Exercise 9 - Mammals (continued)

Fetal Pig (continued)

Dissection already performed will have exposed the major blood vessels visible in the coelom and heart region, as well as the internal organs. Before you go any further, review last week's work: organ-system and function for the organs and direction of blood flow and oxygen content for the blood vessels. If you have any questions, please ask your lab instructor for assistance. (Quickie quiz to see if you are ready to proceed: (1) to which organ-system does the pancreas belong? Name one function of the pancreas? Which way does blood flow in the umbilical artery? What is the oxygen content in the fetal pig's pulmonary vein? What is purpose of the ductus arteriosus and where is it located?)

Excretory System:

The kidneys have already been located. You should now find the ureters, which run posteriorly from the medial side of each kidney and merge into a rather thick-walled sack located between the two umbilical arteries. This is the allantoic sac which extends well out into the umbilical cord prior to birth. After birth, the allantoic sac shortens and is modified into the urinary bladder. You should clean up the region until the coiled ureters can be easily traced from the kidneys completely to their junction with the bladder. If you have a female pig, be very careful not to damage the small oviducts and horns of the uterus which are located just in front of the umbilical arteries. By now the colon should have been isolated, so you can see that it seems to disappear just at the place where the ureters enter the future bladder (in females, the ovaries also seem to go out of sight here too). This is a critical area. On one side, use your pin to free the umbilical artery from the bladder all the way up and then cut it at the base. Then you can locate the exact place where the ureters merge into the bladder. With the artery reflexed out of the way, you can see that the bladder seems to continue on dorsally and posteriorly out of sight as a pale tube. This is the urethra, and drains the bladder to the outside. If you have a male pig, you must be extra careful here to avoid breaking or cutting the sperm ducts (vasa deferentia) which merge from small holes just in front of the upper legs, on the inside, and merge together just in front of the bladder. It appears as a thin strap-like piece of tissue. In most males you can find a tiny artery leaving the aorta just behind the renal artery and extending back to enter the same opening from which the sperm duct emerges. This artery supplies blood to the testicle and is called the testicular artery.

From this point on the excretory and reproductive systems tend to use the same ducts and tubes; hence the two systems are commonly referred to as the urogenital system. Here we will begin our survey of the reproductive system.

Reproductive System: Note: Be sure to examine BOTH genders.

A. Female

The female gonads (ovaries) are two small kidney-shaped, brown structures at the ends of the coiled and looped oviducts (fallopian tubes). The ovaries produce two major things: (1) the ova (eggs) and (2) two types of ovarian hormones - estradiol (estrogen) and progesterone. Look carefully at the ovaries and roll them over with a pin. The oviducts are tiny tubes closely wrapped around the ovaries. The larger coiled tubes are the two horns of the uterus which lead from the single body of the uterus. Each bend in the horn of the uterus will, when the female is pregnant, house a fetal pig. In the adult pig the uterus is about the size of two hands, but when pregnant can stretch enough to accommodate as many as 13 fetal piglets each up to 14-16 inches longs! Notice that the uterus too disappears right along with the urethra at the very farthest end of the coelom. We will now follow the course of the urethra and the vagina. Using the tip of the razor blade, gently push against the pelvic region above where the urethra disappears while simultaneously pushing the legs apart. This should make the necessary cut and is much better than a "sawing" motion. Be careful not to cut too deeply (don't cut into the vagina and urethra. Cut until the legs spread open and the whitish, tube-like urethra and vagina are exposed. Having cut completely through the pubic bone, which closes the front side of the pelvic girdle (refer to human or cat skeleton) you can trace the entire course of the urethra on back to the genital papilla.

For most of its length, the vagina, which is an outward continuation of the uterus, runs closely next to (above) the urethra. The two are covered by the same membrane. Separate this membrane and find the two structures. Which tube is dorsal: the vagina or the urethra?

