Page 263 - Atlas of Histology with Functional Correlations

this type of arrangement is nonlamellar. The woven bone is encountered in the

fetus during skeletal development and in repair of bone fractures. Also, the
woven bone is temporary and is replaced by lamellar or mature bone as the
individual ages.

The lamellar (secondary or mature) bone exhibits organized lamellae that
are either multiple parallel or concentric layers of calcified matrix arranged

around the central canals with the neurovascular bundle, or the osteons. Each
lamella exhibits a parallel arrangement of collagen fibers that follow a helical
course. Also, the bone cells, now called osteocytes, are in lacunae at regular
intervals between the concentric layers of lamellae and are arranged

circumferentially around the central canal. The matrix is more calcified in the
lamellar bone than in the woven bone, and, as a result, the lamellar bone is
stronger than the woven or immature bone.

FUNCTIONAL CORRELATIONS 7.3 ■ Bone Cells

and Their Function

All bones are lined on the internal and external surfaces by osteogenic cells.
Developing and adult bones contain four cell types: osteoprogenitor cells,
osteoblasts, osteocytes, and osteoclasts. Osteoprogenitor cells are
undifferentiated, pluripotent stem cells derived from the connective tissue

mesenchyme. Osteoprogenitor cells line the inner layer of the connective
tissue that surrounds and contacts the bone, the periosteum, and in the thin,
single layer of cells in the marrow cavities, the endosteum. Osteoprogenitor

cells also line the osteons (Haversian system) and the perforating canals with
blood vessels in the bone (see Fig. 7.18). The main functions of the
periosteum and the endosteum are to provide nutrition for the bone and a
continuous supply of new osteoblasts for growth, remodeling, and bone
repair. During bone development, osteoprogenitor cells proliferate by mitosis

and differentiate into osteoblasts, which secrete collagen fibers and the bony
matrix.

Osteoblasts, derived from osteoprogenitor cells, are present on the inner
surfaces of bone. They synthesize, secrete, and deposit osteoid, the organic

components of new bone matrix, which includes type I collagen fibers,
several glycoproteins, and proteoglycans. Osteoid is initially uncalcified and
does not contain any minerals; however, shortly after its deposition, it is
rapidly mineralized and becomes hard bone. Osteoblasts initiate and regulate

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