Fetal Pig Dissection OBJECTIVES Know the external anatomy of the fetal pig. Dissect the digestive, respiratory, circulatory, excretory, and reproductive systems of the fetal pig. Identify and know the functions of the structures associated with digestive, respiratory, circulatory, reproductive and excretory systems of the fetal pig. Key structures you should know for the final are underlined and bold. It is highly recommended that one student create a list of these terms and their respective functions. Use this list to prepare for the 30-question lab test for your final exam, on the fetal pig. There is no write-up associated with this lab! Use the following web site to identify the unlabeled pictures associated with this lab: http://biology.ucok.edu/animalbiology/pigweb/pig.html MATERIALS Preserved fetal pig Dissecting tray Sharpie and 2 rubber bands 2 pieces of string or twine, 40-cm long 2 gal & 1-quart zip lock bag Dissection Tool Kit: Probe (1 metal) Forceps tweezers (2) Dissecting scissors (1) scalpel with fairly clean blade (1) PROCEDURE A EXTERNAL ANATOMY 1. Obtain a fetal pig, a lab manual, a dissecting kit (should match your lab station and/or group #), dissecting tray, a 1-quart size and a 2-gallon size plastic Ziploc bag. 2. DO NOT THROW AWAY THE PRESERVATIVE FROM THE BAG!!!! You will need it to re-soak your pig every day so the pig won't decompose over the next week. If during the coarse of doing the dissection, you find the formalin fumes are too irritating for your eyes, then rinse your pig with tap water to remove excess preservative from the pig's body. Remember to re-soak it like you would do with Shake-n-Bake chicken at the end of each lab. Figure 1 3. Place the pig in a dissecting tray. Locate the following external structures on the pig's body (use below figure 1 as a reference or page 7 of pig manual): umbilical cord (umbilicus), pinna (ears), tail, hind and fore limbs, anus and rooter (it s foremost rim of tough tissue on the upper margin of the snout; used for digging for food in dirt.) 4. Sexing your pig: Find the double row of nipples (mammary papillae) on the pig's ventral surface, (both sexes have these), but male pigs have a slight swelling or loose tissue, which is their scrotal sac at the posterior end, while females have a genital papilla or vulva (small opening with a little Page 1 of 12
nipple-like structure; see Figure 2 or refer to Male page 10 of the pig manual). Be sure you Female can determine the sex of any fetal pig you will be tested on it. Male PROCEDURE B PREPARING FOR THE DISSECTION Figure 2 1. Place the pig on its back in the dissecting tray. Tie a piece of string or twine around the wrist of one forelimb. Then, pass the string under the tray, and tie it to the other wrist with a shoestring knot that can be untied and tightened down after the pig is opened. Repeat tying the pig's hind legs as you did the forelimbs. After the pig is opened you will permanently tie off the legs. DO NOT CUT THE STRING; untie and retie your pig each day. 2. CAUTION: Always cut away from yourself when you use dissecting scissors or a scalpel. Pull up on the umbilicus and use scalpel to make a nick (cut) through the skin and muscles on the ventral side as indicated on Figure 3. Proceed cutting as indicated on the picture, using the scissors (angle the tips of the scissors upwards) to cut through the chest bone. For cut 6, cut along the lung side of Page 2 of 12
the diaphragm, leaving diaphragm inside and attached to the inner, backside of the pig, see figure 4 on the next page. 3. The only blood vessel that you can cut is the umbilical vein. It is just under the skin at the initial cut and it goes from the umbilicus towards the liver. You need to cut this so you can pull the umbilicus and its arteries down towards the tail as shown in figure 4. Figure 3 4. Remove any lateral (side) flaps of tissues. Cut the single umbilical vein coming from the liver in the abdominal cavity leading to the umbilical cord. 5. Drain the fluids from abdominal cavity and if there is a lot of loose, brown, clotted blood, then rinse with running water to remove it. This step can be repeated daily to keep organs fresh, but resoak in preservative solution when you return the pig to its bag. 6. Any muscles, skin or rib cage removed should be thrown away in the garbage. Your group will loose points if your station is dirty or anything is left in and around the sink. 7. Cut and remove all necessary tissue in order to make your pig resemble the picture below. Locate all the organs listed on the picture. (NOTE: As you proceed through the dissection, do not remove any organs unless you are directed to do so.) The respiratory system consists of the trachea (carries O2 to lungs), the lungs (place of gas exchange) and the diaphragm, the thin sheet of muscle that separates the abdominal organs from the chest cavity whose function is to allow air to enter and push air out of the lung tissue. The thymus and thyroid glands are part of the endocrine system. The thymus makes immune cells, while the thyroid regulates our body s metabolism. The rest of the organs will be covered in their respective systems during the rest of the lab. Figure 4 Page 3 of 12
