Regulation of perinatal estrogen levels on primordial follicle formation and activation in mouse

Primordial follicle formation and activation are key for the reproductive ability of females. In mice, primordial follicles are formed and begin to activate during the perinatal period, when the levels of estrogen are fluctuating. Whether estrogen plays a role in primordial follicle formation and activation, and its mechanism are still not fully elucidated. In this study, estrogen remained at high levels before birth and declined after birth. When fetal mouse ovaries (E16.5) were cultured in vitro, higher levels (10 nM) of estrogen maintained the germ cell cysts, prevented primordial follicles from forming prematurely, and promoted the full differentiation of oocytes. Furthermore, it was found that estrogen-regulated JNK-signal pathway through both nuclear and membrane receptors, thereby inhibited the degradation of E-cadherin and maintained the germ cell cysts. After birth, ovarian estrogen concentration decreases and is accompanied by the activation of primordial follicles. Hence, the ovaries of newborn mice (P3) were treated with lower concentrations (0.1 nM) of estrogen to investigate the effect of estrogen on primordial follicle activation. The results demonstrated that estrogen regulated the protein expression of cAMP synthase adenylyl cyclase 3 (ADCY3) through the membrane receptor G-protein-coupled estrogen receptor (GPER), increased the level of cAMP in the ovary, and activated the cAMP-PKA signaling pathway to promote the activation of primordial follicles. This study revealed the regulatory role of perinatal estrogen levels on primordial follicle formation and activation before and after birth, which would help to better understand the potential physiological effect of estrogen in vivo.<b>NEW & NOTEWORTHY</b> In this study, the roles and underlying mechanisms of perinatal estrogen level changes in primordial follicle formation and activation in mice were elucidated. The elevated estrogen levels before birth inhibited the premature formation of primordial follicles and enhanced the quality of oocyte differentiation. Conversely, the reduced estrogen levels following birth promoted the activation of primordial follicles.