01 lab rat dissection PDF

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LABORATORY 1

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Laboratory 1

Digestive System and Body Cavities (Rat) Before attending this class, read these printed notes and make sure you are familiar with the content of this lab.

Intended learning outcomes (ILOs) When you have completed this laboratory you should be able to perform the following tasks in relation to the rat: • Identify the major visceral organs by their colour, shape and position. • Briefly state the function of each organ. • Name the body cavity to which each organ belongs. • Name the slippery membranes which line the body cavities and cover the viscera. • Identify the four passages (for food/fluid and air) which converge at the posterior end of the pharynx, and explain the function of the epiglottis.

Focus questions At the end of this laboratory, the assessment will be in the format of a closed-book, ten-question multiple choice (MCQ) quiz. The following focus questions are aligned with the objectives of the lab, and represent the pool of questions from which the ten in-lab MCQs will be drawn. You may wish to use this as a reference for navigating your studies. A question asking you to: • Identify an organ which has been bolded in the lab guide. • Name the cavity within which a certain organ is found (e.g. thoracic; abdominal). • Name the structure that forms the physical separation between the abdominal and thoracic cavities. • State the location and function of mesentery tissue. • Describe the body cavities. • Describe the location and properties of serous membranes. • Identify organs which are not part of the digestive system. • Identify the gland that produces both digestive enzymes and hormones. • Name the largest gland in the body. • Name the structure(s) found in humans but not in the rat. • Name the two organs (in the rat) that secrete their products directly into the duodenum via a common duct. • Name the site of bile production in the rat. • Describe the key difference(s) between human and rat female reproductive organs. • Describe the functions of the various regions of the stomach. • State where dehydration of digestive contents and reabsorption of water mainly occurs. • List the regions of the small intestine in order from the stomach to the caecum. • Describe the path of nutrient-laden (rich) blood leaving the small intestine. • Describe how the flow of chyme is controlled. • Identify structures that are under partial voluntary versus involuntary control. • Describe the location and function of the epiglottis.

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As you enter the lab 1. Sign the roll 2. Obtain a pair of gloves and a dissecting tray 3. Wrap some tissue paper around your writing equipment to avoid contamination

Introduction The rat is an example of a “generalised” mammal. In many respects, so also are humans; therefore it is not surprising that despite outward differences in appearance, the internal anatomy of Rattus is similar to Homo. Among the relatively few internal differences are the following: •

• •

Teeth: the most notable feature of rodents (the group to which rats, mice, squirrels and guinea pigs belong) is their dentition which is adapted for gnawing. The paired incisors grow throughout life and lack enamel on their posterior surfaces, so that rapid wear of the posterior surface produces a characteristic chisel-shape. Furthermore the lower jaw (mandible) is in two pieces, hinged in the middle. The spacing between the incisors (and hence the width of the bite) can be varied at will. Gall bladder: absent in rats. Reproductive system: in female rats the uterus is “Y” shaped with two “arms” or horns in which the embryos implant. In males the penis is mostly within, (not outside) the abdominal wall.

External anatomy Rattus norvegicus albinus are full-grown (300 gm) at 6-8 months and have a life span of about 3 years. Note the following: • As in humans, the fore and hind limbs are pentadactyle (5 digits). • Rats use their tail as a balancing organ; (human embryos possess a distinct tail but do not retain it through fetal life). • The nares (nostrils) may be closed under water. • Vibrissae (long hairs) on the face are tactile organs which allow the rat to navigate confined spaces in darkness.

MEDSCI 142 LABORATORY GUIDE © 2018

Images C. Quilter, University of Auckland a

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3 Images C. Quilter, University of Auckland a

Examine the external Fig. 1 genitalia and determine if your rat is a male or a female, (see Figure 1). The presence of nipples does not necessarily indicate a female, since (as in humans) both sexes have them.

MALE Find the anus below the base of the tail. In the male, the scrotum, which contains the testes, will be seen near the anus. Anterior to the scrotum is the prepuce, a skin fold concealing the glans of the penis. Whereas in primates the penis is external to the body wall for most of its length, in rats (and many other mammals) the flaccid penis is contained within the abdominal wall.

