1 Department of Community, Occupational & Family Medicine, National University of Singapore and 2 Department of Obstetrics & Gynaecology, Singapore General Hospital, Singapore
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Abstract |
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Key words: daylight/semen volume/sperm density/temperature
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Introduction |
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What causes these seasonal changes in semen quality? Climatic heat was proposed to be a possible causative factor in earlier studies (Levine et al., 1988; Politoff et al., 1989
). However, later reports suggested that heat may not be as important a factor as photoperiod (Synder, 1990
; Levine et al., 1992
). So far, it appears that all the reports on seasonal variation of human semen quality were conducted in the temperate countries. There are no reports of large semen quality data of men from the tropics. Men residing in the tropics are exposed to almost constant hours of daylight throughout the year. Data on variation in semen quality through the year for these men would be useful for testing the importance of photoperiod on spermatogenesis. The objective of this study, therefore, was to determine if there is a circannual rhythm of sperm parameters of men in the tropics.
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Materials and methods |
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Semen collection and analysis
The men were asked to collect their semen at home in the morning by masturbation into a sterile wide-mouth plastic container, after 3 days of abstinence. The samples were brought to the laboratory within 1 h of collection. Time of ejaculation, abstinence period, spillage (if any), and fever during the last 3 months were recorded by the subject. All semen samples were processed and analysed by experienced laboratory technologists at the Fertility Clinic of the Singapore General Hospital immediately upon receiving the samples. Seven laboratory technologists were involved in the semen analysis from 1991 to 1995. Volume, total sperm density, sperm viability, proportion of progressively motile spermatozoa and proportion of normal and abnormal sperm forms were examined according to the World Health Organization's guidelines for the examination of human semen (WHO, 1992). For the objective of this study, only the data for sperm density and semen volume were used for the analysis.
All assays were performed after the semen had liquefied and within 1 h of collection by masturbation. Samples that remained viscous were liquefied by mechanical pipetting with a large-bore disposable pipette. The volume was assessed by aspirating the semen into a 5 ml graduated micropipette with disposable tips (OxfordMacroset, Swedesboro, NJ, USA). Sperm concentration was determined with a Neubauer haemocytometer. A 1:20 dilution was made using 50 µl semen and 950 µl sperm diluent solution. A 10 µl droplet was removed from the well-mixed sample and applied to the slide chamber, which was mounted with a glass coverslip. A second sample was applied to the other chamber. After the spermatozoa had settled, they were counted under phase-contrast microscopy. Only mature spermatozoa (with tails) were counted. Pinhead spermatozoa were excluded. If the difference between the two counts exceeded 10%, another haemocytometer was set up and the counts repeated.
Intra-specimen assays for all the above parameters consistently gave values within a 10% variance.
Statistical analysis
The sperm density distribution was skewed. Log transformation was first used to normalize the data but the transformed data was still slightly skewed to the left. However, with cubic root transformation of sperm density, normality of the distribution was achieved. The cubic root of sperm density data was used in all subsequent analysis involving sperm density. We have also calculated the geometric mean of sperm density as the geometric mean was used for comparison with other studies.
The monthly variations in the semen parameters (i.e. sperm density and semen volume) from 1991 to 1995 were studied. The year-to-year variations in the semen parameters could confound the results since there were significant differences in the mean sperm density for the various years (see Table II). Similarly, the inter-technologists variations and age of the subjects would also need to be considered. Analysis of co-variance (ANCOVA) using the general linear model procedure was used to determine mean sperm density and semen volumes adjusting for age of the subject, the technologist performing the semen analysis and year of examination. The data were then plotted, with the adjusted mean value and the 95% confidence limits, to show the distribution of the various semen parameters over the 12 calendar months. The Bonferroni Post Hoc test was used to compare the semen parameters between the calendar months. Statistical analysis was carried out using the Window version of SPSS 9.1 on a personal computer (SPSS, 1999
).
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Results |
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Figure 2 shows the adjusted (for age of subject, year of test and technologist who performed the analysis) mean semen volume and the 95% confidence interval by calendar months. Although there were month-to-month fluctuations in the mean semen volume these differences were not significant. Similarly, there were no significant differences in the month-to-month adjusted (for age of subject, year of test and technologist who performed the analysis) mean sperm density (Figure 3
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Discussion |
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Our study group consisted of men from couples undergoing initial screening for inability to conceive. Generally, in approximately 30% of the cases, an important abnormality is identified in the man only (Howard, 1995). As such, the data, although taken from an infertility clinic, do not only reflect the sperm parameters of infertile men.
Another possible issue is that of selection bias, i.e. only those who could afford the services would come for the investigation. In some countries only a minor fraction of infertile couples seek medical assistance either because of cultural beliefs or financial reasons; this is not so in Singapore. Health care is affordable and readily accessible. Because of the norm of a two child family, most couples would seek medical assistance if they had a problem trying to conceive. This hospital where the data were obtained is a general hospital which provides governmental financial subsidy for health services.
Effects of frequency of ejaculation on the results of semen analysis were eliminated in this study by the subjects having had a 3 day abstinence period, confirmed by the staff when the semen samples were collected.
