VERTEBRAL COLUMN

Vertebral Column

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Notochord

In all chordate embryos, the first axial endoskeleton to appear is a slender, stiff, unsegmented, gelatinous rod, the notochord. It is present below the nerve cord and above the digestive tract. Its ancestral predecessor is not known but it probably originated from endoderm. Topically, notochord is covered by inner and outer elastic fibrous connective tissue sheaths, called elastica interna and elastica externa, respectively.

In protochordates (amphioxus) and cyclostomes (lamprey), notochord persists throughout life and continues to grow with the animal. But in fishes and higher types, notochord is later on surrounded by cartilaginous or bony rings, called vertebrae. In most fishes and aquatic amphibians, the adult notochord is constricted within each vertebra. It is not constricted in lungfishes and sturgeon. In tetrapods, it is practically obliterated.

Vertebrae

Backbone or vertebral column of all vertebrates is formed of a metamerie series of many small and essentially similar pieces, called vertebrae. Thus, a vertebra is the unit of vertebral column. Vertebrae are named after the region of body in which they occur. Vertebral column of fishes comprises only trunk (abdomen) and caudal (tail) vertebrae. In tetrapods, vertebral column includes five regions : cervical, thoracic, lumbar, sacral and caudal, each having usually several vertebrae. Amphibians have a single cervical (atlas) and only one sacral (9th) vertebra. Morphologically, vertebrae differ in different vertebrates or even in different regions of the same vertebrate, but all vertebrae are built according to a similar basic pattern.

Basic structure of a vertebra

Typically, a vertebra has a cylindrical, spool-like body or centrum, which encloses or replaces the embryonic notochord (Fig. 3C). Above the centrum is a neural arch produced dorsally into a neural spine. Successive neural arches enclose a vertebral or neural canal in which the spinal cord lies. The caudal vertebra in fishes also has a ventral haemal arch enclosing a haemal canal through which the caudal artery and vein pass. Haemal arch also carries a ventral haemal spine.

Types of processes Various kinds of processes (apophyses) arise from the arches or centra of vertebrae.

Zygapophyses :-In vertebrates, from anterior and posterior faces of neural arch project paired articular facets, the pre- and post- zygapophyses. These serve for articulation between adjacent vertebrae. Zygapophyses do not occur in fish vertebrae.
Transverse processes :- Lateral transverse processes arise from centrum and serve for attachment of ligaments and muscles.
Diapophyses :- Each projects laterally from centrum or neural arch and articulates with dorsal head (tuberculum) of thoracic rib.
Parapophyses :- Each projects laterally from centrum and articulates with ventral head (capitulum) of rib.
Basapophyses :-These project ventrolaterally from centrum or haemal arch, or meet ventrally to form haemal arch.
Pleurapophyses :- These are lateral transverse processes fused with short ribs at their tips.
Hypapophysis :- It is a single prominent mid-ventral projection of centrum in certain vertebrae.

Types of centra and vertebrae :-

An intervertebral disc or intercentrum is often present between centra of successive vertebrae in embryo. This may fuse with anterior or posterior end of a centrum changing its shape to convex or flat. On the basis of the particular shape of centra, the following main types of vertebrae occur

Procoelous (pro – in front + coelous – hollow) :- Anterior face of centrum is concave and posterior face convex, e.g. typical vertebrae of frog and most reptiles.
Opisthocoelous (opistho = at the back) :- Centrum is concave posteriorly and convex anteriorly, e.g. cervical vertebrae of some large ungulates.
Amphicoelous (amphi = both) :- Centrum is concave at both ends. e.g. vertebrae of most fishes and tailed amphibians, 8th vertebra of frog.
Acoelous or amphiplatyan (a = absent; platy – flat) :- Centrum is flat at both ends, without a concavity or a convexity, e.g. vertebrae of mammals.
Biconvex (bi – two) :- Centrum is convex at both ends, e.g., sacral or 9th vertebra of frog.
Heterocoelous (hetero = asymmetrical) :-Ends of centra are shaped like a saddle, e.g. vertebrae of modern birds.

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