Quantitative studies of mitotic cells in the chick embryo optic stalk during the early period of invasion by optic fibres.

In addition to mitoses of neuroepithelial cells at the ventricular surface of the chick embryo optic stalk, mitoses in nonventricular stalk zones begin to be observed from stage 19 on. These latter represent the division phase of glioblasts detached from the ventricular surface. Thus, the topographical location of mitotic cells could be considered a morphological marker of neuroepithelial and glioblast populations in the optic stalk. Quantitative analysis of ventricular (VMCs) and extraventricular (EMCs) mitotic cells revealed that the total number of VMCs decreases through the developmental stages studied, while the number of EMCs simultaneously increases exponentially. These results suggest that the glioblast population arises from both division of the early glioblasts and progressive transformation of neuroepithelial cells. The first EMCs in the ventral region of the stalk wall are observed in stage 19, previous to the stages in which the first EMCs appear in the dorsal region. Moreover, EMCs are much more numerous in the ventral than in the dorsal stalk wall in all stages analysed. Keeping in mind that the invasion of the stalk by optic fibre fascicles occurs essentially in the ventral region, these results suggest that EMCs are strongly related to axon fascicle outgrowth in the stalk. Cell division features are different in neuroepithelial cell and glioblast populations, as the proportions of the mitotic phases differ in VMCs and EMCs. In addition, the patterns of mitotic spindle orientation in VMCs and EMCs are also different. In the former, orientations are predominantly longitudinal parallel and transverse parallel, with a smaller proportion of radial mitoses, which are slightly more frequent through stages 23 to 28 than in earlier development.(ABSTRACT TRUNCATED AT 250 WORDS)