PROCEDURE C: DIGESTIVE SYSTEM EXPOSING THE CHEEK AND LOCATING THE SALIVARY GLANDS: 1. The pigs have cuts in their throats in order to inject the plastic dye in their arteries and veins. Turn the pig's head to one side, and carefully remove the skin from the side of its face, above the neck region as shown in figure 5. Pull on an edge of a tear in the throat and run your scalpel parallel to its face, lightly cutting the connective tissue of the skin. 2. You should know that the nerves and salivary ducts are located very close to the surface, so be sure to just to remove the outer skin the face i.e. no striated or globular tissue in the flap of skin that you remove. Also be careful because nerves and ducts aren't injected with rubber dye, so are very fragile and tear easily. 3. The three masses of globular tissue are salivary glands. Note that each gland secretes saliva into a duct that crosses over the masseter muscle (1-chewing muscle) of the jaw to the mouth. Identify the parotid (3), and submaxillary salivary glands (2). Saliva contains an enzyme that helps digest the pig's food. 4. Expose the muscles, glands and their respective ducts, by removing any facial connective tissue as illustrated on Figure 6. EXAMINING THE INSIDE OF THE MOUTH: 5. FIRST, pry the mouth open. Then, insert your scissors into one of the corners of the mouth and cut through the skin, muscles, and bones. Repeat this procedure with the other comer of the pig's mouth. The lower jaw can now be pulled open easily so that you can examine the inside of the mouth, as shown in Figure 6. 6. Notice the ridges on the hard palate (1) on the roof of the pig's mouth. Food is rolled against this hard surface and formed into a ball that is easily swallowed. Behind the hard palate is the soft palate (4), which prevents food from entering the upper nasal passageway. Figure 5 Figure 6 Page 4 of 12
7. Use a probe to locate the epiglottis (2), a flap of cartilage found at the back of the mouth. The epiglottis prevents food from going into the trachea as the pig swallows. 8. At the rear of the mouth, locate the entrance to the esophagus. Separate the trachea (a ribbed tube that sits on top) from the esophagus (just below it) by breaking the connective tissue. Feel the muscular walls of the esophagus with your finger. The esophagus moves food down to the stomach. REMOVE THE DIGESTIVE ORGANS IN ORDER TO TRACE THE ORGAN PLACEMENT. 9. Slide your scissors between the diaphragm and the liver and cut through the thick, dark blue inferior vena cava entering the top of the liver, (see page 63 of lab manual). You will also have to cut towards the back of the pig until you cut through the esophagus, which is lying right behind the vena cava entering the stomach. 10. Move to the anal end of the pig and cut through the large intestine, right before it enters the pelvis (approximately spot 37 on the diagram above. 11. Use your probe or forceps to break any of the connective tissue holding the intestines down. Tear only clear mesentery, do not tear out any glands and don't tear the intestines (small are lighter in color and connected by many pink blood vessels; large appear as a tightly coiled, green roll). Roll the intestinal mass in your hands and move it one way, then the other, from front to back until you free up the GI tract. You should have the liver (with gallbladder attached), stomach, spleen (loosely attached to the bottom side of the stomach), pancreas (tightly connected to small intestine leading from the stomach), and intestines in one ball of quivering flesh. SEPARATE THE LIVER FROM THE REST OF THE GI TRACT. 12. The liver is the largest internal organ and should be dark brown in color, located at the top of the visceral mass that you just removed. Separate the gallbladder from behind the right central lobe of the liver, so it is dangling by the common bile duct to the duodenum. The gallbladder (71 on Figure 7) concentrates and stores bile made by the liver in order to breakdown (emulsify) fat. The liver detoxifies our blood. Locate the duct that leads from the liver and gall bladder to the duodenum (small intestine). 13. The esophagus leads into the large, soft, bag-like, J-shaped stomach (32 on Figure 7) that secretes enzymes and HCl acid to break down proteins. 14. Locate the glandular, light-colored pancreas (42 in Figure 7) behind and slightly posterior to the stomach. It is held onto the stomach/duodenum by mesentery (clear connective tissue). Its function is to secrete digestive enzymes into the duodenum. 15. The (spleen 42 on Figure 7) is a pinkish, glandular organ, attached to the lower, outside portion of the stomach. Its function is to remove worn out red blood cells. STRINGING OUT THE SMALL INTESTINE. 16. This will take two people; one person pulls on the small intestine, exposing the connective mesentery, while the other cuts the mesentery and arteries it with a scalpel. The mesentery and arteries form a fan-like structure, which you can throw away. Don't cut or tear the small intestine; it should be attached to the coiled, large intestine at one end and the stomach at the other end, with the gallbladder hanging off of it by the common bile duct. 17. Although the small intestine is one long tube it has 3 different divisions. The duodenum is the first several inches coming off of the stomach, in which most of the chemical digestion occurs. The middle intestine is the jejunum where most of the absorption of digested food occurs. Finally the ileum is the last couple of inches of small intestine leading to the coiled large intestine (colon.) In order for you to get full points on the lab practical you have to give their exact name or respective functions. Page 5 of 12
