Endocrine changes associated with the growth of pre-vitellogenic oocytes in coho salmon, Oncorhynchus kisutch

It has been hypothesized that in salmonids there are two periods when growth can alter the decision to mature; thus, for a fall spawning species, an 'initiation' period in the fall is followed by a 'permissive' period for continuing maturation in the spring. Research in mammalian and avian models have shown that follicle-stimulating hormone (FSH), steroidogenic acute regulatory protein (StAR) and 17beta-estradiol are key components controlling oocyte growth. In the present study we sampled two-year-old female coho salmon during the proposed fall 'initiation' period. Histological analyses of the ovaries indicated that from August to December (11-15 months prior to spawning) oocytes developed from the late perinucleolar to mid-cortical alveoli stage. The accumulation of cortical alveoli during this period was accompanied by significant increases in GSI, pituitary and plasma FSH, and plasma 17beta-estradiol. In addition, small but significant increases in transcript levels for FSH receptor and StAR occurred during the fall. Such changes in FSH signaling are consistent with a proposed role for FSH stimulation of oocyte growth via steroid production during the fall initiation period.