MORPHOLOGICAL AND IMMUNOHISTOCHEMICAL CHARACTERISTICS OF AXIAL STRUCTURES IN THE TRANSITORY HUMAN TAIL

Ultrastructural relationships between the notochord and neighboring sp inal cord were examined during the regression of the human tail. Also, the presence of certain extracellular matrix components in the notoch ord was immuno-histochemically analysed in the 4th to 12th week old em bryos. At the early stages, a close apposition of the notochord to the spinal cord exists in the entire tail region. The external surface of both structures is covered with a continuous basal lamina. The narrow tissue interspace contains interdigitating cell processes and both am orphous and fibrillar extracellular matrix material. With advancing em bryonic age, separation of the two structures occurs in craniocaudal d irection and the widening interspace becomes occupied by mesenchymal c ells. During tail regression and spinal cord retraction, the appearanc e of large intercellular spaces and cell degeneration takes place in b oth tissues. With age, the extracellular matrix of the notochord, pred ominantly the perinotochordal sheath, increases in amount and antigeni c complexity. While the intensity of laminin, collagen type IV and typ e III expression rises continuously during the period examined, the ex pression of fibronectin begins first at later stages, after the separa tion of the notochord from the spinal cord. The possible developmental significance of the described phenomena in the regression of the post erior end of the human tail remains to be elucidated.