Introduction: We have demonstrated that adult smooth muscle cells in myometrium markedly differentiate during pregnancy, and that this differentiation precedes normal parturition. However, little is known of the molecular events that lead to initial smooth muscle cell fate determination and pattern formation in the embryonic female reproductive tract. Understanding the embryologic lineage of smooth muscle cells in the uterus may provide clues about myometrial differentiation during pregnancy.

Hypothesis: Smooth muscle cell fate determination and pattern formation in the uterus requires spatial and temporal regulation of specific soluble inductive factor genes (Wnt5a), transcription factor genes (MHox), and structural protein genes (smooth muscle alpha actin).

Methods: Timed pregnant C57B16 mice and/or pups were euthanized and mRNA accumulation analyzed by Northern analysis, RNase protection, andin situ hybridization of whole mount and thin sectioned Mullerian duct structures. Protein accumulation was assessed by immunoblot analysis and immunofluorescence.

Results: Wnt5a mRNA can be detected on postnatal day 3 in the coelomic epithelium and peripheral mesenchyme of the postnatal Mullerian ducts, but not in the Mullerian epithelium. The pattern of Wnt5a mRNA accumulation suggests a morphogenetic gradient from coelomic to lumenal epithelium in the Mullerian duct. Subsequently, the emergence of uterine smooth muscle is revealed by immunohistochemistry of smooth muscle alpha actin beginning on postnatal day 5 in the peripheral circular, and on day 8 in the longitudinal smooth muscle of the uterus. Accumulation of mRNA for MHox, a homeodomair transcription factor gene expressed in mesoderm derived tissues, is also detected in these postnatal Mullerian duct structures.

Conclusions: Cell fate determination and pattern formation in the mouse female reproductive tract are postnatal events and involve accumulation of mRNA and protein for the soluble inductive factor, Wnt5a, the homeodomain transcription factor, MHox, and the structural protein, smooth muscle alpha actin. The formation of a Wnt5a gradient may contribute to inductive, morphogenetic signalling in the early postnatal Mullerian duct. Thus, rather than a simple epithelial barrier, the coelomic epithelium that surrounds the Mullerian duct is a rich source of inductive signalling molecules in the developing female reproductive tract.