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Relationships between the number of immunostaining gonadotropes and the plasma concentrations of gonadotrophins in ewes with and without the FecB^B gene

Booroola ewes possess a major gene, FecB^B, that influences their ovulation rate (number of ovulations per oestrous cycle). Homozygous (BB) carriers of the FecB^B gene have higher plasma concentrations of FSH and sometimes LH relative to the non-carriers (++). The aim of this study was to determine whether the plasma concentration differences in FSH or LH between the genotypes were due to a greater number of FSHβ-or LHβ-immunostaining cells in the anterior pituitary gland of BB ewes during the luteal phase of the oestrous cycle. No differences were found between the BB (n = 7) ewes and ++ (n = 8) ewes in total number of pituitary cells, pituitary volume, numbers or diameters of FSHβ- or LHβ-immunostaining cells, notwithstanding significantly higher concentrations of immunoreactive plasma FSH (P < 0.001) but not LH in BB compared with ++ animals. Significant linear relationships were found within each genotype between plasma FSH and number of FSHβ-immunostaining cells. No such relationship was found for plasma LH and number of LHβ cells. For the FSH relationship, the slopes of the regression lines were the same. It is hypothesized that the differences in plasma concentration of FSH between the genotypes is due to a greater output of FSH per pituitary cell in the BB animals.