Clinical Atlas of Sperm Morphology AM Phadke
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1Clinical Atlas of Sperm Morphology2

Introduction to Sperm MorphologyCHAPTER 1

 
DISCOVERY OF HUMAN SPERMATOZOA
Weisman (1941) has given an interesting and lucid account of discovery of spermatozoa. According to him in 1677, Antoj van Leeuwenhoek mentioned configuration of spermatozoa. In a famous letter written on 9th June 1699 to the Royal Society of London, he wrote, “I have discovered Animalcula in the masculine seed”.
Dr Ham first saw spermatozoa under the microscope in August 1677. Subsequently, in March 1678, Nicholas Hartsoeker illustrated human spermatozoa for the first time.
French investigators give credit to this trio for having independently discovered human spermatozoa.
It is incredible that there was a long wait of more than two and half centuries for somebody to describe the acrosome. Williams (1934) described acrosome for the first time.
 
INTRODUCTION TO SPERM MORPHOLOGY
 
GENERAL CONSIDERATIONS
Assessment of sperm morphology is the most difficult aspect of semen analysis. It requires long experience and expertise on the part of the investigator. The subject of sperm morphology not only involves the study of morphology of normal and abnormal spermatozoa but also seeks to investigate the causal relationship between sperm abnormalities and infertility. The identification and characteristics of other cellular elements in semen, though by default is included in sperm morphology, is a separate subject and will be considered in a subsequent volume. The sperm morphology is a dynamic subject and has undergone periodic re-evaluation to keep pace with the ever-increasing scientific data.4
There is unanimity amongst investigators regarding the role of certain isolated sperm abnormalities like spermatozoa with absence of acrosomes or tails and the presence of only “Pin headed” spermatozoa, as a causative factor in infertility. However, it is the relationship between the other sperm abnormalities and infertility, that remains as of now, a gray area beset with continuing debates and conflicting opinions. But it is hardly prudent to debunk this subject entirely on this basis.
 
NOMENCLATURE USED IN ANDROLOGY
Before starting the appraisal of sperm morphology, it is essential that the readers understand the different jargon words used by andrologists to express the sperm variables. Eliason (1970), Mortimer (1994) and WHO manual (1999) provide the requisite explanation of the different terms used by andrologists. Note that the term “spermia” refers to the volume of the ejaculate and “zoospermia” refers to the number of spermatozoa in the ejaculate.
Table 1.1 explains the nomenclature used in andrology.
Table 1.1   Nomenclature used in andrology
1. Aspermia
Absence of the ejaculate.
2. Azoospermia
Absence of spermatozoa in the ejaculate.
3. Hypospermia
Abnormally low volume of the ejaculate.
4. Hyperspermia
Abnormally high volume of the ejaculate.
5. Normozoospermia
Normal ejaculate as per WHO standards (1999)
6. Oligozoospermia: (severe)
Sperm concentration < 10 × 106 /ml.
(mild)
Sperm concentration between 10 × 106 /ml to 20 × 106 /ml.
7. Asthenozoospermia
(a) Less than 50% spermatozoa with rapid progressive motility and slow sluggish motility.
Or
(b) Less than 25% spermatozoa with rapid progressive activity or decreased sperm motility (Usually < 40%)
8. Teratozoospermia
Increased number of spermatozoa with abnormal morphology. (Typically > 50% abnormal forms)
9. Oligoasthenoteratozoospermia
Signifies disturbance of all the three parameters.
10. Necrozoospermia
All the spermatozoa in the ejaculate are dead as confirmed by vital staining.
11. Polyzoospermia
Means a very high concentration of spermatozoa in the ejaculate. (e.g. 250 × 106/ml)
12. Bacteriospermia
Presence of large number of bacteria in the semen.
13. Pyospermia
Presence of large number of leukocytes in the semen.
14. Hemospermia
Presence of blood in semen.
 
