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Sperm morphology in Estonian and Tori Breed Stallions


The standard procedure for assessing the breeding potential of a stallion includes the parameter total number of spermatozoa classified as morphologically normal. This study investigated sperm morphology of fresh semen in randomly chosen Estonian (E, n = 8) and Tori (T, n = 7) breed stallions with proven fertility. Two ejaculates were examined from each stallion. An aliquot from each ejaculate was fixed in 1 mL formol-saline immediately after collection and examined with phase-contrast microscope at a magnification 1000× for all types of morphological abnormalities. Furthermore smears were prepared and stained according to Williams (carbolfuchsin-eosin) for a more detailed examination of the sperm heads with light microscope at a magnification 1000×. Analysis of variance was applied to the data, and results are presented as LSmeans (± SE). One T stallion that had a disturbance in the spermatogenesis and one 22-year-old E stallion were not included in the analyses. The T stallions had on average 57.5 ± 4.1% and the E-stallions 74.4 ± 3.8% morphologically normal spermatozoa (p = 0.012). In 4 of 7 T stallions and 7 of 8 E stallions both ejaculates had >50% morphologically normal spermatozoa. There was a significant difference between breeds in mean percentage of proximal droplets (17.3 ± 2.7% and 2.9 ± 2.5% for T and E stallions, respectively; p = 0.003).


Spermiemorfologi hos hingstar av estländsk och tori ras.

Bedömningen av en hingsts avelsvärde omfattar bland annat spermasamling och beräkning av totalantalet morfologiskt normala spermier i ejakulatet. Denna studie omfattar spermiemorfologisk undersökning av 2 ejakulat från slumpmässigt utvalda fertila hingstar av estländsk (E, n=8) och tori (T, n=7) ras. Omedelbart efter spermasamlingen fixerades en liten mängd sperma i en mL fysiologisk formolsalinlösning för bedömning av samtliga spermiemorfologiska avvikelser i faskontrastmikroskop (1000x). Dessutom gjordes utstryk på objektglas, som färgades enligt Williams metod (carbolfuchsin-eosin), för en mera detaljerad ljusmikroskopisk undersökning av spermiehuvudet (1000x). Den statistiska bearbetningen av erhållna data omfattade variansanalys och resultaten redovisas som LSmeans (±SE). En T hingst, som visade sig ha en störning i spermiogenesen och en 22 år gammal E-hingst inkluderades inte i de statistiska bearbetningarna. T-hingstarna hade i medeltal 57,5±4,1% och E- hingstarna 74,4±3,8% morfologiskt normala spermier (p=0,012). Hos 4 av 7 T-hingstar och 7 av 8 E-hingstar innehöll båda ejakulaten >50% morfologiskt normala spermier. Det var en signifikant rasskillnad avseende procentandelen spermier med proximal cytoplasmadroppe (17,3±2,7% och 2,9±2,5% hos T- respektive E-hingstar; p=0,003).


One criterion used to assess the breeding potential of a stallion is the total number of spermatozoa classified as morphologically normal [14]. Attempts to correlate the percentage of morphologically normal or abnormal spermatozoa with fertility have given conflicting results. Thus [5, 4, 10, 11, 9], reported that sperm morphology is related with fertility to various degrees, while others [24, 7] did not find any relationship between sperm morphology and fertility. Several investigators found a considerable inter-stallion [20, 21, 16] and intra-stallion [21, 12, 16] variation in semen quality.

The standard evaluation of sperm morphology is performed with phase and/or light microscopy [14]. Computer-assisted methods have also been used [2, 6]. However, available computer-assisted methods can only evaluate the sperm head, not count morphological abnormalities of mid-pieces, tails and acrosomes.

The aim of the present study was to investigate the sperm morphology in fresh semen of Tori and Estonian breed stallions.

Materials and methods

The investigation was performed on 15 clinically healthy, randomly chosen stallions of 2 different breeds (7 Tori (T) and 8 Estonian (E) breed stallions) with proven fertility aged between 4–15 years (one E stallion 22 years). The stallions were transported to, and housed at, the Veterinary Clinic of Estonian Agricultural University.

