Connective Tissue Part 2.3 Bone structure PDF

Locomotor system Bone structure Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  General structure of bones Bones are classified according to shape. Long bones are tubular in shape and consist of two ends (proximal and distal e...

Locomotor system Bone structure Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  General structure of bones Bones are classified according to shape. Long bones are tubular in shape and consist of two ends (proximal and distal epiphyses) and a long shaft (diaphysis). Metaphysis is the junction between the diaphysis and the epiphysis. Bone is covered by periosteum, a connective tissue membrane that attaches to the outer surface. Bone cavities are lined by endosteum, a single layer of cells that contains osteoprogenitor (endosteal) cells  Periosteum The periosteum consists of two layers Outer fibrous layer: Outer is an irregular, dense connective tissue type with more collagenous matrix and less number of cells. The outer layer further subdivides into a superficial and deep layer; the superficial layer is more vascular and receives periosteal vessels, while the inner layer is a fibro-elastic layer. The inner cellular layer is made up of osteoprogenitor cells. It is also known as the osteogenetic layer. The Inner layer contains osteoblasts in young developing bones. Although in adult bones, osteoblasts may be absent, they appear whenever required (e.g., fracture healing). Osteoprogenitor cells are multipotent stem cells; they can undergo mitotic division and differentiate into osteoblasts  Endosteum Endosteum: A membrane lining the inner surface of the bony wall also identified as the lining membrane of the Bone marrow cavity is endosteum; The endosteum lines the Haversian canal and all the internal cavities of the bone. The endosteum consists of a layer of flattened osteoprogenitor cells and a type-III collagenous fibers (reticular fibers). The endosteum is noticeably thinner than the periosteum.  General structure of bone tissue Bone tissue formed during development is called immature (woven) bone. It differs from mature (lamellar) bone in its collagen fiber arrangement. Bone tissue is classified as either compact or spongy. Compact bone lies outside and beneath the periosteum, whereas an internal, sponge-like meshwork of trabeculae forms spongy bone.  Cells of bones In addition to osteocytes, four other cell types are associated with bone: Osteoprogenitor cells are cells derived from mesenchymal stem cells; they give rise to osteoblasts. Osteoblasts are cells that secrete the extracellular matrix of bone; once the cell is surrounded with its secreted matrix, it is referred to as an osteocyte. Bone-lining cells are cells that remain on the bone surface when there is no active growth. They are derived from those osteoblasts that remain after bone deposition ceases. Osteoclasts are bone-resorbing cells present on bone surfaces where bone is being removed or remodeled (reorganized) or where bone has been damaged.  Histological structure of bone tissue  Bone formation The development of bone is classified as endochondral (a cartilage model serves as the precursor of the bone) or intramembranous ossification (without involvement of a cartilage precursor). Flat bones of the skull, mandible, and clavicle develop by intramembranous ossification. All other bones develop by endochondral ossification.  Ossification In early fetal development, hyaline cartilage is the precursor of bones that develop by the process of endochondral ossification. During the developmental process, in which most of the cartilage is replaced by bone, residual cartilage at the proximal and distal end of the bone serves as growth sites called epiphyseal growth. This cartilage remains functional as long as the bone grows in length.  Endochondral ossification In endochondral ossification, the hyaline cartilage model is formed. Next, osteoprogenitor cells surrounding this model differentiate into bone-forming cells that initially deposit bone on the cartilage surface (periosteal bony collar) and later penetrate the diaphysis to form the primary ossification center. Secondary ossification centers develop later within the epiphyses.  Endochondral ossification Primary and secondary ossification centers are separated by the epiphyseal growth plate, providing a source for new cartilage involved in bone growth seen in children and adolescents. Epiphyseal growth plate has several zones (reserve cartilage, proliferation, hypertrophy, calcified cartilage, and resorption). Endochondral ossification  Intramembranous ossification In intramembranous ossification, bone formation is initiated by condensation of mesenchymal cells that differentiate into osteoblasts.  Intramembranous ossification Some of the spindle-shaped, pale-staining mesenchymal cells migrate and aggregate in specific areas (e.g., the region of flat bone development in the head), forming ossification centers. This condensation of cells within the mesenchymal tissue initiates the process of intramembranous ossification.  Intramembranous ossification Mesenchymal cells in these ossification centers elongate and differentiate into osteoprogenitor cells. Osteoprogenitor cells further differentiate into bone-secreting cells, the osteoblasts. They begin to deposit unmineralized bone matrix, the osteoid.  Intramembranous ossification The osteoblasts accumulate at the periphery of the ossification center and continue to secrete osteoid toward the center of the nodule. As the process continues, osteoid undergoes mineralization and trapped osteoblasts become osteocytes.