CHARACTERISTICS AND IMPORTANCE OF SPERM MORPHOLOGY
The volume, sperm count and sperm motility are variable parameters of ejaculates in infertile and fertile men and are influenced by various physiological and pathological factors enumerated below.5
Physiological factors
Pathological factors
  • Age
  • Period of abstinence
  • Method of collection
  • Inadequate ejaculations
  • Increased body or scrotal temperatures
  • Climatic conditions
  • Personal habits like consumption of alcohol and smoking
  • Systemic and local infection
  • Systemic diseases like diabetes and anemia
  • Drugs: Nitrofurans, antidepressants, excess of vitamin A
  • Associated hernia, hydrocele and varicocele
  • Filariasis
  • Anxiety and stress
 
RELATIVELY STABLE PARAMETER
Comparatively, sperm morphology is considered a relatively stable parameter in the ejaculate. Even then infection, trauma, other testicular stress situations, certain drugs and hormones influence it. (McLeod 1956, 1966, 1970): David et al., (1972).
 
REFLECTS A COMPREHENSIVE PICTURE OF EVENTS OCCURRING IN THE TESTIS AND EPIDIDYMIS
 
Testicular Factors
Leydig cells dysfunction: Sperm abnormalities are genetically determined (Wyrobeck and Bruce 1978). The first mechanism operates at the level of spermatogonia/spermatocytes, which is influenced by abnormal Leydig cell function. This will result in abnormal acrosomal development.
Sertoli cell dysfunction: The Sertoli cells are susceptible to the environmental factors and stress. The disturbed Sertoli cell function adversely affects spermiogenesis and results in the production of simple reversible elongation of sperm head. The severe elongation of sperm head is irreversible.
The genesis of the other sperm abnormalities is discussed in the narrations of the individual entities.
 
Epididymal Factors
When a spermatid is released from a Sertoli cell during spermiogenesis, a cytoplasmic tag remains attached to it. This cytoplasmic tag is divided into two unequal parts. The larger tag separates gradually from the sperm cell and forms the residual body. During the sperm maturation in the epididymis, the smaller cytoplasmic tag remaining attached to the sperm cell is gradually lost. The presence of large number of spermatozoa in the ejaculate with attached cytoplasmic droplets—denoting their immaturity—is indicative of epididymal dysfunction.
 
SELECTION OF PHYSIOLOGICALLY MATURE SPERMATOZOA
The description of physiologically normal spermatozoa is based on the studies of twin physiological conditions; the post coital sperm migration and binding of spermatozoa to zona pellucida during fertilization in “in vivo and in vitro” situations.
 
POST COITAL SPERM MIGRATION
Mortimer et al., (1982) and Fredricsson and Bjork (1977), in the post coital tests, studied the sperm populations in the cervical mucus at the level of internal os and at the level of the external os. These observers noted that 6during the post coital process, there was significantly higher percentage of abnormal sperms present in the cervical mucus at the level of external os than at the level of internal os. The healthy cervical mucus seemed to possess a mysterious ability of denying access to the abnormal sperms (excepting microsperms and elongated sperms) into the cervical canal. Hence, barring these two sperm abnormalities, spermatozoa recovered from the endocervical mucus in the post coital tests represented functionally normal spermatozoa. However, Menkveld was the first investigator to advocate the concept of taking these spermatozoa as a referral standard for ‘normal’ spermatozoa (1987).
 
BINDING OF SPERMATOZOA TO THE ZONA PELLUCIDA DURING FERTILIZATION
Menkveld et al., (1991) carried out studies, in vitro, regarding the morphology of spermatozoa found tightly bound to the zona pellucida, as seen in hemizona assays. Liu and Baker (1992) on the other hand carried out studies “in vivo”, regarding the morphology of spermatozoa found tightly bound to zona pellucida during fertilization.
The characteristics of ‘normal’ spermatozoa found bound to zona pellucida, both in ‘in vivo’ as well as in ‘in vitro’ conditions were akin to the characteristics of ‘normal’ spermatozoa recovered from post coital endocervical mucus.
 
