The ovary produces estradiol, progesterone, and androstenedione, from which testosterone is derived.

The ovary’s key feature of steroid biosynthesis is its com partmentalization into highly specialized cells. According to the bicellular steroidogenesis model, the interstitial cells of the theca of the follicle are sensitive to LH’s action due to specific receptors. LH in these cells induces the synthesis of androgens, which, in the granulosa cells sensitive to the FSH action, will be converted into estrogens by aromatase activity. It should be noted that granulosa cells cannot synthesize androgens (Fig. 1).

Fig1. Ovarian steroidogenesis. (Copyright EDISES 2021. Reproduced with permission)

The biosynthesis of steroid hormones occurs through the activity of enzymes belonging to the cytochrome P450 superfamily. The precursor common to all biosynthetic path ways is cholesterol, mainly derived from LDL lipoproteins and, to a negligible extent, from ex novo biosynthesis and mobilization of cytoplasmic lipid deposits.

The limiting step in steroid hormone synthesis is cholesterol transport within the mitochondrion. Once localized in the inner mitochondrial membrane, cholesterol is converted to pregnenolone. This can follow two metabolic pathways, depending on cellular localization (Fig. 2). The Δ4 path way represents the main pregnenolone metabolization path way in the ovary and leads to the formation of progesterone. In interstitial cells, including those in the corpus luteum, pregnenolone is converted to progesterone and 17-OH-progesterone by a 17α-hydroxylase. The progressive removal of carbon atoms from progesterone (C21) leads to the C19-androgens  synthesis, particularly Δ4-androstenedione. Hilar cells of the ovary can convert Δ4-androstenedione to testosterone. Δ5-metabolization pathway occurs mainly in the adrenal gland and involves the conversion of pregnenolone to 17-OH-pregnenolone and dehydroandrosterone, which is then converted to androstenedione and testosterone. Estrogen synthesis occurs almost exclusively in the ovary and, to a lesser extent, in the adrenal gland. Estradiol, the primary estrogen synthesized by the ovary, and estrone are derived from Δ4-androstenedione and testosterone by aromatase expressed mainly in the granulosa cells.

Fig2. Pathways of synthesis of androgens and estrogens. In the ovary, the Δ4 pathway is prevalent compared to Δ5, which occurs mainly in the adrenal gland. (Copyright EDISES 2021. Reproduced with permission)

Another important pathway for testosterone metabolism involves dihydrotestosterone synthesis by the 5α-reductase. This conversion also takes place in the ovary, especially medullary interstitial cells, and other districts, such as the skin in the pilosebaceous unit, which has the enzymatic kit for the synthesis and metabolism of androgens; in particular, it expresses the 5α-reductase, necessary for the conversion of testosterone into DHT (dihydrotestosterone, DHT).

The ex novo synthesis of ovarian hormones responds mainly to the control of LH. This hormone induces in the theca cells the expression of P450scc (human cholesterol side-chain cleavage enzyme), the enzyme converting cholesterol into pregnenolone, considered the limiting step of the entire biosynthetic pathway. Although not directly acting on the ex novo biosynthetic pathway, FSH has a critical regulatory role in converting androgens to estradiol by inducing the expression of aromatase in granulosa cells (Fig. 1).

Ovarian hormones are transported in the circulation by vector proteins such as SHBG (Sex Hormone Binding Globulin), which binds estrogens and androgens, corticosteroid- binding globulin, which mainly binds progesterone, and albumin, which is considered an a specific transporter. The synthesis of these transport proteins occurs in the liver and is also modulated by insulin. Both estrogen and progesterone are catabolized in the liver.

The actions of estrogen and progesterone are mediated by interaction with nuclear receptors expressed in target cells, followed by modulation of gene expression. The biological effects of estrogens are ubiquitous. During puberty, they play a decisive role in developing the internal and external genitalia and in the induction of secondary sexual characteristics. In the fertile age, they are involved in fertilization, pregnancy, and childbirth. Estrogens also affect lipid metabolism by regulating the PCSK9 expression in the liver. However, many aspects of the action of estrogen (endogenous or exogenous) on lipoprotein metabolism remain unclear.

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مواضيع ذات صلة

Synthesis and Function of Protein Hormones in Ovary

هرمونات غير نخامية لها علاقة بوظيفة المناسل Non-pituitary Hormones

هرمونات الغدد التناسلية (المناسل) The Gonadal hormones

هرمونات غير نخامية لها علاقة بوظيفة المناسل

الدورة التناسلية

الهرمونات الأنثوية (الاستروجين والبروجسترون)

الهرمونات الذكرية الأندروجينات أو التستسترون

مقدمة الى الغدد التناسلية (المناسل) The Gonads