The effect of NO/cGMP system on sheep somatic cell differentiation in vitro - Literature review Example

(2005) explained that the newly formed blood vessels makes it possible for the hormone-producing cells to collect enough oxygen, nutrients and other substances needed to release some reproductive hormones. During the follicular development, the density of the blood vessels that surrounds the dominant ovulatory follicle increases. In the absence of oxygen, Bianco et al. (2005) explained that cases of hypoxia is likely to happen. The hCG binds to all follicles in vitro whereas in vivo binds to more vascular follicles (Zeleznik and colleagues, 1981).

Since follicle development plays a key role behind the success of both in vitro and in vivo process, several studies considered the process of regulating the development of the blood vessels as a useful strategy in promoting follicular maturation (Barboni et al., 2000; Neeman et al., 1997). Hormones like gonadotrophins help in regulating the development of the ovarian follicle through the classic endocrine mechanism. This is possible since gonadrotrophin can stimulate the initial growth stage of the follicles.

Furthermore, the presence of gonadrotrophin will trigger the inner granulosa and cumulus cells to produce a higher level of VEGF. Neeman et al. (1997) explained that the presence of a high level of VEGF is necessary in controlling the angiogenic activity needed for the CL formation. It is also responsible in regulating the presence of both normal and abnormal angiogenesis (Dvorak, 1995; Ferrara, 1993) and the endothelial nitric oxide synthase (eNOS) expression and the production of nitric oxide (NO) (Reynolds et al., 2002). Furthremore, Shweiki and colleagues (1995) explained that a high level of VEGF mRNA is enough to normalize the oxygen levels in hypoxic cells.

Recently, Ferrara et al. (2003) found out that the VEGF-A is responsible in regulating the formation of the ovarian blood vessels. In the absence of one of the VEGF allelles, embryonic development can be negatively affected (Ferrara et al., 1996). Given that some follicles are heavily dependent on the presence of gonadotrophin, several studies conclude that one of the main reasons why some follicles failed to mature is because of the absence of gonadotrophins (Driancourt, 2001; Goodman and Hodgen, 1983).

The NO can be produced by the ovarian cells (Dave et al., 1997) particularly among the immature ovaries (Ellman et al., 1993) and in the different stages of the follicular development (Hattori et al., 1996). Since the main role of the NO/cGMP is regulating the blood pressure, several studies have closely examined the actual involvement of the cGMP in the transduction of NO in the ovary (Al-Gubory et al., 2005; Ponderato et al., 2000; Matsumi et al., 1998; Jablonka-Shariff and Olson, 1997; Zackrisson et al.

, 1996; Van Voorhis et al. 1995; Van Voorhis et al., 1994). The eNOS and iNOS isoforms expression can be found in the ovarian tissue of some species whereas the eNOS can be present in the ovarian follicles and CL during the oestrus cycle in different species (Al-Gubory et al., 2005). Van Voorhis et al. (1994) reported that the human granulose-luteal cells have eNOS isoform expression