The antral follicle and the corpus luteum (CL), both pivotal structures in the ovary, function as steroidogenic glands. Steroidogenesis in the antral follicle has been meticulously examined in both cellular compartments, adhering to the two gonadotropin theory of ovarian steroidogenesis. This theory precisely elucidates the collaborative efforts of granulosa and theca cells in the synthesis of ovarian steroids (Williams and Erickson, 2015).

Figure 1 provides a comprehensive overview of ovarian steroidogenesis in both theca and granulosa cells. Theca cells house receptors for luteinizing hormone (LH), released from the anterior pituitary upon binding to its receptor. LH signals are crucial for theca cells, stimulating the transcription of genes encoding enzymes necessary for the conversion of cholesterol into androgens, including androstenedione and testosterone. Conversely, granulosa cells possess receptors for follicle-stimulating hormone (FSH), also released from the anterior pituitary (Craig et al., 2011). Similarly, FSH signals play pivotal roles in granulosa cells by upregulating the expression of genes encoding enzymes that convert theca-derived androgens into estrogens, such as 17β-estradiol (E2) and estrogen (Wu et al., 2022). Cholesterol in the theca cell can be acquired through internalization via lipoprotein receptors or synthesized de novo (Arias et al., 2022).

Upon residing in the cytoplasm, cholesterol undergoes transport into the mitochondria facilitated by Steroidogenic Acute Regulatory Protein (STAR) (Christenson and Strauss, 2000; Stocco, 2001). Inside the mitochondria, cholesterol undergoes conversion into pregnenolone through the action of cytochrome P450 cholesterol side-chain cleavage (CYP450scc; CYP11A1) (Craig et al., 2011). Pregnenolone is then translocated from the mitochondria to the smooth endoplasmic reticulum, where it is further converted to progesterone by 3β-hydroxysteroid dehydrogenase (3β-HSD) (Penning, 1997) or to dehydroepiandrosterone (DHEA) by 17α-hydroxylase-17,20-desmolase (CYP45017a; CYP17A1). Both 3β-HSD and CYP17A1 facilitate the diversion of progesterone and DHEA into androstenedione, respectively (Rossetti et al., 2016).

Androstenedione can undergo transformation into testosterone within the theca cell through the action of 17β-hydroxysteroid dehydrogenase (17β-HSD) or be transported into the granulosa cell. In the granulosa cell, aromatase (CYP450arom; CYP19A1) converts androstenedione into estrone, and testosterone is transformed into 17β-estradiol (E2) (Guo et al., 2007). Additionally, within granulosa cells, estrone can be converted into E2 through the action of 17β-HSD (Karman et al., 2012a). E2, the most potent estrogen in female reproduction, undergoes further metabolism into 2-hydroxyestradiol by CYP1A1/2 and CYP3A4 or alternatively into 4-hydroxyestradiol by CYP1B1 (Tsuchiya et al., 2005).

The endocrine system is a complex network of glands that release hormones into the bloodstream, serving as chemical messengers to regulate various physiological functions throughout the body, with a particular impact on the reproductive system (Darbre, 2018). Likewise, the neurosecretory cells, glands, and nonendocrine cells within the neuroendocrine system release hormones into the bloodstream, directly or indirectly influencing the expression of target genes. Lipophilic hormones, such as steroid and thyroid hormones, directly bind to receptors such as the aryl hydrocarbon receptor (AhR), leading to alterations in gene expression (Singh and Verma, 2017; Darbre, 2018).

Hydrophilic hormones, such as peptides or catecholamines, indirectly engage their respective receptors on the cell’s surface. This activation initiates intracellular signal transduction pathways, commonly known as second messenger systems, which subsequently interact with gene expression (Darbre, 2018). Ovarian hormone receptors encompass those specialized for sex steroid hormones like estrogen, progestins, and androgens (Drummond et al., 2002), as well as orphan receptors like the AhR (Pocar et al., 2005), and gonadotropin receptors that bind to LH and FSH. Both sex steroid hormone receptors and AHR function as ligand-attached transcription factors, binding to DNA and regulating the expression of specific genes. On the other hand, LH and FSH receptors are G protein-coupled receptors that modulate various cell functions by initiating second messenger signaling pathways upon binding to their peptide hormone ligand (Althumairy et al., 2020).

Disruption of hormonal functions can occur due to various factors, and among them, an increasing number of environmental contaminants are recognized as endocrine disruptors (EDCs), leading to ovarian toxicity (Costa et al., 2014). In the ovary, the effects of EDCs appear to be linked to estrogen receptors (ESRs), the androgen receptor (AR), and the aryl hydrocarbon receptor (AhR) (Sofo et al., 2015).

