Chapter 4



A complete understanding of the normal anatomy and physiology of the stallion is required in order to evaluate accurately and completely a stallion’s ability to be a successful breeding animal.1–4 The reproductive anatomy of the male horse includes:

The testicles

The testicles are located in the scrotum between the hind legs. They lie in a horizontal plane with the long axis of the body and are oblong in shape (Fig. 4.2). They are quite resilient to palpation, resembling a new tennis ball in character. The testicles are the site of spermatogenesis and the production of the hormones testosterone and dihydrotestosterone. They are surrounded by two connective tissue tunics that are loosely apposed to each other (Fig. 4.3; see also Fig. 4.4):

The testicles comprise:

• The seminiferous tubules, which are lined with germ cells.

• Sertoli (or nurse or sustentacular) cells.5,6 These cells have gap junctions between them, which provide a barrier, known as the blood–testes barrier, between the systemic immune system and the germ cells. The horse’s immune system is never exposed to the developing spermatocytes or spermatids and would therefore see them as foreign cells and destroy them if it was not for the protection of the Sertoli cells. Disruption of this protective barrier may result in subfertility or infertility. The Sertoli cells produce proteins that facilitate sperm production and carry nutrients to the sperm cells.

• Muscle (myoid) cells and fibroblasts line the tubules, which facilitate movement of spermatozoa and fluids along the tubules.

• The interstitium and Leydig cells, which are in the area between the tubules.7 The interstitial tissue is composed of connective tissue, blood and lymphatic vessels and nerves. Testosterone is produced by the Leydig cells and is one of several hormones that facilitate sperm production, as well as supporting the male characteristics of libido, behavior, and masculine appearance.

• The seminiferous tubules empty into the straight tubules, which empty into the rete testis, which empties into the efferent ductules and on into the epididymis (Fig. 4.5). From the epididymis the sperm and associated fluids empty into the vas deferens.

The stallion is a seasonal breeder and thus the size and nature of the testicles change with the season.1–4 There are also changes associated with age.

The scrotum

The scrotum is an outpouching of the skin from the inguinal region. There are two sacs, one surrounding each testicle, and they are separated by a septum. The scrotum has four layers:

• The skin. This has minimal underlying fat and has an abundant supply of sweat glands, which are important in the thermoregulation of the testicles.

• The tunica dartos. This comprises smooth muscle fibers and connective tissue. In warm weather, the muscle cells within the tunica dartos relax and allow the testicles to drop away from the body wall; in cold weather, the dartos muscle contracts, helping to pull the testicles closer to the body.

• The scrotal fascia. This allows the testicles to move freely within the scrotum.

• The common vaginal tunic. Two layers of connective tissue, called tunics, surround the testicles themselves. The outer tunic, the tunica vaginalis or common vaginal tunic, is considered by some to be part of the scrotum and by others to be part of the testicles themselves. The tunica vaginalis is an invagination of the peritoneal lining and passes through the inguinal rings when testicular descent occurs. The tunica albuginea closely adheres to the testicle whereas the tunica vaginalis is separated from the inner tunic by a very thin layer of fluid. This fluid facilitates the movement of the testes in the scrotum.

The spermatic cord

The spermatic cord runs from the internal inguinal ring to the base of the testicles in the scrotum (Fig. 4.6). It consists of:

The testicles are maintained approximately 4–6°C cooler than body temperature for maximal sperm production. The ability to maintain this preferred temperature gradient is provided through the interworkings of several components, including the scrotal skin, the dartos muscle, the cremaster muscle and the pampiniform plexus.

• In warm weather, the many sweat glands in the scrotal skin provide for evaporative heat loss. The dartos and cremaster muscles relax to allow the testicles to hang further away from the body wall, moving them further from the higher core temperature of the abdominal cavity. This relaxation also allows the spermatic cord to stretch out, resulting in a longer time for blood to pass through the pampiniform plexus, thereby allowing more cooling of the blood to occur.

• In the winter, there is no sweating on the scrotal skin and the tunica dartos and cremaster muscles contract. This contraction pulls the testicles closer to the body wall (exposing the testicles to a higher core body temperature). This action effectively shortens the spermatic cord and pampiniform plexus, thereby decreasing the transit time of the blood through the cord and so reduces the alterations in the temperature of the blood as it passes through the cord.