FEMALE There are three openings: (1) anus (2) vaginal opening in front of the anus, and (3) urethral opening on the tip of a papilla anterior to the vagina.

I have a male / female rat.

Opening the body cavity

Images C. Quilter, University of Auckland a

The usual procedure for dissection and examination of the organ systems of a vertebrate is to lie the animal on its back (ie. ventral surface uppermost). Figure 2 shows the pattern of incisions. A mid-ventral incision is the usual starting point, but with males the incision in the pelvic region is made to one side of the mid-line to avoid cutting the penis. Note: 1. Use scissors, not a scalpel. Begin by pinching up a fold of belly skin at point “A” and making a short, deep cut. This must be extended downwards through muscle layers of the abdominal wall. Stop when you break through into a cavity and see the wet, glistening surface of the intestine. MEDSCI 142 LABORATORY GUIDE © 2018

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2. Extend the incision (“B”) by inserting one blade of the scissors (with a rounded tip) into the body cavity and cutting lengthwise. Keep the lower blade of the scissors elevated so that no internal organs are pierced in the process. Stop cutting anteriorly when you encounter resistance with your scissors; you have reached the diaphragm. 3. Two lateral incisions are then made anteriorly and two posteriorly (“C”), and the skin flaps folded back or removed. 4. These steps will expose the abdominal cavity but not the thoracic cavity which lies anterior to the diaphragm and beneath the rib cage. With a finger locate the diaphragm and then with scissors continue the mid-ventral incision anteriorly (“D”) through the rib cage and sternum. Keep the incision shallow as you cross the heart. Stop when you reach a point between the base of the forelegs. Now trim away the ribs on each side (“E”), being careful not to cut through the lungs. Finally cut along the lateral margins of the diaphragm. The heart and lungs should now be fully exposed. Images C. Quilter, University of Auckland a

Fig. 3

Body cavities Run your finger around the inside of the cavity containing the intestine. This wet, slippery surface is actually a very thin membrane bonded to (ie. lining) the abdominal wall. The membrane is called the peritoneum, and it surrounds the peritoneal cavity.

At first you may think that the intestine therefore lies within the Schematic transverse section through the abdominal cavity. The peritoneum is indicated by a dashed line. For clarity, only one cross-section of peritoneal cavity, but see Figure the intestine is shown, and the fluid-filled volume of the peritoneal cavity 3. This shows that the peritoneum is greatly exaggerated. lining the abdominal wall (called the parietal peritoneum) is continuous with a double layer of peritoneum (called a mesentery) suspending the intestine; and is further continuous with peritoneum (called the visceral peritoneum) covering the intestine. The relations of the peritoneum will be explained by your Lab Tutor, in later lectures, and can also be found in Tortora & Derrickson. 14th ed. 19, 890-891 13th ed. 18, 972 12th ed. 18, 927

In addition to the peritoneal cavity, there are three other ventral body cavities. Two of them, the pleural (lung) cavities have been opened when the ventral abdominal and thoracic walls were removed. However the pericardial (heart) cavity may be intact; if it is, you can use forceps to lift up the parietal pericardium from where it overlies the heart. It will be a very delicate, transparent bag which encloses the heart. The human pericardium is much thicker, stronger and more fibrous.

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From: “Essentials of Human Anatomy” RT Woodburne & WE Burkel,

Oxford University Press 1988. ISBN 0-19-504502-5 - original image copied under the CLL

This diagram of the human digestive system is provided for comparison with the rat, and to assist in identifying organs you encounter during dissection. Both Homo and Rattus are omnivores, so it is to be expected that the arrangement and proportions of their digestive systems will be similar.

Images C. Quilter, University of Auckland a

Lungs The rats used in this laboratory were killed humanely using carbon dioxide gas. Unfortunately this causes haemorrhage from small blood vessels in the lungs. It is likely that both lungs will be dark red and blood-filled; quite unlike their normal bright pink colour and spongy texture. Give the specific name of the membrane that lines the right lung.

Heart and thymus gland Later in the course you will be able to dissect a fresh sheep heart, so we will defer cardiac anatomy until then. The anterior end (“base”) of the heart is overlapped by the two lobes of the thymus gland. This is a lymphoid organ which has a role in the formation of T-lymphocytes. It also secretes several hormones important in the development of other lymphoid tissues.

