BIOL 204 W2024 Lab 6 TD1 PDF

Lab 6: The digestive and urogenital system Objectives By the end of this week’s lab, successful students will be able to: 1. Describe the structure(s) and function(s) of the digestive and urogenital systems. 2. Demonstrate how to correctly dissect and reveal the organs of the digestive and...

Lab 6: The digestive and urogenital system Objectives By the end of this week’s lab, successful students will be able to: 1. Describe the structure(s) and function(s) of the digestive and urogenital systems. 2. Demonstrate how to correctly dissect and reveal the organs of the digestive and urogenital system. 3. Compare and contrast the structures of the digestive and urogenital systems in different vertebrate lineages. 4. Discuss the relationship between each of the digestive and urogenital systems and the environment in which an organism’s native habitat. Important Terminology:  peritoneum, mesentery, peritoneal cavity, thoracic cavity,  pleuroperitoneal cavity, transverse septum  oral cavity (buccal cavity), pharynx, esophagus, stomach, rugae, small intestine,  duodenum, ileum, large intestine, pancreas, liver, gallbladder, bile duct, cardiac sphincter, pyloric sphincter, spiral valve, plicae, rectal gland, rectum, anus,  cloaca, spleen, crop, proventriculus, gizzard, colic caeca, caecum  viviparous, euviviparous, ovoviviparous, oviparous, gonads, ovaries, testes,  archinephric duct, oviduct (Müllerian duct), ostium, claspers, uterus, vas deferens, scrotum, epididymis, penis, vagina  accessory urinary duct, urinary bladder, opisthonephric, mesonephric,  metanephric, kidney, ureter, urethra, adrenal gland Body cavities and mesenteries Cavities in the body are spaces that house and offer protection to the organs and tissues within them. Vertebrates have a dorsal body cavity that encases the nervous system (brain and spinal cord) and a ventral body cavity that is further subdivided. In dogfish, the pericardial (anterior) cavity is separate from the more posterior pleuoperitoneal cavity by the transverse septum. In mink and pigeons, the anterior thoracic cavity is separated from the posterior peritoneal cavity by the muscular diaphragm. In mammals and birds, the thoracic cavity is also divided further into a medial pericardial cavity (containing the heart) and two lateral pleural cavities (containing the lungs). The thoracic and peritoneal cavities of mammals are separated by a muscular diaphragm, which we will examine with the respiratory system. Most body cavities are lined with a membrane that encloses the cavity, separating it from other cavities and from other tissues of the body. These membranes are double-layered: the parietal layer lines the wall of the cavity while the visceral layer covers the organs in the cavity. The parietal and visceral layers are continuous with one another and the areas where they connect to one another fuse together to form mesenteries, which allow blood vessels and nerves to innervate the organs of the cavity. 81 In the mink, digestive tract there are two specialized mesenteries, called omenta (sing. omentum), that extend from the stomach. The greater omentum extends from the greater curvature of the stomach and over the intestines, helping to hold the stomach in place. It also contains a large number of fat deposits and lymph nodes. The digestive system The digestive system is responsible for collecting nutrients from our environment in the form of food and breaking the food down into pieces that are small enough to be absorbed by our bodies. Material that cannot be absorbed passes leaves our bodies via defecation, making this system essentially a tube that runs through our body, Food enters the digestive system in the oral (buccal) cavity or mouth and then travels through the pharynx into the esophagus, which transports food to the stomach. As you swallow, the epiglottis covers the opening to the trachea, the glottis, to prevent food from entering the lungs. In the stomach, mechanical and chemical digestion continues before the food mixture moves through a sphincter into the intestines before exiting the body through the cloaca or anus. The digestive system also includes a number of accessory organs, such as the liver, gall bladder, and pancreas, which arise from the embryonic digestive tract and produce secretions that help with digestion. The urogenital system The urogenital system consists of the reproductive and excretory (or urinary) systems, which are often studied together, due to their common location in the caudal region of the body and their structural links. The reproductive system is responsible for producing the gametes that are required to produce the next generation of the species. In the gonads, meiosis occurs to produce the gametes: male testes produce sperm and the female ovaries produce ova. These haploid gametes will combine during fertilization to produce a diploid zygote that will give rise to a new organism, inheriting traits from each of her or his parents. The features of the reproductive system thus support one or more of these functions: producing gametes, exchanging gametes, or nurturing the developing offspring. Oviparous vertebrates lay eggs outside of the body, where they are fertilized and develop externally. Viviparous vertebrates give birth to free-living offspring, typically as a result of internal fertilization. Viviparous vertebrates will either be euviviparous, whereby offspring obtain nutrients from the mother via a placenta, or ovoviviparous, whereby offspring obtain their nutrients from a yolk within an egg capsule that remains in the reproductive tract of the mother. The excretory system removes metabolic wastes (typically nitrogenous wastes) from the body and balances the levels of fluids and dissolved molecules in the blood, thus regulating the pH and osmotic balance of the blood (osmoregulation). The two kidneys are located either side of the aorta and vena cava along the dorsal wall of the abdominal cavity