The epididymis

Maturation and storage of spermatozoa occur in the epididymis. The epididymis is divided into three sections (Fig. 4.7):

The epididymis functions to resorb excess fluid (caput) associated with spermatogenesis, and produces secretions (corpus and cauda) that are important in the maturation of the spermatozoa. Sperm are stored in the terminal cauda epididymis until ejaculation occurs. It takes on average 8–11 days for sperm to traverse the length of the epididymis, although passage may occur as quickly as 3 days in cases of high breeding volume.

The accessory sex glands

The accessory sex glands of the stallion include (Fig. 4.8):

The urethra

The urethra is a muscular tube extending from the bladder to the free end of the penis. The pelvic urethra is surrounded by a thick layer of skeletal muscle, which contracts during ejaculation, resulting in the forceful expulsion of semen through the genitourinary tract. The corpus spongiosum penis, part of the erectile tissue of the penis, surrounds the urethra. The distal end of the urethra terminates in the urethral process.

The penis

The penis of the stallion is musculocavernous in nature. It is composed of three general areas (Fig. 4.10):

The urethra runs through the center of the penis and is surrounded by the corpus spongiosum penis.

• The corpus spongiosum penis contains some erectile tissue, which becomes engorged when sexual stimulation occurs.

• The corpus cavernosum penis surrounds the corpus spongiosum penis.

• The corpus spongiosum penis is not surrounded by any connective tissue and continues distally as the glans (free end) of the penis.

• The corpus cavernosum penis is an intricately sinusoidal structure, which becomes engorged with blood during sexual arousal. A connective tissue covering called the tunica albuginea surrounds the corpus cavernosum penis.

• The glans (free end) of the penis will increase 300–400% in size with engorgement and ejaculation. There are many nerve endings in the glans penis. There is a prominent rim of tissue on the glans (free end) of the penis called the corona glandis, which becomes firm and prominent with penile engorgement.

Engorgement of the penis results in a significant (50%) increase in its length and diameter.


• The fetal testicle is suspended from the abdominal wall.

• The mesonephric duct, which will become the vas deferens and the epididymis, courses caudally towards the pelvic canal.

• The vaginal process invaginates to form the inguinal canal.

• At around day 150 of gestation, the epididymis is pulled towards the inguinal canal by the gubernaculum; however, due to the large size of the testicle, it cannot pass through the inguinal ring.

• The cauda epididymis continues to enlarge as the fetus grows and this stretches the inguinal ring and canal.

• At around day 300 of gestation, the testicle is able to enter the inguinal canal.

• Abdominal pressure from fluid and intestinal contents help push the testicle into and through the canal.

• The right testicle precedes the left testicle in most cases.

The testicles of the male equid are typically descended at birth or descend shortly after birth. However, in some cases, testicular descent may be delayed until 2–6 months after birth.

Failure of normal testicular descent (cryptorchidism)

Failure of testicular descent (cryptorchidism) may be a result of:


• Cryptorchidism is an important clinical and reproductive problem. The condition results in difficulties with diagnosis and with surgery.13 Further, there appears to be some hereditary component to retention of the testicles (cryptorchidism) as it is most common in Quarter Horses and there has been a high incidence along some familial lines.

• It is recommended that cryptorchid horses are not used for breeding animals; they should be castrated.

When atypical male behavior develops in a supposed gelding, a diagnosis of cryptorchidism can sometimes be made by a combination of:


Puberty is defined as the commencement of successful reproductive function in a young animal. In the colt, puberty is usually attained by the age of 18 months, although in some individuals it may be reached earlier (12 months). The pubertal stallion will begin to show male characteristics, including increased muscle mass and increased jowl size, and will begin to display typical male behavior, including vocalization in the presence of females, erection, masturbation and copulation. Libido typically continues to improve with age after the onset of puberty.

