Limb skeletal bones in ontogeny

Two pairs of limbs are typical for almost all vertebrates. Thus, fish have paired pectoral and pelvic fins, which develop from the mesenchyme of the lateral folds.

As vertebrates moved from the aquatic environment to land, their living conditions changed, which led to a significant restructuring of the body. Land animals developed front and back limbs, the skeleton of which is built like bone levers consisting of several links and allowing movement on land. Limb girdles are already present in a rudimentary form in fish, but they reach their greatest development in land species, starting with amphibians. The limbs are connected to the body by means of girdles. The most primitive form of the skeleton of the shoulder girdle can be observed in shark fish, in which it consists of dorsal and ventral cartilaginous arches fused with each other closer to the ventral side of the body. From the place of fusion of these arches on each side, a free part of the fin departs. From the dorsal cartilaginous arch of the primitive shoulder girdle, both in higher fish and in land vertebrates, the scapula is subsequently formed. The glenoid fossa is formed near the scapula for articulation with the skeleton of the free part of the limb.

The ventral cartilaginous arch gives rise to the coracoid, which in amphibians, reptiles and birds fuses with the sternum. In viviparous mammals, the coracoid is partially reduced and grows onto the scapula in the form of a beak-shaped process. Another process called the "procoracoid" develops from the same rudiment, on the basis of which the integumentary bone, the clavicle, is then formed. The clavicle connects with the sternum at its medial end and with the scapula at its lateral end. These bones are developed in mammals whose free part of the limb can move around all axes. In animals whose movements during running and swimming are performed only around one axis (ungulates, carnivores and cetaceans), the clavicles are reduced.

The pelvic girdle in fish is rudimentary and does not connect with the vertebral column, since fish do not have a sacrum. In shark fish, the pelvic girdle is represented by the dorsal and ventral cartilaginous arches. The posterior fins extend from the place where they fuse. The dorsal cartilaginous arch of the pelvic girdle in land animals develops into the ilium. The ischium and pubis, corresponding to the coracoid and procoracoid of the shoulder girdle, originate from the ventral cartilaginous arch. These three rudiments of the pelvic bone are connected to each other at the place where the glenoid fossa is formed for articulation with the free section of the hind limb. In mammals, with age, all three bones merge into one pelvic bone, and the cartilage between them completely disappears. In higher vertebrates, especially in monkeys and humans, the two pelvic bones are connected at their ventral ends, and the sacrum is wedged between them on the dorsal side. This forms a bone ring - the pelvis. In animals, the pelvis is a support for the hind limbs, and in humans - for the lower limbs due to its vertical position. In humans, the iliac bones expand significantly laterally, taking on the function of supporting the internal organs of the abdominal cavity.

The skeleton of the free part of the limbs in fish consists of a number of cartilaginous or bony segments arranged in the form of rays and creating a solid base for the fins. In the skeleton of the limbs of land vertebrates, the number of rays is reduced to five. The skeleton of the forelimbs and hindlimbs in animals, and the upper and lower limbs in humans, has a common structural plan and is represented by three links following one another: the proximal link (humerus and pelvic bones), the middle link (radius and ulna; tibia and fibula) and the distal link (hand, foot). The hand and foot in their proximal part consist of small bones, and in the distal part they are five free rays, called fingers. All of the indicated bone fragments are homologous on both limbs.

Due to the conditions of existence of terrestrial vertebrates, individual bone elements in the distal link of the limbs fused into one bone or underwent reduction. Less frequently, the development of additional bones is observed, most of which are sesamoid (patella, pisiform bone, etc.). In terrestrial animals, not only the anatomy of the limbs changed - their position also changed. Thus, in amphibians and reptiles, the proximal link of the free sections of both pairs of limbs is located at a right angle to the body, and the bend between the proximal and middle links also forms an angle open to the medial side. In higher forms of vertebrates, the free section is located in the sagittal plane in relation to the body, while the proximal link of the forelimb rotates backward, and the proximal link of the hind limb - forward. As a result, the elbow joint is directed backward, and the knee joint of the lower limb is directed forward.

As vertebrates developed further, the forelimbs began to adapt to a more complex function than the hind limbs. In connection with this, their structure also changed. An example is the wing of birds as a flying organ. Climbing mammals developed a grasping limb with an opposable thumb. All four limbs of a monkey have this function.

Man, the only one of all vertebrates, acquired a vertical position, began to rely only on the hind (lower) limbs. The forelimbs of man, which became upper due to the vertical position, were completely freed from the function of moving the body in space, which gave them the opportunity to sometimes make very fine movements. In this regard, the bones of the arm differ from the bones of the leg in greater lightness and fine structure. They are connected to each other by movable joints. Freedom of movement of the upper limb in humans also depends on the presence of the clavicle, which moves the free upper limb to the side. The human hand has adapted to labor activity, namely: the bones of the wrist are small, movably connected to each other; the fingers have lengthened and become mobile; the thumb is located almost at a right angle to the bones of the metatarsus, is very mobile and opposes all the other fingers, which ensures the grasping function of the hand when performing complex work.

The lower limb of a person performs the function of support, holding the body in a vertical position and moving it in space. In this regard, the bones of the lower limb are massive, the joints between individual links are less mobile than those of the upper limb. The different function of the upper and lower limbs in humans has had the greatest impact on the distal link - the hand and foot.

The hand develops and improves as an organ of labor. The foot serves to support the body, it bears all its weight. The toes do not play a significant role in support, they have become very short. The big toe is located in the same row as the other toes and is not particularly mobile.

The foot is a mechanically complex arched structure, due to which it serves as a springy support, on which the smoothing of shocks and vibrations during walking, running and jumping depends.

In human ontogenesis, the rudiments of the limbs appear in the 3rd week of embryonic life as a cluster of mesenchymal cells in the lateral folds of the embryo's body, resembling fish fins. The folds expand and form plates that give rise to the hands and, somewhat later, the feet. In these rudiments, fingers cannot yet be distinguished; they are formed later in the form of 5 rays. The sequence of further development of the elements of the future limbs is observed in the direction from the distal link of the limb to the proximal one.

All bones of the extremities, with the exception of the clavicles, which develop on the basis of connective tissue, bypassing the cartilage stage, go through three stages of development. In this case, the diaphyses of all bones ossify in the utero period, and the epiphyses and apophyses - after birth. Only some epiphyses begin to ossify shortly before birth. In each bone, a certain number of ossification centers are laid, which appear in a certain order. In the diaphyses of tubular bones, the primary ossification center appears at the end of the 2nd - beginning of the 3rd month of intrauterine life and grows in the direction of the proximal and distal epiphyses. The epiphyses of these bones in newborns are still cartilaginous, and secondary ossification centers in them are formed after birth, during the first 5-10 years. The bone epiphyses grow to the diaphyses after 15-17 and even after 20 years. The timing of the appearance of the main ossification points during the formation of individual bones deserves special attention.

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