Structure and chemical composition of bones

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

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

The strength of bone is ensured by the physicochemical unity of inorganic and organic substances and the features of its structure. The predominance of organic substances ensures significant elasticity and flexibility of bone. With an increase in the proportion of inorganic compounds (in old age, with some diseases), the bone becomes brittle and fragile. The ratio of inorganic substances in bone composition varies among different people. Even in the same person, it changes throughout life, depends on nutritional characteristics, professional activity, heredity, environmental conditions, etc.

Most bones in adults consist of lamellar bone tissue. It forms compact and spongy substance, the distribution of which depends on the functional loads on the bone.

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

The walls of the central (Haversian) canals (canales centrales) are formed by concentric plates 4-15 µm thick, as if inserted into each other. Around one canal there are from 4 to 20 such bone plates. The central canal together with the plates surrounding it is called an osteon (Haversian system). An osteon is a structural and functional unit of the compact substance of the bone. The spaces between the osteons are filled with intercalated plates. The outer layer of the compact substance is formed by the outer surrounding plates, which are a product of the bone-forming function of the periosteum. The inner layer, limiting the bone marrow cavity, is represented by the inner surrounding plates, formed from the osteogenic cells of the endosteum.

The spongy (trabecular) substance of bone (substantia spongiosa) resembles a sponge, built of bone plates (beams) with cells between them. The location and size of the bone beams are determined by the loads that the bone experiences in the form of tension and compression. The lines corresponding to the orientation of the bone beams are called compression and tension curves. The location of the bone beams at an angle to each other helps to evenly transfer pressure (muscle traction) to the bone. This design gives the bone strength with the least expenditure of bone substance.

The entire bone, except for its articular surfaces, is covered with a connective tissue membrane - the periosteum. The periosteum is firmly fused with the bone due to connective tissue piercing (sharpei's) fibers that penetrate deep into the bone. The periosteum has two layers. The outer fibrous layer is formed by collagen fibers that give the periosteum special strength. Blood vessels and nerves pass through it. The inner layer is the growth, cambial layer. It is adjacent directly 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 endosteum. Endosteum (endost) in the form of a thin plate tightly adjoins the inner surface of the bone and also performs an osteogenic function.

Bones are characterized by significant plasticity. They are easily rebuilt under the influence of training, physical activity, which is manifested in an increase or decrease in the number of osteons, a change in the thickness of the bone plates of compact and spongy substances. Moderate regular physical activity is preferable for optimal bone development. A sedentary lifestyle, low loads contribute to the weakening and thinning of the bone. The bone acquires a large-cell structure and even partially dissolves (bone resorption, osteoporosis). Profession also affects the peculiarity of the bone structure. In addition to environmental factors, hereditary and sexual factors also play a significant role.

The plasticity of bone tissue, its active restructuring are due to the formation of new bone cells, intercellular substance against the background of the destruction (resorption) of the existing bone tissue. Resorption is ensured by the activity of osteoclasts. In place of the destroyed bone, new bone beams, new osteons are formed.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ]