tion, pubertal development, season of the year, and aging of humans and animals. q 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Spermatogenesis; Species comparisons; Germ cell degeneration
1. Introduction
1.1. Definition and background Spermatogenesis is the sum total of the events that occur within the testis that
Ž .
produce spermatozoa Johnson, 1991b . Spermatogenesis occurs within seminiferous tubules of the testis. It is a lengthy, chronological process by which stem cell
spermatogonia divide by mitosis to maintain their own numbers and to cyclically produce primary spermatocytes that undergo meiosis to produce haploid spermatids
Ž .
which differentiate without further division
into spermatozoa. The efficiency of spermatogenesis is the number of spermatozoa produced per gram of testicular
parenchyma and is not influenced by difference in testicular size among animals. Spermatocytogenesis, meiosis, and spermiogenesis are characterized by division
andror differentiation of spermatogonia, spermatocytes, and spermatids, respectively, Ž
. and are three major divisions of spermatogenesis Fig. 1 . In the bull, these divisions
Ž take 21, 23, and 17 days, respectively, for a total duration of 61 days Fig. 1; Amann,
. 1970 . During spermatocytogenesis, stem cell spermatogonia divide by mitosis to
Ž produce other stem cells that continue the lineage throughout the adult life of males Fig.
. 1 . Stem cells give rise to spermatogonia that cyclically produce committed spermatogo-
nia which proliferate andror differentiate to produce primary spermatocytes that un- dergo meiosis. Meiosis allows exchange of genetic material between homologous
Ž chromosomes of primary spermatocytes and the production of haploid spermatids Fig.
. Ž
. 1 . During spermiogenesis Fig. 1 , spermatids differentiate from cells with spherical
nuclei into mature germ cells shaped like spermatozoa for that species. The flagellum is developed into a tail, and the head of the spermatozoon is composed of the compressed
Ž .
nucleus source of the male genome and an acrosome with its enzymes necessary to penetrate the layers of the egg.
1.2. Kinetics of spermatogenesis Ž
. The spermatogenic cycle cycle of the seminiferous epithelium is superimposed on
Ž the major divisions of spermatogenesis spermatocytogenesis, meiosis, and spermiogene-
. sis; Fig. 1 . The cycle of the seminiferous epithelium is ‘‘a series of changes in a given
Ž .
area region of seminiferous epithelium between two appearances of the same develop- Ž
. Ž
. mental stages steps ’’ Leblond and Clermont, 1952 and lasts for 13.5 days in the bull.
Ž If spermiation release of spermatozoa from seminiferous epithelium and counterpart to
. ovulation in the female is used as a reference point, the spermatogenic cycle would be
all the events that occur between two consecutive spermiations from a given region of the tubule.
Fig. 1. Drawings and classification of germ cells at different developmental steps in the three major divisions Ž
. of spermatogenesis spermatocytogenesis, meiosis, and spermiogenesis combined to make the eight stages of
Ž . the cycle of bull seminiferous epithelium. During the 21 days of spermatocytogenesis, A spermatogonia A
Ž .
Ž . enters cyclic at 13.5-day interval activity during stage III and undergo division to produce intermediate In ,
Ž . Ž .
B B spermatogonia, and leptotene primary spermatocytes L . During the 23 days of meiosis, leptotene Ž .
Ž . Ž .
primary spermatocytes differentiate through zygotene Z , pachytene P , and diplotene D before the first Ž
. meiotic division to produce secondary spermatocytes SS , and the second meiotic division to produce Sa
Ž .
Ž .
spermatids Sa . During the 17 days of spermiogenesis, Sa spermatids differentiate through Sb Sb , Sb
1 1
2
Ž .
Ž .
Ž .
Ž .
Sb , Sc Sc , Sd Sd , and Sd
Sd steps of development before spermiation as spermatozoa. The letters
2 1
1 2
2
indicate the developmental step, and the numbers associated with each germ cell step indicate the developmen- tal age of each cell type in the middle of each spermatogenic stage. The cycle length is 13.5 days, and the
Ž .
duration of spermatogenesis is 61 days in the bull. Modified from Johnson et al. 1994 .
