Ž . Fig. 6. Species comparison in the number of Sertoli cells and in the germ cell:Sertoli cell ratio. a The number
Ž . of Sertoli cells per gram parenchyma or per testis for the rat, bull, boar, horse, and human, and b the number
of germ cells per Sertoli cell. The bull and human have fewer germ cells supported by each Sertoli cell than Ž
. does the rat, horse, or boar from Johnson, 1986b; Johnson et al., 1994, 1996b; Okwun et al., 1996 .
Stereological methods of histologic images have illustrated seasonal variation in Ž
number of Sertoli cells and the germ cell:Sertoli cell ratio in the horse Figs. 5 and 6; .
Ž Johnson, 1986a . Seasonal variation in number of Sertoli cells Johnson and Thompson,
. 1983; Johnson, 1986a have characterized the annual cycle of the Sertoli cell population
Ž .
Ž in the horse Fig. 5 . There is significantly more Sertoli cells in the summer natural
. Ž
. breeding season , and there is a dose month effect based on the time of the year the
Ž .
samples were taken Fig. 5 . The ratio of spermatogonia, spermatids, or all germ cells in Ž
. stage VIII tubules was greater in the breeding season of the horse Johnson, 1986a .
Hence, Sertoli cell function appears to be enhanced during the breeding season with seasonal modulation of spermatogenesis in the horse.
2. Germ cell degeneration
2.1. General considerations Degeneration of germ cell occurs largely during spermatocytogenesis and meiosis
Ž .
Johnson, 1986b . Losses during spermatocytogenesis include 25 in mice, 11 in Sherman rats, and 75 in adult Sprague–Dawley rats. Greater degeneration of sper-
Ž .
matogonia in rams occurs following long day illumination Johnson, 1986b . Degenera- tion of B spermatogonia in horses is greater in the breeding season when the number of
Ž .
A spermatogonia is doubled Fig. 4; Johnson, 1985 . Meiotic divisions account for a 13 loss of potential production in mice, 2 in Sprague–Dawley rats, 27 in Sherman
Ž rats, 25 in rabbits, and 6–15 in stallions depending upon season Johnson, 1985,
. Ž
. 1986b . In rams, fewer 40–50 spermatids were found after long day illumination
Ž .
Johnson, 1986b, 1991b . Germ cell degeneration during spermiogenesis was noted in Sherman rats and mice.
Ž .
Ž .
A loss of 6 or less depending on season occurred in horses Johnson, 1985 . Bulls Ž
. Ž
. Amann, 1970 and adult Sprague–Dawley rats 400 g had no significant germ cell
Ž .
loss during spermiogenesis Fig. 2; Johnson, 1986b .
No significant degeneration of human germ cells occurred between type B spermato- Ž
. gonia and secondary spermatocytes or during spermiogenesis Fig. 2; Johnson, 1986b .
A 30–40 loss from germ cell degeneration occurred during the meiotic divisions in humans. The loss of spermatozoan production late in meiosis is significantly, negatively
Ž .
correlated r s y0.86 with daily sperm production per gram parenchyma in humans. Likewise, serum concentrations of FSH in men are positively correlated with the
percentage of germ cell degeneration during post prophase of meiosis. Degeneration Ž
detected late in meiosis occurred during the second meiotic division in young men Fig. .
2 and is greater in aged men. It is not known why humans have a greater degeneration Ž
. rate late in meiosis than other species Fig. 2 , but it appears to be a critical step in
human spermatogenesis that could be exploited to increase efficiency of spermatogene- sis or to devise contraceptive strategies. There was no similar loss during meiosis in
bulls, horses, or rats, but boars have a significant loss of potential for sperm production
Ž .
during post prophase of meiosis Fig. 2 .
3. Conclusions