MORPHOLOGICAL CHARACTERISTICS OF NORMAL SPERMATOZOA
Human spermatozoa belong to a select group of species in whom there is marked heterogeneity of spermatozoa in semen. This is in contrast with the situation in other species, including subhuman primates, where there is uniformity of form and structure of spermatozoa (Zamboni et al., 1971).
 
WHO CRITERIA
An ideal method for studying normal spermatozoa would be to study in in vivo condition, the morphological characteristics of those spermatozoa that are engaged in ovum penetration. As this is not possible in humans, an alternative model has to be relied upon. Hence the physiologically normal spermatozoa, present in the endocervical mucus at the level of internal os in the post coital process, are taken as representative of normal forms. Papanicolaou-stained endocervical normal sperm population, despite some inevitable shrinkage, is used for studying the morphological characteristics of normal forms.
There are dual reasons as to why the so-called ’Normal’ spermatozoa observed in the semen of fertile men cannot be relied upon to serve as a referral standard, firstly because in the semen smears a homogenous sperm population of normal spermatozoa is seldom available for studying their morphological characteristics and secondly the physiological status of such normal forms remains unproven.
 
TYGERBERG STRICT CRITERIA
Strict criteria for sperm morphology, originally described by Kruger et al., (1986) were based on the data obtained from IVF results. These authors observed that there were no pregnancies where the semen samples contained < 14% normal forms but the overall pregnancy rate was 25.8% where the semen samples contained > 14% normal forms.
However in the next year Kruger et al., (1987) reported that a fertilization rate of 49.4% per oocyte was noted where the semen samples contained <14% normal forms. With the semen samples having > 14% normal forms an overall fertilization rate of 88.3% per oocyte was achieved.7
 
Method of Study
The stained slide is examined under an oil immersion (1000 x) with a light microscope and an ocular micrometer, calibrated with a stage micrometer, is used for the exact morphometric measurements.
Table 1.2 Summarizes the criteria of morphologically normal spermatozoa as per “strict” criteria and WHO criteria.
Table 1.2   Criteria for morphologically normal spermatozoa
Papanicolaou stain
Strict criteria (1991)
WHO criteria (1999)
Sperm head, neck, midpiece and tail should be normal.
Head
Smooth oval configuration
Oval with regular smooth outline. A slightly different shape, recognized as normal, is oval in shape but is slightly tapered at the post acrosomal end.
Acrosome
Acrosome should comprise about 40 to 70% of anterior sperm head.
Acrosomal area should comprise 40 to 70% of the head area.
Head length
3–5 μm
4–5 μm
Head width
2–3 μm Head width should be between three-fifth and two-thirds of the normal head length.
2.5 — 3.5 μm The length to width ratio should be 1.5 to 1.75
Borderline normal head forms
Regarded as abnormal.
Considered abnormal.
Presence of vacuole Micrometric measurements
Presence of solitary vacuole considered normal. Necessary.
Neck
No abaxial implantation and must be normal.
Midpiece
Slender (axially attached). Approximately 1 μm in width and approximately 1 ½ times the head length. No cytoplasmic droplets of more that half the head size.
Slender, less that 1μm in width and about 1½ times the length of the head. Attached centrally to the head. The cytoplasmic droplets should be less than half the size of sperm head.
Tail
Uniform, slightly thinner than the midpiece, uncoiled, approximately 45 μm long.
Uniform, straight, thinner than the midpiece and un coiled. It should be approximately 45 μm in length.
 
