Lacunae, osteoclasts/osteoblasts

Subsequently, the sister cell of an osteoclast, which is called an osteoblast, is also transported through the vascular and capillary system to the side of the lacuna and fills the crater by depositing new living bone (see Fig. 6). 

Figure 1. Diagram of a thin bone trabecula (5) showing the four types of bone cells. Osteoblasts (8) and their precursors (7) are shown on the upper surface over a layer of uncalcified osteoid matrix (9), os-teocytes (6) are shown in their lacunae, an osteoclast (1) and a bone lining cell (3) are shown on the lower surface. Capillaries (4), containing red blood cells in their lu-mina, and a fibroblast (2) are shown near the trabecula. Adapted from Krstic (1978).

Bone tissue also contains osteoclasts, which are multinucleated cells that are derived from the hematopoietic (granulocyte-monocyte) cell line located in bone marrow. Osteoclasts are primarily responsible for bone resorption and they secrete enzymes and hydrochloric acid that break down collagen matrix and help dissolve the bone. The area where osteoclast cell membrane lies adjacent to bony tissue is known as a Howship s lacunae. The osteoclast cell membrane that lies in close proximity to bone can contain numerous villous extensions and form a ruffled border. These areas are also known as resorption pits. The plasmalemma border of the osteoclast cell in this region forms a specialized seal with the underlying bone to prevent the release of enzymes and hydrochloric acid. This process also results in the release of growth factors previously deposited in bone by osteoblasts, which are responsible for maintaining the process of regenerating new bone. 

Fig. 10.1 Osteoclasts tunneling. Osteoclasts differentiate at cracks in the bone and form a tunnel (lacuna) containing a central capillary surrounded by connective tissue. Fibroblast-like precursors of osteoblasts are activated to become osteoblasts and new bone is formed (Modified from Fig. 22.54 in The Molecular Biology of the Cell. B. Alberts et al., 4th Ed. 2002. Garland Science, Taylor Francis Group, New York)...

FIGURE 61-6 The bone remodeling cycle. Osteoclast precursors fuse and are activated to resorb a lacuna in a previously quiescent surface. These cells are replaced by osteoblasts that deposit new bone to restore the integrity of the tissue. 

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