Morphogenetic features in the tail region of the rat embryo.

The secondary (direct) body formation is a mechanism of development in which morphogenesis of various organs occurs directly from a mass of undifferentiated mesenchymal cells (blastema) without previous formation of germ layers. It is characteristic of the posterior end of the embryonic body, i.e. of the tail bud of tailless and the tail of tailed mammals. Development of the neural tube occurring by this mechanism (secondary neurulation) has been previously explained. We investigated the morphogenetic mechanism by which two other axial structures in the rat tail develop: the tail gut and the notochord. Both structures develop from an axial condensation of undifferentiated mesenchymal cells (tail cord) of tail bud origin. The tail gut forms in a similar way to the secondary neural tube: cells in the ventral part of the tail cord elongate, acquire an apicobasal polarity and form a rosette-like structure around a lumen in the centre. The notochord forms by detachment of a group of cells of the tail cord dorsally to the developing tail gut. The peculiarities of this morphogenetic mechanism in comparison with those in other parts of the embryo are discussed. Causal (including evolutionary) explanations of this mechanism are ruled out.