The cycle length and frequency at which spermatozoa are released both are deter- mined by the rate at which committed spermatogonia enter the process of spermatogene-
Ž sis. The cycle length and duration of spermatogenesis from the production of committed
. Ž
. spermatogonia to spermiation are species-specific Swierstra et al, 1974; Amann, 1986 .
The cycle length in days for the prairie vole is 7.2, hamster 8.7, mouse 8.9, rhesus monkey 9.5, rabbit 10.7, stallion 12.2, bull 13.5, beagle dog 13.6, and man 16
Ž .
Clermont, 1963; Swierstra et al, 1974; Amann, 1981, 1986 . The stage represents an association of 4–5 germ cells, each of which is in a specific,
chronological, developmental step in spermatogenesis. The assignment of stages repre- sent man-made divisions of naturally cyclically occurring cellular associations. In bulls,
Ž .
Ž the cycle has been divided into eight stages Amann, 1970 or 12 stages Berndtson and
. Ž
. Desjardins, 1974 . Fourteen stages have been described in rats Clermont, 1972 , eight
Ž .
Ž .
in the horse Swierstra et al, 1974 , and only six stages in humans Clermont, 1963 . The Ž
. Ž
. bull Amann, 1970 and horse Swierstra et al, 1974 , like most species, have mainly
only one stage of the cycle represented in a cross-section of the seminiferous tubule. Ž
. Humans have more than two stages per cross-section Clermont, 1963 .
1.3. Testis Quantitative approaches have been extended to calculate daily sperm production, a
quantitative measure of spermatogenesis to express the total number of spermatozoa Ž
produced per day by a testis or paired testes Kennelly and Foote, 1964; Amann, 1970; .
Johnson, 1986b . Considering the life span and theoretical yield of a specific germ cell, a daily
expression of spermatozoan production can be obtained from the number of germ cells Ž
. of that type in the testis Kennelly and Foote, 1964; Amann, 1970 . The life span of a
germ cell is the duration of stages of the cycle in which that cell type occurs. Theoretical yields are calculated by 2
n
, where n is the number of cell divisions between that cell type and spermatids.
Daily sperm production per gram of testicular parenchyma is a measure of efficiency Ž
of spermatogenesis, and it is useful in species comparisons Fig. 2; Amann et al, 1976;
Fig. 2. Efficiency of spermatogenesis in various species based on potential daily sperm production per gram parenchyma at different developmental steps in spermatogenesis of the rat, bull, horse, boar, and human.
Potential daily sperm production per gram is calculated from numbers of B spermatogonia, pachytene primary Ž
. Ž
spermatocytes, Golgi and cap phase spermatids round spermatids , and maturation-phase spermatids elon- .
Ž .
gated spermatids . Adult rats 400 g experienced no significant loss during these different steps in
Ž spermatogenesis. Adult horses had early germ cell degeneration in spermatogenesis end of spermatocyte
. formation with no subsequent losses. Younger adult humans had significant losses during the second meiotic
division. Humans and bulls have much lower efficiency of spermatogenesis than do other species. Boars and humans have significant losses in potential production at the end of meiosis. Bulls and horses have no
Ž .
Ž .
significant losses at the end of meiosis. Modified from Johnson et al. 1981, 1984a,b, 1994 , Johnson 1986b , Ž
. and Okwun et al. 1996 .
. Ž
6
. Johnson, 1986b . Humans have a much lower efficiency 4–6 = 10 rg of spermatogen-
Ž .
esis Johnson et al., 1981 than do other species tested, including rhesus monkeys Ž
6
. Ž
6
. Ž
6
. Ž
6
. 23 = 10 rg , rabbits 25 = 10 rg , rats 20–24 = 10 rg boars 23 = 10 rg , stal-
Ž
6
. Ž
6
. Ž
6
. lions 16–19 = 10 rg , rams 21 = 10 rg , hamsters 24 = 10 rg , and even bulls
Ž
6
. Ž .
