Development and age characteristics of the bones of the trunk
Last reviewed: 23.04.2024
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In the development of the skeleton of vertebrates, including humans, three stages are distinguished: connective tissue (membranous), cartilaginous and bone. First, a spinal cord is formed, which occupies an axial position and is gradually surrounded by an embryonic connective tissue. Thus, the primary connective tissue skeleton, which is found in the lancelet, begins to form.
In cyclostomes (lampreys, myxins) and in lower fish (sharks, sturgeons), the chord exists simultaneously with primitive cartilaginous vertebrae. In higher vertebrates the spinal cord is present only in the embryonic period.
In the process of development in most representatives of the chordates the membranous skeleton is replaced by cartilaginous. In the embryonic connective tissue surrounding the chord and the neural tube, there appear islands of cartilaginous cells - the germs of future cartilaginous vertebrae. The third stage of development of the skeleton - bone - in higher animals follows the cartilaginous. Bone tissue develops on the site of cartilage displaced by it.
The complex process of development of the skeleton in phylogeny is repeated in the main features in the embryonic period in man. After the formation of the spinal cord around it and between the embryonic sheets, embryonic embryonic connective tissue spreads, which is gradually replaced by cartilaginous tissue. In place of cartilage, the skeleton is subsequently formed.
The bones of the human body evolve from primary segments (somites) - derivatives of the dorsal section of the mesoderm. Evacuated from the medioventral part (sclerotome) of each somite, the mesenchyme envelops the chord and the neural tube, resulting in the formation of primary (membranous) vertebrae. At the 5th week of the development of the human embryo, separate nests of the cartilaginous tissue appear in the bodies and in the emerging dorsal and ventral arcs of the vertebrae, which later merge with each other. Surrounded by a cartilaginous tissue, the chord loses its function and is retained only in the form of a gelatinous nucleus of intervertebral discs between vertebral bodies. The dorsal arcs of the vertebrae, growing, form at the confluence of unpaired spinous processes, paired articular and transverse processes. The ventral arches grow in the form of strips to the sides and penetrate into the ventral parts of the myotomes, forming ribs. The front ends of the nine upper cartilaginous ribs expand and on each side merge into the cartilaginous (thoracic) striae. By the end of the second month of the embryo's life, the upper ends of the right and left breast strips merge, forming a sternum arm. A little later, the lower sections of the chest strips are joined together - the body of the sternum and the xiphoid process are formed. Sometimes these strips do not merge all the way, then the xiphoid process at the bottom remains bifurcated.
At the beginning of the 8th week, the replacement of the cartilaginous skeleton with bone begins. In each rib, the point of ossification occurs at the site of the future angle, from which the bone tissue spreads in both directions and gradually occupies the entire body of the rib. The head of the rib has a point of ossification at the 15-20th year of life. In the 10 upper ribs, in the 15-20th year of life, the ossification point also appears in the tubercle of the rib.
In the sternum, up to 13 points of ossification are laid, and in the handle one or two are already in the 4-6th month of intrauterine life. On the 7th-8th month, ossification points appear in the upper body (often paired), in the middle section - before birth, and in the lower one - on the 1st year of life. Individual parts of the sternum are fused into a single bone body in the 15-20th year. The oblong process begins to ossify in the 6-20th year and fuses with the sternum after 30 years. The handle with the body fuses later than all parts of the sternum or does not grow together at all.
The vertebrae begin to ossify at the end of the 8th week of embryogenesis. In each vertebra there are 3 points of ossification: one in the body and two in the arc. The points of ossification in the arc merge in the 1st year of life, and the arc fuses with the vertebral body in the 3rd year or later. Additional points of ossification in the upper and lower parts of the vertebral bodies appear after 5-6 years, but grow to the body in 20-25 years. Independent nuclei of ossification are formed in the processes of the vertebrae. Cervical vertebrae (I and II) differ in development from the remaining vertebrae. Atlas has one ossification point in future lateral masses, hence the bone tissue grows into the posterior arch. In the anterior arc, the point of ossification appears only in the first year of life. The part of the body I of the vertebra is separated from it at the stage of the cartilaginous period and connects with the body II of the vertebra, turning into a tooth-shaped process (tooth). The latter has an independent point of ossification, which merges with the bony body of the second vertebra in the 3-5th year of the child's life.
The sacral vertebrae develop in the same way as the rest of the three main points of ossification. The three upper sacral vertebrae at the 6-7th month of intrauterine life appear additional points of ossification, due to which the lateral parts of the sacrum develop (rudiments of the sacral ribs). On the 17-25th year the sacral vertebrae fuse into a single bone. In the coccygeal (rudimentary) vertebrae, one ossification point appears at different times (from 1 year to 20 years).
The human embryo contains 38 vertebrae: 7 cervical, 13 thoracic, 5 lumbar and 12-13 sacral and coccygeal. During the growth of the embryo, the following changes occur: the 13th pair of ribs is reduced and fused with the transverse processes of the corresponding vertebra. The last thoracic vertebra turns into the first lumbar, and the last lumbar vertebra becomes I sacral. Later, most of the coccygeal vertebrae are reduced. Thus, at the time of the birth of the fetus, the vertebral column has 33-34 vertebrae.
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