Semen collection and processing

Semen was collected during the non-breeding season (October – January). One ejaculate of semen was collected daily for 10 subsequent days from each stallion. Semen was collected using a Missouri type artificial vagina while the stallion was mounting a teased mare in oestrus. The ejaculate was filtered through gauze to remove the gel fraction. The semen was immediately put into an incubator at +34°C, and all manipulations were conducted using warmed glassware. Two ejaculates, one among the first 5 ejaculates and one among the last 3 ejaculates, were chosen for evaluation of sperm morphology. An aliquot from the ejaculates was fixed in 1 mL formol-saline immediately after collection. One drop of semen was also placed on each of 3–4 glass slides, and smears were prepared and air-dried.

Morphological examination of spermatozoa

Morphological abnormalities of spermatozoa were studied in wet preparations made from the formol-saline fixed samples [8] under a phase-contrast microscope at a magnification of 1000×. Altogether 200 spermatozoa were counted in each preparation and all different abnormalities (see below) in each spermatozoon were recorded. The abnormalities were classified according to a system developed by [3].

For a more detailed examination of the sperm heads, smears were prepared as described above, stained with carbolfuchsin-eosin according to the method described by [25] and modified by [15]. Five hundred spermatozoa were counted in each smear at a magnification of 1000× in a light microscope. The head abnormalities were classified according to [15]. If the percentage of head abnormalities recorded in the spermatozoa stained with carbolfluchsin-eosin deviated from the percentage recorded in the formol-salin fixed samples, the former was used in the calculations.

The morphological abnormalities were counted as a percentage of the total number of counted spermatozoa. Morphological categories used in this study were: I. Abnormal heads (including pear shaped, narrow at the base, abnormal contour, undeveloped, loose abnormal head, narrow, big, little-normal, short-broad), II. Loose heads (including both those with normal and abnormal head morphology), III. Acrosome defects, IV. Proximal cytoplasmic droplets, V. Abnormal midpieces, VI. Abnormal tails (including double folded, single bent and coiled tails under the head) (see also Fig. 1a, 1b).

Figure 1
figure 1

a. Some sperm abormalities in stallion. b. Some sperm abormalities in stallion.

Presence of spermatogenetic cells or debris of spermatogenetic cells was recorded in smears stained according to a modification of a method originally described by [18] in a light microscope at a magnification of 250×.

Statistical analyses

Statistical analyses were performed using SAS Version 8 software (SAS Institute Inc., Cary, NC, USA). Analysis of variance was performed using the MIXED-procedure according to a statistical model including the fixed effect of breed (2), collection number (2) and the interaction between breed and collection number. The statistical model also included the random effect of stallion nested within breed (6 Tori; 7 Estonian).


One of the T stallions had signs of a moderate disturbance in the spermatogenesis (high percentage of morphologically abnormal spermatozoa and presence of spermatogenetic cells in both ejaculates) and one E stallion was 22 years old. Their results were not included in the analysis of variance, but are presented separately. The LS means ± SE and the ranges of morphological categories in the ejaculates of the remaining 6 T and 7 E stallions are presented in Table 1. There were significant differences in the mean values between breeds in percentages of proximal droplets (p = 0.003) and of morphologically normal spermatozoa (p = 0.012). The LSmean ± SE of percentage proximal droplets were 17.3 ± 2.7% in T stallions and 2.9 ± 2.5% in E stallions and the percentage of normal spermatozoa 57.5 ± 4.1% and 74.4 ± 3.8% in T and E stallions, respectively. The percentages of abnormal sperm heads were 12.6 ± 1.7% and 13.9 ± 1.5% in T stallions and in E stallions, respectively. The T stallion with a disturbance in the spermatogenesis had 23.7 ± 10.8% normal spermatozoa, and the 22-year-old E stallion had 49.1 ± 8.2% normal spermatozoa.

Table 1 LSmean ± SE and range of percentage of different sperm abnormalities in the ejaculates of 6 Tori and 7 Estonian breed stallions.

There was a significant interaction between breed and collection for percentage of abnormal heads (p < 0.01). Differences in least squares means show that for E stallions, this abnormality decreased with time (by 2.6% units) whereas for T stallions this abnormality increased with time (by 2.8% units).

