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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Spermatogenesis occurs in testicular seminiferous tubules and is the process of:
    • Mitotic proliferation of spermatogonia
    • Meiotic divisions of primary (first meiotic division) and secondary (second meiotic division) spermatocytes to form spermatids
    • Maturation of spermatids into spermatozoa capable of motility and fertilization
    • Equids—sequence takes approximately 55–57 days
  • Spermiogenesis is the portion of spermatogenesis that involves maturation of round spermatids into spermatozoa
  • Epididymal matura or passage of sperm through the epididymis takes 9 days and is thought to be necessary for spermatozoa to achieve normal motility and fertilizing ability

Pathophysiology!!navigator!!

Endocrine Considerations

  • Testosterone concentrations in the testis seem to be positively associated with spermatogenic activity:
    • Tissue concentrations of testosterone are related to blood concentrations
    • Difficult to assess testosterone activity in the testis or circulating levels with a single blood sample
  • Modulation of spermatogenesis involves LH, FSH, GnRH, testosterone, estrogen, inhibin, prolactin, and other paracrine/autocrine factors not completely characterized

Seasonal Patterns

  • Some studies report a higher testicular concentration of testosterone during the breeding season
  • Sperm production during the winter months may be half of that observed during the maximum photoperiod
  • Normal stallions potentially fertile year-round with little variation in sperm motility and percent of normal morphology

Age Related

  • At puberty, positive correlation of testis weight with:
    • Potential DSO based on histologic evaluation of seminiferous tubules
    • Quantitative measures of spermatogenesis
  • In 1–3-year-old horses, spermatogenic efficiency does not attain normal adult levels of 14–18 million spermatozoa/g of testicular tissue/day until the individual testis weight reaches 70–80 g, or a combined testicular volume of 133–152 mL (or cm3)
    • Some 3-year-olds may reach this testicular volume without producing sufficient spermatozoa to rank them as satisfactory during a BSE
    • After puberty, sperm production may continue to increase for years. Sperm production can continue to increase past 12 years of age. Full maturity of stallions (sperm production) is often considered to be 6 years
  • Intratesticular testosterone (produced/g of testicular tissue) increases with age; related to sperm production
    • Differences between stallions may explain differences in sperm production
  • In general:
    • Testicular size increases with age
    • Sperm production and sperm output increase with testicular size

Nutrition

Supplementation with omega-3 and omega-6 fatty acids, as well as their precursors (e.g. docosahexaenoic acid), especially when semen is to be frozen or cooled

Signs!!navigator!!

Historical Findings

1 or more of the following:
  • Abnormal or changes in sexual behavior
  • Appearance and/or size changes
  • Sperm numbers decrease
  • Sperm motility or morphology changes
  • Conception or foaling rates decrease

Physical Examination Findings

  • Azoospermia or the absence of spermatozoa
  • Oligospermia or oligozoospermia
  • Small testes
  • Abnormally large testes, especially with neoplasia or following trauma
  • Softening or increased firmness of the testes
  • Possibly altered sexual behavior
  • Poor semen characteristics:
    • Reduced motility
    • Increased morphologic defects
    • Reduced total sperm numbers per ejaculate or DSO

Causes of Arrested Or Disrupted Spermatogenesis!!navigator!!

Congenital

  • Testicular hypoplasia due to hypogonadotropic hypogonadism
    • Abnormally low GnRH, LH, and FSH result in low testosterone
    • Probably rare
  • Primary testicular degeneration
    • Unknown if part of a congenital lesion or acquired
  • Chromosomal anomalies such as XXY
  • Intersex conditions
  • Cryptorchidism

Acquired

  • Rare, testicular neoplasia
  • Testicular heating due to inflammation/infection, fever, hydrocoele, ventral edema, trauma, exercise at elevated environmental temperatures
  • Testicular degeneration associated with testicular metabolic changes is incompletely characterized; might be associated with either:
    • Anabolic steroid or medications with steroid-like effects, i.e. androgens, estrogens, or progestins for behavioral or medical conditions; interfere with normal endocrine feedback loops, decreasing GnRH and LH
    • Interference with normal metabolism by nutritional deficiencies, or exposure to toxic substances. Challenging diagnosis; few cases have been specifically documented in the stallion

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Failure to ejaculate
  • Abnormal ejaculation
  • Excurrent duct system blockage
  • Behavioral disturbances

Laboratory Tests!!navigator!!

Assessment of Overall Health and Wellbeing

  • Complete physical examination including lameness
  • CBC, serum biochemistry

Semen Evaluation

  • Evaluate a set of 2 ejaculates collected at a 1 h interval:
    • Minimum sample size to assess fertility and/or sperm production
    • Evaluate spermatozoid numbers, motility (total, progressive), and morphology. Spermatozoid numbers and motility—by spectrophotometer (concentration); or computer-assisted semen analysis—concentration, motility (including velocity, direction), and morphology
    • Differential interference contrast or phase contrast microscopy—much superior to normal bright-field light microscopy to evaluate motility and morphology
  • DSO—daily collections until sperm numbers stabilize
  • Rule out:
    • Bilateral blockage of the excurrent duct system
    • Ejaculatory disturbances
    • Behavioral abnormalities that cause failure of normal ejaculation
  • If azoospermia or oligospermia is thought to be transient, reevaluate the stallion's semen no less than 60 days after cessation of the insult or disease

Imaging!!navigator!!

