3.1. Structure and regeneration of bone tissue
Bone structure
Bone is a connective tissue. Lamellar bone tissue can form a compact and cancellous bone substance. Two mechanisms of bone formation have been evolutionarily developed - direct and indirect.
- Direct (primary, intramembrane) osteohistogenesis: the bone is formed directly from the cells of the skeletal mesenchyme. This is the way how the bones of the skull roof, part of the collarbone are formed.
- Indirect (secondary, enchondral) osteohistogenesis: the cartilaginous "models" of bones are first formed from the skeletal mesenchyme, which are then replaced by bone tissue during ontogenesis.
Based on the modern definition of "tissue", bone tissue is a system of interacting cells and intercellular substance.
Osteoblasts are the most functionally active cells in osteohistogenesis. In an adult organism, the cells of the osteogenic layer of the periosteum, endosteum, are the source of cells that support the osteoblast population; they are found among the elements of the stroma of the bone marrow, perivascular cells. By their phenotype, osteoblasts are typical, actively synthesizing and secreting cells, while the secretion can be carried out by the entire cell surface. Osteoblasts secrete the overwhelming majority of the organic bone matrix components - type I collagen, alkaline phosphatase, osteocalcin, bone morphogenetic proteins, transforming growth factors.
Osteoblasts play a leading role in the mineralization of the organic basis of the bone matrix. It is believed that the process of bone matrix mineralization begins with the deposition of amorphous calcium phosphate. Calcium cations enter the extracellular matrix from the bloodstream, where they are bound to proteins.
Osteocytes represent the terminal stage of differentiation of cells belonging to this histogenetic series; their proliferation is irreversibly blocked. The most well-established opinion about the function of osteocytes says that they ensure the integrity of the bone matrix due to their participation in the formation of protein and polysaccharide components of the extracellular substance, in the regulation of bone mineralization, osteocytic osteolysis and provision of a response to mechanical stimuli.
Osteoclasts are large multinucleated cells that resorb bone tissue. Together with osteoblasts, they participate in the remodeling of bone structures in embryonic, postnatal, and regenerative osteogenesis.
Intercellular substance - bone matrix, which forms 50% of the dry weight of the bone and consists of organic (25%), inorganic (50%) parts and water (25%). Almost 95% of it consists of type I collagen, and 5% of non-collagen proteins. In addition, the bone matrix contains glycosaminoglycans. The organic substances of the bone matrix are synthesized by osteoblasts and delivered by tissue fluid.
The skeletal organs - bones are built from bone tissue. There are tubular, flat, voluminous and mixed bones.
Long tubular bones form the basis of the appendicular skeleton. In tubular bones, there is a diaphyseal zone represented by a compact substance, epiphyseal zones represented by a spongy substance, and transitional metaepiphyseal zones (Fig. 3.1).
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Fig. 3.1. Zones of the tubular bone: a) epiphysis; b) metaepiphysis; c) diaphysis
The diaphyses are constructed from lamellar bone tissue covered externally with a periosteum.
In the periosteum, two layers are distinguished:
- outer - fibrous, consisting mainly of fibrous connective tissue;
- inner - osteogenic, or cambial, which is of great importance for the consolidation of the fracture.
In addition, there is a network of arterial vessels in the periosteum that carries out the tropism of the surface third of the cortical substance of the diaphysis.
Under the periosteum, the outer surrounding (general, common) bone plates are located, which cover the entire diaphysis in a circle. Under them, a layer of osteons begins, which are a system of concentrically arranged bone plates around the central channel, in which the vessels of the microcirculatory bed, myelin-free nerve fibers, lymphatic capillaries pass. Osteon channels, called Volkmann channels, anastomose to each other, circulating the intercellular fluid of the bone tissue.
From inside, bone marrow cavity is lined with an endostome, which is a dynamic structure formed by a thin connective tissue layer, including the cells lining the bone, osteogenic cells, and osteoclasts. In places of active osteogenesis, under the layer of osteoblasts, there is a thin layer of non-mineralized matrix - osteoid. The medullary cavity contains fatty yellow bone marrow and red hematopoietic bone marrow.
Bone growth occurs in childhood and adolescence. Growth in thickness occurs due to the functioning of the periosteum. In this case, the cells of the inner layer proliferate, differentiate into osteoblasts, synthesize the intercellular matrix, which is gradually mineralized and walls up the cells that synthesized it. Since the cells of the periosteum are actively dividing, this process is repeated many times. The growth that occurs in this way is called appositional growth.
The longitudinal growth of bones occurs due to the presence of a metaepiphyseal growth plate in the transition zone between the diaphysis and the epiphysis.
Flat bones on the saw are represented almost exclusively by a spongy substance. They can be extensive in area, but their thickness is insignificant (pelvis, shoulder blade, sternum, rib).
Bone tissue regeneration
Types of bone regeneration
There are physiological and reparative regeneration.
Physiological processes occurring in healthy tissues are based on maintaining a balance between physiological degeneration (as they function, permanent wear and tear of cells occurs) and physiological regeneration (replacement of degenerated cells with new ones). Physiological regeneration of bone tissue occurs by cell division.