and are surrounded by fat. You will see large blood vessels feeding into and out of the kidneys, as these organs filter the blood. This blood volume flows through a large number of capillaries that wrap around the nephrons of the kidney, removing metabolic wastes, including nitrogenous compounds, such 82 as urea. These wastes pass through the ureter and are collected in the bladder, where they are stored until excretion through the urethra. Dissections Dogfish Digestive system 1. Using scissors, cut just lateral of the ventral midline from the cloaca to just before the transverse septum. From each end of the incision, cut laterally out to towards the pectoral and pelvic fins and deflect the flaps to open up the pleuroperitoneal cavity. You should see the falciform ligament (a remnant of the ventral mesentery) in the anterior region of the cavity, extending from the midventral region of the liver. In order to see the interior of the cavity, you will likely have to cut this ligament, but wherever possible, try to leave the mesenteries intact. You will also be able to see the lesser omentum over the liver, stomach and small intestine, the greater omentum over the esophagus and stomach, the reproductive mesentery over the reproductive organs, and the dorsal mesentery over the intestines. In the oral cavity, you will see the tongue, teeth, and pharynx, which begins past the spiracle, gill slits, and tongue. 2. Cut through the left jaw, along the centre of gills to the anterior border of the pectoral fin. Use a paper towel to cover the teeth and extend the incision through the pharynx, so that you can open the entire mouth. 3. From the end of the incision above, cut horizontally across the body to other pectoral fin. Keep your incision posterior of the transverse septum, so that you do not open the pericardial cavity and continue through the esophagus. Open the jaw and remove any debris. 4. Cut into esophagus to see the papillae (projections which help keep water out of the digestive system) and into the stomach to observe the rugae (longitudinal folds). The dogfish stomach is shaped like a “U”, with an anterior cardiac limb and a posterior pyloric limb. The pyloric sphincter separates the stomach from the intestines. The anterior region of the intestines is the duodenum, while the posterior region is the ileum (Figure 6.1). 5. Cut into the ileum to see the spiral valve which separates the duodenum from the ileum. The large intestine of the dogfish is shorter than the small intestine and is connected to the rectal gland, which functions in salt excretion (see below). The posterior end of the large intestine is the rectum, which ends in the anus and then projects into the cloaca. 83 Digestive organs The dogfish liver is comprised of three lobes that extend posterior from the transverse septum. You should be able to see the gall bladder as a greenish gland on the central lobe that is connected to the duodenum via the bile duct. The pancreas has two lobes: a dorsal lobe that lies between the pyloric stomach and the duodenum and a ventral lobe that extends over the duodenum. You will also be able to see the spleen posterior from the stomach. While the spleen is not part of the digestive system (it functions in the circulatory system), its location here merits its mention. Figure 6.1. Digestive system of Squalus acanthias. Image courtesy of UBC Vancouver. Excretory system The opisthonephric kidneys of the dogfish are flat, grey bands found along the dorsal body wall, behind the peritoneum. Dogfish maintain high levels of urea in their system, making their bodies hyperosmotic with respect to their saltwater environment, preventing water loss. They do still 84 regulate salt levels in their bodies, by excreting excessive salt via the rectal gland, which empties into the large intestine (Figure 6.2). In the female, the archinephric ducts carry urine from the kidney to the cloaca, while the accessory urinary ducts perform the same function in male dogfish. Reproductive system In the male dogfish, the testes are found along the dorsal body wall and may resemble liver lobes. The testes are connected to the vasa efferentia and the archinephric duct along the top of the kidney, before widening and straightening into the seminal vesicle. The seminal vesicle empties into the sperm sac, which stores the mature sperm until they are released into the cloaca. The external claspers function during internal fertilization. Note that the urine bypasses the archinephric ducts through the accessory urinary ducts. In the female dogfish, the two ovaries are located anterior of the kidney and resemble lobes of the liver. The ovaries release eggs into the body cavity that will enter the ostium (opening) of the oviduct, which are silver-coloured tubes that lay on top of the kidneys. The shell gland of the oviduct provides the albumen and the egg case to the eggs, which will develop inside the reproductive tract for 20-24 months before giving birth, as dogfish are ovoviviparous. 85 Figure 6.2. Urogenital system of Squalus acanthias. Image courtesy of UBC Vancouver. Mink Digestive system Before starting your dissection, examine the head of your mink and locate the salivary glands (parotid and mandibular or submaxillary). In the mouth, take note of the differentiated teeth and the papillae on the tongue. 