**3. Placenta steroidogenesis**

A key function of the placenta is the secretion of hormones. Like other steroid hormones, T is derived from cholesterol and the synthesis involves several enzymatic steps. The first and fundamental step in its biosynthesis involves the oxidative breakdown of the cholesterol side chain by the enzyme P450scc (side-chain cleavage), a mitochondrial cytochrome oxidase, resulting in the loss of six carbon atoms to give rise to pregnenolone. Only certain cell types in humans are capable of pregnenolone synthesis, including testicular Leydig cells, ovarian theca and corpus luteal cells, placental trophoblast cells, cells of the adrenal cortex, and specific cells in the brain, such as the pyramidal and granular neurons of the hippocampus and the Purkinje cells from the cerebellum [9]. The resulting pregnenolone is either converted to progesterone or 17 α-hydroxypregnenolone via 3β-hydroxysteroid dehydrogenase (HSD3B) or cytochrome P450 17A1 (CYP17A1), respectively. Progesterone is secreted into maternal circulation, and 17α-hydroxypregnenolone can be metabolized to DHEA via CYP17A1. DHEA is oxidized into A4 via HSD3B. A4 is then reduced to 5 α-androstenedione via 5 α-reductase (SRD5A). DHEA and A4 can be converted by 17 β-hydroxysteroid dehydrogenase into androstenediol and T, respectively. Subsequently, T is converted into DHT via 5 α-reductase (SRD5A). T and 5 α-androstenedione can further be metabolized to estrogens via aromatase (CYP19A1) [10]. A summarized overview of placental steroidogenesis is provided in **Figure 2**.

Androgens are synthesized in tissues where 17α-hydroxylase/17,20-lyase cytochrome P450 (P450c17) exists. This enzyme is located in different tissues such as fetal and maternal adrenal glands, fetal ovaries and testes, and the corpus luteum, depending on to the animal species. In non-pregnant woman, 50% of all DHEA is secreted by the adrenal glands, 20% from the ovarian theca and 30% is derived from metabolism of circulating DHEA sulfate [11]. Adrenal glands and ovaries produce equal amounts of A4, with the total daily production rate 1.4-6.2 mg/ day [12]. 50% of T is synthesized in the ovaries and adrenals and the other half is produced from A4 in the peripheral tissues. Daily production rate of T in nonpregnant women is in the order of 0.1–0.4 mg/day. Finally, the conversion of T to

#### **Figure 2.**

*Summarized overview of the functional interaction between the placental, maternal and fetal compartments for the biosynthesis of progesterone, estrogens and androgens by the human placenta. Progesterone is produced mainly from maternal cholesterol. Progesterone can be metabolized into DHEA by the maternal and fetal adrenal gland. DHEA can be converted into T. Subsequently, T can further be metabolized to estrogens via aromatase (CYP19A1). In horses, the placenta does not appear to express P450c17 and thus cannot convert* de novo *c21 progestogens (pregnenolone and progesterone) to the c19 androgens (DEHA and A4). It is for this reason that the reaction occurs in the fetal adrenal.*

DHT occurs in peripheral tissues, such as ovaries and skin, with a daily production rating between 4.3 and 12.5 mg/day.

During pregnancy, an additional source of androgens comes from the fetus and placenta (**Figure 2**). Androgens are principally synthesized in the corpus luteum during early stages of gestation in rats and dogs and this function is taken over by the placenta in late stages of gestation in rats [13]. In sheep and goats, P450c17 is present in the placenta [14]. Some studies revealed that the absence of P450c17 in human and horse placentas results in negligible androgen synthesis [15]. However, protein and mRNA levels of CYP17A1 have been detected in primary human trophoblast cells and the human trophoblast cell line JEG-3 and trophoblast cells were able to generate testosterone *de novo* [16]*.* Placentas associated with a male fetus at term have increased expression of 5 α-reductase compared to a female fetus [17]. This enzyme is involved in reducing T to DHT, a potent androgen with a higher binding affinity to AR than T, suggesting the placenta may play a role in the hormonal differences between pregnancies between female and male fetuses.

In horses, the placenta does not appear to express P450c17 and thus cannot convert *de novo* c21 progestogens (pregnenolone and progesterone) to the c19 androgens (DEHA and A4). In this case, the fetal gonads are the main androgen source as estrogens precursor during mid to late gestation in the horse. Removal of fetal gonads results in an immediate fall in maternal plasma concentrations of conjugated and unconjugated estrogens whereas progestogens levels remain unchanged [18].
