The Kit system, composed of Kit ligand (KL) and its tyrosine kinase receptor, cKit, has been well characterized in mammals. Studies have shown that it is involved in signaling between the oocyte and somatic cells during the process of follicle maturation. We characterized KL mRNA expression during follicle maturation in the domestic hen, examined regulation of KL and a possible function of the Kit system. KL mRNA expression was assessed using quantitative PCR (n=4 replicates) in follicles of various sizes (1, 3, 5, 6-12 mm, F1). Expression of KL mRNA decreased significantly (p<0.01) with follicle development and was highest in <1 mm follicles, which contained the theca as well as granulosa layers, with high levels also found in the granulosa layer of 3 mm follicles and ovarian stroma. To study regulation of KL mRNA, granulosa cells from 6-8 mm follicles (n=4 replicates) were plated in M199 plus 0.1% BSA in the presence of various treatments including: oocyte conditioned medium (OCM), Vitamin D(3), FSH, estradiol, progesterone and testosterone. OCM caused a dose-related increase (p<0.05) in expression of KL mRNA; Vitamin D(3) increased and FSH decreased expression of KL mRNA. cKit was detected (at the expected size) in the theca layer of 3-5 mm follicles and in a lysate of whole <1mm follicles. Culture of granulosa cells in the presence of OCM resulted in a decrease of P4 secretion, an effect blocked by pre-incubation of OCM with cKit antibody. Although OCM caused a dose-related increase in E2 secretion from theca, this was not blocked by cKit antibody.
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