Scanning and transmission electron microscope studies of interkinetic nuclear migration in the cerebral vesicles of the rat

Abstract

A scanning electron microscope (SEM) was used to examine the morphology and surface texture of neuroepithelial cells during interkinetic nuclear migration in the cerebral visicles of the rat at 12, 13 and 14 days of gestation. Serial sections of embryonic material of the same age were also prepared for the transmission electron microscope (TEM). Particular attention was paid to the SEM and TEM appearance of mitotic neuroepithelial cells which occur exclusively along the ventrivcular border of the neural epithelium. Three distinctly‐shaped classes of mitotic cells were recognised in sacnning micrographs. (1) Pyriform cell. This type of mitotic cells was characterised by the presence of very long, fin procsses rediating from the tip and shoulders of a short external (basal) process. These fine processes were temed “intramitoticfilopodia” Microvilli were found on the surface of most pyriform cells. (2) Conical cells. Thee lacked an external process but there were large numbers of intramototic filopodia at the basal pole of the cell body, and the perkikaryal surface was rich in microvilli. (3)Globular cells. It was possible to subdivide this class of cell into large and small types. An external process and microvilli were absent in globuar cells of both sizes, but usually a few short intramitotic filopodia were absent in globlar cells of both sizes, but usually a few short intramitotic filopodia were present at the bnasal pole. The perikaryal surfced of the globular population were reised in coarse liums and bubble‐like protrusions. By pooling TEM and SEM information we were able to deduce that pyriform cells probably possess a prphase or prometaphase chromosome morphology, While conical cells exhibit a chromosome morphologyu somewhere between prometphase and early anaphase. Lrge globular mitotic cells were found to be between metaphase and late anaphase and small glbular cells were indentified as early telophase cells. On the basis of these findings we have proposed that as a bipolar neuroepithelial cell rounds up for motosis it passes first through a pyriform state during which the external process is retracted or broken down, and then through a conical stage when the cell consolidates its position on the verntricular surface. Finally, the cell enters a large gobular stage before dividing into two small filopodia play in this procss of rounding up.