Notice that before they reach the genital papilla, the two merge into a common tube for about the last centimeter or so. In other mammals, such as the human, the two tubes remain separate for their entire lengths and the vagina and urethral openings are separate. By pulling the hind legs apart even more, you can determine that dorsal to the vagina is the outer part of the colon, here called the rectum. In summary, then, we find that in the pelvic region there are three parallel tubes all lying close together. The ventral most is the urethra (draining the bladder), the middle vagina (opening into the reproductive system), and the dorsal rectum (lower portion of the digestive system).

B. Male

The male gonads (testes) start life well up in the coelom, close to the kidneys. As time passes, the migrate downward (posteriorly) and disappear into two tunnels (inguinal canals) which are extensions of the coelom into the scrotum. Originally fairly large, they readily admit the testes (one per inguinal canal) which move down into them, pulling with them the testicular artery and vein, a nerve, and the sperm duct (vas deferens) which are all together in a bundle of tubes. Then the canal closes down, leaving each testis isolated in a small pouch on the rear side of the pelvic region. The pouch is one-half of the scrotum. As the testes descend into these pouches, they are removed from the excessively high heat of the internal body cavity. If the testes fail to descend (a condition called "cryptorchidism" or hidden-gonad), the individual will be sterile. In your pig, the testis may be anywhere from the abdominal cavity to the scrotal sac, depending on the age of the embryo. To find all of these structures, first locate the inguinal canal on one side of the body wall near one of the rear legs. Carefully insert a tip of a pin into the canal and carefully cut along the edge of the pin to open up the inguinal canal. In life, if this canal is ripped open by excessive exertion or lifting, a coil or two of the intestine may descend into the scrotal sac resulting in an "inguinal hernia". This is easily corrected by suturing the tear shut. As you continue the cut on back, you will come to the testis itself. Each testis contains a very long, coiled seminiferous tubule (microscopic) which actually produce the sperm cells. In the spaces between the coils of the tubules, there is interstitial tissue which produces testosterone, the male sex hormone. This is taken back to the body by the testicular vein (which is usually not visible in the fetus). The seminiferous tubules are coiled on the surface of the testes in a structure called the epididymis from which emerges the sperm duct (vas deferens). The sperm cells, of course, travel to the urethra via the vas deferens.

Now having found where the two sperm ducts merge on the front side of the bladder, you can cut the umbilical artery on one side, reflex it, and see how the urethra proceeds down and out of sight. What is different here (from the female) is that the sperm ducts empty into the urethra itself, which thus serves a dual function: (1) it carries urine from the bladder out through the penis and (2) it carries the sperm cells (in a fluid called semen) from the testes out through the penis. Carefully cut dead-center through the pelvic bone to follow the urethra from the anterior end of the bladder through the pelvic canal and up through the skin along the belly to the base of the umbilical cord. Firmly squeeze and roll between your fingers the skin and muscles next to the belly to find the urethra. Cut away all of the tissue, etc. to expose the urethra from the bladder clear to the genital opening. The last part of the urethra will later be transformed into a penis by the incorporation of erectile tissue, muscles, etc. around the urethra. At this stage of development, there is only a urethra. With care in your dissection, you should be able to follow the entire length of this tube. In the pelvic region, just dorsal to the urethra, is the lower end of the colon, here called the rectum.

In males of most kinds of mammals, the penis is normally carried hidden in a sheath of skin in the abdominal region. Perhaps this is a way of protecting the penis against damage (e.g. from running through brush etc.). In primates, and especially higher primates, the penis is completely free from the abdomen, perhaps as a result of animals going about on the hind legs with less likelihood of injury in an upright posture.

When you have finished your dissection. Take some time to review all of the structures you have located and be able to find them in your pig and on the human model. When you are ready, you may take the practical quiz.

Note about quiz: You will use your own pig and the human model for your practical quiz. Be sure all structures are adequately dissected so that you can point to each one if asked. You will not be able to do additional dissection during the quiz.