7 The Drosophila Ovary: An In Vivo Stem Cell System

Abstract

STEM CELLS: A SPECIAL CASE OF PATTERNED GROWTH REGULATION
Adult multicellular organisms are massively larger than the eggs from which they arise. To a significant degree, the final shapes of tissues, organs, and, ultimately, bodies are dictated by highly regulated cell growth processes that spatially and temporally control the rates of cell division within tissue primordia. Biologists have made great advances in understanding this fundamental problem during the last ten years. The molecular regulation of the cell cycle has been largely deciphered, and many aspects of intercellular signaling, which regulate growth spatially, have also been clarified. Now, in favorable systems such as the Drosophila embryo and larval imaginal discs (for review, see Edgar and Lehner 1996), as well as in vertebrate organ primordia (see Clarke and Tickle 1999), complex mechanisms that integrate patterning and growth-promoting signals are being learned. The major signaling systems mediated by hedgehogs, epidermal growth factors, bone morphogenetic proteins, fibroblast growth factors, and wingless/wnts frequently stimulate, modulate, or repress the growth rates of receptive cells. Eventually, proliferation ceases when a programmed size and morphology has been achieved, presumably due to changes in the signals and in cellular responsiveness.

On the basis of the studies described below, we have come to regard adult stem cells as a special case of developmental growth regulation, albeit a particularly interesting and potentially informative one. We define an adult stem cell as a cell residing within an adult tissue that divides, either autonomously or in response to regulated signals, to produce cells...