The notochord of the newborn opossum and its fate during postnatal development.

The notochord was examined by light, transmission and scanning electron microscopy, in 25 pouch young opossums (Didelphis virginiana) ranging in age from 6 hr to 25 days post natum. In the newborn opossum, the notochord forms a thin cylinder within developing vertebral centra and presents diamond-shaped expansions in regions of the developing intervertebral discs. The opossum notochord differs from that of other species in that it appears to consist of a heterogeneous population of light and dark cells, and also in the thinness of the notochordal sheath which never attains the degree of development seen in other animals. The sheath is made up of thin cytoplasmic processes which arise both from the light and dark cells. The processes cross a mucopolysaccharide-filled space and then expand to form a thin cytoplasmic sheath that forms the external boundary of the space. The matrix of the cartilage immediately adjacent to the cytoplasmic sheath shows an increased concentration of microfilaments as compared to the cartilage further from the notochord. During postnatal development the notochordal tissue in the centra of the vertebral bodies decreases, while in the regions of the developing discs, there is a corresponding increase. At the end of the first week, notochordal tissue in the vertebral centra is represented only by a thin cylinder of mucopolysaccharide which contains cell fragments. In subsequent weeks, as ossification of the centra occurs, both the cartilage and notochordal tissue are replaced by bone. Notochordal tissue in the intervertebral regions contributes to the formation of the nucleus pulposus of the intervertebral discs. Despite its short gestation period of only 12 1/2 days, the basic structure and fate of the notochord in the opossum is similar to that of other mammals.