Objective
To test whether bone morphogenetic protein 4 (BMP4) directly regulates differentiation of adult mouse ovary-derived oogonial stem cells (OSCs) in vitro. Design: Animal study. Setting: Research laboratory. Animal(s): Adult C57BL/6 female mice. Intervention(s): After purification from adult ovaries by fluorescence-activated cell sorting, OSCs were cultured without or with BMP4 in the absence or presence of the BMP4 antagonist, Noggin. Main outcome measure(s): Rates of in vitro-derived (IVD) oocyte formation and changes in gene expression were assessed. Result(s): Cultured OSCs expressed BMP receptor (BMPR) 1A (BMPR1A), BMPR1B, and BMPR2, suggesting that BMP signaling can directly affect OSC function. In agreement with this, BMP4 significantly increased the number of IVD oocytes formed by cultured OSCs in a dose-dependent manner, and this response was inhibited in a dose-dependent fashion by cotreatment with Noggin. Exposure of OSCs to BMP4 was associated with rapid phosphorylation of BMPR-regulated Smad1/5/8 proteins, and this response was followed by increased expression of the meiosis initiation factors, stimulated by retinoic acid gene 8 (Stra8), muscle-segment homeobox 1 (Msx1), and Msx2. In keeping with the IVD oocyte formation data, the ability of BMP4 to activate Smad1/5/8 signaling and meiotic gene expression in OSCs was abolished by cotreatment with Noggin.