Figure 7 18. Where the small intestine connects to the large intestine, there is a small pouch-like structure forking off of the large intestines called the cecum as shown in figure 7. Remove the connective tissue holding the cecum to the large intestine. 19. DO NOT UNCOIL THE LARGE INTESTINE! Notice how the large intestine spirals through the abdominal cavity, loops through the upper portion of the small intestine, and then forms a straight tube, the rectum. PROCEDURE D CIRCULATORY SYSTEM 1. Continue the dissection by opening the chest cavity as described below. If you haven't already cut through the chest cavity, make incision as shown in the diagram on page 2. Use your dissecting scissors to cut through the ribs, breastbone, and collarbone. (NOTE: In order to avoid puncturing the delicate lung and heart tissues pull up on your scissors to keep them tightly pressed against the inside of the rib cage.) Remove the flaps of skin and chest bones. 2. Free the diaphragm from the body wall by clipping the diaphragm around the edges where it joins the ribcage. Do not completely remove the diaphragm! Page 6 of 12
3. Locate the heart in the middle of the chest cavity between the lungs. Note that the heart is enclosed in a transparent membrane, called the pericardium. The heart may be partially covered by a portion of the thymus gland. This gland is a dull white color and may resemble a mass of fatty tissue. 4. Mammalian hearts are divided into four chambers. The top chambers the atria collect blood as it comes into the heart (right atrium gets blood from the body, while left atrium gets blood from the lungs). The bottom chambers the ventricles pump blood to the lungs (right ventricle) and body (left ventricle). GROUPS 1-5 5. Remove the heart from the chest cavity. Once you remove the heart from the chest cavity, bring it to your instructor to determine your next cut. Make a longitudinal cut of the heart along the midline from top to bottom of the heart. You should be able to locate the right atrium (#46 on Figure 8a & 8b), left atrium (#48 on Figure 8b & 8a), right ventricle (#47 on Figure on Figure 8a & b) and the left ventricle (#49 on Figure 8a & b). Clean out any excess blue or red plastic dye from the chambers, but leave any dye in the blood vessels. Your will loose points if I can't see at least 3 out of 4 of the chambers. GROUPS 6-12 will leave their hearts inside the pig, but should clean up the chest/neck/heart area so it resembles 9a or 9b of this writeup. This means I should be able to see the blood vessels leaving the heart towards the head and lower extremities. You will loose 1 point for every vessel that is not cleaned up, i.e. carotid arteries (#82 on Figure 9a), external (#87 on figure 9a) and internal jugulars (#88 on figure 9), precava (# 54 on Figure 9b) and postcava, (#55 on Figure 9b) and aorta (#50 on Figure 8b). See Figure 9 on the next page. Figure 8a above Figure 8b below 6. Notice the blood vessels leading to and from the heart. The precava enters the right atrium from above and the postcava enters the right atrium from below. The aorta, located underneath the pulmonary artery, carries the blood from the left ventricle to the rest of the body. Locate the branches of the aorta. Locate as many of the labelled arteries and veins illustrated in figure 8. 7. The chambers and major vessels that lead to and away from the heart as well as the blood flow through the heart is required knowledge for everyone taking the final exam. Page 7 of 12
Figure 9a Figure 9b Page 8 of 12
8. Locate the blood vessels in the umbilical cord. The umbilical arteries supplies blood rich in oxygen and nutrients from the placenta to the fetus. The umbilical vein carrys away waste products from the fetal pig's body (See Figure 4 on page 3 of this writeup). Use a probe to trace the umbilical arteries and vein to and from the heart. PROCEDURE F EXCRETOR SYSTEM 1. CLEANING UP THE ABDOMINAL CAVITY: There is a membrane that encloses the abdominal cavity, called the peritoneum. Under the peritoneum, on either side of the spine are two large, dark red, bean-shaped kidneys (109 on Figure 10). The kidneys filter wastes from the blood. 2. Notice the whitish, crescentshaped organs attached to the anterior edge of each of the kidneys. This pair of adrenal glands (#44 on figure 10) is not part of the excretory system, but the endocrine system. The adrenal glands produce adrenaline that affects the behavior and metabolism the animal. GROUPS 6-10: 3. Use your probe or your forceps to tear the connective tissue holding a kidney to the body wall. Then, cut the blood vessels and ureter that lead from this kidney and remove the kidney. Cut the kidney longitudinally from the lateral to the medial surface. Notice that the kidney is divided into three sections. The light-colored renal cortex is the outer section of the kidney (#114 of Figure 10b). The renal medulla is darker in color (#115 on Figure 10b) and is made of many tiny tubules that project into the renal pelvis. The renal pelvis (# 116 on figure 10b) drains the incoming and outgoing fluids. Figure Figure 10b Page 9 of 12