Due to its lipophilic nature and slow metabolism, TCDD has a prolonged environmental half-life and can persist in humans for over 10 years following a single exposure (Karman et al., 2012b). TCDD has been detected in human adipose tissue, blood serum, breast milk, and ovarian follicular fluid (Humblet et al., 2011; Ulaszewska et al., 2011). Recognized as one of the most potent carcinogenic compounds, TCDD exerts significant toxic effects on various tissues and organs (Karman et al., 2012a). Regarding reproductive toxicity, previous research has demonstrated that TCDD directly and indirectly affects ovarian functions, leading to disruptions in the estrous cycle (Brown et al., 1998). TCDD exposure has been associated with delayed puberty and early onset of menopause in women (Warner et al., 2007). Similarly, in female rodents, TCDD exposure results in early puberty, irregular estrous cycles, reduced or blocked ovulation, decreased circulating estradiol levels (E2), and premature reproductive senescence (Franczak et al., 2006; Jablonska et al., 2010). Given that disruption of steroid hormone synthesis, activity or metabolism can lead to follicular dysfunction and atresia, thereby affecting the functions of female reproductive system (Sanderson, 2006), female exposure to TCDD influences the production of estrogens and progesterone by porcine granulosa cells (Jablonska et al., 2011; Jablonska et al., 2014). In this regard, it was reported that TCDD disrupted endocrine signaling pathways, potentially causing significant impairments in growth and reproduction (Baldridge et al., 2015). Mechanistically, this effect was induced by altering the expression of transcripts involved in follicular atresia, as well as in cell proliferation and cell cycle regulation (Ruszkowska et al., 2018; Nynca et al., 2019). Further investigations have focused on the impact of TCDD on enzymatic activities within the estrogen biosynthesis pathway. It has been observed that TCDD directly affects antral follicles and inhibits the production of steroid hormones (Karman et al., 2012a). Table 1 summarizes the information on the models, species, molecules used and effects of dioxins on mammalian females.

Several studies have revealed inconsistencies in the effects of TCDD on folliculogenesis and preimplantation embryos. Son et al. (1999) conducted a study on pre-pubertal IHR rats following oral administration of TCDD, which resulted in altered gene expression of the CYP1A1 gene but no change in the expression of the CYP1B1 gene. This was associated with a significant decrease in the rate of ovulation and the number of developing follicles in the ovary (Son et al., 1999).

Yokoi et al. (1993) demonstrated that assessing the gene expression of AHR, ARNT, and CYP1A1 in TCDD-exposed embryos at different stages could help identify stage-specific effects of TCDD on preimplantation embryos. Conversely, Wu et al. (2002) found that TCDD exposure did not alter cell proliferation, differentiation, or survival in embryos. Similarly, Blankenship and co-workers reported that TCDD stimulated blastocyst differentiation without affecting embryo viability or the number of embryo cells (Blankenship et al., 1993).

Several studies also indicated that the sensitivity of each stage of embryonic development of rats differs to TCDD. The changes in gene expression of CYP1A1, AhR and Arnt genes at specific stages of life of fetuses of rat mothers were evaluated as a response to TCDD, namely, the single-cell stage (fertilized egg), the two-cell stage, the eight-cell stage and the blastocyst stage, and it was found that at single-cell and two-cell stages, the expression of these genes did not change, while the expression of CYP1A1 increased in the eight-cell stage, while no change was observed in the blastocyst stage (Penning, 1997).

Another study revealed that injecting TCDD compound into female mice can directly impact the embryo’s implantation stage, leading to reduced viability. Embryos from injected mothers exhibited poorer survival rates compared to those from control mothers (Hanukoglu, 1992). This finding was corroborated by research involving pregnant NIH mice exposed to increased TCDD doses from days 1–8 of gestation. The study demonstrated a decrease in the number of implantation embryos when dosed on days 1–3 of pregnancy, just before the implantation stage, compared to mothers dosed on days 4–8. This highlights the heightened sensitivity of endometrial implantation areas during early developmental stages (pre-implantation) to TCDD compared to the post-implantation stage (Tsuchiya et al., 2005).

Additionally, administering TCDD orally to pregnant rats of the same strain with doses of 100 or 500 ng/kg from gestation days 8–14 resulted in poor ovulation and a decrease in the number of ovarian follicles, particularly primary and secondary follicles in females of the second and third generation (Britt and Findlay, 2002). Karman et al. (2012c) suggested that species’ sensitivity to TCDD could be linked to their biological capacity to metabolize TCDD and the level of AHR expression. While the effects of TCDD on folliculogenesis vary by species, exposure to TCDD has been shown to decrease or block ovulation in rodents in vivo (Jung et al., 2010). The mechanism by which TCDD blocks ovulation likely involves reducing the numbers of granulosa cells in the S-phase and inhibiting the levels of cyclin-dependent kinase 2 (Cdk2) (Jung et al., 2010). Furthermore, TCDD exposure was reported to diminish ovarian reserve in rats and impede follicular development in adult female offspring, effects observed across the F1 and F2 generations. Changes in ovarian Anti-Mullerian hormone (AMH) levels may contribute to these adverse effects, shedding light on the multigenerational impacts of TCDD on follicular development and ovarian quality (Yu et al., 2020).