INDIVIDUALISM IN THE SEMEN PICTURES OF INFERTILE MEN
The morphological characteristics of semen show marked variations in different individuals, but there is a relative constancy of the same in a given individual, so much so, that it has been stated that his semen may identify a man. Thus Moench (1934) stated that the sperm morphology serves as a means of identifying an individual and repeated his claim in 1952. Joel and Pollack (1940) substantiated Moench's claim and added that such identification may have forensic value. MacLeod (1962) even stated that the cytology of the ejaculate might be considered analogous to the fingerprint of that individual.
However, this relative constancy of the morphological features in semen of any man may alter suddenly in response to toxic chemical substances, increased exposure to heat, and during an episode of chickenpox and pneumonia or following severe allergic reactions.8
 
SPERM MORPHOLOGY AND NATURAL FERTILIZATION
Moench and Holt (1932) had claimed that the morphology of sperm head seems to be the most reliable indicator of fertilizing power of the cells.
However, several investigators have stated that sperm morphology played little or no role in natural fertilization (Singer et al., (1980), Aitken et al., (1982), Zaini et al., (1985) and Van Zyl et al., (1990)).
Nevertheless, Van Zyl et al., (1990) have also stated that there are poor chances of in vivo fertilization if the percentage of normal forms is less than 4%.
 
THE STUDY OF SPERM MORPHOLOGY
The study of sperm morphology has undergone successive evolutionary stages regarding its aims and methodology.
 
ERA OF RECORDING OF SPERM ABNORMALITIES
Maddox (1984) described the abnormalities of spermatozoa for the first time. Martin et al., (1902) not only illustrated abnormal spermatozoa but also described measurements of different parts taken with the aid of screw micrometer. Carry (1916) summarized the literature to that date.
 
CORRELATION BETWEEN SPERM MORPHOLOGY AND INFERTILITY
 
Percentage of Abnormal Sperms in Fertile and Infertile Men
Initially the investigators strived to find out the relationship between sperm morphology and infertility on the basis of the incidence of abnormal spermatozoa present in the semen of fertile and infertile men. Three representative studies are considered here. Thus Hotchkiss (1944) noted that the incidence of abnormal sperms was 15.8% in the semen of 105 infertile men as against 10.9% in the semen of 200 fertile men.
Aitken et al., (1982) studied a selected group of unexplained infertile couples, in which the sperm concentration and motility were within normal range and concluded that increased percentage of abnormal sperms was possibly the etiological factor.
MacLeod and Gold (1952) carried out an exhaustive comparative study, based on the detailed statistical data, of the semen quality in 1000 men of known fertility and 800 cases of infertile marriages. Regarding the sperm morphology in these two groups the authors observed that abnormalities in morphology increased with decreasing sperm counts and they were more frequently found in infertile men with low sperm counts.
Investigators soon realized that it was futile to attempt to establish correlation between the sperm abnormalities and infertility for several reasons. The criteria as to what constitutes a ‘normal’ sperm were not standardized and each laboratory had its own norms in this regards. Several other vexing questions also remained unanswered. Out of several abnormal forms, which were to be identified for such a comparative study? How the question of multiple sperm abnormalities was to be resolved? Were the concerned abnormal forms viable and motile? What were the permissible limits of sperm abnormalities in cases of known fertility?
In the reference values of fertility potential WHO manual (1999) has refrained from stating any percentage of normal and abnormal forms pending the results of multicentric population based studies that are now in progress.
The most important drawback of such studies was that no cognizance was taken of normal physiological post coital processes.9
Thus one is reminded of Hunner (1921) who wrote, “If there are many actively moving spermatozoa present, per microscopic field, sperm morphology is of no importance and the presence of many abnormal spermatozoa will have no influence on the outcome of the post-coital process.”
While endorsing the opinion of Hunner, Cary and Hotchkiss (1934) had stated “Abnormal forms may possess motility, but are rarely if ever, found in the upper layer of the cervical mucus in the post-coital process and we must consider them ineffectual in fertilization.”
These difficulties forced the investigators to focus their attention on ‘normal’, rather than ‘abnormal’, forms for carrying out the comparative studies of semen of ‘fertile’ and ‘infertile’ populations.
 