There are other limitations inherent in this study. The clinical status of each of the subjects was not available. Different types of diseases, which this study was not able to look into, may influence the sperm parameters. As the samples were collected over a 5 year period, it was not feasible for only one technologist to perform all the seminal analysis. Inter-technologist biases could be introduced in the process. Similarly, there may be year-to-year fluctuations arising from inter-observer variation or laboratory measurement fluctuations. To minimize these possible confounders, data were adjusted for these factors when the month-to-month mean sperm parameters were studied.
No significant monthly differences were found in the mean semen volume and sperm density. These results were in contrast to those in a prospective study in San Antonio, Texas, USA, in which Levine et al. reported that men had significantly higher sperm density and percentage of normal sperm morphology in winter than in summer (Levine et al., 1990). Similar findings were reported in another prospective study of similar design but conducted in New Orleans, Louisiana, USA (Levine et al., 1992
). Other retrospective studies, all in the temperate cities (Edinburgh, Lille, Bologna, Minnesota), have also reported reduction in sperm density during summer (Mortimer et al., 1983
; Politoff et al., 1989
; Campaniello et al., 1990
; Fisch et al., 1997
).
The causes of these seasonal changes in semen quality observed in the temperate countries but not in the tropics are unknown. Climatic heat and hours of daylight have been postulated as possible causes for seasonal fluctuation in the temperate countriesas temperatures are higher in summer and the daylight hours are much longer. If these hypotheses are true, there will not be any significant monthly difference in the semen quality in the tropics, given that temperature and daylight hour fluctuation are not large.
Normal spermatogenesis requires a temperature below that of the abdomen. The intrascrotal temperature is 23°C lower than the rectal temperature (Synder, 1990). High ambient air temperatures may inhibit thermal loss through the scrotum, leading to a rise in testicular temperature. Infertile men without varicocele have been noted to have significantly higher mean intrascrotal temperatures than men without fertility problems (Zorgniotti and Sealfon, 1988
). Sperm production in humans is known to decrease when testicular temperature is raised by experimental techniques (Mieusset et al., 1987
). In an epidemiological study by Chia et al. (1994), it was reported that `plant and machine operators (PMO) had an odds ratio (OR) of 1.93 (95% CI 1.123.30) for oligospermia compared with the non-PMO'. Workers exposed to excessive heat were found to be associated with a higher risk of oligospermia, OR = 2.72 (95% CI 1.127.41), in the PMO group. In a study to investigate the effects of climatic heat, Levine et al. (1992) collected semen from two groups of men, one engaged in predominantly indoor and the other in predominantly outdoor activity, both in summer and in winter. They reported there was no correlation between the number of hours worked outdoors during the summer and summer time measures of semen quality or any differences in these measures between summer and winter. They dismissed the hypothesis that the heat of the summer is detrimental to male reproductive capacity (Levine et al., 1992
). There were no other reports to substantiate Levine's observation.
The duration of daylight may affect semen quality. Under laboratory conditions, rhesus monkeys' testis volume increases after exposure to shorter daylight (8 h of light, 16 h of dark) and decreases after exposure to longer daylight (16 h of light, 8 h of dark), with concomitant increases and decreases in plasma testosterone respectively (Chik et al., 1992). Levine has suggested that the summer suppression of spermatogenesis observed in the temperate countries may be due in part to the photoperiodic exposure affecting an endogenous circannual rhythm (Levine, 1994
).
Singapore is located between latitudes 1° 09'N and 1° 29'N and longitudes 103° 36'E and 104° 25'E. The temperature is relatively stable throughout the year due to its close proximity to the equator (80 km north). The average maximum and minimum temperatures are around 32°C and 25°C respectively. The range for the daily mean bright sunshine hours for the last 10 years was from 5.15.9 h (Singapore, 1998). Whether the findings in the current study that there was no significant month-to-month variation in semen quality among Singapore men was related to lack of seasonal variation in climatic temperature and daylight duration is an intriguing question. If the conclusion that summer climatic temperature is not a determining factor of semen quality in man (Levine, 1994) is accepted, then the findings of this study lend support to the hypothesis that photo duration may be an important factor in human spermatogenesis.
In photoregulated, seasonally breeding mammals, the pineal gland and melatonin are key components in the neuro-endocrine pathway coupling day length to gonadotrophin release (Bronson, 1995). There is no doubt that melatonin secretion in humans is sensitive to light (Lewy et al., 1980
). However, studies conducted on human melatonin secretion over the different seasons have been inconclusive (Lewy et al., 1980
). Further studies on photoregulation of human spermatogenesis and the possible underlying neuro-endocrine pathways are needed.
As the current study is based on, so far, data obtained from only one country in the tropics, it may not truly reflect the actual situation for individuals who are residing in the tropics. There is a need for a multi-centre study involving other andrology centres to confirm these findings.
In summary, it has been shown that there were no significant month-to-month fluctuations in semen volume and sperm density among men who reside in the tropics. These results may be related to the minimal changes in temperature and daylight hours that the men were exposed to in the tropics.
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Acknowledgments |
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Notes |
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References |
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Submitted on June 21, 2000; accepted on November 7, 2000.