Human osteoblasts of endosteum.
The bone is completely covered by a hexagonally packed layer of plump osetoblasts (scanning electron microscopic image, SEM). The osteoblast surfaces are studded with many tiny microvillus-like projections, and they display numerous intercellular cytoplasmic processes.
The osteoblast cells are located on the bone surfaces and differentiated from osteoprogenitors in the periosteum and bone marrow elements which are affected by hormones, growth factors and cytokines. Osteoblasts produce alkaline phosphatase, type I collagen, bone morphogenetic proteins, osteocalcin, osteopontin, osterix and osteonectin etc., and are responsible for the synthesis and mineralization of bone matrices. Osteoblast cells also regulate activities of osteoclasts and osteoclastogenesis. Finally the osteoblasts become trapped in the bone matrix which become osteocytes and cease to generate osteoid and mineralized matrix. Osteocytes communicate with each other and with osteoblasts on the bone surfaces by extensive cytoplasmic processes via gap junction or other pathway.