Vertebra

The vertebra refers to each of the bones or cartilages that make up the vertebral column, a structure that protects the spinal cord and supports vertebrate animals.

In humans there are 33 vertebrae during the fetal stage and in childhood (7 cervical + 12 thoracic + 5 lumbar + 5 sacral + 4 coccyx), and during the adult stage, there are only 24 because the vertebrae The bones of the sacrum and the coccyx unite, becoming one bone each. Each of them is separated from the one immediately below it by means of a vertebral disc, except for the 5 vertebrae of the sacrum and the 4 of the coccyx, due to their Union.

The vertebrae are aligned with each other by the so-called vertebral bodies and by their articular processes. Between one vertebra and another there are nuclei of loose connective tissue called intervertebral discs.

With the exception of the first and second cervical vertebra, the so-called true or movable vertebrae (belonging to the aforementioned three upper regions) present certain common features that are best recognized by examining a vertebra in the middle of the thoracic region.

General structure

Vertebrae show a similar general structure: a body and a dorsal (or posterior in humans) neural arch composed of two vertebral laminae, two pedicles, a spinous process, and articular processes. Fish vertebrae also have hemal arches in the tail region that protect the blood vessels, with a structure similar to the neural arches but ventral position. Tetrapods also have transverse processes and one or two specialized neck vertebrae called Atlas and Axis. In tetrapods, the hemal arches become the chevron bones.

Types of vertebrae according to their position

Fish

In fish, the vertebrae are divided into truncal and caudal. The trunk vertebrae have basiapophyses that articulate with the ribs, while the caudal processes have hemal arches.

Tetrapods

In non-mammalian tetrapods there are cervical vertebrae, related to the neck, trunk, articulated with the ribs, sacral, modified for articulation with the pelvic girdle, and caudal. The differentiation of the trunk vertebrae into thoracic and lumbar vertebrae is characteristic of mammals.

Mammalian Vertebrae

Cervical vertebrae

They are generally small and dainty. Its spinous processes are small and bifid (C7 is the first vertebra whose spinous process can be palpated). They can be differentiated by having a hole at the base of the transverse processes (hole for the vertebral artery). Numbered from top to bottom C1 through C7, they are the vertebrae that allow rotation of the neck. Specifically, the atlas (C1) allows the skull to go up and down, and the axis (C2) is responsible for the upper part of the neck turning from left to right, then there is the rixi vertebra (C3) which is the pattern vertebra, from it all the vertebrae are practically the same. The C6 vertebra has what is known as the "Chassaignac tubercle". The C7 vertebra is known as the "Vertebra Prominente". In addition, they have a very wide spinal canal, because it coincides with the beginning of the spinal cord. The intervertebral discs of the cervical region create what is called the cervical lordosis (dorsal concave curvature) of the spine.

Thoracic vertebrae

Their spinous processes point downward almost vertically, and are smaller relative to those of the other regions. They have articular facets on their lateral faces (costal pits), which articulate with the head of the ribs, and another articular facet on their transverse processes intended to articulate with the costal tubercle. They have a small degree of rotation between them but, being articulated with the ribcage, they become almost immobile. The intervertebral discs in the thoracic region create what is called the thoracic kyphosis (dorsal convex curvature) of the spine.

Lumbar vertebrae

They are much more robust vertebrae than the previous ones since they have to support greater weights. It has a triangular vertebral foramen, its processes are long and thin. They allow considerable flexion and extension, moderate lateral flexion, and a small degree of rotation (5º). The intervertebral discs in the lumbar region create what is called the lumbar lordosis (dorsal concave curvature) of the spine. In addition, its spinous process is quadrilateral and lies almost horizontally.

Sacral Vertebrae

The sacral vertebrae present the transverse processes, and in many cases the bodies, fused together, forming a single structure called the sacrum.

Types of vertebrae according to the morphology of their bodies

• Acelics or amphiplanes: the surface between the bodies is flat or slightly concave or convex. This is the typical mammal vertebrae.
• Procelics: the previous part of the bodies is concave and the later is convex. Typical of lizards and snakes (diapsies).
• Opistocelics: the previous part of the bodies is convex and the later is concave. Also typical of deapsides.
• Anphicelics: the ends of the bodies are concaves. Typical of condrictios, bonefish and primitive amphibians.
• Heterocelics: shape of a saddle, appear in cervical and troncales of birds except in penguins.

Embryogenesis

During embryonic development in most vertebrates, sclerotome cells migrate to surround the spinal cord and notochord. The sclerotome is formed from the paraxial mesoderm and originates from the ventromedial part of the somites. The column of tissue surrounding the notochord and medulla has a segmented appearance, with alternating parts of dense and less dense areas, which in some fish become separate masses of tissue called sclerotomites. This division never occurs in tetrapods. Each pair of dense and less dense zones accompanies a myotome. Finally, a posterior part will combine with an anterior part, in a process known as resegmentation, which causes the vertebrae to have an intermediate position between two successive myotomes. As the sclerotome develops, it becomes more condensed and begins to form the vertebral body on one side and the arches on the other. The arches appear primarily in the evolution of chordates, and in chondrichthyans and some fish they are formed from paired structures, four on each side of the notochord, called arcualia. In many arcualia fish, the notochord is persistent throughout life. The body or center appears later and can be formed in different ways: from sclerotomous cells belonging to the arches (we speak of arcocentre), from sclerotomous cells that invade the interior of the notochord (cordacentre) and from cells that strangle the notochord from the outside in (autocenter or perichondral center). This last form is the only one present in amniotes, although in other groups two or even all three types of development may be present. In turn, the body can ossify directly without going through a cartilage stage (intramembranous ossification, actinopterygians, some amphibians), or first form a cartilage mold (endochondral ossification, chondrichthyans, amniotes). The arches of extant osteichthyans (including tetrapods) develop from the anterior dorsal pair of arcualia (called basidorsals). In teleosts and amniotes, the arches and bodies are always fused together, a condition called holospondyly. Primitive tetrapods have diplospondylar vertebrae, with two bodies, one cranial and one caudal (both perichondral), which take different names according to the greater development of one or the other. While current tetrapods have monospondylar vertebrae, with only one body. These two centers of primitive vertebrates, called the pleurocentre and the hypocentre (or intercentre) were formerly homologated to arcualia by the German naturalist Hans Gadow, who had studied the development of chondrichthyans. This theory is currently abandoned, considering that both centers are perichondral centers.

The development of the proper shapes of the vertebral bodies is regulated by HOX-like genes. The notochord disappears in the sclerotome (vertebral body) segments, but will persist in the intervertebral discs.