Page 62 - Headlines Histology2024_Neat

• The trabecullae are thin and composed of bone matrix with lacunae
containing osteocytes, which exchange metabolites via canaliculi with
blood sinusoids in the marrow cavity.

• The trabecullae are lined by endosteum that contains osteogenic cells,
osteoblasts and osteoclasts. Cancellous bone forms the epiphysis of the
long bone and the cores of flat and irregular bone.

Bone development

• The fetal development of bone occurs in two ways:
Intramembranous ossification

• The bones of the vault of the skull and maxilla develop by
intramembranous ossification that occurs within “membrane” of
condensed primitive mesenchymal tissue. Mesenchymal cells
differentiate into osteoblasts that begin to synthesis and secrete the
organic matrix (osteoid), such areas are called centers of ossification.

• The amount of osteoid is gradually increased followed by complete
mineralization and some osteoblasts become trapped in lacunae to
become osteocytes.

• Progressive bone formation results in the fusion of adjacent bony centers
to form trabecullae similar to those of cancellous bone. The primitive
mesenchyme remaining between the trabecullae of developing bone
differentiates into bone marrow.

• As the bony trabecullae increase in width and length by addition of new
lamellae, all the mesenchyme is replaced by cancellous bone that is
transformed into compact bone later.

2) Endochondral ossification

• The bones of the extremities, vertebral columns develop by
endochondral ossification that involves the transformation of cartilage
models into bone.

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