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Penis and Scrotum of the Horse

First, anatomy. The normal stallion has two testes. Their prime job is to produce spermatozoa and testosterone. At birth, each testis normally will weigh between five and 10 grams. It will remain at that weight and size during the first 10 months of life. A slight growth occurs between 11 and 16 months of age, and rapid development of both testes starts at about 18 months of age. In some cases, the testes don't reach final maturation until the stallion is 12 to 13 years old.

Each testis is covered by a thick lining of connective tissue called the tunica albuginea. Fused to the outer surface of this capsule is the thin visceral vaginal tunic. Supporting strands of connective tissue extend from the tunica albuginea to divide the testis into lobules.

The non-capsular part of the testis is called the parenchyma. It consists of seminiferous tubules (capable of producing and conveying semen) and interstitial tissue situated between the seminiferous tubules. Located within the seminiferous tubules are Sertoli's cells. They are involved with the production of spermatozoa. Located in the interstitial tissue between the seminiferous tubules are Leydig's cells. They are involved in the production of testosterone, which causes the stallion to be sexually aggressive.

During the first 18 to 24 months after birth, the testes do not produce sperm and the colt is infertile. At about 18 months of age, the testes grow and develop rapidly. Several months later, they gradually begin to produce sperm. In the adult stallion, billions of spermatozoa are produced daily in the convoluted seminiferous tubules. This figures out to be at the rate of about 70,000 per second.
Although there is this prolific production of sperm on a second-by-second basis, the development and maturation of each individual spermatozoa require between 54 and 57 days. During this time frame, a great many things can go awry and result in the sperm's being unable to carry out the fertilization role.

The process of sperm development is called spermatogenesis. It is basically divided into three phases.

  • Phase one is known as spermatocytogenesis. This is the initial differentiation and subsequent division of the germ cells known as spermatogonia to increase their number.

  • The second step is meiosis--the process where genetic rearrangement occurs between homologous chromosomes and where spermatogonia are reduced in chromosome number by division to form spermatids.

  • The final step in the process is referred to as spermiogenesis, where spermatids differentiate into mature spermatids.
    Once spermatids are released from the seminiferous epithelium into the lumen of the seminiferous tubule, they are referred to as spermatozoa. Each of the above three phases requires 18 to 19 days.

    Once spermatozoa are released within the seminiferous tubule, they travel through a series of ducts to the epididymis, which is lightly attached to the upper surface of each testis. It is here that spermatozoa complete the maturation process and are stored.

    The epididymis is divided into three sections--head (caput), body (corpus), and tail (cauda). It is while traveling through the epididymis, from head to tail, that the spermatozoa reach maturity and acquire the ability to fertilize an egg. The migration through the epididymis takes approximately eight days. Once the migration and maturation process is completed, the spermatozoa are stored in the tail of the epididymis.

    During breeding season, the spermatozoa are released through ejaculation. When the stallion is not being used in the breeding shed, he will, nevertheless, produce sperm. When the epididymis is filled to overflowing, the excess sperm are voided in the urine.

    The minimum number of spermatozoa produced within a 24-hour time period is known as daily sperm production. This will vary among stallions and is strongly influenced by testicular size. Daily sperm output refers to the number of spermatozoa that can be collected per 24-hour period from a stallion and is determined by collecting the stallion daily for seven days.

    The number of spermatozoa produced varies from stallion to stallion. Contrary to some long-held beliefs, frequent ejaculations do not stimulate a more rapid production of spermatozoa. In other words, whether a stallion is collected daily or every other day for a week will not have an effect on the number of spermatozoa produced during that week. The stallion, quite simply, has an individual productive capability and the number of times he is collected or used in live cover has no bearing on sperm production.

    It also is significant to note that the entire process involved in the development and maturation of spermatozoa takes approximately 65 days. If something negative should occur during that process, it could have harmful effects on the spermatozoa being produced, but might not harm those that are in storage in the epididymis. Thus, if a stallion suffers an injury to the testes, the full implications might not be manifested until two months down the line.

    Before getting into that, however, let's take a quick look at the route the spermatozoa travel from epididymis to the female's vagina. The reason for doing so is that much can happen to the spermatozoa during this trip.

    The Reproductive Trail

    From the tail of the epididymis, the spermatozoa, during ejaculation, are delivered to the deferent duct, which is a continuation of the epididymal duct. As the deferent duct approaches the pelvic urethra, it widens into a structure termed the ampulla of the deferent duct. The ampulla is about 18 millimeters in diameter as compared to four to five millimeters for the deferent duct. During ejaculation, spermatozoa are moved through the ducts and into the urethra by muscular contractions.

    Along the route through the ducts that the spermatozoa will travel are the two vesicular glands, which are elongated, hollow pouches about 15 to 20 centimeters long and five centimeters in diameter.
  • Fluid secreted by the vesicular glands contributes a major portion of the seminal plasma in an ejaculate.
  • Next along the route is the prostrate gland. The secretion of the prostate gland is thin and watery and likely helps cleanse the urethra during ejaculation. The secretion also contributes to seminal plasma.
  • Next are the bulbourethral glands. Their secretions also contribute to seminal plasma.

    As the sperm complete their journey through the deferent ducts, fortified along the way with seminal plasma, they ultimately arrive at the urethra. The urethra is a long mucous tube that extends from the bladder to the free end of the penis. At this point, the ejaculate is called semen. This is the term used to describe the whitish fluid of the male reproductive tract consisting of spermatozoa suspended in the secretions of the accessory glands.

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