Kimberly D. Tremblay

Mouse Development and Organogenesis

In our lab we study the development of the definitive endoderm, one of the 3 primary germ layers that arises during gastrulation. The definitive endoderm produces the entire gastrointestinal tract as well as accessory digestive and respiratory organs. These organs include lung, liver, pancreas, thyroid, parathyroid and thymus. Although much is known about the genes involved in the function of these organs in the adult, relatively little is known about how these tissues are initially patterned and organized. An overall goal of the lab is to understand the morphological and molecular mechanisms that give rise to endodermal organs, focusing on liver and pancreas. Towards this end, we are taking two broad approaches to study the early stages of endoderm organogenesis in the mouse. They can be defined as embryological approaches, utilizing whole embryo culture, and genetic approaches, using transgenic and homologous recombination to create favorable environments to study this fascinating tissue layer.

Identification and Manipulation of Liver and Pancreas Precursors

The endoderm is a uniform epithelial layer that covers the ventral surface of the pre-somitic mouse embryo. From 8-15 somites (S), the accessory organs appear, in a temporally and spatially characteristic manner, as thickenings in this epithelium.

 Organ-specific gene expression follows or is coincident with the onset of these morphological processes and as a result, little is known about the location of these organ progenitors in the endodermal sheet, the morphological processes leading to organogenesis, or the molecular mechanisms that initiate organogenesis. Because of the lack of genes or promoters expressed specifically in the endoderm, more traditional approaches, such as knock-out and transgenic experiments, have had limited success in tackling these issues. We have decided to use a whole embryo culture system in which we can successfully culture whole embryos, and their accompanying extraembryonic tissues, from early somite stages (~day 8.25) until day 10 (Fig. 2). Culturing allows us to manipulate the pre-specified (d8.25) endoderm and follow these changes through d10, when the liver and pancreas buds have formed and begun to differentiate. We have used viable dyes to identify precursor populations in the early somite embryo and are now using electroporation to identify genes that disrupt early organogenesis.

Clonal Analysis of Organ Growth 

Little is known about the growth or developmental potential of early individual endodermal cells. Is an individual endodermal cell committed to a specific organ or is it capable of giving rise to cells contributing to multiple organs? Similarly, it is unknown whether individual cells expressing organ-specific markers are capable of giving rise to all differentiated cell-types within the organ or are excluded from certain lineages. To answer these questions we perform experiments with transgenic or knock-in mice that produce embryos that have had individual endoderm cells marked with either the fluorescent markers EGFP or the histological marker LacZ. By retrospectively analyzing the clonal descendants of these cells, we will elucidate the normal processes that give rise to endodermal organ, further define organ precursors and understand the morphological processes that produce the mature organs.