Studies have documented that TCDD alters the steroidogenic structure of both granulosa and theca cells, likely mediated by a cholesterol-transporter protein found within the mitochondria known as steroidogenic acute regulatory protein (STAR) (Murray, 2001). Several studies have highlighted the negative effects of dioxin in inhibiting estrogen biosynthesis, which is considered one of the most significant hormones in the female reproductive system (Petroff et al., 2001). In rats, TCDD exposure reduces the number of antral follicles without any marked changes in atresia, suggesting that TCDD has an antiproliferative effect on the rat ovary (Heimler et al., 1998). In the same context, Li et al. (2006b) reported that TCDD at very low concentrations (2 ng/kg) significantly reduced serum progesterone levels but had no effect on serum estradiol.

Furthermore, the level of TCDD in the uterus was found to be similar to its levels in the liver but lower than in adipose tissue. These findings suggest that TCDD sensitivity may be influenced by its localized accumulation in the uterus (Li et al., 2006a). Additionally, immature hypophysectomized rats (IHR) exposed to TCDD exhibited significant ovarian dysfunction, leading to blocked ovulation (Son et al., 1999). Karman et al. (2012c) suggested that the effects of TCDD on ovarian steroidogenesis are likely attributed to its inhibitory effects on specific steroidogenic enzymes such as Hsd17b1 and Cyp19a1, resulting in steroidogenic defects in antral follicles. Moreover, exposure of mice to 90 ng TCDD/kg/day markedly increased the testosterone-to-estradiol ratio (Maranghi et al., 2013), while TCDD exposure in chicken ovaries led to decreased estradiol levels (Sechman et al., 2014). Moreover, Grochowalski et al. (2000) demonstrated that treatment of porcine thecal and granulosa cell co-cultures with 0.1 nM or 10 nM TCDD resulted in reduced estradiol levels, while progesterone levels were significantly decreased with 10 nM TCDD treatment. Similarly, isolated mouse antral follicles exposed to TCDD (0.1–100 nM) showed reduced levels of progesterone, androstenedione, and estradiol (Karman et al., 2012c; Karman et al., 2012d). These hormonal deficiencies were restored with the addition of pregnenolone, suggesting that TCDD may act prior to pregnenolone formation to decrease hormone levels (Karman et al., 2012b). Consistent with this, female mice exposed to TCDD exhibited low levels of estradiol (Huang et al., 2011). A similar scenario was observed in Sprague-Dawley rats after oral administration of TCDD for a long period (Chen et al., 2009). Other studies have revealed that TCDD inhibits estrogenic signaling in the kidney and liver but paradoxically promotes estrogenic signaling in the pituitary gland within the same individual (Yoshida et al., 2020). In conclusion, the effects of dioxin on the female reproductive system have shown considerable variability in the literature, likely attributed to differences in TCDD concentration, duration of exposure, animal species, and age of the animal. Understanding the impact of dioxins on the ovary is crucial for refining existing policies aimed at addressing dioxin-induced ovarian toxicity. This knowledge can also facilitate the development of novel therapies to address dioxin-induced abnormalities in both reproductive and non-reproductive health (Patel et al., 2015).

Endometriosis is characterized by the presence of stromal and/or endometrial glandular epithelium implants outside the uterine cavity, often leading to symptoms such as pelvic pain, dysmenorrhea, dyspareunia, urinary symptoms, and sometimes presenting asymptomatically (Soave et al., 2015). Normally, the immune system clears endometrial tissue from the peritoneal cavity. However, dioxin-like environmental pollutants have been implicated in altering the inflammatory processes responsible for tissue clearance, thereby promoting the development of endometrial tissue (Soave et al., 2015). As a marker for endometriosis, the expression of matrix metalloproteinase (MMP) system, which degrades extracellular matrices, has been investigated (Alali et al., 2018). In healthy tissue, progesterone typically suppresses the expression of MMPs (Soave et al., 2015). However, exposure to TCDD can disrupt this balance in endometrial tissue, leading to elevated production of matrix metalloproteinases (MMPs) (Soave et al., 2015). Additionally, TCDD exposure can reduce the levels of progesterone, further exacerbating the increased expression of MMPs in the endometrium (Bruner-Tran et al., 2008).