

Longitudinal Section)

During endochondral ossification, the bone is first formed as a model of

embryonic hyaline cartilage. As bone development progresses, the cartilage is
replaced by bone. The process of endochondral ossification can be followed by
examining the upper part of the illustration and proceeding downward.

In the upper part, the hyaline cartilage is surrounded by the perichondrium

(13). The zone of reserve cartilage (1) shows chondrocytes in their lacunae
distributed singly or in small groups. Below this region is the zone of
proliferating chondrocytes (2) where the chondrocytes divide and become
arranged in vertical columns. Chondrocytes in lacunae (14) increase in size in
the zone of chondrocyte hypertrophy (3) as a result of swelling of the nucleus

and cytoplasm. The hypertrophied chondrocytes degenerate, forming thin plates
of calcified cartilage matrix (15). Below this region is the zone of ossification
(4), where a bony material is deposited on the plates of calcified cartilage matrix

(15).

Blood sinusoids (20) or capillaries invade the calcifying cartilage. Lacunar
walls and the calcified cartilage (15) are eroded, and the red bone marrow
cavity (16) is formed. The connective tissue around the newly formed bone is
now called periosteum (5, 6, 17) and is now the zone of ossification (4). In this

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