The Incidence of Sperms with Normal Morphology in the Semen of Fertile and Infertile Men
Glezarman and Batoov (1993) reported that in the fertile semen samples the percentage of normal forms varied in the range of 4 to 45%. Haidl and Schill (1993) found a mean of 29.5% normal spermatozoa in fertile population with Düsseldorf criteria. On the other hand Menkveld et al., (1990) noted a mean of 16.7% normal spermatozoa in infertile population using strict Tygerberg criteria.
Similarly, reduced fertility and longer interval to first pregnancy was found to be associated with reduced percentage of normal forms (Bartoov et al., 1982; Bostole et al., 1982).
Likewise higher percentage of sperms with normal morphology was found in fertile semen donors in A.I.D. programs (Edwinson et al. 1983: McGoven et al., 1983).
However, Zaini et al., (1985) and Polansky and Lamb (1988) found no predictive value in sperm morphology, as well as other semen parameters, on the existence of male infertility.
 
Data Obtained from Intrauterine Inseminations
In intrauterine inseminations with oligozoospermic and asthenozoospermic semen, there was a correlation between the percentage of normal forms and pregnancy rate (Francavilla et al., 1990).
 
Inputs Received from IVF Results
Tygerberg hospital IVF results were reported by Kruger et al., (1986). With greater than 15% morphologically normal spermatozoa, the fertilization rate was 82.5% with a pregnancy incidence of 25.6%. When the percentage of normal spermatozoa was less than 14%, the fertilization rate dropped drastically to 37% and no pregnancies were recorded.
In Norfolf IVF center, Oehninger et al., (1988) reported the data regarding sperm morphology and pregnancy successes. These authors reported the pregnancy rate of 32% where the percentage of normal sperms was > 14%; the pregnancy rate of 25% where the percentage of normal sperms ranged between 4 — 14% and the pregnancy rate of only 7% where the percentage of normal forms was < 4%.
WHO manual (1999) has summed up the current situation as follows: “Data from assisted reproductive technology programs suggest that, as the percentage of sperm morphology falls below 15% normal forms, using the methods and definitions described in this manual, the rate of fertilization in ‘in vitro’ decreases.”
 
