Name______Per ______

RAT DISSECTION PROCEDURE

Please read all lab instructions carefully and follow them step-by-step!

  1. Obtain an apron, goggles, and gloves. Label a ziplock bag with your period number and lab group number.
  2. Place your rat, ventral side up, in your dissection pan. You may need to break the scapulas so that your rat lies flat in the dissecting tray. To do this, gently grab the arms and pull them out and down toward the bottom of the pan.
  3. Look at the posterior end of your rat to determine if it is male or female. To identify a male rat, look for the presence of the scrotum – a large sac of skin on the posterior end containing the testes. To identify a female rat, look for the anal opening at the base of the tail, and then look for the vagina opening ventral to the anus. Be sure to examine a rat of the opposite sex carefully as you will need to identify structures from both a male and female rat for your lab practical!

Structures of the Head & Thoracic Cavity:

  1. Start just above the sternum. This bone, along with the costas (ribs), protects the heart and lungs of the rat. Use your scissors to puncture the skin, then lift the skin with the forceps or fingers and cut along a center axis up to the mandible. Lift up as you cut with the scissors so that you do not damage important organs.
  2. Use the probe to carefully move the skin away in the neck and cheek region. You may need to use your fingers to gently tear the skin away from the cheeks. You will expose 2 large muscles at the base of the mandible, one on each side of the face. These large cheek muscles are called the masseter muscles, and are powerful muscles used for closing the jaw during mastication. Underneath the mandible are the digastricus muscles, used for opening the jaw during mastication. Look at the rat’s incisors, which are especially prominent!
  3. In the neck region, locate the mandibular salivary glands. Rats have one large pair, and two smaller pair of salivary glands. Locate the larger pair. They may still be connected together by connective tissue. Use your scissors to gently cut the connective tissue apart so that you can separate the salivary glands. Use your probe to move them out of the way... DO NOT cut them out!
  4. With the salivary glands moved out of the way, use your probe to slit the tissue in the neck region just under the digastricus muscles. Locate the trachea. Look for the white stripes on the trachea – these are bands of cartilage to keep the trachea from collapsing.
  5. Continue the center cut through the thoracic cavity (towards the abdomen). Use caution – lift your scissors as you cut so that you do not damage the thymus gland, which is located on top of the heart. Cut through the sternum carefully – when you reach the end of the sternum, cut laterally down the sides of the rat. Be careful - at the end of the sternum is the diaphragm - you want to try to keep this skeletal muscle in tact. You may cut away the costas (ribs) on each side to better expose the thoracic cavity.
  6. Locate the lungs. The left lung has one lobe and the right lung has four lobes.
  7. Locate the heart. The two small sacs on either side of the anterior (top) portion of the heart are the atria. The atria are usually dark red or maroon in color and look like little flaps on top of the heart. The larger posterior (bottom) portion of the heart consists of the two ventricles. Muscle tissue is the densest of tissues – which chambers should (and do!) feel more muscular? Now, look for the small vessels embedded in the muscles of the ventricles. The redder ones are coronary arteries and the bluer ones are coronary veins. These vessels carry blood from the heart to the heart to oxygenate the myocardium (muscle layer) of the heart.
  8. Look for the descending aorta and inferior vena cava below the heart. The descending aorta is a dark colored vessel on the lower left side of the rat’s heart (on your right) leading down into the abdominal cavity. The inferior vena cava will be on the lower right side of the rat’s heart (on your left) leading down to the liver. Do not break these vessels – just identify them.
  9. Locate the thymus, a hat-shaped gland on top of the heart.
  10. Locate the diaphragm, a sheet of muscle that separates the thoracic and abdominal cavities. The diaphragm attaches all the way around the abdominal wall. Carefully cut it all the way around to the back so that it will fall freely down onto the liver.

