before, at, or after the beginning of diameter deviation. However, results of studies in mares suggested that LH does not influence growth of the dominant follicle until after the beginning of
deviation. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Dominant follicle; Horses; Follicle selection; Follicular waves; Subordinate follicles
1. Introduction
Follicle selection is the mechanism whereby only one of the many available follicles becomes the ovulatory follicle in monovular species and has been a long-time mystery
in reproductive biology. Primarily because of the transrectal ultrasound technique, changes in follicle populations have been well characterized in the large farm species,
and progress is being made toward resolution of the follicle-selection mystery. The ultrasound technique is being used for characterizing follicle-population changes, track-
ing individual follicles from examination to examination, monitoring the effects of treatment, and eliminating, treating, or sampling specific follicles at specific times
during a follicular wave. Among farm species, cattle and horses have the most effective selection mechanism as indicated by a greater frequency of single ovulations than for
swine, sheep, and goats. The purpose of this report is to examine the current scientific status of the follicle-selection phenomenon by comparing research results between cattle
and horses. Most of the review considers the morphologic extrafollicular aspects of
Ž selection
changes in follicle diameters and circulating gonadotropin and estradiol .
concentrations ; less is known about the intrafollicular biochemical aspects.
2. Follicular waves
Cattle and horses have similar follicle-selection characteristics. The two species make good comparative research models because of suitability for different experimental
approaches, utilizing, for example, a consistent early diestrous follicular wave in cattle and two- to threefold larger follicles in mares.
2.1. Cattle Based on histologic study, two waves of antral follicular development were proposed
initially for the bovine estrous cycle, and each wave resulted in a follicle of preovulatory Ž
. diameter Rajakoski, 1960 . Subsequently, results of various experimental approaches
Ž were interpreted to agree or disagree with the two-wave proposal for reviews see
. Matton et al., 1981; Fortune et al., 1991; Ginther et al., 1996b . Transrectal ultrasonic
imaging for the study of bovine ovarian follicles was introduced in Pierson and Ginther Ž
. 1984 . Using this technology, it was reported that, on average, two waves of follicular
Ž growth for various diameter categories occurred during the estrous cycle Pierson and
. Ginther, 1987b . It was concluded that each wave involved a selection process, wherein
a large follicle grew to a preovulation diameter, and other follicles began to regress
several days before the largest follicle reached maximum diameter. Functional selection against nonovulatory follicles approximately 5 days before ovulation was confirmed by
Ž a reduced response to a superovulatory gonadotropin regimen Pierson and Ginther,
. Ž
1988a . Ultrasonic tracking of individual follicles from day-to-day Fortune et al., 1988; Pierson and Ginther, 1988b; Savio et al., 1988; Sirois and Fortune, 1988; Knopf et al.,
. 1989 confirmed these interpretations and provided additional and more detailed charac-
terization. In various herds, a predominance of either two-wave or three-wave estrous cycles was found, accounting for earlier reports of two versus three waves. Each wave is
characterized by emergence of a group of follicles at 4 mm, growth of all follicles for a few days, and then dissociation into a large follicle that continues to grow and smaller
follicles that regress. The anovulatory wave which begins as 4-mm follicles during the periovulatory period will be featured in this report because it has the most consistent
characteristics and has been most extensively studied. In the earlier studies, the follicles
Ž .
Ž .
were defined as largest F1 and second largest F2 , followed by the terminology Ž
. Ž
dominant Goodman and Hodgen, 1983 , dominant and nondominant
Ireland and .
Ž Roche, 1987; Sirois and Fortune, 1988 , or dominant and secondary Ireland and Roche,
. 1987; Savio et al., 1988 . Thereafter, the terms dominant and subordinate follicles have
been used most frequently. 2.2. Mares
Ž The types of follicular waves that develop in mares are major waves characterized
. Ž
by dominant and subordinate follicles and minor waves largest follicle does not attain .
the diameter of a dominant follicle . Based on transrectal palpation, a single major Ž
. follicular wave was proposed initially for the equine estrous cycle Ginther, 1979 . The
wave of follicles dissociated about 6 days before ovulation into a single growing preovulatory follicle and several regressing follicles. The palpation work was subse-
quently substantiated by ultrasound, based on grouping of follicles into diameter
Ž .
categories Palmer, 1987; Pierson and Ginther, 1987a
and tracking of individual Ž
. follicles Sirois et al., 1989; Ginther, 1990 . There are profound breed differences in
Ž .
wave patterns during the estrous cycle for review see Ginther, 1992 . In some breeds Ž
. quarter horses, ponies , usually only one major wave develops in late diestrus and
Ž .
culminates in the estrous ovulation. In other breeds thoroughbreds , a secondary major wave frequently develops in early diestrus, and the dominant follicle may be anovula-
tory, as in cattle, or ovulatory. The secondary-wave phenomenon accounts for the earlier Ž
. intriguing discovery Hughes et al., 1972 of diestrous ovulations. Minor follicular
Ž waves have been demonstrated statistically in mares
Ginther, 1993; Ginther and .
Bergfelt, 1992 . The selection phenomenon of the major ovulatory wave that begins at midcycle for all breeds will be used for this report and will be compared to the selection
aspects of the anovulatory wave that begins near ovulation in cattle.
3. Stimulation of waves by FSH surges
