The vertebral column is derived from the somites. The somites develop in a craniocaudal sequence from the 20th to 30th day of development. Each somite develops a cavity called somito-cele, which contains looselyarranged core cells.
In the 4th week of development the somites begin to rupture and the core cells of each somite along with the cells of its ventromedial wall form a mass of proliferating mesenchymal cells which is called a sclerotome. The sclerotomes derived from each somite pair migrate toward the notocjiord and surround it. Gradually, the notochord becomes surrounded by a series of sclerotomic masses (also called scle-rotomic blocks or sclerotomic segments). Each sclerotomic mass is derived from the ventromedial walls of a somite pair. Successive sclerotomic masses are separated from each other by the intersegmental arteries (which are the branches of the dorsal aorta).
By the end of the 4th week, the mesenchymal cells in the caudal part of each sclerotomic mass proliferate rapidly and, consequently, this part condenses into densely packed cells (the cranial part of the sclerotomic mass still consists of loosely packed cells). Following this, the two parts of every sclerotomic block become separated from each other. As the sclerotomic masses split, the denser caudal part of each mass joins the looser cranial part of the ; sclerotomic block caudal to it. These recombinations (and not the original sclerotomic blocks) are the primordia of the vertebrae. Since a vertebra devel-ops from parts of two adjacent sclerotomes, the original intersegmental arteries now come to pass midway across the body of a vertebra.
Recombination of sclerotomes causes the myotomes to bridge the intervertebral discs.This alteration allows the myotomic muscles to move the vertebral column. The segmental (ie, spinal) nerves lie at the level.of intervertebral discs and leave the vertebral column through the intervertebral foramina.
Each recombined mesenchymal mass forms the centrum (central mass of the body) of a vertebra. The centrum Is also called precartilaginous vertebral body.
Recombination of the sclerotomic masses establishes intervertebral fissures between the developing vertebral bodies.Mesenchymal tissue de- rived from the denser portions of the vertebral primordia fills the inter-vertebral spaces and contributes to the formation of the intervertebral discs (for further details on the development of the intervertebral discs see below).
As development progresses, processes arise from the precarti-laginous vertebral body. Two mesen chymal processes grow in a dorsomedial direction flanking the developing jral tube. Eventually, these processes meet and fuse in the midline j, thus, form the vertebral arch (also lied neural arch) that encloses the l cord. Ventrolateral growth from half of the centrum produces costal processes (in the thoracic region processes form the ribs).
During its formation, each de-)ing vertebra passes through three stages :(1) precartilaginous stage, (2) idrification stage, and (3) ossifica-i stage.
The precartilaginous stage has been described above. During this stage, the developing vertebra (consisting of the centrum and its processes) is formed of condensed mesenchyme.
The chordrification stage begins in the 6th week. During this stage Imjmber of centers of chordrification y in the mesenchymal vertebrae,such centers appear in the centrum. These centers fuse with each other by the end of the 8th week to form a cartilaginous centrum. Simultaneously, four centers of chondri-fication are seen in the vertebral arch. These centers fuse with each other as well as with the centrum. Extensions of chondrification centers in the vertebral arch give rise to the spinous and transverse processes.
The ossification stage begins during the embryonic period and ends at about 25 years of age. Three primary centers of ossifications are seen by the end of the embryonic period: one in the centrum and one in each half of the neural arch. Thus, at birth each vertebra consists of three bony parts connected by cartilage. During postnatal development the vertebral arch halves fuse by the end of the first year of life, and the vertebral arch fuses with the centrum at *3 to 6 years of age. After puberty five secondary centers of ossification appear in each vertebra: one for the tip of spinous process, one for the tip of each transverse process, and two annular (ring-shaped) epiphyses for the vertebral body: one on the upper surface and another on the lower surface. All the secondary centers of ossification unite with rest of the vertebra by the 25 years of age.
The above given account holds good for the vertebral development in general. However, a few deviations from the general pattern of development also occur which are described below:
(1) The body (centrum) of the first cervical vertebra (atlas) fuses with that of the second cervical vertebra (axis) and forms the odontoid process (dens) of the axis. Consequently,the atlas is left only with the neural arch which grows anteriorly and eventually fuses in the midline to form the characteristic ring-like first cervical vertebra.
(2) The sacral and coccygeal vertebrae represent types with reduced vertebral arches. Between puberty and about 25 year age, the sacral vertebrae unite progressively into a single bony mass.The rudimentary coccygeal vertebrae also undergo a similar fusion with each other.In later life, the coccyx often fuses with the sacrum.