Formation and migration of neuroblasts in the spinal cord of the chick embryo

Abstract

To determine the origin, migration pattern and final localization of the neurons in the spinal cord, chick embryos ranging in age from two to eight days were treated with two doses of tritiated thymidine with an interval of two to four hours and sacrificed at various days thereafter. As a result of this labeling procedure, all neuroepithelial cells were labeled; hence all neuroblasts arising from the neuroepithelial cells after the treatment were labeled, while those formed earlier were unlabeled. By examining the position of the unlabeled neuroblasts at daily intervals, the migration route and final localization of these neurons could be determined.Neuroblasts formed on days two and three are initially found along the inside of the basement membrane in the ventrolateral as well as the dorsolateral region of the cord. During the following days, however, the neuroblasts found in the dorsolateral region disappear and by day 8 the majority of the two‐and three‐day neuroblasts is concentrated in the lateral region of the anterior horn. Since no evidence for degeneration of neuroblasts was found in the alar plate region, it is probable that „alar plate neuroblasts”︁ migrate to the region of the anterior horn.The majority of the neuroblasts formed on days 3 and 4 finds its final position in the lateral region of the anterior horn; the majority of those formed on days 5 and 6, however, occupies a position in the medial region of the anterior horn. Hence, the majority of the motor neurons innervating the muscles of the extremities is formed earlier than those innervating the trunk musculature.The neuroblasts of the internuncial zone are formed on days 2 through 7, that is, during the entire period that neuroblast formation in the spinal cord occurs. The majority of the neuroblasts for the posterior horn arises on days 6 and 7, while the last ones are formed on day 8. Despite the fact that the majority of the neuroblasts for the anterior horn is formed before those of the internuncial zone and these in turn before those of the posterior horn, none of the cell groups in either region is formed within a sharply delineated time period.