Bovine somite development and vertebral anlagen establishment.

Abstract

AbstractSerial sections of seventy four bovine embryos from 18 through 38 days gestation were studied to determine the processes and sequence of somite development and differentiation.Somites are formed from day 20 through day 32 or 33 depending upon the individual. Each newly formed somite consists of a discrete block of cells organized into a densely aggregated outer layer enclosing a core of loose mesenchymal cells. Each somite differentiates about 12 hours after its formation into sclerotome and dermamyotome. The ventral and medial surface layer becomes loosely mesenchymal continuous with the mesenchymal core while the rest of the surface layer remains unchanged as dermamyotome. Further change occurs during the next 24 hours as the dermamyotome flattens with somite growth to form a distinct surface layer of perpendicularly arranged cells, the dermatome, and an inner layer of longitudinally arranged cells, the myotome. By day 4 after a somite is formed a transitory artifact, the so‐called “myocoele”, is often produced during dehydration for sectioning. The shrinkage artifact occurs progressively caudal through the sacral region until the cellular density of the myotomes equals that of the dermatomes.Vertebral development begins ventro‐medial to somite pairs 6 through 12 on day 23 as sclerotomic cells form a continuous band beneath the notochord. Band production continues caudad progressively. The sclerogenic band is transformed into a continuous sclerogenic tube around the notochord at first ventro‐medial to somite pairs 15 through 20 and then progressively cranial to somite pair 6 and caudal to somite pair 25 during day 26 as the dorsolaterally attached sclerotomes develop dense caudal halves. During day 27, sclerotomic cells from the caudal dense sclerotome halves form perichordal rings within the sclerogenic tube of the cervival and thoracic regions alternating with the blastemal centra which become vascularized during day 28. Neural and transverse processes develop during day 29 from the caudal dense sclerotome halves connected to the perichordal rings and cranial portion of the centra. Sclerotomes of the cervical through sacral regions disappear by day 31. The blastemal centra become procartilaginous (new term) during day 32 through 34. Precartilage is produced in the ribs during day 33, cervical and thoracic neural processes during day 35, lumbar and sacral neural processes and cervical and lumbar transverse processes during day 36, and sacral transverse processes during day 37.