The corpus luteum is a transient ovarian endocrine gland generated after ovulation. It produces progesterone, necessary for pregnancy establishment and maintenance. In humans, once it has developed, the placenta becomes the primary source of progesterone. In contrast, the corpus luteum is the main producer of progesterone throughout gestation in rodents (1, 2). Progesterone exerts a crucial role during pregnancy by regulating different processes including the differentiation of the endometrium, the quiescence of the myometrium and the modulation of the immune system (3). Therefore, alterations in the levels of progesterone are associated with difficulties in conceiving and an increased risk of early pregnancy loss or miscarriage (4, 5).

Several studies have shown that lifestyle significantly influences both physiological and pathological events. Moreover, the perinatal environment could impact the quality of gestation, as well as fetal and offspring health (6). In particular, we reported that exposing mice to an enrichment protocol which includes social, physical, and sensory stimuli that enhance their natural behavior, has a beneficial effect on gestation (7). We observed that maternal environmental enrichment regulates uterine physiology, promotes vascular remodeling during early gestation, and prevents pregnancy loss in mice (7). Furthermore, we demonstrated that enrichment of the maternal environment prevents preterm birth in mice, and improves offspring’s health (8, 9). Also, other authors informed that environmental enrichment affects the placental response to stress mediators and protects the fetus against an inflammatory stimulus (10). Altogether, this evidence suggests that modifying the maternal environment could improve pregnancy outcome.

Based on our previous results, and considering that many uterine processes are regulated by ovarian activity, we propose that maternal enrichment could also modulate ovarian physiology. We hypothesize that environmental enrichment may enhance oocyte development or the activity of corpora lutea, thereby contributing to improved pregnancy success. Therefore, we decided to study the effect of maternal environmental enrichment on ovarian morphology and progesterone production during early gestation.

First, the effect of maternal environmental enrichment on ovarian structure and size was analyzed. For this, the ovaries were extracted and sectioned for histological studies. The structure of the ovary was conserved in both experimental groups ( Figure 1A ). The ovaries exhibited distinct borders, well-developed follicles and corpora lutea, and no signs of fibrotic or hemorrhagic tissues. Besides, no differences in size were observed (control: 4.2 mm² ± 0.2; enriched: 4.3 mm² ± 0.2).

The number of primordial, primary, preantral, antral, and atretic follicles was analyzed and no differences were detected between the groups ( Figures 1B–F ). However, ovaries from enriched females presented more corpora lutea than control females (p=0.017) ( Figures 1G, H ). Also, the size of the corpora lutea was larger in enriched mothers (p=0.049) ( Figure 1I ).

Subsequently, the progesterone level was determined in maternal serum. Enriched females presented higher progesterone levels than the control group (p=0.038) ( Figure 2A ).

The ovarian levels of proteins involved in steroidogenesis were determined by western blot. StAR, a rate-limiting protein in steroid production, did not change its expression (p=0.066) ( Figure 2B ). Nevertheless, the level of 3β-HSD, responsible for progesterone synthesis, was found to be elevated in enriched mothers (p=0.049) ( Figure 2C ).

In the present study, we demonstrated that female mice exposed to an enriched environment present more corpora lutea, higher progesterone levels, and increased protein expression of 3β-HSD on day 7 of gestation. These results suggest that maternal lifestyle can affect the physiology of the ovary and modulate the production of progesterone, a crucial hormone for maintaining pregnancy.

Results in other animal models support our observations. It has been reported that housing conditions could affect ovarian physiology. Social interaction modulates progesterone levels in non-pregnant mice (13). Also, it was reported that the environment in which gilts are housed modulates the mRNA levels of enzymes involved in steroidogenesis, like StAR and 3β-HSD (14).

In line with these observations, in the present work, we report that enriched females exhibit a higher number of corpora lutea compared to the control group. In mice, the corpus luteum is the principal source of progesterone throughout pregnancy, and it is well-described that progesterone is necessary for maintaining gestation (1). Previously, we reported that enriched females exhibit a higher pregnancy rate than control mothers (7). We described that maternal environmental enrichment improves the maintenance of gestation and prevents pregnancy loss. Therefore, we hypothesize that the higher progesterone serum levels could explain this increase in pregnancy rate, as progesterone is essential to prevent the contraction of the uterine muscles that could interfere with embryo implantation and early gestation. Also, progesterone stimulates decidualization and the secretory activity of the uterine glands necessary for the developing embryo (15).

Additionally, steroid hormones promote uterine vascular remodeling and modulate uterine artery contractility, impacting uterine blood flow during pregnancy (16). Specifically, uterine arteries express progesterone and estrogen receptors and are reactive to these steroids (17). Previously, we observed that these steroids promote the acquisition of the human first-trimester trophoblast endovascular phenotype (18). Also, progesterone stimulates nitric oxide synthase activity and, specifically, endothelial nitric oxide synthesis in endothelial cells (19). Recently, we reported that the maternal environmental enrichment protocol used in the present work induces changes in the micro and macrovasculature that are paralleled by the modulation of molecules involved in vascular processes, like VEGF-A, nitric oxide, and endoglin (7). Therefore, the increase in serum progesterone levels accompanies the stimulation of the vascular adaptations previously observed in enriched females.

Progesterone metabolism could be regulated, among others, by StAR and 3β-HSD proteins. For its part, StAR plays a pivotal role in steroidogenesis, as it is the rate-limiting step in steroid production. It is responsible for transporting cholesterol to the inner membrane of the mitochondria, where the enzymes involved in steroidogenesis are located. This protein is primarily expressed by granulosa and theca cells in the follicle (20). In our work, no differences in the levels of this protein were detected. However, enriched females presented higher protein levels of 3β-HSD. This enzyme converts pregnenolone into progesterone and is expressed by granulosa and theca cells in the follicle and luteal cells in the corpus luteum (20). The increase in the protein levels of 3β-HSD could explain the increment in progesterone levels observed in the present work.

The modulation of ovarian physiology is regulated by the hypothalamus and the limbic system, which are sensitive to external stimuli. They respond by releasing neurotransmitters that modulate the hypothalamus-pituitary-gonadal axis. This axis regulates reproductive processes by hormones that affect the ovaries and stimulate follicular development. The intricate interplay between the hypothalamus, the limbic system, and the hypothalamus-pituitary-gonadal axis is vital to maintaining proper reproductive function (21). Another possible mechanism linking how changes in the environment affect ovarian physiology could be the regulation of neurotrophins such as VIP or BDNF. The nervous system produces neurotrophins in response to environmental stimuli. Concerning this, neurotrophin levels are modified by exercise and other lifestyle-related factors (22). Moreover, it is reported that they participate in follicular development, oocyte maturation, ovulation, and maintenance of the corpus luteum. Also, they contribute to the stimulation of steroid hormone production in the ovaries (23, 24). Therefore, we propose that these mechanisms might be involved in the modulation of the ovarian physiology in our model.

In conclusion, our results demonstrate that the enrichment of the maternal environment regulates ovarian physiology. The increase in corpora lutea number and size, as well as progesterone serum levels, provide support for the idea that exposure to an enriched environment exerts a protective role in gestation, preventing embryo loss. Even considering the limitations of an animal model, the positive effects of the enriched environment support the idea that a maternal enrichment protocol could be recommended for women seeking pregnancy. This non-pharmacological intervention might be advantageous, particularly for women undergoing fertility treatments.