Ultrastructure of secondary neurulation in the chick embryo.

Formation of the future lumbosacral level of the spinal cord was studied in two-day-old chick embryos by light and electron (transmission and scanning) microscopy. A neurulation overlap zone occupied this level. The dorsal portion of the neural tube formed by bending of the neural plate and approximation and fusion of neural folds (i.e., by primary neurulation), and the ventral part formed during secondary neurulation by cavitation of an initially solid, compact mass of cells, the medullary cord, derived from the tail bud. Secondary neurulation involved four morphogenetic processes: (1) segregation of the cells of the prospective medullary cord from cells of adjacent regions, (2) formation of a precisely delimited medullary cord, (3) cavitation of the central portion of this cord, and (4) coalescence of all lumina into a single, central cavity. Cell segregation was associated with the formation of a layer of primarily extracellular materials between adjacent organ rudiments. The source and composition of these materials are unknown. Formation of the medullary cord entailed considerable elongation of the peripheral cells of this developing structure and the fabrication of small intercellular juctions, first at the basal (outer) ends of the elongating peripheral cells, and then at their apical (inner) ends. These arranged, columnar cells, having characteristics similar to those of the neural plate, and an inner cluster of irregularly shaped and arranged cells. Cavitation always occurred first at the junction between these two cellular populations. The central cells of the medullary cord also eventually elongated, like the peripheral cells, and may have been intercalated into the lateral walls of the developing neural tube as lumina coalesced.