4. You must remove the peritoneum (semi-transparent covering) from the surface of the renal arteries, renal veins and adrenal glands. You must remove any connective tissue from the descending aorta and postcava, thereby separating the two major vessels. Also clean away any connective tissue covering the iliac arteries and umbilical arteries. 5. LOCATING STRUCTURES OF THE EXCRETORY SYSTEM: Each kidney has a white tube called a ureter (#111 on Figure 10a) that carries the urine from the kidney to the urinary bladder. Remove any connective tissue on the ureters all the way to the urinary bladder (#112 on Figure 10a). The urinary bladder is located on the inside of the ventral flap that contains the umbilical cord. Urinary bladder is responsible for storing urine to be removed from the body. 6. LOCATING LOWER STRUCTURES OF THE EXCRETORY SYSTEM: After birth urine leaves the body through the urethra. You must make an incision down the middle of the hips, but not too deep. To location of the urethra varies slightly depending on the sex of your pig. In male pigs it goes from the bladder loops back towards the umbilical cord in the penis. In females you must make a deep incision with your scalpel down the middle of the hips and spread the hind legs as far apart as you can until the pelvis snaps. Probe with your finger until you find the cartilage that makes up the pelvic bone. Cut through the cartilage and expose the urethra. The urethra leads to the urogenital opening through which urine passes out of the body. PROCEDURE G REPRODUCTIVE SYSTEM Follow the directions for dissecting the reproductive system of your pig. You will be expected to answer questions on both male and female urogenital structures; therefore I recommend you examine other groups pigs. Female Reproductive System 1. Below the kidneys, and above the rectum, locate the small, off-white, almond-shaped ovaries (#101 on Figure 11) which are about 0.5 cm in diameter. Ovaries produce eggs and estrogen. 2. Use a probe to locate the thin, threadlike oviducts or Fallopian tubes (#103 on Figure 11), that loop behind the ovaries. In a mature female pig, eggs break through the ovary walls and pass into the oviduct. Tiny cilia line this passage, sweeping the egg through the coiled tube. If fertilization occurs, it usually takes place in one of the oviducts. Page 10 of 12 Figure 11
3. The uterus is broken down into three regions: the Horns of the uterus (#104, Figure 11) where the coiled fallopian tubes (left and right) come together, the uterus (#105, figure 11) and at the end of the tube is the vagina (#106, Figure 11). 4. Notice how the urethra (#113, figure 11) merges with the vagina near the urogenital opening. This common tube is called the urogenital sinus. Locate the urogenital sinus of your fetal pig. Male Reproductive System 1. Locate the scrotum on either side of the anus. Refer to figure at the top of page 2. If you haven't already cut posterior to the umbilicus, continue the posterior incision through the scrotum (structure 9, Figure 2) on one side of the anus. You may need to cut through the cartilage of the pelvic bones. Sometimes prying open the pelvis with your hands is more effective, see page 110 of the lab manual. 2. Cut through the lining of the scrotum, and determine if the testis is present. A male has two testes Figure 12 Page 11 of 12
small, hard oval masses about 0.5 cm in length. In a young male fetus the testes may not yet have descended into the scrotal sacs from the abdominal cavity. You may have to cut through the region above the scrotal sac called the inguinal canal to locate the descending testis (notice the difference between 90 and 91 remove the connective tissue coverings so they both look like 91, Figure 12). Testes produce sperm and testosterone. 3. You will have to remove the connective covering of the testes to observe the following structures. You will be graded on this step. Along one side of the testis, find the coiled mass of tiny tubes the epididymis (#92, Figure 12) that store the sperm. The epididymis leads into the vas deferens (#94, Figure 12), a tube that passes through the inguinal canal, loops over the ureters, and joins the urethra near the entrance to the urinary bladder. From this point the excretory and reproductive systems share the same passageway, the urethra. 4. Trace the urethra posterior until it enters the cord like cylinder under the skin and anterior to the umbilical cord, which becomes the penis (#96 Figure 12b). Notice that the urogenital opening from the urethra in the penis is located just posterior to the umbilical cord. Figure 12b Page 12 of 12