Ultrasonography

  • Testicular dimensions, volume, texture, presence of fluid within the space between the parietal and visceral vaginal tunics
  • Evaluate ampullae for blockage or sperm accumulation

Other Diagnostic Procedures!!navigator!!

  • Measure serum/plasma concentrations of FSH, LH, estrogen, inhibin, and testosterone
    • Daily blood samples for 3 days
  • GnRH stimulation test:
    • Single dose of 25 μg GnRH given IV at 9:00AM
    • Blood collected—baseline and 30 min intervals (i.e. 0, 30, 60, 90, 120 min)
    • Assay serum/plasma LH and testosterone
  • Triple pulse GnRH stimulation test (nonbreeding season):
    • 3 IV doses of 5 μg of GnRH given 1 h apart
    • Samples taken 1 h before and 6 h after GnRH administration
    • Assay LH
  • hCG stimulation test
    • Give 10 000 IU hCG IV
    • Blood samples 1 h before and 6 h after hCG
    • Assay testosterone and estrogen
  • Anti-Müllerian hormone
    • Testicular biopsy—3 tissue samples: 1 sample fixed in Bouin's fluid or modified Davidson's solution for histopathologic evaluation and 2 placed in phosphate-buffered saline and snap-frozen for assay of paracrine/autocrine factors
  • Flow cytometric procedures such as the sperm chromatin structure assay or sperm ubiquitin tag immunoassay
    • Detect alterations in sperm integrity
    • Available in some specialized research centers

Pathologic Findings!!navigator!!

Azoospermia in the Ejaculate

  • Absence of germ cells in the seminiferous tubules (rare)
  • Arrested spermatogenesis
  • Bilateral blockage of the epididymis or ductus deferens
  • Stallions with congenital absence of spermatogenesis or arrested spermatogenesis will have smaller testes than normal

Oligospermia in the Ejaculate

  • Small testicular volume, reduced sperm-producing tissue
  • Congenital malformations
  • Testicular degeneration/disruption of spermatogenesis owing to toxicants, metabolic disorders, or testicular heating
  • Trauma
  • Neoplasia
  • Inflammation of the testes or epididymides
  • Partial blockage of the duct system
  • Typically, stallions with chronic oligospermia have smaller than normal testes

Poor Fertility Following Cryopreservation and/or Cooling

  • Ability to tolerate these special procedures varies by stallion
  • Causes susceptibility of spermatozoa to damage during cryopreservation and/or cooling best described as idiopathic
    • Subtle disruptions in spermatogenesis cannot be ruled out
    • Further diagnostics might be indicated

Treatment ⬆ ⬇

Outline

TREATMENT

  • Resolve underlying disease condition, if identified
  • Hemicastration indicated if 1 testis is irreversibly affected by trauma, inflammation, infection, or neoplasia
  • Treatments of infertility due to abnormal spermatogenesis are speculative at best, if the etiology and underlying pathologic process remain unidentified
  • Use centrifugation strategies to manage semen from selected stallions, especially when semen is being frozen or cooled

Nursing Care!!navigator!!

Indicated in instances of systemic disease or hemicastration.

Activity!!navigator!!

  • Stallions need exercise in a stress-free environment
  • Can be transient reproductive impairment while actively involved in competitions, especially with elevated environmental temperatures

Diet!!navigator!!

Ensure proper levels of energy, protein, vitamins, and minerals.

Client Education!!navigator!!

Knowledge of spermatogenic cycle length; recovery may require 60–70 days.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

BSE of an injured or affected stallion—approximately 60 days from date of correction of underlying process.

Prevention/Avoidance!!navigator!!

  • Control fever
  • Manage minor scrotal edema and underlying etiology
  • No anabolic steroids or progestins and caution in medications or supplements and to stallions currently or eventually used for breeding
  • Controversial—avoid exposure of breeding stallions to endophyte-infected fescue, especially during the breeding season

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Synonyms!!navigator!!

  • Infertility
  • Oligozoospermia
  • Shooting blanks
  • Sterility
  • Subfertility

Abbreviations!!navigator!!

Suggested Reading ⬆ ⬇

Amann RP, Graham JK. Spermatozoal function. In: McKinnon AO, Squires EL, Vaala WE, Varner DD, eds. Equine Reproduction, 2e. Ames, IA: Wiley Blackwell, 2011:1053–1084.

Ball BA, Applications of anti-Mullerian hormone (AMH) in equine reproduction. Proc of hte Society for Theriogenology, July 2016 (Asheville, NC).

Johnson L, Griffin CE, Martin MT. Spermatogenesis. In: McKinnon AO, Squires EL, Vaala WE, Varner DD, eds. Equine Reproduction, 2e. Ames, IA: Wiley Blackwell, 2011:1026–1052.

Love CC. Modern techniques for semen evaluation. Vet Clin North Am Equine Pract 2016;32(3):531–546.

Author(s) ⬆

Author: Tim J. Evans

Consulting Editor: Carla L. Carleton

Acknowledgment: The author and editor acknowledge the prior contribution of Rolf E. Larson.