Development and age-specific features of bone joints in ontogeny

In lower vertebrates (living in water), parts of the skeleton are connected by continuous joints (using fibrous, cartilaginous or bone tissue). These joints are slightly mobile or immobile. With the exit to land, movements become more complex. Therefore, they provide greater mobility of bone levers and enable animals to perform movements with a greater scope and variety, necessary for movement on land. Along with joints, transitional forms of connections (symphyses, or semi-joints) are also formed.

In humans, all joints are initially formed as continuous during embryogenesis. Later, the mesenchymal layer between the bones is gradually replaced by fibrous or cartilaginous tissue. Joints (synovial joints) are formed starting from the 6th to 11th week of embryogenesis. Gaps are formed in the mesenchymal layers. The joint capsule and ligaments are formed from the mesenchyme surrounding the joint primordium. The deep layer of the capsule is transformed into the synovial membrane. Two joint gaps arise in the areas of the knee, temporomandibular and other complex joints. The mesenchyme between the ends of the articulating bones is transformed into an intra-articular disc or menisci. The cartilaginous articular lip is formed from the intra-articular cartilage. The center of this cartilage is resorbed, and the peripheral part grows to the edges of the articular surface of the bone. When symphyses are formed, cartilage is formed from the mesenchymal layer between the articulating bones, and a narrow gap is formed within it.

In newborns, all anatomical elements of the joints are basically formed. However, their differentiation continues. The epiphyses of the connecting bones at this age are represented by cartilage. At the age of 6-10 years, the structure of the synovial membrane becomes more complex, the number of villi and folds increases, vascular networks and nerve endings are formed in the synovial membrane. Collagenization of the joint capsule occurs. At this time, the capsule and ligaments thicken, their strength increases. The formation of all joint elements ends at the age of 13-16 years. With an optimal functional load, the joints do not experience obvious involutional changes for many years. With prolonged excessive physical exertion, as well as with age, structural and functional changes in the joints are observed. This may be thinning of the articular cartilage, sclerosis of the joint capsule, ligaments, the formation of osteophytes (bone outgrowths) on the edges of the articular surfaces. A common sign of these changes is a decrease in joint mobility.

Aging of joints is related to body type. With a brachymorphic body type, the rate of aging of the hand, foot, large joints (shoulder, elbow, etc.) is usually somewhat more intense than in people with a dolichomorphic body type. Aging of the articular ends of most bones in women is also accelerated compared to men.

Involutional changes occur in a certain sequence. The most frequent in terms of timing and activity of occurrence of such changes are the knee, hip joints and joints of the lumbar spine, sacroiliac joint. These joints bear a significant load when standing, walking, which accelerates their "wear and tear". Next in frequency of occurrence are often changes in the joints of the cervical spine, joints of the upper limb.

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