What is inside the pericardial cavity?

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The digestive system: oesophagus to rectum

LABORATORY 1 Images C. Quilter, University of Auckland a

Most of the contents of the abdominal cavity belong to the gut. The gut tube in this region is convoluted in order to be long (about 5 times the body length of the rat); it is long in order to gain increased surface area for absorption and secretion. The gut has three tubular portions (oesophagus, small intestine, large intestine); and two sacs (stomach and caecum) where the gut contents temporarily come to rest. Absorption and secretion occur everywhere downstream of the oesophagus. In addition, secretion occurs in two large glands (liver and pancreas) which are derived embryologically from the gut lining and which shed their secretions into the small intestine. Does the oesophagus lie dorsal or ventral to the trachea?

Stomach The stomach, as in humans, lies mostly on the left side. It has a greater and a lesser curvature; the oesophagus enters part-way along the lesser curvature. Pull the stomach posteriorly until you can see the oesophagus emerging from a small hole in the diaphragm. The oesophagus travels from the throat through the thoracic cavity, dorsal to the heart and lungs, to the diaphragm. The lining of the rat stomach is divided into two distinctly different types. The anterior and lateral portion has a wall which is thin and translucent so that the gut contents are often visible. This part is specialised for food storage. Its lining resembles that of the oesophagus, with no glands and a protective stratified squamous epithelium. Note the human stomach does not have such a gland-less portion comparable to what you are observing in the rat. The pyloric region (medial and posterior) is glandular and opaque. Here numerous simple tubular glands release acid and enzymes which break large molecules down into small ones (digestion). A constriction at the outlet of the stomach indicates the pyloric sphincter, a collar of smooth muscle which controls the flow of stomach contents (“chyme”, pronounced “kyme”) into the first section of the small intestine, the duodenum. What are the respective functions of the transparent and opaque regions of the rat stomach?

Small intestine The main function of this part of the gut is the absorption of small molecules resulting from digestion. The small intestine is a tube of uniform diameter, about four body-lengths from inlet to outlet. Histologists divide it into three regions, (duodenum leaving the stomach, jejunum, and ileum entering the caecum) but these are hard to distinguish with the naked eye. The small intestine is irregularly coiled in order to allow it to fit within the abdomen. Lift up one or two coils; note that they are attached to the dorsal body wall by a transparent mesentery which is actually a double-layer of peritoneum (Figure 3). Blood vessels (often shrouded with fat) fan out through the mesentery to supply and drain the intestinal wall. The arteries carrying high-pressure oxygenated blood to the intestinal wall are two branches of the dorsal aorta (coeliac a. and superior mesenteric a.); the veins draining deoxygenated, nutrient-laden blood from the wall are tributaries of the hepatic portal vein which leads to the liver, (not directly back to the right heart as do most systemic veins). MEDSCI 142 LABORATORY GUIDE © 2018

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7 Images C. Quilter, University of Auckland a

Pancreas This gland releases an alkaline digestive juice containing enzymes (lipases, amylases, and proteases) into the duodenum. It is a “dual purpose” gland, also containing endocrine cells which secrete hormones (eg. insulin) into the bloodstream. In both rats and humans the pancreas sits close to the greater curvature of the stomach, with the pancreatic duct extending from the medial end of the gland towards the duodenum. The rat pancreas is a more diffuse organ than its human counterpart. To find it, lift up the stomach and look carefully in the mesentery posterior to the greater curvature. The pancreas is made of small, scattered pink lobules suspended by mesentery. The lobules resemble grain-like deposits of fat, but they are pink in colour (unlike fat which is white).

