Gonadotropins oocyte

There is ever more evidence that the results obtained with low doses can equal those earlier obtained with larger doses, while in principle reducing the risk of over-stimulation. Where human chorionic gonadotropin is concerned, a dose of 3300 IU now appears to be sufficient, at least in high responders, to provide adequate oocyte maturation and fertilization (29), whereas doses of 5000 or 10 000 U have often been used. 

Wang, Y., Ock, S. A. and Chian, R. C. (2006) Effect of gonadotropin stimulation on mouse oocyte quality and subsequent embryonic development in vitro. Reprod BioMed Online 12, 304-314. 

The ovaries perform two major functions (i) the storage, maturation, and expulsion of healthy haploid germ cells (i.e., oocytes) for fertilization, and (ii) the synthesis and secretion of hormones to prepare the reproductive tissues for the establishment and maintenance of pregnancy, to properly regulate gonadotropin secretion from the hypothalamic-pituitary axis, to induce appropriate sexual behaviors, and to provide lactation. The two primary functional units in the ovaries are the maturing follicles and the corpora lutea. 

One rare phenomenon occurring in patients undergoing IVF is empty follicle syndrome, the failure to aspirate oocytes from follicles. This can be predicted by measuring beta human chorionic gonadotropin levels 36 h after hCG administration (i.e., at the time of oocyte retrieval) (N4). Women with empty follicle syndrome have significantly lower levels of beta-hCG (range 0-9 mU/mliter) than controls without the empty follicle syndrome (range 106-290 IU/liter) (N4). The fact that this syndrome occurs only very rarely, and that at the time the beta-hCG level is known, the laboratory also has concluded that no oocytes are present in the follicular fluid, makes this test not very practical for everyday use. 

If, however, the oocyte is fertilized and begins to divide, the corpus luteum persists past its normal two-week lifespan. It is prevented from degenerating by the human chorionic gonadotropin (HCG), a hormone produced by the chorion of the embryo as early as eight to 12 days after fertilization. HCG acts like LH in stimulating the secretory activity of the corpus luteum and the presence of hCG in maternal blood or urine is an indicator of pregnancy. 

The ovary expresses multiple isoforms of NOS that are expressed in a cell-specific manner and are regulated by gonadotropins in distinct ways. Due to the potential multiple roles for ovarian NO, one may wonder if each isoform synthesizes NO for a particular function. Some hints regarding the isoform specific functions of ovarian NO have been derived from experiments utilizing NOS inhibitors, which vary in their selectivity for the NOS isoforms. Two studies have documented that aminoguanidine, at doses that selectively inhibit iNOS (Misko et al., 1993), was as effective at reducing the number of ovulated oocytes as nonselective NOS inhibitors (Shukovski and Tsafriri, 1995 Yamauchi et al., 1997). These inhibitor studies imply that iNOS-derived NO may be more critical than ecNOS-derived NO for the ovulatory process. 

ENDOCRINE SYSTEM A number of endocrine tissues respond to PGs. In a number of species, the systemic administration of PGE increases circulating concentrations of adrenocorticotropic hormone (ACTH), growth hormone, prolactin, and gonadotropins. Other effects include stimulation of steroid production by the adrenals, stimulation of insulin release, and thyrotropinlike effects on the thyroid. The critical role of PGF in parturition relies on its ability to induce an oxytocin-dependent decline in progesterone levels. PGE works as part of a positive-feedback loop to induce oocyte maturation required for fertihzation during and after ovulation. 

Nuttinck F, Charpigny G, Mermillod P, Loosfelt H, Meduri G, Freret S, Giimard B, Heyman Y. 2004. Expression of components of the insulin-like growth factor system and gonadotropin receptors in bovine cumulus-oocyte complexes during oocyte maturation. Domest Anim Endocrinol 27(2) 179-195. 

Jensen JT, Zelinski-Wooten MB, Schwinof KM, Vance JE, Stouffer RL. 2005. The phosphodiesterase 3 inhibitor ORG 9935 inhibits oocyte maturation during gonadotropin-stimulated ovarian cycles in rhesus macaques. Contraception 71(l) 68-73. 

As indicated earlier, both gonadotropic and steroidal hormones have been implicated in the processes of maturation as well as ovulation. Both effects can be induced with either type of hormone in intact animals, as well as isolated ovarian fragments, ovarian follicles, or oocytes maintained in vitro. However, these two events (ovulation and matma-tion) are not mutually dependent on each other, and in fact are affected differentially by gonadotropins and steroids (Subtelny et al., 1968 Schuetz, 1971), depending partly on seasonal variations and the uncertain contribution of follicle cells in hormone interactions. Thus some confusion can arise in deciding which kind of hormone is more immediately involved in the processes of maturation and ovulation. 

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