Sheep Brain Dissection Objectives: 1. List and describe the principal structures of the sheep brain 2. Identify important parts of the sheep brain in a preserved specimen Materials: Dissection tools, lab glasses, lab gloves, preserved specimen Lab Information: The mammalian brain has three primary subdivisions: the cerebrum (including the outer, wrinkled cortex), cerebellum, and brainstem. The brain-stem is further divided into the diencephalon, midbrain, pons, and medulla. The human brain is about 85 percent cerebrum, 11 percent cerebellum, and 4 percent brainstem. The human brain has more than 100 billion neurons, with 14 to 16 billion in the cerebral cortex and nearly 100 billion in the cerebellum alone. In addition, there are perhaps nine times as many glial cells, whose exact roles are unclear, but which help to support and maintain neurons. Most neurons are present shortly after birth, and as the brain continues to grow, the number and complexity of neuronal connections increases. These neurons are arranged into gray matter and white matter. Gray matter composes areas rich in neurons, their dendrites, and synapses. White matter is tissue rich in axons (nerve fibers), but with a few cell bodies or dendrites. It gets its color from an insulating wrap called myelin around the nerve fibers. The high lipid content of white matter makes it light and easily distinguished from gray matter in fresh, unstained tissue. The human brain weighs roughly 1.5 kg and has an average volume of 1130 cm 3. A sheep s brain weighs in however at 0.140 kg. The cerebrum and cerebellum each have a multilayered sheet of cells on the surface called the cortex, composed of gray matter. The white matter lies deep to this and consists of axons that send information to and from the cortex or connect different regions of the cortex to each other. Deeper masses of gray matter are also found embedded in the white matter.
Pre-Lab: Complete the following questions before lab. You will also need to pass the pre-lab quiz on BrainHoney before being able to participate in lab. 1. Define the following terms (1 pt each) a. Posterior: b. Lateral: c. Medial: d. Ventral: 2. Draw a horizontal and frontal plane on the picture below and label (4 pts) Draw and label a sagittal plane on the picture below (2 pts)
Dissection Proper: Section A - External Sheep Brain: The sheep brain is quite similar to the human brain except for proportion. The sheep has a smaller cerebrum. Also the sheep brain is oriented anterior to posterior whereas the human brain is superior to inferior. Place a check mark in each box when done with that step. 1. The tough outer covering of the sheep brain is the dura mater, one of three meninges (membranes) that cover the brain. You will need to remove the dura mater to see most of the structures of the brain. Use your tweezers and be very careful because the brain is soft and easily damaged. Dura Mater a. Describe what the dura mater feels like
2. The most prominent feature of the brain is the cerebrum - which is divided into nearly symmetrical left and right hemispheres by a deep longitudinal fissure. 3. The surface of the cerebrum is covered with large folds of tissue called gyri. Draw the gyri of the cerebrum. 4. The grooves between the gyri are sulci. The deeper sulci are often termed fissures. The fissures are used as landmarks to divide the surface of the cerebrum (the cerebral cortex) into regions: frontal lobes / parietal lobes / occipital lobes / temporal lobes * Locate each of the lobes of the brain. 5. Closely examine the cerebrum. Locate the longitudinal and central fissures. a. Describe the location of the longitudinal fissure with reference to at least 3 other brain structures. Use at least 1 term from the pre-lab terminology. 6. Use a ruler and measure along the frontal plane, and horizontal plane and height (use mm). a. Frontal Plane b. Horizontal Plane c. Height
7. The smaller, rounded structure at the back of the brain is the cerebellum. The cerebellum has smaller gyri that are roughly parallel to one another. Draw the gyri of the cerebellum 8. Compare the gyri of the cerebellum to that of the cerebrum. 9. Use a ruler and measure along the frontal plane, horizontal plane, and height of the cerebellum (use mm). a. Frontal Plane b. Horizontal Plane c. Height 10. Removing the dura mater from the cerebellum can be tricky business. Look for areas on the side of the brain that you can snip to peel the dura mater off.
11. Turn the brain over so that the cerebrum is down. The most prominent structure visible on the ventral side of the brain is the optic chiasm, where the two optic nerves cross over each other and form an X shape. Locate and draw the optic chiasm. 12. The pituitary gland is a large round structure under the chiasm. If you removed this area with the dura mater, you may need to replace it to see the chiasm and pituitary gland. 13. Toward the front of the brain are two prominent round structures; find the olfactory bulbs. 14. Toward the back of the brain, in order from the optic chiasm are bulges that indicate the midbrain, the pons, and the medulla. 15. Just behind the optic chiasm is a raised area or bump that indicates the infundibulum (also known as the pituitary stalk). This is where the pituitary was attached to (which was probably removed with the dura mater).
