Cells of Osseous Tissue

Specialized Cells of Bones

OsteocytesMature bone cells that maintain bone tissue. are the main bone cell found in both compact and spongy boneA porous bone tissue found inside bones, providing lightweight support.. Other bone cellsThe basic structural and functional units of life. are specialized and found only in bone tissue. Osteocytes have a star shape and are trapped in lacunae between concentric lamellae. Their purpose is to maintain the matrix, including the calcium salts and the collagen-rich osteoid. The marrow of the medulla cavity contains cells called osteogenic cells. These stem cells are capable of giving rise to the other three types of bone cells. These bone cells are quite active when young and continue to be active into adult life. Their ability to create new bone cells starts to slow down as you enter older ages.

The osteogenic cells are in the endosteumA thin connective tissue lining the marrow cavity.. Osteoblasts Bone-forming cells that secrete osteoid. are found along the superficialNear the surface of the body. face of the compact boneDense, strong bone tissue forming the outer layer of bones. of the bone collar. Osteoblasts secrete the osteoid. They produce the collagen-containing protein fibers of the matrix. These fibers will later be calcified in ossification. In a fetus, osteoblasts are more numerous than any of the other types of cells. Osteoblasts secrete the matrix against the superficial face of the bone. They push themselves outward as they do so. This action expands the bone’s diameter. This is called appositional growthThe process of bone widening and thickening by adding layers to the surface. and occurs along the outer edges of the bone. Sometimes an osteoblast will be outpaced in its matrix production by the other cells next to it. In this case, the underperforming osteoblast becomes trapped in between two concentric lamellae of the matrix. Because of its new location in a lacunaSmall spaces in bone tissue where osteocytes reside., the osteoblast transforms into an osteocyte. Instead of secreting the matrix, it starts to maintain it.

Homeostasis of Blood Calcium

Cells called osteoclasts are also scattered among the osteogenic cells. They are located at the borderline between spongy bone and the marrow of the medullary cavityThe hollow space inside bones that contains bone marrow..  They are part of that endosteum that lines the marrow cavity of bone.  OsteoclastsBone-resorbing cells that break down bone matrix. do not arise from the osteogenic stem cells.  In factA statement based on direct observation that is repeatedly confirmed., they arise from a myeloid stem cell. This stem cell is the precursor for erythrocytes(RBCs): Red blood cells responsible for oxygen transport. and neutrophils. It is also the precursor for that cell that fragments to give rise to platelets: a megakaryocyte.  An osteoclast has a very different lineage than the other bone cells. I think that these cells look like the ghosts in pac man. These cells do osteolysis which is called the breakdown of bone tissue. Contained within osteoclasts are lysosomes or vesicles containing enzymesProteins that speed up chemical reactions in the body. that are capable of breaking down spongy and compact bone.