Osteoclasts are the cells that degrade bone to initiate normalbone remodeling and mediate bone loss in pathologic conditions byincreasing their resorptive activity. They are derived fromprecursors in the myeloid/ monocyte lineage that circulate in theblood after their formation in the bone marrow. These osteoclastprecursors (OCPs) are attracted to sites on bone surfaces destinedfor resorption and fuse with one another to form the multinucleatedcells that resorb calcified matrixes under the influence ofosteoblastic cells in bone marrow.functions for OCPs andosteoclasts in and around bone other than bone resorption. Forexample, they regulate the differentiation of osteoblast precursorsand the movement of hematopoietic stem cells from the bone marrowto the bloodstream; they participate in immune responses, andsecrete cytokines that can affect their own functions and those ofother cells in inflammatory and neoplastic processes affectingbone.
2)During intramembranous ossification, compact and spongy bonedevelops directly from sheets of mesenchymal (undifferentiated)connective tissue
The osteoblasts secrete osteoid, uncalcified matrix consistingof collagen precursors and other organic proteins, which calcifies(hardens) within a few days as mineral salts are deposited on it,thereby entrapping the osteoblasts within. Once entrapped, theosteoblasts become osteocytes . As osteoblasts transform intoosteocytes, osteogenic cells in the surrounding connective tissuedifferentiate into new osteoblasts at the edges of the growingbone.
Several clusters of osteoid unite around the capillaries to forma trabecular matrix, while osteoblasts on the surface of the newlyformed spongy bone become the cellular layer of the periosteum .Theperiosteum then secretes compact bone superficial to the spongybone. The spongy bone crowds nearby blood vessels, which eventuallycondense into red bone marrow .The new bone is constantlyalso remodeling under the action of osteoclasts.
3)Intramembranous ossification begins in uteroduring fetal development and continues on into adolescence. Atbirth, the skull and clavicles are not fully ossified nor are thejunctions between the skull bone (sutures) closed. This allows theskull and shoulders to deform during passage through the birthcanal. The last bones to ossify via intramembranous ossificationare the flat bones of the face, which reach their adult size at theend of the adolescent growth spurt.
