Death by Design: How an early embryo takes form

Abstract

Extensive modification is a normal and important part of embryogenesis. Disruption of these events can lead to catastrophic malformations or death, so it is important for us to understand developmental processes. In a part of the throat called the pharynx, aortic arch arteries form and are then removed or altered, pouches are remodeled as their closing plates rupture, and a duct to the emerging thyroid gland is removed. Differential cell proliferation is part of pharyngeal remodeling (Miller, et al, 1993), but involvement of programmed cell death (apoptosis) has not been conclusively demonstrated in many of the research models for human developent.

I used chick embryos, a classic model for understanding development, and applied immunohistochemical markers to determine if apoptosis correlated with morphogenetic events in the pharynx. Apoptotic cells are present among the cells that surround aortic arch arteries 1 and 2 at the time they regress via a capillary plexus. Extensive apoptosis is apparent in the thyroid rudiment and in the thyroglossal duct of later embryos. Apoptotic cells were also marked in the plates of cells that temporarily close off pharyngeal pouches 2 and 3. These results suggest that programmed cell death acts in conjunction with cell proliferation, cell shape changes and possible cell migration during remodeling of the chick embryo pharynx.