FETAL PIG DISSECTION
Background information and procedures
Background informationMammals are warm-blooded vertebrates (animals with internal backbones made of individual bones called vertebrae) that have hair or fur, bear live young, and nourish their young with milk produced in mammary glands. The placental mammals are the largest group of mammals. Placental mammals develop completely inside their mother’s uterus before they are born. During this time of development, these mammals are connected by the umbilical cord to the placenta, a capillary-rich organ that attaches to the wall of the uterus. There, nutrients and oxygen from the mother’s blood are exchanged with wastes and carbon dioxide from the blood of the developing young, or fetus. In this investigation, you will be studying the fetal pig. An in-depth examination of this organism will help you to better understand the features and body systems of all placental mammals, including humans.
External features of the pig
1. Put on gloves if you will be handling the pig.
2. Obtain a fetal pig and lay it on its side in a dissecting tray. Point out (touch with a blunt probe or a finger tip as you name the structure) the major body areas of the pig to your partner(s). Label them on Figure 1.
· head
· neck
· trunk (including chest and abdomen)
· tail
3. Point out and label each of the following on Figure 1.
· anterior end
· posterior end
· dorsal surface
· ventral surface
4. Examine the pig’s head. Point out the eyelids, the external ears, and the nostrils.
5. Study the pig’s appendages (legs). Point out the toes, “ankle,” “knee,” “wrist,” and “elbow.”
6. Study the ventral surface of the pig. Point out the tiny bumps (nipples). These are present in both sexes. In the female, these structures connect to the mammary glands.
7. Measure your pig from the tip of the snout to the tail. Record your measurement in mm. Refer to the following table to determine its gestational age. Record the age below Figure 1.
Measurement (mm) / Age (days)11 / 21
17 / 35
28 / 49
40 / 56
200 / 100
300 / 112-115 (full term)
8. Locate the umbilical cord. With scissors, cut across the cord to leave about 1 – 2 cm of it attached to the body. Examine the three openings visible on the cut surface of the umbilical cord. The largest (which may have blue plastic in it) is the umbilical vein, which carries blood from the placenta to the fetus. The two smaller ones (which may have red plastic in them) are the umbilical arteries, which carry blood from the fetus to the placenta. Point out these three blood vessels.
9. Examine the ventral and posterior surfaces of the pig. Point out the anus, which is in the same position in both sexes. Then locate the urogenital opening, following the instructions below. (The urogenital opening is the opening which allows urine and reproductive cells to leave the body). This will allow you to determine the sex of your pig. Record the sex of your pig on Figure 1.
· In a female, the urogenital opening is located just ventral to the anus; a small fleshy flap called the papilla surrounds this opening.
· In a male, the urogenital opening is located just posterior to the umbilical cord. (Another external clue that a fetal pig is a male is a slightly swollen region on the trunk posterior to the hind legs – this is the undeveloped scrotum.)
· Point out the urogenital opening in your pig. Then find a lab group whose pig is a different sex than yours, and point out the urogenital opening in that pig. Also, point out the papilla in the female and the scrotum in the male.
Abdominal cavity – focusing on digestive organs
10. Place the fetal pig ventral side up in the dissecting tray. Tie a string securely around one of its “wrists,” run the string under the tray, pull the string tight, and tie it to the other “wrist.” Repeat this procedure with another string, tying it to the pig’s “ankles.” This will help hold the pig in place as you work on it and examine it.
11. Study Figure 3. The lines show the first set of incisions you will make.
· Begin by pulling upward on the umbilical cord and inserting the point of your scissors through the skin and muscle layer just to one side of the umbilical cord. Cut next to the row of nipples on each side of the umbilical cord. You will know if you have cut deeply enough if you can see a fairly deep opening or some liquid (which may be brownish or bluish). Once you have cut deeply enough to be through the abdominal wall, keep the tips of your scissors pointed upward to avoid cutting too deeply.
· After you have cut around both sides of the umbilical cord as Figure 3 indicates, cut towards the sides of the body near the hind legs and below the rib cage to form two flaps. Open these flaps, and cut the umbilical vein which seems to be holding the umbilical cord to the liver.
· You should now be able to see the organs of the abdominal cavity. If there is brown or blue material in the abdominal cavity, rinse it out with running water (hold the entire dissecting pan in the sink).
12. Point out the following abdominal structures:
· diaphragm – a sheet of muscle that separates the abdominal cavity from the chest cavity.
· liver – a large brown structure (the most obvious structure in the abdominal cavity)
· stomach – a soft sac found beneath the liver on the pig’s left side
· small intestine – find the point where it connects to the stomach. Then note that this coiled, narrow tube is held tightly together by a filmy layer called the mesentery. The mesentery’s blood vessels absorb digested food from the small intestine, and the mesentery itself keeps the small intestine coils from tangling with each other.
(continued)
· large intestine – the first part of this tube is very tightly coiled and bound together; further along, it straightens out as it travels along the midline of the abdominal cavity just inside the back. This straight section of the large intestine is called the rectum.
· appendix – a blind pouch located where the small intestine joins the large intestine
· spleen – a long, flat, reddish organ wrapped partly around the stomach on the pig’s left side. The spleen is one of the main locations where white blood cells hide out to wait for pathogens to show up and filters out old red blood cells.