SPERM MORPHOLOGY AND ACCIDENTS OF PREGNANCIES
Several investigators sought to investigate if there was any relationship between sperm morphology and accidents of pregnancy like miscarriages, ectopic pregnancies and stillbirths. Bender, S. (1952), Swyer (1953), Joel (1955), MacLeod and Gold (1957), and Hartman (1965), concluded that there was no relation between the poor sperm morphology and accidents of pregnancy.10
BIBLIOGRAPHY
  1. Aitken RJ, Best, FSM, Richardson, DW, et al. An analysis of sperm function in case of unexplained infertility: Conventional criteria, movement characteristics and fertilizing capacity. Fertil Steril 1982; 30: 212.
  1. Bartoov B, Eites F, Langsam J, Synder M, Fisher J. Ultrastructural studies in morphological assessment of human spermatozoa. Int J Androl 1982; 5: 81.
  1. Bender S. The end results of primary sterility. Brit Med J 1952; 2: 409.
  1. Bostole E, Serup J, Raffe H. Relation between morphologically abnormal spermatozoa and pregnancies obtained during a 20 year follow up period. Int J Androl 1982; 5: 379.
  1. Cary HW. Examination of semen with reference to gynecological aspects. Am. J. Obstet. Dis women child 1916; 74: 615.
  1. Carry WH, Hotchkiss RS. Sperm appraisal, A differential stain that advances the study of sperm morphology. J Am Med Assos 1934; 102: 587.
  1. David G, Bisson JP, Jouannet, et al. Les teratospermies. In Thiboult C (Ed.) La sterilitie due male. Aquisions recents. Masson et cie,  Paris,  1972.
  1. Edwinson A, Bergman P, Steen Y, Nilsson S. Characteristics of donor semen and cervical mucus at the time of conception. Fertil Steril 1983; 30: 327.
  1. Francavilla, Romano R, Santucci R, Poccia G. Effects of sperm morphology and motile sperm count on outcome of intrauterine insemination in oligozoospermia and/or asthenozoospermia Fertil Steril 1990; 53: 892.
  1. Eliasson R, Hellinga G, Luebcke F, et al. Empfehlungen zur Bomenklatur in der Andrlogie. Androllegia, 1970; 2: 186.
  1. Eliason R. Standards for investigation for human semen. Andrologie 1971; 3: 49.
  1. Fredricson B, Bjork R. Morphology of post-coital spermatozoa in cervical secretions and its clinical significance. Fertil Steril 1977; 28: 841.
  1. Glezerman, Marek, Bartoov Benjamin. Semen analysis in Infertility Male and Female. (ed,) Insler, VS and Luenfield, B Churchill Livingstone,  London  1993; 295.
  1. Haidl G, Schill WB. Sperm morphology in fertile men. Arch Androl 1993; 31: 153.
  1. Hartman CG. Correlation among criteria of semen quality. Fertil Steril 1965; 16: 662.
  1. Hotchkiss RS. Fertility in men. William Hieneman (Medical Books) Limited.  London.  1944; 130.
  1. Huhner M. Methods for examining spermatozoa in the diagnosis and treatment of sterility. Int J Surg 1921; 34: 91.
  1. Joël CA. The role of spermatozoa in habitual abortion. Fertil Steril 1955; 6: 459.
  1. Joël K, Pollak OJ. Die Spermaauntersuchung nach dem heutigen Stander Forschung. Helvet Med Acta 1940; 7: 70.
  1. Kruger TF, Menkveld R, Stander FSH et al. Sperm morphology features as a prognostic factor in in vitro fertilization. Fertil Steril 1986; 44: 118.
  1. Kruger TF, Acosta AA, Simmons KF, Swanson RJ, Matla JF, Oehminger S. Predictive value of abnormal sperm morphology in in vitro fertilization. Fertil Steril 1988; 49: 112.
  1. Kruger TF, Acosta AA, Simmons KE, Swanson RJ, Matta JF, Veek LL, Morshedi M, Burgo S. Urology 1987; 30:(3) 248.
  1. Liu DY, Baker HWG. Morphology of spermatozoa bound to the zona pellucida of human oocytes that failed to fertilize in vitro. J Reprod Fertil 1992; 94: 71.
  1. Mc Gowan MP, Baker MWG, Kosias GT, Rennie G. Selection of high fertility donors for artificial insemination programmes. Clin Reprod Fertil 1983; 2: 269.
  1. MacLeod J. Human semen. (Current Reviews) Fertil. Steril, 1956; 7: 368.
  1. MacLeod J. A possible factor in the etiology of human fertility: Preliminary report. Fertil Steril 1962; 13: 29.
  1. MacLeod J. The clinical implications of deviations in human spermatogenesis as evidenced in seminal cytology and experimental production of these deviations. Ex Med Int Cong Ser No. 133 P563, Proceedings of the Fifth Congress on Fertility and Sterility, Stockholm, 1966; 16–22.
  1. Macleod J. The significance of deviation in human sperm morphology in human testis. Adv Exp Med Biol 1970; 10: 481.
  1. Macleod J, Gold RZ. The male factor in fertility and infertility, IV Sperm morphology in fertile and infertile marriage. Fertil Steril 1952; 2: 394.
  1. MacLeod J, Gold RJ. The male factor in fertility and infertility. IX Semen quality in relation to accidents of pregnancy. Fertil Steril 1957; 8: 36.
  1. Maddox RL. Alquras observaciones sobre varias formas del espermatozoo de Cadez, Gac Med de cadez 1894; 11: 252.
  1. Martin E, Carhett JS, Levi MF, Pennington MF. Study of morphology of human spermatozoa, Univ Penn Med Bull 1902; 2:15.

