THE MOUSE OVARIAN SURFACE EPITHELIUM IS HETEROGENEOUS AND CONTAINS A POPULATION OF BETACATENIN SIGNALING AND CYTOKERATIN-14 POSITIVE CELLS

Abstract

Wnts are a family of secreted signaling molecules involved in a number of developmental processes including the establishment of cell fate, polarity and proliferation. Recent studies also implicate wnts in epithelial adult stem cell maintenance, renewal and differentiation. We have shown that a number of wnts are expressed in the ovary, specifically in the membrana granulosa and the ovarian surface epithelium (OSE). Wnts transduce their signal through one of three signaling pathways. The best studied, the wnt/β-catenin pathway, leads to an increase in intracellular β-catenin which acts as a co-transcription factor with members of the Tcf/Lef family. We have used transgenic mice, bearing lacZ-reporter constructs responsive to β-catenin/Tcf binding, to assess wnt/β-catenin-signaling in the ovary. Hence, cells in which wnt/β-cateninsignaling is activated will express β-galactosidase, which can be detected with X-gal staining. Ovaries were obtained from transgenic mice at various ages (1, 5, 10, 15, and 21 days of age), fixed, sectioned and processed for X-gal staining. In addition, sections from adult mice ovaries were assessed for cytokeratin-8 (K8) and −14 (K14) expression using antibodies directed against these microfilaments. Expression of these cytokeratins has been associated with less differentiated (K14) and differentiated (K8) cells in other epithelia. The evaluation of ovarian sections showed β-galactosidase positive cells only in OSE. The proportion of β-galactosidase positive (presumptive wnt/β-catenin-signaling) OSE cells decreased with age. Almost all OSE in new-born ovaries, approximately 40% in 10-day-old but <10% in 21-day-old ovaries were βgalactosidase positive. These age-related changes in staining were confirmed by X-gal staining of intact ovaries. All OSE cells were positive for K8 except for a small population (<10%) that were positive for K14. Interestingly, the K14 positive population was increased by exogenous hormone stimulation of ovulation and, in this case, we also observed cells that appeared to be in transit from K14 positive to K8 positive. We have not determined if the K14 positive and wnt/β-catenin-signaling cells constitute the same population. Based on the fraction of β-galactosidase positive cells and an estimate of the number of OSE cells at each of the ages examined, it appears that the number of wnt/β-catenin-signaling cells remains relatively constant regardless of age. This raises the possibility that the original wnt/β-catenin-signaling cells may give rise to a replacement as well as an expanding population of non-signaling progeny. Taken together, our results indicate that the mouse OSE is heterogeneous and may contain a population of stem/progenitor cells that are needed to generate the definitive OSE and to allow wound repair of this tissue at ovulation. (poster)