Charles H Sullivan

Cell Biology / Developmental Biology

I am interested in the formation of sensory structures during neural development in vertebrate embryos. Most of my work with students at Grinnell has focused on lens formation in chicken embryos (a lens from a four day old embryo is shown in Figure 1). We have used a tissue culture approach to separate tissues that are normally in contact in the embryo or to grow tissues in novel combinations. At the end of the culture period, expression of the lens protein delta-crystallin in elongated lens cells is measured by western blots or immunohistochemistry. Using these approaches, we have identified both positive and inhibitory tissue interactions that determine the position of the lens and we have detected reciprocal tissue interactions between the lens and adjacent eye tissues that lead to full development of several eye structures. During a recent sabbatical leave with Dr. Sally Moody at George Washington University Medical Center, I began studying formation of the preplacodal region (PPR) within the head of frog embryos. The PPR is a U-shaped region that develops at the border of the neural plate (the future brain) as show by the purple staining of a ~24 hour old embryo in Figure 2. Later in development, PPR tissues give rise to the lens, ear, nose, and other cranial structures. We have used microinjection of different mRNAs into particular cell lineages of the 16-cell stage frog embryo to better understand the early steps in the formation of sensory structures. Embryos are then processed by whole mount in situ hybridization for expression of target genes. We have discovered the same major themes mentioned above but at the level of genetic interactions. That is, there are positive and inhibitory genetic interactions that pattern the ectoderm into brain, sensory structures, neural crest, or skin, and there are reciprocal genetic interactions too that expand or reduce the size of these regions. For example, over-expression of a particular gene (shown by the asterisk on the left side of the head of the embryo shown in Figure 3) blocks formation of the developing ear. The blue arrow shows the position of the normal ear on the control side of the tailbud-stage embryo that is about two days old.