Physiologically at puberty, spermatogenesis begins as a result of increased testosterone production. Leydig cells produce increasing amounts of dihydrotestosterone, which is converted to testosterone, providing a very high local concentration of testosterone surrounding the germ cells and their nurturing Sertoli cells. Dihydrotestosterone also stimulates the transformation of the stem cells (spermatogonia) into spermatozoa. Surges of luteinizing hormone occur with greater frequency and amplitude, thereby increasing testosterone concentrations. Prolonged stimulation of the Leydig cells is believed to be required for their maturation.16

It has been suggested that, in the prepubertal stallion, production of estradiol results in a negative feedback on the hypothalamus and its production of gonadotropin-releasing hormone.17,18 When production of gonadotropin-releasing hormone increases in frequency and amplitude at puberty, there is also an increased concentration of estradiol receptors, which results in a change from negative to positive feedback. This results in increased amounts of gonadotropin-releasing hormone, which in turn results in increased luteinizing hormone production. Sustained increases in concentrations of the gonadotropic and steroid hormones result in increased testicular growth and development of spermatogenesis.


Hormonal control

Spermatogenesis is a complicated process involving communication between the testicles and two parts of the brain: the hypothalamus and the pituitary. Several hormones are involved in the production and maturation of spermatozoa, the male sex drive and copulation (Fig. 4.12).19,20

• Hormones produced in the hypothalamus flow through the portal vessels to the pituitary gland, specifically the pars distalis.

• The hypothalamus produces short pulses of gonadotropin-releasing hormone, which is the driving force for the rest of the hormonal cycle.

• Gonadotropin-releasing hormone exerts its effect on the anterior pituitary gland and stimulates the release of two hormones: follicle-stimulating hormone and luteinizing hormone.

• Both luteinizing hormone and follicle-stimulating hormone are released in a pulsatile fashion, although, during the breeding season, pulses of luteinizing hormone may occur so close together that they are physiologically indistinguishable.

The Leydig cells produce the steroid hormones in the testicle:22


Spermatogenesis is the process of maturation of the testicular germ cells from stem cells to spermatozoa.24 The maturation process requires both mitotic and meiotic division of the germ cells. Spermatozoa are produced by the progression of germ cells.

Spermatogenesis occurs in cycles, or waves. Depending on the location in the seminiferous tubule one may find a different stage of the cycle. In a cross-section of a tubule, 4–5 generations are arranged in definitive cellular associations. There are eight stages of the cycle of the seminiferous epithelium (Fig. 4.13):25

• Stage 1: from the disappearance of mature spermatids lining the tubular lumen to the beginning of spermatid nuclei elongation.

• Stage 2: from the beginning to the end of spermatid nucleus elongation.

• Stage 3: from the end of spermatid nuclei elongation to the start of the first meiotic division.

• Stage 4: from the start of the first meiotic division to the end of the second meiotic division.

• Stage 5: from the end of the second meiotic division to the first appearance of type B-2 spermatogonia.

• Stage 6: from the first appearance of type B-2 spermatogonia to the onset of migration of elongated spermatids towards the tubular lumen.

• Stage 7: from the onset of migration of elongate spermatids towards the lumen to the conclusion of their migration and disappearance of all B-2 spermatogonia.

• Stage 8: from elongate spermatids all residing at the tubular lumen and the appearance of preleptotene primary spermatocytes until all spermatids have disappeared from the lumen.

A single cycle of the seminiferous epithelium takes 12.2 days in the stallion. Different stages of the cycle can be detected along the length of the tubule. This change in the stages is termed ‘a wave of the epithelium’. It takes around 57 days for spermatogenesis to be completed from the stage of spermatogonia until a mature spermatozoon is formed.

There are three phases of spermatogenesis:

Spermatogenesis changes with the season of the year.26,27

Spermatogenesis is also influenced by the age of the stallion.29,30

Spermatozoa are continuously produced after the onset of puberty.31 There is continuous production of differentiated and committed spermatogonia, which go on to become spermatocytes and then on to spermatids and eventually spermatozoa. Uncommitted spermatogonia are replaced so that the germinal epithelium always has a supply of stem cells from which spermatozoa can be produced.


Thermoregulation of the testicles is an important function of the scrotum and spermatic cord.32

Erection and ejaculation

Erection, emission and ejaculation are complex neurological and physical processes.35,36


Erection results from penile engorgement; the penis lengthens and increases in diameter as a result of venous distension of the corpus cavernosus penis and the corpus spongiosum penis.

