LAB 6.1 Male reproductive system

Males

• The paired testes of male farm mammals are located in a muscular bag called the scrotum where they can be maintained several degrees below body temperature for the efficient production of spermatozoa or sperm. Here are two boar testes, the one on the left sliced open.   
• Each testis can be raised by the cremaster externus muscle.{Evidence of cremaster muscles may appear in carcasses of sows and gilts and so cannot be used to identify male carcasses!}.
• The connective tissue round the testis is called the tunica albugenia: it is white with a good blood supply. 
• Spermatozoa are produced in seminiferous tubules tightly packed into the oval shape of the testis. The image above shows one tubule under the microscope. The higher power image below shows part of the tubule (lumen downwards) with meiotic divisions leading to the formation of spermatozoa, whose tails can be seen faintly at the bottom of the image.
• The many seminiferous tubules in each testis open into a labyrinth of tubes called the rete testis. Immature spermatozoa from the rete testis pass in a number of efferent ducts to a further tubular system, the epididymis, located on the surface of the testis, as shown in the image below.
• Spermatozoa mature during storage in the epididymis and are carried to the urethra during mating by peristalsis of the vas deferens.
• The urethra is located ventrally in the penis.
• Seminal fluid to carry the spermatozoa is produced by the paired seminal vesicles, by the prostate gland, and by the paired bulbo-urethral glands (= Cowper's glands). The glands are located along the urethra, near to the bladder. The image below shows the seminal vesicles of a boar.
• The penis has a sigmoid flexure or S-shaped bend along its length. The sigmoid flexure is straightened out when the penis is extended for mating. This occurs when a pair of muscles, the ischio-cavernosus muscles, compress the veins which drain the blood from the penis. Arterial blood pressure then expands the volume of vascular tissue in the penis.
• The ischio-cavernosus muscles are attached to the ischium and the trimmed stump of the muscle may be seen on dressed sides of beef as a pizzle eye. The pizzle eye is poorly developed in steer carcasses and is larger and darker in bull carcasses.
• During the embryonic development of both mammals and birds, the testes are formed from tissue located near to the kidneys. In male mammals, the testes normally move to the scrotum outside the body cavity, and they pass through the body wall in the inguinal canal. The testes are attached to the inside of the scrotum by the gubernaculum which is contractile in fetal animals and is responsible for pulling the testis through the inguinal canal.
• The layers of connective tissue that cover each testis are the (1) tunica vaginalis communis and (2) the inner layer formed by the tunica vaginalis propria. Both layers are derived from modified layers of peritoneum gathered by the testes during their migration. The inner layer supports the blood vessels and nerves to the testis.
• In cryptorchid pigs or ridgelings, movement of the testes along the inguinal canal is incomplete and they do not reach the scrotum. This abnormality causes infertility, but an older cryptorchid pig may still develop boar taint like a normal boar. The cause of the condition is not fully known but the normal mechanism of testicular movement appears to involve the action of testosterone on the genito-femoral nerve which then produces a peptide that activates the gubernaculum.