· pancreas – a thin, somewhat reddish structure located underneath the stomach. It is covered by a filmy membrane which you will need to remove to see the pancreas clearly. The texture of the pancreas is different from other organs you’ve seen; it seems to be made of many tiny spheres stuck together.
· gall bladder – a small sac attached underneath the liver. Note the tiny tubes which carry bile to the gall bladder and then to the small intestine.
Abdominal cavity – focusing on excretory and reproductive organs
13. Move the digestive organs gently to one side of the body cavity if necessary so that you can study the organs of the excretory (urinary) and reproductive systems. Do not remove any organs completely from the body. Study figures 4 and 5 as you continue with the following steps.
14. Point out the following structures:
· kidneys – the large, bean-shaped structures lying against the dorsal body wall. Note that they are covered by a membrane.
· bladder – the long tubelike structure which lies inside the abdominal wall. The two red umbilical arteries are attached to either side of the bladder. The end of the bladder nearest the umbilical cord has no opening; the opening of the bladder is at its most posterior end.
· ureters – tubes extending from the kidneys to the bladder. You will need to carefully remove the membrane covering the ureters to see them clearly.
15. If you have a female pig, point out the following structures:
· ovaries – small bean-shaped structures
· Fallopian tubes – coiled; attached to the ovaries
· uterus – the structure where the two Fallopian tubes meet. Some of the uterus is “hidden” behind the bladder and urethra.
16. If you have a male pig, point out the following structures:
· vas deferens – thin tubes emptying into the urethra at the base (posterior end) of the bladder. In most male pigs, the other end of each vas deferens will pass through the abdominal wall.
· testes (if they are still in the abdominal cavity) – these always develop in the abdominal cavity at first. At some point, usually but not always before birth, the testes move down into the scrotal sacs through openings in the abdominal wall. Most of the male fetal pigs we use will already have had the testes descend into the scrotum, but some males will still have testes in the abdominal cavity.
17. If your pig is a female, continue with steps 17 and 18, and skip steps 19 - 21. If it is a male, skip steps 17 and 18 and continue with steps 19 – 21. In either case, you will be examining how the excretory, reproductive, and digestive systems connect to their external openings (the urogenital opening and the anus).
18. If your pig is a female, make an incision along the midline of the body from the abdominal cavity to the anus. (Do not cut into the anus or the urogenital opening.) You will need to cut deeply through muscle and the semi-solid pelvic bones. A scalpel often is helpful in making this incision.
19. At the posterior end of the female pig, point out two tubes:
· the rectum, which connects to the anus and lies closest to the dorsal surface of the pig
· the urogenital sinus, which connects to the urogenital opening. Follow this toward the abdominal cavity. Point out where it divides into two tubes:
§ the tube which lies closest to the ventral surface of the pig is the urethra; it connects to the bladder.
§ the tube between the urethra and the rectum is the vagina; it connects to the uterus.
20. If your pig is a male, observe the skin of the abdomen posterior to the umbilical cord. Point out a muscular tube lying just below the skin. This tube is the penis. In the male pig, as in many mammals, the penis is normally retracted inside the skin; it only extends outside (through the urogenital opening) when the male is trying to mate.
21. Make an incision along the midline of the male’s body from the abdominal cavity to the anus. (Do not cut into the anus or the urogenital opening, and do not cut through the penis.) Point out the two scrotal sacs, one on each side of the body in the scrotum. Slit the membrane covering one of the sacs and point out the testis and the coiled epididymis inside the sac.
22. At the posterior end of the male pig, point out two tubes:
· the rectum, which connects to the anus and lies closest to the dorsal surface of the pig
· the penis, which makes a “U-turn” inside the body and connects to the bladder. The part of the tube which runs from the bladder toward the posterior end of the pig is usually called the urethra; the part of the tube which runs from the posterior end of the pig to the urogenital opening near the umbilical cord is called the penis.
23. Find a lab group whose pig is a different sex than yours, and point out all the structures in steps 13 – 21 that are present in their pig.
Chest cavity – focusing on circulatory and respiratory organs
24. Make an incision along the ventral midline of the pig, beginning just above the diaphragm and extending to the “wart” on the pig’s chin. Then, just above the diaphragm, cut towards each side of the body; make similar cuts toward the pig’s “armpits;” this will form upper flaps on each side of the body. Open these flaps to observe the organs in the chest cavity. (You may need to cut through some membranes to see some of the organs.)
25. Point out the following structures:
· lungs
· heart
· pericardial sac (the membrane which surrounds the heart)
· diaphragm
· rib cage
· left and right atria (the small, dark brown chambers of the heart)
· left and right ventricles (the large, muscular, light brown chambers). You can’t see the internal wall separating these two chambers, but you can tell where it’s located by observing the blood vessels forming a diagonal line on the outside of the heart. These blood vessels are the coronary vessels (artery and vein).
· coronary vessels (These blood vessels supply food and oxygen to the heart muscle cells; if they get blocked, the heart muscle cells will die, causing a heart attack.)
· pulmonary artery. This is one of the large blood vessels at the top of the heart.