FIGURE 8.6 • Photomicrographs of decalcified immature and mature bone. a. Decalcified immature bone, stained with H&E, showing the relationship of cells to the extracellular matrix. The immature bone has more cells, and the matrix is not layered in osteonal arrays. 130. b. This cross section of decalcified mature compact bone stained with H&E shows several osteons (O) with concentric lamellae. The Haversian canals contain blood vessels and connective tissue. Osteocytes undergo considerable shrinkage during routine slide preparation, revealing empty lacunae with a small nucleus attached to their walls. Mature bone has fewer osteocytes per unit area than immature bone. Note the presence of interstitial lamellae between neighboring osteons. 160.
active inactive granulocyte/monocyte osteoclasts osteoclast progenitor (GMP, CFU-GM) endosteal cells
FIGURE 8.7 • Schematic drawing of cells associated with bone. All cells except osteoclasts originate from the mesenchymal stem cells, which differentiate into osteoprogenitor cells, osteoblasts, and finally osteocytes and bone-lining cells. Bone-lining cells on external bone surfaces are part of the periosteum, hence the term periosteal cells. Bone-lining cells on internal bone surfaces are frequently called endosteal cells. Note that osteoprogenitor cells and bone-lining cells have a similar microscopic appearance and are often difficult to distinguish from each other. Osteoclasts originate from hemopoietic progenitor cells, which differentiate into boneresorbing cells. The specific details of osteoclast differentiation are illustrated in Figure 8.13.
The osteoprogenitor cell is derived from mesenchymal stem cells.
Osteogenesis, the process of new bone formation, is essential to normal bone function. It requires a population of renewable osteoprogenitor cells (osteoblast precursor cells) that are responsive to molecular stimuli that transform them into bone-forming cells. Osteoprogenitor cells are derived from mesenchymal stem cells in the bone marrow that have the potential to differentiate into many different cell types including fibroblasts, osteoblasts, adipocytes, chondrocytes, and muscle cells. The key factor that triggers differentiation of osteoprogenitor cells is a transcription factor called core binding factor alpha-1 (CBFA1). This protein prompts the expression of genes that are characteristic of the phenotype of the osteoblast. As noted on page 219, bone morphogenic proteins also play a role in the differentiation of osteoblasts.
The osteoprogenitor cell is a resting cell that can differentiate into an osteoblast and secrete bone matrix.
Osteoprogenitor cells are found on the external and internal surfaces of bones and may also reside in the microvasculature supplying bone. Morphologically, they comprise the periosteal cells that form the innermost layer of the periosteum and the endosteal cells that line the marrow cavities, the osteonal (Haversian) canals, and the perforating (Volkmann’s) canals. In growing bones, osteoprogenitor cells appear as flattened or squamous cells with lightly staining, elongate, or ovoid nuclei and inconspicuous acidophilic or slightly basophilic cytoplasm. Electron micrographs reveal profiles of rough-surfaced endoplasmic reticulum (rER) and free ribosomes as well as a small Golgi apparatus and other organelles. The morphology of the osteoprogenitor cell is consistent with the finding that its stimulation leads to differentiation into a more active secretory cell, the osteoblast.
The osteoblast is the differentiated bone-forming cell that secretes bone matrix.