16. Occulomotor nerves may be visible to each side of the pituitary gland (or stalk). Or in some cases you may find them stuck to the dura mater that you removed with the pituitary gland. 17. Carefully bend the cerebellum to get an inside glimpse of the brain. The bumps you see (kind of resemble a butt ) are the superior colliculi. The smaller ones underneath are inferior colliculi. 18. If you gently push those structures down, you can see the tiny nub of the pineal gland. 19. Now that you have had a chance to look at these structures you must now label them. Get some making tape from the front of the class and dissection pins. Create flags on each push pin and label them pons, medulla, and longitudinal fissure. Gently push these into the structures that you want to label. Then get Mr. Fernandes and have him see if you got it right. He will put your grade here. Then move on to Section B.
Section B. Internal Sheep Brain. 1. Use a knife or long-bladed scalpel to cut the specimen along the longitudinal fissure. This will allow you to separate the brain into the left and the right hemisphere. Lay one side of the brain on your tray to locate the structures visible on the inside. You should also cut through the cerebellum. 2. The corpus callosum had been connecting the two cerebral hemispheres and can now be clearly seen in the brain section. 3. The tiny space within the corpus callosum (which hold cerebrospinal fluid) is called the lateral ventricle. Underneath it, you can find the third ventricle. There are other ventricles within the brain, but those are the easiest to locate in a preserved specimen. The white area between those two ventricles is the fornix. The fourth ventricle is the space under the cerebellum. 3. Inferior to the corpus callosum is a round structure known as the thalamus. It seems it almost perfectly centered. Just behind the thalamus is the pineal body (gland). The hypothalamus is also round shaped but is lower and toward the front of the brain. 4. The pons, medulla, and spinal cord are also visible in the side view of the brain. Together these form the brain stem. Locate these structures. 5. Use a ruler and measure along the frontal plane, horizontal plane and height of the brain stem (use mm). a. Frontal Plane b. Horizontal Plane (multiply x2 because brain is cut in half) c. Height
6. Within the cerebellum, locate the arbor vitae, named such because the white lines resemble a tree. 7. Use a scalpel to cut a cross section of the cerebrum in the occipital lobe area. You should be able to see the color and texture differences of the white matter and the gray matter. 8. Now that you have had a chance to look at these structures you must now label them. Get some making tape from the front of the class and dissection pins. Create flags on each push pin and label them arbor vitae, white matter, brain stem. Gently push these into the structures that you want to label. Then get Mr. Fernandes and have him see if you got it right. He will put your grade here. Then move on to analysis.
Section C - Sheep Brain Dissection Analysis: Match the structure to the description 1. Arbor Vitae a. looks like a butt 2. Lateral Ventricle b. leathery covering over the entire brain 3. Optic Chiasm c. cauliflower, the area toward the back of brain 4. Superior Colliculi d. behind the colliculi, looks like a little nub 5. Dura Mater e. looks like a tree 6. Cerebellum f. the rounded part of the brain stem 7. Pineal Gland g. shaped like an X 8. Thalamus h. large area under the corpus callosum 9. Pons i. space for fluid between the corpus callosum and the fornix 10. Olfactory Bulb j. contains nerves, connects to the far front of the brain 1. Name some similarities between the Human Brain and the Sheep Brain. 2. Discuss some differences between the Human Brain and the Sheep Brain. 3. Describe the Cerebrum (appearance, shape, color, etc.). 4. Describe some differences between the Cerebrum, Cerebellum, and the Medulla. 5. Analyze your measurements taken from the dissection. Create a volume for cerebellum, cerebrum, and brain stem (LxWxH). Include units. Cerebellum Cerebrum Brain Stem
6. Graph the results from #5 7. Find the percentage for each of the 3 parts of the brain (use the volume). Cerebrum Cerebellum Brain Stem
8.) By percent how much of a human brain is cerebrum, cerebellum, brain stem. Some outside research will be needed. Cerebrum (%) Cerebellum (%) Brain Stem (%) 9. Graph the percentage for each part of the brain for human AND sheep. Use red for human and blue for sheep. A bar graph is probably the best for this.
10. Given what you know about the uses for the different parts of the brain what can you tell me about sheep versus humans (i.e. if the brain stem is bigger in humans what kinds of behaviors will you see in them vs. a sheep). Use evidence from the lab to support your answer.