• Sensory stimulus of the glans penis and psychological stimulation of the cerebral cortex initiate erection.

• Parasympathetic impulses from segments S2–S4 pass to the penis resulting in dilation of the penile arterioles.

• There is an increase in the amount of blood entering the corpus cavernosus penis and the corpus spongiosum penis.

• The muscles of the penile root simultaneously contract and compress the veins of the penis against the ischial arch; this blocks the venous return from the penis, leading to penile engorgement. Cardiac output also increases, resulting in increased blood flow and pressure in the caudal aorta.

• Erection is lost when sympathetic impulses dominate again, resulting in arteriolar contraction, and the penile root muscles relax, resulting in the restoration of normal venous outflow.

Daily sperm output

• Daily sperm output can be determined by collecting the stallion daily for 5–7 days. At this time, the extragonadal sperm reserves are stabilized. When the stallion is collected on day 8, an accurate assessment of daily sperm output can be made. It can also be estimated by using testicular dimension measurement (see below).

• When stallions are collected daily, the daily sperm output will vary from 3.2 to 6.6 billion spermatozoa.

• The number of spermatozoa per milliliter of semen differs between first and second ejaculates, collected 1 hour apart, by approximately 55%.37 There is a minimal reduction in gel-free semen volume for the second ejaculate.

• In stallions collected once versus twice daily, the sperm output is similar, regardless of the frequency of ejaculation. Therefore, a defined number of sperm is available on a daily basis for ejaculation and once the daily sperm output is reached there is no benefit of collecting twice daily for artificial insemination programs.

• In stallions being utilized in natural service programs, adequate time between services must be provided such that extragonadal sperm reserves are replenished.

Factors affecting sperm production/output

The primary factors that influence daily sperm output include:

Other, secondary, factors that may affect sperm output include:

Season of the year

There is a significant effect of season on daily sperm production.39

• The normal breeding season in the northern hemisphere is May–August. In the southern hemisphere the season extends from September to December.

• Semen volume is approximately 40% higher during the breeding season than during winter. The average number of spermatozoa per ejaculate is about 50% higher during the breeding season than during the nonbreeding season.

• If stallions are exposed to decreasing day length in autumn and then stimulated by an artificial photoperiod (16 light hours, 8 dark hours), the seasonal increase in sperm production can be stimulated earlier in the breeding season such that maximal sperm production in the northern hemisphere will occur in April rather than in June.40 However, in these stallions, sexual recrudescence will occur earlier in the breeding season. Therefore, if the majority of mares are to breed in February–May, placing stallions under artificial lighting will be of benefit. However, if the majority of mares will be bred during May–July, artificial lighting is not recommended.

Age of the stallion

Age is an important factor to consider when determining the potential daily sperm output and the book of mares to a particular stallion.

Frequency of ejaculation

Frequency of ejaculation will affect total volume and number of spermatozoa per ejaculate until the daily sperm output is reached (Table 4.1).14

Sperm stored in the ampulla, vas deferens and cauda epididymis can be released during ejaculation.

Testicular size and consistency

Testicular size affects daily sperm production since daily sperm output depends on the number of spermatozoa produced per gram of testicular tissue.

Testicular measurements can be made using calipers (Fig. 4.14) or by using an ultrasonographic measurement system.

Testicular size, daily sperm production and extragonadal sperm reserves continue to increase for several years following puberty.

Testicular consistency should resemble a new tennis ball. The epididymis should be compliant to compression.


There are many factors that affect the inherent ability of an individual stallion to reproduce successfully. This may include the stallion’s age and genetic make-up, his current and past health status, his body condition, the season of the year, and the environment in which he is housed. Assessment of the stallion’s ability to breed successfully includes:

Evaluation may need to be performed:

The evaluation should at a minimum consist of two semen collections taken 1 hour apart. For more in-depth evaluations, semen may be collected daily for 7–10 days in order to determine the daily sperm output.

