OSTEOBLAST

large cell responsible for the synthesis of new bone during both initial formation and later remodeling of bone. Osteoblasts form a closely packed sheet on the surface of the bone, from which cellular processes extend through the developing bone. They arise from the differentiation of osteogenic cells in the periosteum, the tissue that covers the outer surface of the bone, and in the endosteum of the marrow cavity. This cell differentiation requires a regular supply of blood, without which cartilage-forming chondroblasts, rather than osteoblasts, are formed. The osteoblasts produce only the organic, unmineralized component of the bone, called osteoid, which consists of two substances secreted in different regions of the cell: mucopolysaccharides from the Golgi apparatus and collagen fibres from the endoplasmic reticulum. Mucopolysaccharides are the basic component of osteoid, while the collagen strengthens the matrix just as steel rods are used to reinforce concrete. Eventually the osteoblast is engulfed by the growing bone matrix, and as the material calcifies, the cell is trapped and can no longer lay down new bone. Thus entrapped, it becomes an osteocyte (q.v.), or stable bone cell. See also bone.