VAN 211: Veterinary Histology and Embryology (2+2)

VERTEBRAL COLUMN

• The primitive axial support of all vertebrates is the notochord or chordadorsalis. It is a transient structure in mammals and consists of a cylindrical rod of mesodermal cells from the Sessel’s pocket to the tip of the tail.
• The notochord degenerates soon except at the intervertebral discs within which it persists as the swollen mucoid nuclei pulposi.
• The axial skeleton differentiates from the mesenchyme that traces its origin to serially arranged pairs of mesodermal segments - the somites.
• The ventro-medial part of a somite develops into the sclerotome and cells proliferate from this mass and migrate towards the notochord. The sclerotomes are destined to from the vertebrae and ribs.
• Each sclerotome presents and anterior loose and a posterior dense part.
• The anterior part of a sclerotome fuses with the posterior part of the sclerotome in front of it, and these reconstituted sclerotomes form the primordia of the definite vertebra.
• From each of these sclerotomes growth takes place in three directions
• medially to surround the notochord to from the body,
• above to form the neural arch.
• ventrolaterally to provide costal processes or primordia of ribs.
• Following this blastemal stage, chondrification sets in during the seventh week and ossification during the tenth week. When the atlas forms its body, it is soon appropriated by the axis as the dens.
• In the thoracic region, the costal processes enlarge and from the ribs. Here the original union of costal process with the vertebrae is replaced by joints (costo-central and costotransverse articulations). Chondrification and ossification follow to form the ribs. The costal processes remain diminutive and are fused with the transverse processes in the cervical and lumbar regions.
• The sternum arises from a pair of mesenchymal bands on the ventrolateral aspects of the body wall. These unite progressively from before backwards and obtain connection with the ribs and cartilages.
• The mesenchyme of the sternum undergoes chondrification at nine weeks and ossification at about five months. The segmentation of the sternum is a feature secondarily acquired.