For both the percentage of abnormal tails and the percentage of normal spermatozoa there were significant interactions between breed and collection time (p < 0.05). Differences in least squares means show that for E stallions, percentage of abnormal tails decreased with time by 4.7% units, whereas for T stallions it increased with time by 1% unit. For the percentage of normal spermatozoa, the corresponding differences were: E stallions: -5.3% units; T stallions:-0.7% units.

In 4 out of 7 T stallions and 7 out of 8 E stallions both ejaculates contained >50% morphologically normal spermatozoa. Six T stallions and all E stallions had >50% morphologically normal spermatozoa in at least one of 2 examined ejaculates.


The results of the present study gave information about the sperm morphology in ejaculates of randomly chosen Tori and Estonian breed stallions with proven fertility. A morphological examination of the spermatozoa is widely used in the evaluation procedure of semen in many mammalian species, including stallion. Various stains have been used for stallion spermatozoa [24, 10, 17]. [10] reported that some staining techniques might induce morphologic changes in the spermatozoa. In the present study, smears were made from raw semen and stained according to the method described by [25] and modified by [15]. This method is well established and is outstanding for evaluation of sperm head abnormalities in light microscope. Morphological abnormalities of the acrosome, the midpiece and the tail as well as presence of proximal cytoplasmic droplets were studied in unstained wet preparations made from formol-saline fixed samples under a phasecontrast microscope. The advantage of this method is that sperm morphology remains intact, which is not always the case when staining techniques are used (see above). The disadvantage associated with the buffered-formol saline method is that sperm head abnormalities can be difficult to evaluate in wet preparations. This disadvantage was compensated for by checking the occurrence of sperm head abnormalities in both stained smears and wet preparations (see above).

There were significant differences in one or 2 morphological parameters between the 2 ejaculates exmined from a stallion. This indicates the need of a morphological examination of at least 2 ejaculates (not the first one collected) for evaluation of the morphological quality of the semen.

The mean percentages of proximal cytoplasmic droplets were 17.3% and 2.9% in T and E stallions respectively (p = 0.002). Similar large differences between breeds within studies and between stallion population from different countries have earlier been reported. Thus [7, 11] found mean percentages of 13.1% and 15.5% proximal cytoplasmic droplets, while [24] reported 0.5%–1.4% of proximal cytoplasmic droplets. The reason for this wide variation is not known. [7] recommended a careful interpretation of percentages of this defect in relation to fertility, because stallions appear to differ from other species in which excess numbers of cytoplasmic droplets are considered to be indicative of immature spermatozoa and deleterious to fertility (e.g. in bulls, [23, 1]).

The mean percentages of abnormal sperm heads did not differ between the 2 breeds (12.6% and 13.9% for T- and E-stallions, respectively). In previous studies [24, 7, 11] the mean percentages of head abnormalities varied between 6.4%–21.5%. The present investigation of stallions with proven fertility showed higher mean percentages of head abnormalities than reported for normal stallions (9%), but lower than reported for stallions with testicular degeneration (17%) (Pickett 1993). One T-stallion in the present study had a semen picture of both ejaculates indicating a current testicular degeneration (>30% morphologically abnormal sperm heads plus spermatogenetic cells in the ejaculate). This stallion had given acceptable foaling rate after natural mating during the previous breeding season. At the time of examination this stallion must have suffered from testicular degeneration and was therefore excluded from the mean values of the sperm morphology of randomly sampled stallions.

The mean percentages of loose sperm heads in the present study were approximately at the same level as reported previously by [[24, 4] and [11]].

The percentages of abnormal midpieces of the spermatozoa from T and E stallions (5.1% and 2.4%, respectively) were comparable with corresponding results of [11] who reported a frequency of 7.4%. [24] on the other hand reported a mean of 25.3% of midpiece defects in ejaculated semen from 3 stallions, which seems extremely high for normal stallions. No explanation for the high level is given in their report.