The Society of Theriogenology has established guidelines for evaluating fertility potential of stallions.47 The results of the breeding soundness examination should be recorded carefully, preferably using a standardized form.48 These results are often required for legal and medical reasons and errors and omissions should be avoided. The breeding soundness examination is used to make recommendations on the number of mares that a stallion may mate successfully each season.

A stallion can be rated as:

Prospective purchasers, mare owners and stud managers need to understand the implications of this classification. Stallions that are considered satisfactory and given reasonably good management and mares of good fertility are expected to render at least 75% of 40 or more mares pregnant when bred naturally, or 120 mares pregnant when bred artificially in one breeding season.

However, even poor breeders can be useful stallions and some that are classified as satisfactory have problems in other areas and so may be less effective breeders than the other classes. The whole status of the stallion should be considered carefully at the start of a breeding program. The role of the veterinarian in enhancing breeding potential of a stallion cannot be over-emphasized.

The breeding soundness examination represents a single assessment of the animal’s breeding status at the time of the examination. Events and physiological status may change with age, management and medical manipulations.

The breeding soundness examination identifies (and eliminates) most stallions with genetic defects and also alerts owners to potential problems.

The standard examination comprises the following:

1. The stallion should always be positively identified from documents provided. The identification should include:

2. A complete history that details medical problems, injuries, drug therapy the stallion has been receiving over the past 6 months, and vaccination status needs to be taken. It should also include:

• The dates of the athletic performance career (beginning and end).

• The present use of the stallion (if used other than for breeding).

• Previous breeding performance, including first cycle conception rates.

• The pregnancy rate for the previous seasons and the current year.

• Libido, mating behavior record.

• Abnormalities encountered during breeding.

• Results of previous fertility examinations and past breeding management should be evaluated and compared with the present findings, especially if the stallion has decreased fertility.

• Any positive or negative experiences should be noted, especially if the stallion has a behavioral problem.

• Mismanagement, inadequate nutrition, or a concurrent medical disorder may lead secondarily to deterioration in semen quality.

3. Physical examination of the whole horse to assess general health, including blood and serological testing for infectious or other disorders that may be directly involved in breeding (e.g. dourine, equine viral arteritis):

• The examination must be thorough and the results properly recorded: reports and results are often required for sale purposes, for obtaining fertility insurance or for making recommendations on breeding management.

• Particular attention should be directed to auscultation of the heart, evaluation of the eyes and the ability of the stallion to move freely.

• Observation of the stallion moving in a small paddock may help to identify any musculoskeletal problems such as lameness due to chronic degenerative joint disease, laminitis or back disorders.

• Potentially heritable traits such as cryptorchidism, brachygnathia, or cervical vertebral instability (‘Wobbler’) syndrome need to be recorded.

• Rectal examinations are not routinely performed due to the inherent risks involved to both the examiner and the stallion.

4. Detailed evaluation of external genitalia (and possibly the accessory sex glands):

• Examination of the reproductive tract of the stallion is conducted primarily to evaluate fertility potential and to diagnose gross abnormalities of the external genitalia.

• The scrotum and its contents should be evaluated for symmetry and size:

image Total scrotal width or testicular volumes are measured using calipers or testicular ultrasonography, respectively.

image The size of the testicles is directly related to the male’s daily sperm output and the number of mares he is able to breed during one breeding season (see p. 52).

image Testicular consistency is assessed. The testicles and epididymides are palpated for tone, symmetry, and the presence of any abnormally firm or soft areas (see p. 55).

image Both testicles should be present in the scrotum.

image The spermatic cords are palpated for the presence of the vas deferens, sperm granuloma, varicocele (dilation of the blood vessels), excessive fat or masses in the cord.

5. Evaluation of libido and sexual behavior requires observation of mating behavior when presented with an estrus mare.

6. Evaluation of mating ability requires direct observation of mating.

7. Semen quality requires collection of a truly representative semen sample:

Many of these parameters are relatively well correlated to fertility, but not all subfertile stallions will be identified by the examination.

Technological advances in semen preservation have changed breeding management strategies.


External genitalia are examined to rule out anatomic or pathologic conditions that will interfere with fertility.

Jun 18, 2016 | Posted by in EQUINE MEDICINE | Comments Off on THE STALLION
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