Structures of the Abdominal Cavity

  1. Continue the cut from the diaphragm downward to the anus to expose the abdominal cavity of the rat. If your rat is a male, cut to the right of the penis down to the teste… and then do the same thing by cutting to the left of the penis down to the other teste. This should allow you to pull the penis back to see into the lower abdominal cavity.
  2. Make a few lateral cuts through the extra skin in the abdominal cavity – you may even cut away the extra skin. This allows for easier viewing of the abdominal structures.
  3. Locate the liver – it is divided into four lobes. Notice that the rat does not have a gall bladder under its liver. Why???
  4. Reach your probe toward the dorsal wall of the rat just above or below the diaphragm and gently pull up the soft tube. This is the esophagus. How does it compare in appearance and structure with the trachea?
  5. Trace the esophagus and find where it leads to the stomach. The stomach feels very dense when you touch it with the probe because it is a muscular sac. Do not poke into the stomach.
  6. Look just to the right of the stomach and locate the spleen. The spleen is a slender, elongated, aggie-maroon colored organ attached to the outer curve of the stomach by mesentery – the membrane that holds the various organs within the abdomen in place.
  7. Gently lift up the stomach and spleen and locate the pancreas. The pancreas is tannish-yellow colored and is composed of very glandular (bumpy) tissue. It is tucked under the stomach and extends to the duodenum of the small intestine.
  8. Follow the stomach to locate the small intestine. The small intestine is divided into the duodenum, jejunum, and ileum. The duodenum is the smoother, first portion of the small intestine coming directly off of the stomach. The duodenum leads to the more coiled jejunum and ileum. You may gently separate the small intestine from the mesentery to observe the length. Take care not to rip it.
  9. Follow the small intestine to locate the large intestine. The large sac within the first part of the large intestine (where the small and large intestines meet) is called the caecum. Cellulase, an enzyme missing in humans, is secreted into the caecum. What molecule (that humans cannot digest) is chemically digested in the caecum of rats by the action of cellulase?
  10. The large intestine leads to the rectum, which terminates with the anus – the external opening. Find both of these structures. The rectum may have balls of undigested food that were making their way down to the anus.

Structures of the Urinary & Reproductive Systems

  1. Lift up the abdominal organs and look back against the dorsal body wall of the abdominal cavity for the kidneys. They are two bean-shaped organs embedded in a layer of fat against the back wall.
  2. Next, look just above the kidneys for the adrenal glands. These glands are two small, rounded glands buried in the fat just above the kidneys.
  3. Carefully move some of the fat around just below the kidneys to locate the ureters. The ureters are very small, white, almost threadlike vessels that extend from the kidneys to the urinary bladder. The bladder may be quite small and clear or yellowish in color.
  4. Examine the center area of the dorsal wall just below the kidneys and locate two large vertical blood vessels, one a reddish color and one a bluish color. These vessels are the descending aorta and the inferior vena cava. The vessels have been injected with blue dye if they normally contain blood low in oxygen and red dye if they normally contain blood high in oxygen. Which vessel is the descending aorta – blue or red? The inferior vena cava?
  5. Obtain a male rat and locate the male reproductive organs:
  1. Scrotum – a large sac of skin, muscle, and connective tissue on the exterior body containing the testes
  2. Testes – Use the scissors and cut open the scrotum and locate the testes. You will have to make two cuts – one for the right testis and one for the left testis. You should see blood vessels running through the testes.
  3. Seminal vesicles or vesicular glands – prominent structures on either side of the urinary bladder. They lie just below the bladder and resemble “wrinkled question marks”.
  4. Prostate gland – large glands located on either side of the urethra. The urethra runs along the inside of the penis, so look at the base of the penis.
  5. Vas/Ductus deferens – the tubes that run from the epididymis of the testes to the penis.
  6. Epididymis – yellow looking structures located at the top/extending down the back of each teste. Stores sperm and carries it to the vas deferens.
  1. Obtain a female rat and locate the female reproductive organs:
  2. Ovaries – the adult non-pregnant female rat is usually in breeding condition with mature or well-developed follicles in the ovaries. Thus, the ovaries appear as a mass of follicles just posterior to the kidneys. They are usually buried in fat, which you can remove. The mature follicle contains an egg or ovum.
  3. Uterus/Uterine Horns – actually divided into two separate uteri in the rat, each leading to the body of the uterus.
  4. Body of the uterus – where the two uterine horns meet. Leads to the vagina, which leads to the outside of the body.

Rat Dissection Clean-Up Procedure:

Please complete EVERY step of the following cleanup procedure everyday! Failure to do so will result in a significant point deduction in your lab grade.

  1. Place your rat in the labeled ziplock bag. Make sure the bag is well sealed before placing it in the appropriate tray for your class period. Use a paper towel to wipe off any excess juice that is on the outside of the bag.
  2. Place all handouts, worksheets, and Rat Manual on a lab chair. This prevents them from getting wet! If you get the Rat Manual wet and cause water damage, you will be charged a replacement fee. Cost of manual = $45.00
  3. Place all solid contents (rat skin, fat, bones) from your dissection tray in the trashcan. No rat parts in the sinks!
  4. Clean all dissecting equipment (scissors, probe, forceps) with SOAP and water.
  5. DRY all equipment (scissors, probe, forceps) COMPLETELY with the cloth towel.
  6. Spray disinfectant spray on tabletop and in the dissecting tray. Dry them with a paper towel. Make sure your table and tray are completely dry!!!
  7. Throw gloves in trashcan.
  8. Wash your hands and dry them with a paper towel.
  9. Put away goggles and apron.
  10. Push in chairs completely.