Connective and supporting tissues

Osseous tissues

• are crucial for the bone skeleton formation in fetuses and in adults
• contain cells of osteoblastic line (precursor cells, osteoblasts, osteocytes), osteoclasts, and calcified intercellular substance (matrix)
• mineral components (calcium phosphate, hydroxyapatite) prevail (up to 70%) in the matrix to ensure the bone strength; meanwhile, organic components (30-35%) include collagen type I and provide for elasticity
• are rich in blood vessels, and the metabolic activity is high
• are classified into woven and lamellar bone

Woven bone

• in the matrix, collagen fibers are not well ordered, the tissue is poorly mineralized resulting in low mechanical strength
• it is the principal bone tissue type in the fetus; later woven bone is replaced by lamellar bone
• in adults, it is only retained at fusion sites of the cranial sutures and at the site of tendon attachmentto bones

Lamellar bone

• is the main tissue of the bone skeleton in adults
• replaces woven bone that is present in the fetal skeleton
• the matrix is mineralized and made up of bone lamellae, which contain parallel collagen (ossein) fibers to form complex spatial structures inside the bones
• there are cell bodies of osteocytes in thr lacunae between the bone lamellae. Processes of the cells penetrate the lamellae via a system of bone canaliculi
• osteogenic cells, osteoblasts and osteoclasts are situated in the periosteum and loose connective tissue that surrounds the supplying blood vessels

Lamellar bone constitutes compact and spongy bones, which also involve the periosteum and endosteum, bone marrow, blood vessels, and nerves

Lamellar bone tissue

Bone development

At different sites of the skeleton bone formation may proceed via direct or indirect osteogenesis

Direct (intramembranous) ossification is typical of embryonic development of flat bones, clavicles, and distal phalanges. This histogenetic vector results in woven bone formation through differentiation of cells of osteoblastic lineage. This process starts directly in the mesenchyme and takes several steps:

• ossification center formation
• cell differentiation in the ossification center with the formation of an organic bone matrix (osteoid)
• osteoid calcification

Conversion of mesenchyme into bone (direct osteogenesis)

Indirect (chondral) ossification is typical of the majority of bones in the skeleton (tubular bones, pelvic bone, vertebrae). Vertebral rudiments first emerge from hyaline cartilage tissue, which is further replaced by woven bone via several steps:

• formation of a cartilage model of the bone
• formation of a perichondral bone collar in diaphysis
• formation of endochondral bone in the diaphysis
• formation of endochondral bone in the epiphyses

Replacement of cartilage with bone (indirect osteogenesis). Stage of perichondral and endochondral ossification in diaphysis

Replacement of cartilage with bone (indirect osteogenesis). Stage of epichondral ossification in epiphyses