Morphological evidence for secondary formation of the tail gut in the rat embryo.

The secondary body formation is a developmental mechanism occurring in the caudal part of the embryo in which embryonic structures arise from a mass of mesenchymal cells without previous formation of germ layers. The formation of the tail gut by this mechanism was investigated on transverse serial semithin and ultrathin sections of 12-, 13-, 14- and 15-day rat embryo tails. The tail gut, together with the tail portion of the notochord, originates from an axial mass of condensed mesenchymal cells named tail cord. Formation of the tail gut involves the appearance of large intercellular junctions among tail cord cells, and rearrangement of these cells around a newly formed lumen. Mesenchymal characteristics of these cells are gradually lost, and they simultaneously acquire the morphology of epithelial cells. Some cells of the tail cord, located ventral to the tail gut, do not participate in the tail gut formation and form a separate mass of cells without any definitive morphogenetic fate. This surplus group of cells is first evident in 12-day embryos, and it increases in mass during the following 3 days. In 15-day embryos, after the tail gut has completely disappeared, the surplus cells represent all that remains of the tail cord. The mesenchymal-epithelial transformation of the tail cord cells into the cells of the tail gut, and the appearance of the surplus cells, could be considered as the main morphological arguments for the secondary formation of the tail gut.