Fetal Pig -  Urogenital

 
                                               
       
       
       
       
       
       
       
       
                                       
  Introduction
The urinary and genital systems have distinct and unique functions. The first serves to remove nitrogenous and other wastes and to maintain the body’s water balance, while the other functions in the reproduction of the species. However, due to the similarities of their developmental origins and the sharing of common structures, they are usually considered as a single system, the urogenital system. We will first study the urinary system, which is similar in males and females. We will then proceed to study the reproductive systems of the two sexes. You are responsible for learning the reproductive systems of both male and female pigs. After studying your specimen, examine the reproductive system of a specimen of the opposite sex. Remove the liver, spleen, stomach, and intestines. Leave the last two inches of the large intestine intact.

Urinary System Kidneys — They are large bean-shaped structures on either side of the vertebral column at the level of the third to fifth lumbar vertebrae. Although they bulge into the abdominal cavity, they lie beneath the peritoneum, or retroperitoneally, often surrounded by fat. The adrenal glands are narrow band-like structures lying median to the anterior region of the kidneys. Clear the kidneys to expose the renal arteries, renal veins, and the ureters. Some of the parts of the kidney are the:

Hilus — This is a central depression in the medial surface of the kidney. The ureters exit the kidney at the hilus. In order to observe the following structures it is necessary to cut one of the kidneys in frontal section as in the diagram below.

Renal Sinus — This is a central cavity which contains fat, branches of the renal vessels and the renal pelvis. The pelvis is the funnel-shaped expanded portion of the ureter within the renal sinus. Renal Cortex This is the outer layer of kidney tissue.

Renal Medulla — This is the more central portion of the kidney, beneath the cortex.

Renal Papilla — This is a cone-shaped projection of the medulla enclosed by the pelvis. In the pig’s kidney there is only one papilla, in man there are many. Trace the ureter from the hilus to the urinary bladder. Do this on both sides. Do not injure the reproductive structures. Lift the urinary bladder and find the urethra, which transports urine from the bladder. To this point only the urinary structures have been examined. They are alike in males and females. If your specimen is a female, continue the dissection as directed here. If your specimen is male, continue in the next section. However, whether your specimen is male or female, you are responsible for knowing the reproductive structures of each. Therefore, work closely with a student whose pig is of the opposite sex of your specimen.

FEMALE GENITAL SYSTEM

Ovaries — These are the female gonads. They are paired, small bean-shaped structures located posterior to the kidneys. The oviducts, or fallopian tubes, and extremely narrow, usually located dorsal to the anterior portion of the ovary. Use a hand lens to observe them more closely. Also observe the expanded ends of the openings, the ostium, fringed by small finger-like projections, termed fimbriae. These guide the ova into the ostium.

Uterine Horns ( Or Cornua) — Trace the oviducts to the dorsal surface of the ovary where they join the much wider uterine horns. Trace these caudally to where they join to form the body of the uterus, which lies dorsal to the urinary bladder and urethra. In pigs and other mammals, the fetus does not develop in the body of the uterus, as in man, but in the horns extending from the uterus. This permits the development of more fetuses at one time and the birth of a litter. In humans, development of the fetus in the body of the uterus makes multiple births a rarity.

Membranes — The ovaries are suspended from the dorsal body wall by a peritoneal membrane called the mesovarium. An ovarian ligament connects the ovaries to the uterine horns. Each horn is supported by a peritoneal fold the mesometrium. These three membranous suspensions are part of the broad ligament. This ligament extends into the pelvic area serving to hold the body of the uterus and vagina to the body wall. Another support, the round ligament, extends from the dorsal body wall to the middle of each uterine horn. In order to continue the dissection it is necessary to cut through the pubic symphasis and spread the pelvic bones apart. This will expose the rest of the genital organs lying below the pelvic bones.

Uterus and Vagina — You are now ready to expose the entire urethra and the body of the uterus. The vagina is a continuation of the uterus and lies dorsal to the urethra. The cervix is a constricted area between the body of the uterus and the vagina.

Urogenital Sinus — Separate the urethra from the vagina. Posteriorly the vagina and urethra unite to form a common passageway which opens to the exterior, the urogenital sinus, or vestibule. In human females the vagina and urethra are separate throughout their lengths and the vestibule is a much reduced area, a part of the external genitalia.

