Margaret H. Baron

Research

One area of investigation is the formation and cell fate specification of mesoderm, the embryonic germ layer that gives rise to blood and endothelial (blood vessel) cells as well as muscle, cartilage and bone.  This work utilizes various transgenic mouse and embryonic stem (ES) cell lines to dissect out genetic and molecular pathways involved in these processes. 

A second area of investigation that has occupied much of our attention for the past few years is the development of embryonic ("primitive") erythroid (red blood) cells.  These "EryP" form in the yolk sac, the first site of hematopoietic development in the mammalian embryo.  We have developed transgenic mouse models to monitor and isolate the three major yolk sac cell populations:  EryP, vascular endothelial and visceral endoderm.  In the mouse, EryP arise in large numbers from yolk sac-derived progenitors at the end of gastrulation, entering the circulation as nucleated cells soon thereafter, and continue to mature in a stepwise and synchronous fashion. By taking advantage of a transgenic mouse line in which fluorescent proteins are expressed exclusively in the EryP lineage, we have been able to follow and purify not only circulating EryP but also the very first erythroid-committed cells that in the yolk sac, as early as embryonic day (E) 7.5. We generated global gene expression profiles at 24 hour intervals from E7.5 through E12.5 that revealed two abrupt changes in transcript diversity that coincided with the entry of EryP into the circulation and with their enucleation, respectively.  These changes were paralleled by expression of critical regulatory factors.  Experiments designed to test predictions from these data have allowed us to identify key regulatory pathways and factors involved in EryP progenitors and in their more differentiated descendants.  Our analysis uncovered not only well studied red blood cell genes but also some surprises.  These genes form the basis for ongoing studies.  To our knowledge, this is the first transcriptome assembled for a single lineage of the embryo over the course of its differentiation.  While we have focused initially on the development of EryP, insights from our microarray studies can also be applied tothe emergence of the first adult type (definitive) hematopoietic stem/progenitor cells. 

The transgenic mouse lines we have created can be crossed to generate multiply transgenic lines in which different color fluorescent proteins are expressed in various lineages (for example, in blood cells, endothelial cells, or endoderm cells).  We are taking advantage of these lines to image the development of the embryo in real time.

Finally, we are beginning to apply what we have learned from our studies on embryonic red blood cells to the development of adult ("definitive") red blood cells in human patients with hematological diseases.