Some Aspects of the Early Development of the Hippocampal Formation in Certain Insectivorous Bats

Abstract

Description is given of the development of the hippocampal formation for each stage considered in a series of Tadarida brasiliensis mexicana embryos ranging from 6 mm to somewhat over 12.5 mm CR [crown-rump] length. In 6 to 8 mm embryos, the cornu ammonis is foreshadowed by the migration of a few cells from the ependymal layer into the narrow marginal layer. Ventral to these cells, the future gyrus dentatus is indicated by an even narrower cell-free marginal layer, fa the 9 mm embryos a distinct cell layer constitutes cornu ammonis whereas only a few scattered cells migrating into the marginal layer foreshadow gyrus dentatus. Adjacent to gyrus dentatus, many cells (designated marginal cells) migrate from the ependymal layer to the outer part of the marginal layer leaving a concavity in which the fimbria will later develop. In the 10 mm embryo, between the better defined primordial pyramidal cell layer of cornu ammonis and the fimbria, a large mass of migrating cells constitutes the primordial gyrus dentatus. fa this cell mass the granular layer and the polymorphic layer begin to develop adjacent to cornu ammonis at 12 mm. Differentiation of the definitive granular layer, occurring in the periphery of the cell mass, spreads medially and is completed at a later stage (19 mm). The central cells become the polymorphic layer. The marginal cells do not significantly contribute to gyrus dentatus. All layers of cornu ammonis are recognizable in the 12 mm embryo. The subdivisions of cornu ammonis are clearly identifiable in the embryo somewhat over 12.5 mm in length. || ABSTRACT AUTHORS: Author