12 = 10 rg Amann et al, 1976; Amann, 1981; Johnson, 1986b . Comparisons of
daily sperm production per gram parenchyma vs. potential daily sperm production based on germ cells in different developmental steps of spermatogenesis facilitate detection of
Ž species differences in sites of germ cell degeneration in spermatogenesis Fig. 2;
. Johnson et al, 1999a,b .
Ž .
Lower efficiency of spermatogenesis in human testes compared to other species Ž
. Ž
results from a longer duration of spermatogenesis 74 days , longer cycle length 16 . Ž
. Ž
days Clermont, 1972 , and lower density of germ cells in human testes Johnson,
.
6
1986b . In humans, the number of round spermatids per gram parenchyma is 49.2 = 10 Ž
. Johnson, 1986a,b . In comparison, in the rat, whose cycle length is 12.9 days, and the
horse whose cycle is 12.2 days, the number of round spermatids per gram is 190 = 10
6
Ž .
6
Ž .
Johnson et al, 1984a and 162 = 10 , respectively Johnson, 1985 . The percentages of the human testis occupied by seminiferous tubules and seminiferous epithelium are
Ž .
lower than that for bulls, horses, or rats Fig. 3; Johnson, 1986b . The reason humans differ from other species in their lower testicular germ cell density and longer cycle
length remains a mystery. However, it does not appear to be a problem for the survival of the human species, considering the population explosion on the earth.
Germ cell degeneration at specific developmental steps of spermatogenesis has been quantified by comparing daily sperm production per gram parenchyma based on germ
Ž .
cell types in different steps of development Fig. 2; Johnson, 1986b . This has been done Ž
. Ž
. Ž
. in the boar Kennelly and Foote, 1964 , bull, Amann, 1970 , rat Johnson et al, 1984a ,
Ž .
Ž .
horse Johnson, 1985 , and human Johnson, 1986b . However, among breeds of boars, the efficiency of spermatogenesis and potential daily sperm production based on germ
Ž .
Fig. 3. Composition of testes in various species expressed as the percentage volume density of the testicular parenchyma occupied by seminiferous tubules or seminiferous epithelium. In the human, the volume density of
Ž .
seminiferous epithelium is less than 50 of the testicular parenchyma. Modified from Johnson 1986b and Ž
. Johnson et al. 1994 .
Ž .
cell at different steps of spermatogenesis are similar Fig. 2
and reflect similar degenerative rates throughout spermatogenesis.
For species comparisons, daily sperm production has been estimated at different Ž
. development steps throughout spermatogenesis
Fig. 2 . While humans and boars experienced a 30–40 reduction in potential sperm production during the end of
meiosis, there was no comparable loss or degeneration during meiosis of the bull or stallion. In the breeding season of the stallion, there was a larger number of A
spermatogonia than whose progeny could be sustained. This resulted in significant
Ž .
degeneration of B spermatogonia at the end of meiosis Johnson, 1985 . In the bull, a significant number of degenerated germ cells have been noted during spermatocytogene-
Ž .
sis Berndtson and Desjardins, 1974 . Another way to evaluate degeneration of germ cells at different developmental steps is by the ratio of more advanced germ cell per type
A spermatogonium.
1.4. Season Ž
. Stallions unlike bulls, boars, rats, and humans modulate or regulate daily sperm
production with season while continuing to produce spermatozoa throughout the year. Germ cell degeneration during meiosis and seasonal modulation of the number of A
w spermatogonia which is twice as large in the breeding season as in the non-breeding
Ž .x
season of the horse testis Johnson, 1986b are mechanisms that seasonally regulate
spermatogenesis. The number of A plus B spermatogonia per testis in the breeding
1
Ž
9
. Ž
. season 5.1 0.2 = 10
was 71 higher p - 0.01 than the number in the non-breed- Ž
9
. Ž
. ing season 3.0 0.3 = 10 . Daily sperm production per testis was 84
p - 0.01 higher in the breeding season. Season affects the number of different subtypes of
Ž .
spermatogonia per testis Fig. 11 . Seasonal modulation of A spermatogonia in a species may result from proliferation of
renewing stem cells. The reserve stem cell spermatogonium is the youngest form of Ž
germ cells which may be dormant in testes active in spermatogenesis Ao stem cell; .