Abnormal tails were found in 6.6% and 6.4% of the spermatozoa in T and E stallions respectively. These results are comparable with earlier investigations done by [7] (10.9 %), [11] (2.4%) and [24] (4.0%–5.5%)

The overall percentages of morphologically normal spermatozoa were 57.5% and 74.4% for T and E stallions respectively (p < 0.05). These mean values correspond to earlier findings by [11] (52.5%), [20] (60.8%), but lower than findings by [4] (85%) who presented sperm morphology of stallions with high fertility, and with at least 60% motile spermatozoa in their ejaculates.

In the Netherlands it has been recommended that minimal values of semen quality of young (3 years old) stallions for registration in the studbook is a mean total number of progressive motile morphologically normal spermatozoa of 2 × 109 and a mean of 50% for motility and 50% of morphology of 2 ejaculates collected at one h interval [19]. The mean number of spermatozoa per ejaculate of the 3-year-old Dutch Warmblood stallions was 11.3 ± 7.1 × 109 spermatozoa. In the present study 4/7 T stallions fulfilled the Dutch criteria. Of E stallions 6/8 had more than 50% morphologically normal spermatozoa and at least 50% motile spermatozoa in both ejaculates. However, not all of these E stallions had at least 2 × 109 morphologically normal and motile spermatozoa per ejaculate, because the size of the Estonian horse is much smaller than the Tori horse. In a previous study, [13] showed that the daily sperm output (DSO) was 12.9 ± 0.8 × 109 for T stallions compared with 4.5 ± 0.3 × 109 for E stallions (p < 0.001). Therefore 6 out of 7 E stallions (the 22 year old stallion did not ful-fill these criteria) must be considered to have an acceptable production of morphologically normal and motile spermatozoa.


The T stallions had 57.5% ± 4.1% and the E-stallions 74.4% ± 3.8% morphologically normal spermatozoa (p = 0.026). In T stallions 4 of 7 stallions had more than 50% of morphologically normal spermatozoa in both ejaculates; one ejaculate was under the limit in 2 stallions and both ejaculates in one stallion (testicular degeneration). In E stallions 7 of 8 stallions had more than 50% of morphologically normal spermatozoa in both ejaculates, and all 8 in at least one of 2 ejaculates.


  • Amann RP, Seidel GE, Mortimer RG: Fertilizing potential in vitro of semen from young beef bulls containing a high or low percentage of sperm with a proximal droplet. Theriogenology. 2000, 54: 1499-1515. 10.1016/S0093-691X(00)00470-2.

    Article  CAS  PubMed  Google Scholar 

  • Ball BA, Mohammed HO: Morphometry of stallion spermatozoa by computer-assisted image analysis. Theriogenology. 1995, 44: 367-377. 10.1016/0093-691X(95)00191-A.

    Article  CAS  PubMed  Google Scholar 

  • Bane A: Acrosomal abnormality associated with sterility in boar. Proc 4th Int Congr Anim Re-prod & AI, The Hague IV. 1961, 810-817.

    Google Scholar 

  • Bielanski W, Dudek E, Bittmar A, Kosiniak K: Some characteristics of common abnormal forms of spermatozoa in highly fertile stallions. J Reprod Fert. 1982, 21-26. Suppl 32

  • Bielanski W, Kaczmarski F: Morphology of spermatozoa in semen from stallions of normal fertility. J Reprod Fert. 1979, 39-45. Suppl 27

  • Casey PJ, Gravance CG, Davis RO, Chabot DD, Liu IKM: Morphometric differences in sperm head dimensions of fertile and subfertile stallions. Theriogenology. 1997, 47: 575-582. 10.1016/S0093-691X(97)00015-0.

    Article  CAS  PubMed  Google Scholar 

  • Dowsett KF, Pattie WA: Characteristics and fertility of stallion semen. J Reprod Fert. 1982, 1-8. Suppl 32

  • Hancock JL: The morphology of boar spermatozoa. J R Microsc Soc. 1957, 76: 84-97.

    Article  CAS  PubMed  Google Scholar 

  • Hellander JC, Samper JC, Crabo BG: Fertility of a stallion with low sperm motility and a high incidence of an unusual sperm tail defect. Vet Rec. 1991, 128: 449-451.

