Df. Salamone et al., Changes in the cumulus-oocyte complex of subordinate follicles relative tofollicular wave status in cattle, THERIOGENOL, 52(4), 1999, pp. 549-561
We investigated factors that affect cumulus-oocyte complex (COC) morphology
and oocyte developmental competence in subordinate follicles on different
days after follicular wave emergence in beef heifers. In Experiment 1, heif
ers (n = 13) were assigned at random to COC aspiration during the growing/s
tatic (Days 1 to 3) or regressing (Day 5) phase of subordinate follicle dev
elopment (follicular wave emergence = Day 0). Follicular wave emergence was
induced by transvaginal ultrasound-guided follicular ablation, ovaries wer
e collected at slaughter, all follicles greater than or equal to 2 mm excep
t the dominant follicle were aspirated, and COC were microscopically evalua
ted for morphology. There was a greater percentage of COC with expanded cum
ulus layers on Day 5 (42.4%) than on Days 1 to 3 (2.2%). In Experiment 2, h
eifers (n = 64) at random stages of the estrous cycle had all follicles gre
ater than or equal to 5 mm ablated and 4 d later, 2 doses of PGF were injec
ted 12 h apart; heifers were monitored daily by ultrasonography for ovulati
on (Day 0 = follicular wave emergence). Heifers were assigned to the follow
ing time periods for oocyte collection from subordinate follicles: Days 0 a
nd 1 (growing phase), Days 2, 3 and 4 (static phase), and Days 5 and 6 (reg
ressing phase). Ovaries were individually collected at slaughter, and all f
ollicles greater than or equal to 2 mm except for the dominant follicle wer
e aspirated. The COC were morphologically evaluated and then matured, ferti
lized and cultured in vitro. Expanded COC were more frequent during the reg
ressing phase (53.4%) than the growing or static phase (14.4 and 17.8%, res
pectively; P<0.05). While the proportions of COC with greater than or equal
to 4 layers of cumulus cells and denuded oocytes were higher (P<0.05) in t
he growing and static phases, the production of morulae was highest (P<0.05
) with COC collected from subordinate follicles during the regressing phase
. In Experiment 3, heifers (n = 18) were assigned at random to oocyte colle
ction from subordinate follicles 3 and 4 d (static phase) or 5 and 6 d (reg
ressing phase) after follicular wave emergence. The heifers were monitored
ultrasonically for ovulation (Day 0 = follicular wave emergence); COC were
collected from all follicles (greater than or equal to 5 mm) except for the
dominant follicle by transvaginal ultrasound-guided follicle aspiration 3
to 6 d later. Recovered oocytes were stained and examined microscopically t
o evaluate nuclear maturation. A higher proportion of oocytes collected on
Days 5 and 6 showed evidence of nuclear maturation (50%) than on Days 3 and
4 (8.3%; P<0.05). Results support the hypothesis that COC morphology and o
ocyte developmental competence change during the growing, static and regres
sing phases of subordinate follicle development. (C) 1999 by Elsevier Scien
ce Inc.