Page 585 - Concise Pathology for Exam Preparation ( PDFDrive )

570 SECTION II Diseases of Organ Systems

Spongy
bone

Epiphysis

Epiphyseal plate

Compact
bone

Diaphysis

Metaphysis

Secondary
ossification
centre
FIGURE 21.1. Parts of a long bone.

(b) Woven bone: Woven bone is immature bone, in which collagen fibres are arranged
in irregular random arrays and contain smaller amounts of mineral substance and
a higher proportion of osteocytes than lamellar bone. Woven bone is temporary
and is eventually converted to lamellar bone.

Microscopic Structure of Bone

1. Bone cells
(a) Osteoblasts: Line the surface of bone or osteoid and synthesize collagen, proteo-
glycans and glycoproteins. Osteoblasts also synthesize alkaline phosphatase, an
enzyme needed for the mineralization of osteoid. The cell has an eccentrically
located nucleus with a prominent nucleolus and a perinuclear halo similar to a
plasma cell but lacks the cartwheel-like chromatin pattern that is typical of the
latter. An inactive osteoblast has a flattened shape and low alkaline phosphatase
activity.
(b) Osteocytes: An osteoblast gives rise to an osteocyte which lies in a lacunar space
and is connected to other osteocytes by dendritic processes through tunnels called
“canaliculi”.
(c) Osteoclasts: Osteoclasts are thought to be derived from the monocyte–macrophage
system and are responsible for bone resorption. They are multinucleated cells with
fine, finger-like cytoplasmic processes. An increased number of osteoclasts may be
seen in diseases with increased bone turnover.
2. Bone matrix: Bone matrix consists of organic and inorganic components. The asso-
ciation of organic and inorganic substances gives bone its hardness and resistance.
The organic component or osteoid forms 35% and the inorganic or mineral compo-
nent forms 65% of the bone. Osteoid is composed of collagen fibres with predomi-
nately Type I collagen (95%) and amorphous material, including glycosaminoglycans

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