Читать книгу Anatomy of bone system. The manual for medical students / Анатомия костной системы. Учебное пособие для медицинских вузов - Г. И. Ничипорук - Страница 12

The osseous tissue appears in the human embryo in the middle of the second month of fetal development, when all other tissues have been already formed. The development of bones may proceed in two ways: on the basis of connective tissue and on the basis of cartilage. It should be noted that connective tissue never turns directly into cartilaginous tissue or osseous tissue. Osseous tissue is capable of developing by the way of growth along the surface of connective tissue or cartilage (appositional bone growth), or it develops to replace a resorbed cartilage.

Bones developing on the basis of connective tissue are termed primary. They are calvarial bones, and viscerocranial bones. Ossification of the primary bones is termed endesmal. It occurs in the following way: within the anlage of connective tissue, an ossification centre, punctum ossificationis (centrum ossificationis), appears and then expands into the depth and across the surface. From the ossification centre, osteal trabeculae start to form along the radii. They are interconnected with bone rods. In these spaces between the rods there are red bone marrow and blood vessels. In most of primary (membrane) bones, not one but several ossification centres are formed. They gradually grow and merge with each other. Eventually, only the most superficial layer of the initial connective tissue stratum remains unchanged. Then this layer turns into the periosteum.

Bones developing on the basis of cartilages are termed secondary. They pass through connective, cartilaginous and, in the last turn, osseous stages. Secondary bones are the skull base bones, trunk bones and bones of extremities. Let`s study the development of a secondary bone on the example of the long tubular bone. By the end of the second month of the fetal period, the cartilaginous anlage appears; it resembles a definite bone by shape. The cartilaginous anlage is covered by perichondrium. In the area of the future diaphysis of the bone, the perichondrium transforms into the periosteum. Lime salts are accumulated in the cartilaginous tissue under the periosteum, and cartilaginous cells die away. The osteal cells – osteoblasts – come from the periosteum to replace the dead cells. They start to produce an organic matrix of osseous tissue which endures calcification. The osteoblasts enclosed in the intercellular substance transform into osteosytes. Thus, the osteal cylinder termed periosteal or perichondral bone is formed in the diaphysis area. This stage of ossification of secondary bones is termed perichondral. Subsequently, new bone layers overgrow from the periosteum. Bone lamellae evolve, i.e. Haversian systems (osteons) start to develop around the vessels growing from the periosteum. The vessels sprouting from the periosteum are directed to the midst of the cartilaginous anlage. The cartilage located in the center of the diaphysis accumulates lime salts, dissolves and is substituted by spongy bone. This process is termed enchondral ossification of the diaphysis. The medullary canal is absent at first. It is formed in the process of transformation of spongy tissue of the enchondral bone inside the diaphysis and during red bone marrow development inside it.

In the epiphyses, ossification starts later, some bones are ossified even after the birth. Ossification begins from the ossification centre which appears within the cartilaginous anlage of the epiphysis. Such ossification process is called enchondral. It occurs in the following way: firstly, from the periosteum, blood vessels sprout into depth of the cartilage along the radii. In the midst of the epiphysis, the cartilage accumulates lime salts, dissolves and is substituted by osseous tissue. Later, the periosteal (perichondral) bone develops from the periosteum along the edge of the cartilaginous anlage of the epiphysis. The periosteal bone is comprised by a thin layer of compact tissue. The perichondral lamina is absent only in the areas of future articular surfaces – a quite thick layer of cartilage remains there. The cartilaginous layer also remains between the epiphysis and diaphysis – this is a metaepiphysial cartilage. It is the area of bone growth in length, and it disappears (transforms into osseous tissue) only after bone growth stops.

In long tubular bones, individual ossification centres appear in each epiphysis. Fusion of the epiphyses with the diaphysis usually occurs after birth. For example, in the tibia, the lower epiphysis merges with the diaphysis by the age of about 22 years, and the upper epiphysis – by 24 years. Short tubular bones normally have the ossification centre only in one epiphysis, the other epiphysis is ossified from the diaphysis. Some tubular bones have several ossification centres in their epiphysis at the same time. For example, in the upper epiphysis of the humerus, three centers appear; in the lower epiphysis of the humerus – there are four centers.

Volumetric bones are ossified like the epiphyses of long tubular bones i.e. enchondral ossification precedes periostal ossification. In flat bones, this process occurs vice versa, i.e. periostal ossification precedes ehchondral ossification.

It should be noted that, besides the main ossification centres, additional ossification centres may exist. They appear much later than the main centres. With the coming puberty, metaepiphysial cartilages become thinner and are replaced with osseous tissue. In the skeleton, synostoses start to develop. The distal epiphysis of the humerus and the epiphyses of the metacarpals are the first to fuse with their diaphyses. Formation of the synostoses comes to an end by about 24–25 years. Bone growth terminates when all the main and additional centers merge into one solid mass, i.e. when the cartilaginous layers separating the bone parts from each other disappear.

Significant individual differences in ossification rates are observed. The process of skeleton ossification in a child may be accelerated or decelerated; it depends on the genetic, hormonal and environmental factors. The term «osteal age» is used to assess the process of skeleton development in children. It is determined according to the number of ossification centres in bones and according to the time of their fusion. To evaluate ossification, roentgenography of the hand is made, because the age dynamics of ossification centre appearance and formation of synostosis are very clearly manifested in this part of the body. Ossification centres in carpal bones appear in the following periods:

– at birth, carpal bones are cartilaginous;

– ossification points emerge in the capitate and hamate bones during the first year of life;

– in the triquetral bone – during the third year of life;

– in the lunate bone – during the fourth year of life;

– in the scaphoid bone – during the fifth year of life;

– in the trapezium and trapezoid bones – during the sixth-seventh years of life;

– in the pisiform bone – in the tenth-fourteenth years of life.

Ossifcation of free parts of limbs is given in fig. 1.5.

V.S. Speransky distinguishes the following characteristic features in the ossification process:

1) ossification starts earlier in the connective tissue than in the cartilage;

2) ossification of the skeleton occurs in the cranial-caudal direction;

3) skull ossification spreads from the viscerocranium to the neurocranium;

4) in the free parts of extremities, ossification occurs from the proximal regions to the distal areas.

Fig. 1.5. Periods of ossification of free parts of limbs in male (Alexina L.A., 1985, 1998)

Osteal age does not always coincide with the passport age. In some children, the ossification process terminates 1–2 years earlier than it happens normally, in others – 1–2 years later. Starting from the age of 9 years, sexual differences of ossification can be distinguished clearly – in girls this process occurs more rapidly.

Body growth in girls terminates mainly at the age of about 16–17 years, in boys – at about 17–18 years. After this age, body growth in length is no more than 2 %.

In old age, bone rarefication termed osteoporosis occurs in different parts of the skeleton. In tubular bones, osseous tissue dissolves inside the diaphysis, as a result, the medullary cavity becomes wider.

At the same time, calcium salts are accumulated, and osseous tissue is developed on the outer surface of the bone, under the periosteum. Quite often, in places of attachment of ligaments and tendons, bony outgrowths called osteophytes develop. They also emerge along the edges of articular surfaces. In elderly persons, bone durability considerably decreases, and even minor injuries may cause fractures.

Skeleton ageing is characterized by individual changeability. In some persons, ageing symptoms appear as early as at the age of 35–40 years, in other persons – only after 70. Skeleton ageing symptoms are more manifested in women than in men. But this process significantly depends on the complex of factors: genetic, climatic, hormonal, alimentary (nutritive), functional, ecological etc.