Clermont, 1972 or actively involved in producing other stem cells or proliferating Ž
. spermatogonia
As stem cells; Huckins, 1971 . These cells carry on the lineage throughout the life of adult males. Seasonal variation in the number of renewing stem
cells has been found in other seasonal breeders such as rams and red deer stags Ž
. Hochereau-de Reviers, 1981 . In the horse, the number of the most primitive spermato-
Ž .
Ž .
Ž .
gonia A was 25 greater p - 0.05 in the breeding season Fig. 4 . Hence, seasonal
1
differences in number of more primitive spermatogonia contribute significantly to seasonal differences in total number of spermatogonia in the horse.
Season influences the developmental steps or spermatogonial subtypes that degener- Ž
ate. The yield of B rA or B rA was greater in the breeding season of the horse Fig.
2 1
2 2
. 4; Johnson, 1991a . However, the yield of conversions of B
spermatogonia to early
2
primary spermatocytes was greater in the non-breeding season. The greater degeneration of B
spermatogonia in the breeding season results form an overpopulation of A
2
spermatogonia beyond the increased number of Sertoli cells in the breeding season. Ž
. Researchers Johnson, 1985, 1986a found a positive relationship between the number of
A spermatogonia and the amount of degeneration that occurred in A
and A
1 2
3
Fig. 4. Number and yield of spermatogonia in adult equine testes in the breeding and nonbreeding seasons. The Ž
. number of horse A , A , A , B , and B spermatogonia A , A , A B , and B , respectively per testis and
1 2
3 1
2 1
2 3
1 2
the yield of specific spermatogonial subtypes to B spermatogonia are different between the breeding or
2
Ž . Ž
. non-breeding seasons. a The number of A spermatogonia is 25 p- 0.05 greater in the breeding season
1
of the horse. The numbers of A , A , B , and B spermatogonia and preleptotenerleptotenerzygotene
2 3
1 2
Ž .
Ž .
primary spermatocytes PS are 39, 83, 91, 110, and 49 greater p- 0.01 , respectively, in the
Ž . Ž
. breeding season. b Although the conversions of A to B and A to B spermatogonia are similar p 0.05
3 2
1 2
Ž .
between seasons, the conversions of A to B and A to B are greater p- 0.01 in the breeding season. The
1 2
2 2
Ž .
Ž conversion of B to early primary spermatocytes is less p- 0.01 in the breeding season from Johnson et al.,
2
. 1991 .
spermatogonia. As the result of a higher yield of early spermatogonial subtypes in the breeding season, the number of late spermatogonial subtypes was significantly increased
Ž .
Fig. 4 . This increased yield early in spermatogenesis appeared to make the greatest contribution to the significantly increased spermatogonial numbers in the breeding
season. Although not seasonal, the bull had a significant loss of potential daily sperm Ž
. production regulatory degeneration or trimming of spermatogonial progeny between
Ž .
spermatocytogenesis and meiosis Fig. 2 .
Fig. 5. Effect of season on the number of Sertoli cells in the equine testis as viewed at different times of the year. Number of Sertoli cells found in 43–48 adult horses during each 3-month period throughout 1 complete
Ž .
Ž year illustrates more
p- 0.05 Sertoli cells per gram parenchyma in May–July the natural breeding season .
of the horse than in other periods. The number of Sertoli cells per testis is greater in May–July compared with Ž
. the value in August–October or February–April p- 0.05 or compared with the value for November–January
Ž . Ž
. p- 0.01
from Johnson and Nguyen, 1983 .
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