  1. 11 Menkveld R. An investigation of environmental influences on spermatogenesis and semen parameters. PhD. Dissertation Faculty of Medicine, University of Stellenbosch, South Africa 1987.
  1. Menkveld R, Stander FSH, Kotz TJ vW, Kruger TF, Van Zyl JA. The evaluation of morphological characteristics of human spermatozoa according to stricter criteria. Human Reprod 1990; 5: 586.
  1. Menkveld R. Appendices in Menkveld R, Oettle EE, Kruger TF, Swanson RJ, Oehninger S (eds.) Atlas of Human Sperm Morphology. p. 115. Williams and Wilkins.  Baltimore  1991.
  1. Menkveld R, Kruger TF, Oettle EE, Swanson RJ, Oehninger RJ, Oehninger S. (eds) Atlas of human sperm morphology, p 1. Williams and Wilkins.  Baltimore  1991.
  1. Moench GL, Holt H. Biometric studies of head lengths of human spermatozoa. J Lab Clin Med 1932; 17: 297.
  1. Moench GL. Sperm morphology and biometrics as a means of identification of the individual. M. Times, New York 1934; 62:33.
  1. Moench GL. Männliche Fruchtbarkeit In Biologie and Pathologie des Weibes (ed 2), L Seitz, (Ed). Berlin, 1952; 476.
  1. Mortimer D, Leslie EE, Kelley RW and Templeton AA. Morphological selection of human spermatozoa in Vivo and in Vitro. J Reprod Fertil 1982; 64: 391.
  1. Mortimer D. Practical laboratory Andrology. P.25 (Oxford University Press)  Oxford,  1994.
  1. Oehninger S, Acosta AA, Kruger TF, Veeck LL, Flood J, Jones HW. Failure of fertilization in in vitro fertilization: “Occult” male factor. J. In Vitro Fertil Embryo Transfer 1988; 5: 181.
  1. Polansky FF, Lamb EJ. Do the results of semen analysis predict future fertility? A survival analysis study. Fertil Steril 1988; 49: 1059.
  1. Singer R, Sagiv M, Barnet M, Segercich E, Allalaiouf F, Landav B, Servadio C. Motility, viability and percentage of morphologically abnormal forms of human spermatozoa in relation to sperm counts. Andrologia 1980; 12: 92.
  1. Swyer GIM. Discussion of male infertility. Proc R Soc Med 1953; 46: 835.
  1. Van Zyl JA, Kotze TJvW and Menkveld R. Predictive value of spermatozoa morphology in natural fertilization. In Acosta, A. A., Swanson RJ, Ackerman SN, Krger TF, Van Zyl JA, Menkveld R. (eds): Human Spermatozoa in Assisted Reproduction. P.319. Williams and Wilkins,  Baltimore  1990.
  1. Weisman, Abner I. Spermatozoa and sterility. Paul B Hoeber Inc Medical book department of Harper Brothers.  New York.  1941; 1–2.
  1. Williams WW. Spermatic abnormalities. New Eng J Med 1937; 217: 946.
  1. Williams WW, Mac Guigan A, Carpenter HD. The staining and morphology of human spermatozoa. J Urol 1934; 32: 201.
  1. World Health Organization. WHO Laboratory manual for the examination of human semen and sperm-cervical mucus interaction. 4th edn. (Cambridge University Press:  Cambridge),  1999.
  1. Wyrobeck AJ, Bruce WR. Induction of sperm shape abnormalities in mice and humans. In Hollandare, A, de-Serres, FJ. (eds) Chemical mutagens. Volume 5. Plenum,  New York,  1978.
  1. Zaini A, Jennings NG, Baker MWG. Are conventional sperm morphology and motility assessments of predictive value in subfertile men? Int. J Androl 1985; 8:427.
  1. Zamoboni L, Rabbe H, Hamman R. The fine structure of monkey and human spermatozoa. Analyt Rec 1971; 169: 129.
  1. Zaneveld LJD, Polakashi KL. Collection and physical examination of the ejaculate. In Hafez ESE (ed): Techniques of Human Andrology. Elsevier/North Holland,  Amsterdam  1977; 147.