Structure and chemical composition of bones

Bones occupy a strictly defined place in the human body. Like any organ, bone is represented by different types of tissues, the main place among which is bone tissue, which is a kind of connective tissue.

Bone (os) has a complex structure and chemical composition. In a living organism, up to 50% of water, 28.15% of organic and 21.85% of inorganic substances are present in the bone of an adult person. Inorganic substances are represented by compounds of calcium, phosphorus, magnesium and other elements. Macerated bone by 1/3 consists of organic substances, called "ossein", 2/3 - from inorganic substances.

Strength of bone is provided by physicochemical unity of inorganic and organic substances and features of its design. The predominance of organic substances provides significant elasticity, elasticity of the bone. With an increase in the proportion of inorganic compounds (in old age, with certain diseases), bone becomes brittle, fragile. The ratio of inorganic substances in the composition of bone varies from person to person. Even for the same person, it changes throughout life, depends on the characteristics of nutrition, professional activity, heredity, environmental conditions, etc.

Most of the bones of an adult person consists of lamellar bone tissue. It forms a compact and spongy substance, the distribution of which depends on the functional load on the bone.

The compact substance (substantia compacta) of bone forms diaphyses of tubular bones, in the form of a thin plate covers the outside of their epiphyses, as well as spongy and flat bones constructed of spongy substance. The compact substance of a bone is penetrated by thin channels in which pass blood vessels, nervous fibers. Some channels are located mainly parallel to the surface of the bone (central or havers, channels), others open on the surface of the bone with nutrient holes (foramina nutricia), through which arteries and nerves penetrate into the thickness of the bone, and veins come out.

The walls of the central (havers) channels (canales centrales) are formed by concentric plates 4-15 microns thick, as if inserted into each other. Around one channel from 4 to 20 such bone plates. The central canal, together with the surrounding plates, is called osteon ( Havers system). Osteon is a structural and functional unit of a compact bone substance. The spaces between the osteons are filled with intercalating plates. The outer layer of a compact substance is formed by outer surrounding plates, which are the product of the bone-forming function of the periosteum. The inner layer that delimits the bone marrow cavity is represented by internal surrounding plates, which are formed from the osteogenic cells of the endosteum.

The spongy (trabecular) substance of the bone (substantia spongiosa) resembles a sponge built of bone plates (beams) with cells between them. The location and dimensions of the bony beams are determined by the stresses experienced by the bone in the form of stretching and compression. The lines corresponding to the orientations of the bony beams are called the compression and extension curves. The location of the bone beams at an angle to each other contributes to the uniform transmission of bone pressure (muscle traction). This design gives bone strength at the least expense of bone substance.

The entire bone, except for its articular surfaces, is covered with a connective tissue membrane - the periosteum. The periosteum firmly fuses with the bone by means of connective tissue perforating (sharpeic) fibers penetrating deep into the bone. The periosteum is separated into two layers. The outer fibrous layer is formed by collagen fibers, which impart a special strength to the periosteum. Blood vessels and nerves pass through it. The inner layer is germinal, cambial. It is directly attached to the outer surface of the bone, contains osteogenic cells, due to which the bone grows in thickness and regenerates after injury. Thus, the periosteum performs not only protective and trophic, but also bone-forming functions.

From the inside, from the side of the bone marrow cavities, the bone is covered with an endosteum. Endost (endost) in the form of a thin plate densely attached to the inner surface of the bone and also performs osteogenic function.

Bones are markedly plastic. They are easily rebuilt under the influence of training, physical exertion, which is manifested in an increase or decrease in the number of osteons, changes in the thickness of bone plates of compact and spongy substances. For optimal development of the bone, moderate regular exercise is preferred. Sedentary life, low loads contribute to weakening and thinning of the bone. Bone acquires a coarse-celled structure and even partially resorces (bone resorption, osteoporosis). The profession also influences the peculiarity of the structure of the bone. An essential role, in addition to the external, is also played by hereditary-sexual factors.

The plasticity of bone tissue, its active reconstruction is caused by the formation of new bone cells, intercellular substance against the background of destruction (resorption) of the existing bone tissue. Resorption is provided by the activity of osteoclasts. On the site of the collapsing bone, new bone beams, new osteons are formed.

[1], [2], [3], [4], [5], [6], [7], [8], [9]