TY - CHAP M1 - Book, Section TI - Bone A1 - Mescher, Anthony L. PY - 2021 T2 - Junqueira's Basic Histology Text and Atlas, 16e AB - Bone SUMMARY OF KEY POINTSBone is a type of connective tissue with a calcified extracellular matrix (ECM), specialized to support the body, protect many internal organs, and act as the body’s Ca2+reservoir.Major Cells & Matrix Components of BoneOsteoblasts differentiate from (stem) osteoprogenitor cells and secrete components of the initial matrix, called osteoid, that allow matrix mineralization to occur.Important components of osteoid include type I collagen, the protein osteocalcin, which binds Ca2+ and matrix vesicles with enzymes generating PO4−.High concentrations of Ca2+ and PO4− ions cause formation of hydroxyapatite crystals, whose growth gradually calcifies the entire matrix.Osteocytes differentiate further from osteoblasts when they become enclosed within matrix lacunae and act to maintain the matrix and detect mechanical stresses on bone.Osteocytes maintain communication with adjacent cells via a network of long dendritic processes that extend through the matrix via narrow canaliculi radiating from each lacuna.Osteoclasts are very large cells, formed by fusion of several blood monocytes, which locally erode bone matrix during osteogenesis and bone remodeling.Periosteum & EndosteumPeriosteum is a layer of dense connective tissue on the outer surface of bone, bound to bone matrix by bundles of type I collagen called perforating (or Sharpey) fibers.Regions of periosteum adjacent to bone are rich in osteoprogenitor cells and osteoblasts that mediate much bone growth and remodeling.The endosteum is a thin layer of active and inactive osteoblasts, which lines all the internal surfaces within bone; osteoblasts here are also required for bone growth.Types & Organization of BoneDense bone immediately beneath the periosteum is called compact bone; deep to the compact bone are small bony trabeculae or spicules of cancellous (or spongy) bone (Table 8–1).In long bones of the limbs, these two types of mature bone tissue occur in both the knobby, bulbous ends, called epiphyses, and in the intervening shaft or diaphysis.Immature bone, called woven bone, is formed during osteogenesis or repair and has a calcified matrix with randomly arranged collagen fibers.By the action of osteoclasts and osteoblasts, woven bone undergoes rapid turnover and is remodeled into lamellar bone with new matrix deposited in distinct layers with parallel collagen bundles; both compact and cancellous bone is lamellar bone.Most lamellar bone consists of lamellae organized concentrically around small central canals containing blood vessels and nerves; this organization is called an osteon or Haversian system.Within each osteon, osteocytic lacunae occur between the lamellae, with canaliculi radiating through the lamellae, which allow all cells to communicate with the central canal.OsteogenesisBones of the skull and jaws form initially by intramembranous ossification, with osteoblasts differentiating directly from progenitor cells in condensed “membranes” of mesenchyme.All other bones form by endochondral ossification, in which osteoprogenitor cells surround and then invade hyaline cartilage models of the skeletal elements in the embryo.Primary ossification centers in diaphyses of fetal long bones form when chondrocytes die after enclosure of the cartilage within a collar of woven bone, creating an initial cavity that is entered by periosteal osteoblasts and vasculature.Later, secondary ossification centers develop similarly within the epiphyses, with cartilage of the epiphyseal growth plate between the primary ... SN - PB - McGraw Hill CY - New York, NY Y2 - 2024/10/08 UR - accessmedicine.mhmedical.com/content.aspx?aid=1184199904 ER -