Steroidogenesis in X. laevis Ovary
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Steroidogenesis in X. laevis Ovary
Ovarian sex steroid production is essential for Follicular growth and subsequent Ovulation in Xenopus laevis. In the Xenopus ovary, Oocytes are arrested in an immature form in the Cell Cycle at Prophase-I boundary of the first meiotic cycle, and grow to their maximal size. Prophase-I arrest is terminated by specific signals, often Steroid Hormones, that cause release from Cell Cycle arrest and induce progression  of the Meiotic Cell Cycles to an arrest point during the second Meiotic division, often at Metaphase-II. This process, referred to as “Oocyte Maturation” transforms the immature Oocyte into a fertilizable egg. Two Steroid Hormones: Progesterone and Androgens are the primary mediators of Xenopus oocyte maturation. These hormones are synthesized in the Ovary itself, at the time of Ovulation and, are present at higher levels in ovulating Xenopus. Both Progesterone and Androgen are produced from a single precursor, Pregnenolone, via two pathways: the Delta4/Progesterone Pathway and the Delta5/Pregnenolone Pathway (Ref.1 & 2).

Steroid production in the Xenopus Ovary requires both Follicular cells and Oocytes. Pregnenolone is produced in Follicular cells. It is converted to Progesterone by the action of the enzyme, 3Beta-HSD (3-Beta-Hydroxysteroid Dehydrogenase).  But, since Pregnenolone is a poor substrate for 3Beta-HSD; a very small portion of Pregnenolone is converted to Progesterone, and a major portion of it is transported to the Oocyte. In the Oocyte, Pregnenolone is rapidly converted to 17Alpha-Hydroxypregnenolone and then DHEA (Dehydroepiandrosterone) by CYP17 (Cytochrome P450 steroid 17-Alpha-Hydroxylase/17,20 Lyase). Ovarian CYP17 activity is present exclusively in Oocytes, whereas other important steroidogenic enzymes, including 3Beta-HSD and 17Beta-HSD, are located primarily in the surrounding Follicular cells. This compartmentalization explains the "two-cell" model whereby Xenopus Ovarian Steroid Hormone production for the purpose of Oocyte maturation requires both Follicular cells and Oocytes themselves. CYP17 exhibits two enzymatic activities: it catalyzes both 17-Alpha-Hydroxylase and 17,20-Lyase reactions and is active both in the Pregnenolone and Pregnenolone pathways. Its 17-Alpha-Hydroxylase activity converts Pregnenolone and Progesterone which enter the Oocyte (from the Follicular cells), into their respective 17-Alpha-Hydroxylated products: 17-Alpha-Hydroxypregnenolone and 17Alpha-Hydroxyprogesterone. The second 17,20-Lyase activity cleaves the steroid side chains of 17Alpha-Hydroxypregnenolone and 17-Hydroxypregnenolone to yield DHEA and AD (Androstenedione), respectively. DHEA and AD are then transported back to the Follicular cells, where 3Beta-HSD and 17Beta-HSD (17Beta-Hydroxysteroid Dehydrogenase) act on them to complete Testosterone synthesis. DHEA is converted to AD by the action of 3Beta-HSD, whereas, conversion of AD to Testosterone is aided by 17Beta-HSD. These two Androgens subsequently are transported back to the Oocyte to promote maturation. However, relative to AD, Testosterone has higher potency, and is the primary Androgen for the induction of Oocyte maturation in Xenopus Oocytes (Ref.1, 3 & 4).

The two Androgens: Androstenedione and Testosterone, produced in the Follicular Cells, and Progesterone, produced in the Oocyte itself act on the Oocyte to promote its maturation. These steroid hormones bind to their respective receptors and initiate Oocyte maturation, during which, the Oocytes enter Meiosis, undergo nuclear envelope disassembly/GVBD (Germinal Vesicle Breakdown), complete Meiosis-I with the extrusion of a polar body, and arrest at Metaphase-II of the second Meiotic division until fertilization. Thus, Oocytes play a critical role in the production of the steroids used for their own maturation, a crucial process transforming the immature Oocyte into a fertilizable egg (Ref.4 & 5).