Liver The liver is the largest gland in the body, and like the pancreas functions as both an exocrine and an endocrine gland, (see T&D for definitions). It performs numerous functions concerned with metabolic regulation and as a by-product of these functions produces bile which contains mainly excretory products. The bile salts aid in digestion and absorption of fats and vitamins. In humans and other mammals bile is stored in a gall bladder whose duct (cystic duct) joins the bile duct to discharge into the duodenum. The rat lacks a gall bladder but like humans, the pancreatic ducts join the hepatic duct to discharge through a common duct into the duodenum a short distance downstream of the pyloric sphincter. You will probably not be able to see these tiny ducts. Note the large size of the liver, and the fact that it has (about) four lobes. The dark red colour indicates its rich blood supply (vascular), and the fact that its connective tissue capsule is very thin. It has a soft, jelly-like texture. The liver is vulnerable to traumatic injury, which causes severe internal bleeding. As already noted, the liver receives venous blood from the gut wall (via the hepatic portal vein). It also receives oxygenated blood from the aorta (via the hepatic artery, a branch of the coeliac artery). How do the pancreas and liver contribute to the digestive system, respectively?

Spleen (Lymphatic system) The spleen is NOT part of the digestive system, (it is part of the lymphatic system where immune responses occur), but you should look for it now. Lift up the stomach and look beneath its greater curvature, on the extreme left side of the abdominal cavity. The spleen is dark red, elongated and leaf-shaped. In colour and texture it resembles the liver, because both organs lack tough capsules and both have a generous blood supply. The human spleen is a more compact organ than in the rat, being fist-shaped rather than leaf-shaped, but it sits in the same position, (Figure 4). MEDSCI 142 LABORATORY GUIDE © 2018

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Large intestine

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Images C. Quilter, University of Auckland a

This region of the gut has four parts: caecum, colon, rectum and anus. The large intestine is concerned primarily with reabsorption of water and formation of faeces from the undigested food residue. Resident bacteria contribute to putrification of this material. Faeces are retained in the rectum and voided periodically through the anus.

Caecum A flabby, thin-walled bag within which the brown intestinal contents are easily seen. Handle it gently. Here bacteria act on the gut contents while they are slow-moving. The human caecum terminates in a vermiform (worm-like) appendix which contains lymphoid tissue, but rats lack a true vermiform appendix. What material in particular is broken down here?

Colon The rat colon is usually said to have three parts: one ascending (anteriorly) on the right, one transverse, and one descending (posteriorly) on the left towards the rectum. However these three parts form a rather untidy arc and are much less geometric than their human counterparts. Observe that the colonic contents upstream (leaving the caecum) are semi-fluid, but downstream (approaching the rectum) they are formed into more or less firm faeces as water is withdrawn. The presence of faeces makes the colon easy to distinguish from the small intestine in case of doubt. What is the key function of the colon?

Rectum A short passage leading to the anal outlet, only visible after extensive dissection which we will leave for the present.

Anus A sphincter under partial voluntary control which allows the faeces to be voided.

Kidneys (Renal system) Lift aside the intestine to search for the kidneys, buried in fat on the dorsal wall of the abdominal cavity. The ventral surface of each kidney is covered with parietal peritoneum; the dorsal surface is attached to the body wall. For this reason the kidneys are said to be “retroperitoneal” (= behind the peritoneum) but of course most abdominal organs (gut etc) are also “behind” the peritoneum in the sense that they are excluded by peritoneum from the peritoneal cavity, (Figure 3). Note the colour of the kidneys, indicating their rich blood supply. The depression (hilus) on the medial surface is the point where the renal artery, renal vein and ureter access the kidney; of these only the renal vein will be obvious.

Adrenal glands (Endocrine system) Small brown bodies embedded in fat near the anterior pole of both kidneys. The outer crust (cortex) secretes several steroid hormones; the inner core (medulla) secretes adrenalin. MEDSCI 142 LABORATORY GUIDE © 2018

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9 Images C. Quilter, University of Auckland a

Urinary bladder (Renal system) On the midline near the pelvis, pale cream or yellow in colour, and varying in size from a match-head (when empty) to a grape (when full). If you tug gently on the bladder, the thread-like ureters can sometimes be seen leading anteriorly towards the kidneys.

Male reproductive organs These will be covered in a later laboratory session devoted to the reproductive system. However you should identify two of the reproductive glands; they are the seminal vesicles (elongated, curved and with a hard lumpy texture), and the prostate gland (soft, pale-coloured, wrapped around the base of the urinary bladder).

Female reproductive organs The parts easily visible are the paired “horns” of the uterus. At ...