Ultrastructure of secondary neurulation in the chick embryo

TLDR

Primary neurulation forms the dorsal neural tube, while the ventral lumbosacral region develops by secondary neurulation through cavitation of a medullary cord derived from the tail bud, though the source and composition of the extracellular material involved remain unknown. The study aimed to delineate the four morphogenetic steps of secondary neurulation: segregation of medullary cord cells, precise cord delimitation, central cavitation, and luminal coalescence into a single cavity. Using light and electron microscopy on two‑day‑old chick embryos, the authors observed medullary cord formation via peripheral cell elongation, basal and apical intercellular junctions, and the sequential development of a pseudostratified outer layer and an inner irregular cell cluster. They found a neurulation overlap zone at this level, cell segregation creating an extracellular layer between organ rudiments, cavitation initiating at the junction of the outer and inner cell populations, and eventual elongation of central cells that may intercalate into the neural tube as lumina merge.

Abstract

Abstract 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. (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 junctions, first at the basal (outer) ends of the elongating peripheral cells, and then at their apical (inner) ends. These events resulted in the formation of an outer pseudostratified layer of radially 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.