Development and age-specific features of the torso bones

In the development of the skeleton of vertebrates, including humans, three stages are distinguished: connective tissue (membranous), cartilaginous and bone. At first, the dorsal string is formed, which occupies an axial position and is gradually surrounded by embryonic connective tissue. Thus, the primary connective tissue skeleton, which is present in the lancelet, begins to form.

In cyclostomes (lampreys, hagfish) and in lower fish (sharks, sturgeons), the notochord exists simultaneously with primitive cartilaginous vertebrae. In higher vertebrates, the dorsal string is present only in the embryonic period.

During the development of most representatives of chordates, the membranous skeleton is replaced by a cartilaginous one. In the embryonic connective tissue surrounding the notochord and neural tube, islands of cartilaginous cells appear - the rudiments of future cartilaginous vertebrae. The third stage of skeletal development - bone - follows the cartilaginous one in higher animals. Bone tissue develops in place of the cartilage it displaces.

The complex process of skeletal development in phylogenesis is repeated in its main features in the embryonic period in humans. After the formation of the spinal cord, embryonic embryonic connective tissue spreads around it and between the germ layers, which is gradually replaced by cartilage. The bone skeleton is subsequently formed in place of the cartilage.

The bones of the human trunk develop from primary segments (somites) - derivatives of the dorsal part of the mesoderm. The mesenchyme emanating from the medioventral part (sclerotomes) of each somite envelops the notochord and neural tube, resulting in the formation of primary (membranous) vertebrae. In the 5th week of development of the human embryo, separate nests of cartilaginous tissue appear in the bodies and emerging dorsal and ventral arches of the vertebrae, which subsequently merge with each other. The notochord, surrounded by cartilaginous tissue, loses its purpose and is preserved only in the form of a gelatinous core of the intervertebral discs between the bodies of the vertebrae. The dorsal arches of the vertebrae, growing, form unpaired spinous processes, paired articular and transverse processes upon fusion. The ventral arches grow in the form of stripes to the sides and penetrate the ventral sections of the myotomes, forming the ribs. The anterior ends of the nine upper cartilaginous ribs expand and merge on each side into cartilaginous (pectoral) stripes. By the end of the 2nd month of the embryo's life, the upper ends of the right and left pectoral stripes merge, forming the manubrium of the sternum. Somewhat later, the lower sections of the pectoral stripes also join together - the body of the sternum and the xiphoid process are formed. Sometimes these stripes do not merge along their entire length, then the xiphoid process remains bifurcated below.

At the beginning of the 8th week, the replacement of the cartilaginous skeleton with a bone one begins. In each rib, at the site of the future angle, an ossification point appears, from which the bone tissue spreads in both directions and gradually occupies the entire body of the rib. The head of the rib has an ossification point at the 15-20th year of life. In the 10 upper ribs, at the 15-20th year of life, the ossification point also appears in the tubercle of the rib.

Up to 13 ossification centers are formed in the sternum, with one or two in the manubrium already in the 4th-6th month of intrauterine life. In the 7th-8th month, ossification centers appear in the upper part of the body (usually paired), in the middle part - before birth, and in the lower part - in the 1st year of life. Individual parts of the sternum grow together into a single bone body in the 15th-20th year. The xiphoid process begins to ossify in the 6th-20th year and fuses with the body of the sternum after 30 years. The manubrium fuses with the body later than all parts of the sternum or does not fuse at all.

The vertebrae begin to ossify at the end of the 8th week of embryogenesis. Each vertebra has 3 ossification centers: one in the body and two in the arch. The ossification centers in the arch merge in the 1st year of life, and the arch fuses with the vertebral body in the 3rd year or later. Additional ossification centers in the upper and lower parts of the vertebral bodies appear after 5-6 years, and grow onto the body at 20-25 years. Independent ossification centers are formed in the processes of the vertebrae. The cervical vertebrae (I and II) differ in development from the other vertebrae. The atlas has one ossification center in the future lateral masses, from where the bone tissue grows into the posterior arch. In the anterior arch, the ossification center appears only in the 1st year of life. Part of the body of the first vertebra separates from it at the stage of the cartilaginous period and joins with the body of the second vertebra, turning into a odontoid process (tooth). The latter has an independent ossification point, which merges with the bone body of the second vertebra in the 3rd-5th year of the child's life.

The sacral vertebrae develop in the same way as the others, from three main ossification centers. In the three upper sacral vertebrae, additional ossification centers appear in the 6th-7th month of intrauterine life, due to which the lateral parts of the sacrum (rudiments of the sacral ribs) develop. In the 17th-25th year, the sacral vertebrae fuse into a single bone. In the coccygeal (rudimentary) vertebrae, one ossification center appears at different times (in the period from 1 year to 20 years).

The human embryo has 38 vertebrae: 7 cervical, 13 thoracic, 5 lumbar, and 12-13 sacral and coccygeal. During the embryo's growth, the following changes occur: the 13th pair of ribs is reduced and fuses with the transverse processes of the corresponding vertebra. The last thoracic vertebra turns into the first lumbar, and the last lumbar vertebra becomes the first sacral. Later, most of the coccygeal vertebrae are reduced. Thus, by the time of birth, the spinal column has 33-34 vertebrae.

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