The early development of the neopallial wall and area choroidea in fetal rats. A light and electron microscopic study.

The telencephalic wall was studied by light and electron microscopy in 11-13 day old fetal rats (E11-13). A few specimens from E14-16 were also included for comparisons. Two areas were selected: the dorso-lateral convexity of the hemispheric vesicles, called the neopallial wall, and the area choroidea, the posterior part of the telencephalic roof which unites the two hemispheres. Our observations and a review of the literature have shown that on E11-12 the neopallial wall, the telencephalic roof, and the hippocampal anlage between them form a continuous, nonstratified, cohesive monolayer of columnar and mitotic cells, which essentially is similar to epithelial monolayers elsewhere in the body. This simple structure is modified late on E12 or early in E13 in the neopallial wall when postmitotic neurons appear and migrate in order to form the cortical plate. However, bipolar radially oriented cells, which span the entire width of the wall, still predominate. These cells, now called radial glial cells, increase greatly in number and length during the period of neuronal migration. The cuboidal cells in the neural tube, the columnar cells in the early neopallial wall, and the radial glial cells in the period of neuronal migration have the same basic structure. They are axially polarized epithelial cells which are characterized by the following basic features. They have an elongated bipolar shape which is maintained by a cytoskeleton of longitudinally oriented microtubules. Opposite ends are different structurally and functionally. Thus, the apical ends, connected by tight junctions, face the fluid-filled cavity while the outer ends, covered by a basal lamina, face mesenchymal tissue including blood vessels. A polarization of cytoplasmic organelles is also evident, e.g. the Golgi apparatus has always a supranuclear position. During the early development of the telencephalon this basic epithelial structure is maintained but is modified locally in order to serve various functions. The columnar/radial glial cells in the neopallial wall are elongated and slender, have a narrow Golgi apparatus, profiles of RER and vesicles, relatively few ribosomes, and show a few examples of micropinocytosis. These cells grow continuously in length during development. On the other hand, the cells in the area choroidea have a low columnar or cuboidal shape, which does not change during development. The inner portion (between the nucleus and the ventricle) contains a voluminous Golgi apparatus, many mitochondria, RER cisternae which contain electron-dense material, SER, and many vesicles. The inner ends of the cells project into the ventricular cavity as bulbous or apical protrusions which contain many organelles, especially MVBs.(ABSTRACT TRUNCATED AT 400 WORDS)