    Article  CAS  PubMed  Google Scholar 

  • Hurtgen JP, Johnson LA: Fertility of stallions with abnormalities of the sperm acrosome. J Reprod Fert. 1982, 15-20. Suppl 32

  • Jasko DJ, Lein DH, Foote RH: Determination of the relationship between sperm morphologic classifications and fertility in stallions: 66 cases (1987–1988). J Amer Vet Med Assoc. 1990, 197: 389-394.

    CAS  Google Scholar 

  • Jasko DJ, Lein DH, Foote RH: The repeatability and the effect of season on seminal characteristics and computer-aided sperm analysis in the stallion. Theriogenology. 1991, 35: 317-327. 10.1016/0093-691X(91)90282-I.

    Article  CAS  PubMed  Google Scholar 

  • Kavak A, Lundeheim N, Aidnik M, Einarsson S: Testicular measurements and daily sperm output of Tori and Estonian breed stallions. Reprod Dom Anim. 2003, 38: 167-169. 10.1046/j.1439-0531.2003.00418.x.

    Article  CAS  Google Scholar 

  • Kenney RM, Hurtgen J, Pierson R, Withersponn D, Simons J: Manual for clinical fertility evaluation of the stallion. J Soc Theriogenology. 1983, 9: 3-100.

    Google Scholar 

  • Lagerlöf N: Morphologische Untersuchungen über Veränderungen im Spermabild und in den Hoden bei Bullen mit vermindeter oder aufgehobener Fertilität. (Researches concerning morphologic changes in the semen picture and in the testicles of sterile and subfertile bulls.). Acta Path Microbiolog Scand Thesis, Stockholm. 1934, Suppl 19

  • Love CC, Varner DD, Thompson JA: Intra- and inter-stallion variation in sperm morphology and their relationship with fertility. J Reprod Fert. 2000, 93-100. Suppl 56

  • Malmgren L: Assessing the quality of raw semen: a review. Theriogenology. 1997, 48: 523-530. 10.1016/S0093-691X(97)00268-9.

    Article  CAS  PubMed  Google Scholar 

  • Papanicolaou GN: A new procedure for staining vaginal smears. Science. 1942, 95: 438-439. 10.1126/science.95.2469.438.

    Article  CAS  PubMed  Google Scholar 

  • Parlevliet JM, Kemp B, Colenbrander B: Reproductive characteristics and semen quality in maiden Dutch Warmblood stallions. J Reprod Fert. 1994, 101: 183-187.

    Article  CAS  Google Scholar 

  • Pattie WA, Dowsett KF: The repeatability of seminal characteristics of stallions. J Reprod Fert. 1982, 9-13. Suppl 32

  • Rousset H, Chanteloube PH, Magistrini M, Palmer E: Assessment of fertility and semen evaluations of stallions. J Reprod Fert. 1987, 25-31. Suppl 35

  • SAS Institute Inc: SAS/STAT Users Guide, Version 8. Cary, NC. 1989

    Google Scholar 

  • Söderquist L, Janson L, Larsson K, Einarsson S: Sperm morphology and fertility in A.I. bulls. J Vet Med. 1991, 38: 534-543.

    Article  Google Scholar 

  • Voss JL, Pickett B, Squires E: Stallion spermatozoal morphology and motility and their relationship to fertility. J Amer Vet Med Assoc. 1981, 178: 287-289.

    CAS  Google Scholar 

  • Williams WW: Technique of collecting semen for laboratory examination with a review of several diseased bulls. Cornell Vet. 1920, 10: 87-94.

    Google Scholar 

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The authors are grateful to Mrs. K. Selin-Wretling and Mrs. A. Rikberg for the excellent technical assistance. This study was financially supported by Swedish Institute, AGRIA, Estonian Scientific Foundation Grant no. 4111.

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Reprints may be obtained from A. Kavak, Department of Obstetrics and Gynaecology, Faculty of Veterinary Medicine, Estonian Agricultural University, Kreutzwaldi 62, 51014, Tartu, Estonia. E-mail:, tel: 372 7 422 482, fax: 372 7 422 259.

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Kavak, A., Lundeheim, N., Aidnik, M. et al. Sperm morphology in Estonian and Tori Breed Stallions. Acta Vet Scand 45, 11 (2004).

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