Removal of Follicle Wall Components from Ovarian Oocytes of the Brook Trout, Salvelinus fontinalis

Fully grown brook trout (Salvelinus fontinalis) ovarian follicles, preincubated in Ca2+-free media, were dissected from other ovarian tissue by use of watchmaker forceps. Manual defolliculation and further Ca2+-free treatments were used to obtain oocytes freed of one or more follicular investments. Combinations of these treatments ultimately resulted in oocytes denuded of somatic cells as demonstrated by scanning electron microscopy. Mechanically denuded brook trout oocytes maintained the ability to undergo progestogen-induced meiotic maturation in vitro. This technique, which avoids use of enzymes that may alter cell surfaces, provides a useful system for the study of interactions between follicle wall components and the oocyte during hormonally induced meiotic maturation in vitro.

Electrophysiology of oocytes during meiotic maturation and after ovulation in brook trout (Salvelinus fontinalis)

Brook trout oocytes were stripped of their follicular layers and were impaled with both voltage-measuring and current-passing microelectrodes. The membrane resting potential (Vm) was −46 ± 1.2 mV (mean ± SE, n = 50, bath grounded) in control denuded oocytes with an intact germinal vesicle. Vm was reduced to −38 ± 1.8 mV (n = 16) in eggs that had ovulated in vivo. Accompanying the potential drop was a marked increase in membrane resistance (Rm) from 26 ± 1.7 in denuded oocytes to 1640 ± 150 kΩ∙cm2 in ovulated eggs and a reduction in membrane current (Im) from 2380 ± 260 to 25 ± 2.6 nA∙cm2. The increased resistance in ovulated eggs was in part the result of reduced Na+ and K+ membrane conductances; there was no significant change in Cl− conductance. Treatment of denuded oocytes with 17α-20β-dihydroxyprogesterone produced germinal vesicle dissolution; in denuded oocytes that had undergone germinal vesicle dissolution in vitro, Rm increased and Im decreased, comparable at least qualitatively to the changes seen after ovulation in vivo. The results indicate that major electrophysiological changes accompany germinal vesicle dissolution and ovulation in brook trout oocytes.