External Genitalia — Follow the urogenital sinus caudally to its opening on the outside of the body. Use your scissors to make a ventral longitudinal cut along the urogenital sinus in an anterior direction from the external genitalia, through the vagina beyond its union with the urethra, as in the accompanying photo. Find the urethral orifice on the ventral surface of the vagina at its junction with the urethra. Near the opening of the urogenital sinus, again in the ventral wall, locate the clitoris, the homolog of the penis. On either side of the urogenital aperture, at the external opening of the urogenital sinus, just ventral to the anus, are folds of skin called the labia majora. These, together with the urogenital aperture, constitute the vulva. The prominent, fleshy, conical urogenital papiila, which readily identifies the pig as female, projects beyond the urogenital aperture.
 

MALE GENITAL SYSTEM

Testes — These are the male gonads. Locate the scrotum, the swollen double sac ventral to the anus. It contains the testes. Carefully cut the skin of the scrotum. It is lined with peritoneum and is divided into two compartments by a medial septum.

Epididymis — This is an extremely coiled tubular structure lying on the dorsal surface of each. testes. It consists of a head on the anterior part of the testes where it is connected to the testes by numerous microscopic efferent ductules. It also has a body, the middle portion, and a posterior portion, the tail. Follow the epididymis tail cranially. Its convoluted ducts are continuous with the duct that exits the scrotum into the abdominal cavity.

Vas Deferens (Ductus Deferens) — It is through this tube that sperm and seminal fluid leave the testes. It exits the scrotum into the abdominal cavity.

Note: In order to study the remainder of the male reproductive system cut the pelvic bone at the pubic symphasis and spread the pelvis apart.

Spermatic Cord — The ductus deferens is only one of the tubes leaving the testes. Blood vessels, nerves, and lymphatic vessels supplying the testes also pass from the scrotum into the abdominal cavity. They are united by a tough outer fascia to form the spermatic cord.

Urogenital Canal — Follow the spermatic cord cranially through the external inguinal ring located at the juncture of the scrotum and the abdominal wall. Continue further cranially through a short channel in the abdominal wall, the inguinal canal and out into the abdominal cavity through the internal inguinal ring.

During embryological development the testes are at first located within the abdominal cavity, below the kidneys. During later development they descend through the inguinal canal into the scrotum. Inhuman males the condition of inguinal hernia is common. It is a weakening of the inguinal rings permitting a loop of intestine to be pushed through the inguinal canal into the scrotum. This condition is due to man’s upright, two-legged position. Pigs do not suffer from this malady.

Upper Spermatic Cord — Continue to follow the spermatic cord within the abdomen. The blood vessels, the spermatic vein and internal spermatic artery soon separate from the ductus deferens. The right spermatic vein enters the posterior vena cava below the level of the kidney, while the left spermatic vein generally enters the renal vein near the top of the kidney. It thus does not enter the vena cava directly. The right and left internal spermatic arteries enter the abdominal aorta next to one another at the level of the posterior region of the kidneys. This can clearly be seen in the photo. Trace these blood vessels in your specimen.

Upper Vas Deferens — The vas deferens loops dorsally over the base of the ureter near the urinary bladder and continues caudally. The urethra emerging from the urinary bladder, together with the vas deferens, pass posteriorly and penetrate the prostate gland at the proximal end of the penis.

Urogenital Canal — From this point on, the urethra continues as a merged tube, the urogenital canal, carrying sperm and seminal emissions from the testes and prostate gland, plus urine from the urinary bladder.

Seminal Vesicles — These are very small glands in the fetal pig and are difficult to locate. They are found dorsally on either side of the prostate gland. They too contribute to the seminal fluid.

Bulbo Urethral Glands (Cowper’s Glands) — These are located on either side of the urethra as it passes through the pelvic girdle.

Penis — Follow the urethra, or urogenital canal, caudally to the beginning of the penis. The penis is the cylindrical copulatory organ of males. Remove the skin and trace the penis to its attachment in the region of the pubic symphasis. Locate the crus of the penis and a small muscle the ischiocavernosum. Both are lateral projections at the proximal end of the penis anchored to the ischium bone. The crus is the proximal end of the corpus cavernosum, a cylindrical mass of vascular erectile tissue which, together with a second corpus cavernosum lying side by side, form the dorsal part of the penis. A third cylindrical mass of vascular erectile tissue lies ventrally within the penis in a groove between the corpora cavernosum. This is the corpus spongiosum. The urethra passes through this mass of vascular tissue. The urethra continues through the penis to its distal end and emerges at the ventral abdominal wall just below the umbilical cord. The urethral orifice on the surface of the body is known as the urogenital opening. Its presence readily identifies the specimen as a male.

Each student is responsible for learning the reproductive structures of both male and female fetal pigs.