6. Use your probe to feel the roof of the oral cavity and the hard and soft palates. Posterior of the mouth is the pharynx and the larynx, which then connects to the esophagus, which carries food to the stomach via peristalsis. Due to its location in the thoracic cavity, we will examine the esophagus next week. 7. Using your scissors, make an incision just lateral of the midventral line from just posterior of the sternum to ~3-5 cm anterior of external genitalia (penis in males, vagina in females). From each end of this incision, cut laterally to create flaps and deflect these 86  flaps to be able to see the peritoneal cavity. The esophagus is separated from the stomach by the cardiac sphincter, while the pyloric sphincter separates the stomach from the duodenum of the small intestine. The duodenum forms a long curve, with ducts entering it from the accessory organs of the digestive system (see below). Further along the small intestine are the jejunum, which loops back and forth as it descends (moves posterior), and the ileum, which is the most posterior and loops as it ascends. The ileum connects to the large intestine or colon to form a T-intersection with the blind end of the large intestine forming the caecum and food moving through the ascending, transverse, and descending regions of the colon, before ending in the rectum and the anus. Digestive organs The liver of the mink consists of five lobes, with the gall bladder located within the middle of the liver, close to the bile duct that empties into the duodenum. The pancreas of the mink has two lobes that resemble bubble gum, with one extending past the pyloric sphincter towards the spleen and the other laying along the duodenum. The spleen can also be seen in the mink to the left of the stomach and functions in the lymphatic system Excretory system The metanephric kidneys of the mink are bean-shaped organs located on the dorsal wall of the abdomen on either side of the aorta and vena cava that run down the midline of the body. From their centres, the ureters run down towards the urinary bladder in the pelvis, which itself empties into the urethra that excretes the urine outside of the body (Figure 6.3). 8. Use a scalpel to cut longitudinal section of kidney, separating it into two halves. Compare the kidney to the models on display and note the different layers: the outer renal cortex, where the glomerulus and Bowman’s (or renal) capsule of the nephron are located, the inner renal medulla, where the loop of Henle (or nephron loop) extends, and the central renal pelvis, where the urine drains into the urethra. 87 Figure 6.3. Mammalian urogenital system. Image courtesy of UBC Vancouver. Reproductive system Depending on the amount of damage done to the body of the male mink during the skinning process, you may or may not be able to find the testes, which are typically held within the external pouch of the scrotum. On the dorsal surface of each testis is the epididymis, which receives the sperm from the testis and is the site of sperm maturation and storage. During fertilization, the sperm will travel along the vas deferens and into the urethra before exiting the body through the penis. In the female mink, the two ovaries are located posterior of the kidneys within the adipose tissue along the dorsal body wall and are held in place by mesentery. Eggs are released into the body cavity and pass through the ostium (opening) into the oviduct, which carries the egg to the uterus. Mink have two uterine horns that join at the main body of the uterus. At the posterior end of the uterus is the cervix, which is the passage to the vagina. The vagina, as well as the urethra, are both connected to the urogenital sinus, which exits the body. 88 Pigeon Digestive system 9. Locate the posterior of the sternum using your fingers. Keeping the tips of the scissors pointing up and away from the body and make a small incision in this thin muscle. Use your fingers to widen this incision and then cut laterally from each edge from below the sternum to the leg. 10. Gently hold one hand over the lower portion of the body and use your other hand to lift the sternum. This may crack some of the ribs. 11. Lift the posterior edge of the sternum. Take care not to damage the organs under the ribs and carefully use the scissors to cut the ribs towards the base of the wing on each side. You will be able to lift the sternum to view the internal organs. 12. From the ventral view, the two lobes of the liver will cover most of the other organs. Make sure to show the liver to students dissecting other organisms in the lab and then carefully remove the liver. The intestine will be visible at the posterior of the abdominal cavity. Gently pull it out and follow towards the posterior end. From posterior to anterior, you will be able to find (see also Figure 6.4):  The expanded cloaca,  A relatively short large intestine,  Small, paired colic caeca on the large intestine that harbor bacteria that break down cellulose in the pigeon’s diet,  A longer, thinner small intestine,  A long, flat, brownish pancreas within the anterior most loop of the small intestine (this is the duodenum), and  The round, hard gizzard, which carries out both chemical digestion via enzymes, and mechanical digestion via swallowed pieces of gravel. 13. Use your scissors to cut through the gizzard wall. Observe the digestive surface and gravel within.  The proventriculus, anterior of the gizzard, is the anterior chamber of the stomach and secretes hydrochloric acid and digestive enzymes,  The spleen, which lies between the proventriculus, gizzard and right lobe of the liver, can be seen by rotating the proventriculus. 89 Figure 6.4. Digestive system of Columba livia. Image courtesy of UBC Vancouver. Excretory system Along the dorsal walls of the body cavity, you will be able to see the two metanephric kidneys. In pigeons, each kidney is separated into three lobes by blood vessels (Figure 6.5). The adrenal glands can be seen anterior of the kidneys. The ureters (metanephric ducts) extend from the medial side of the kidneys, starting between the anterior and middle lobs and running straight posteriorly to connect to the cloaca. In females, the left ureter is located dorsal of the oviduct. Reproductive system In each sex, the gonads are located at the anterior end of the kidney. In males, the lightly-coloured, paired testes are bean-shaped. Their size varies with age: as the individual ages and becomes mature, they become larger. In younger individuals, the testes may be difficult to find. 14. Use your forceps to gently move one of the testes. The vas deferens will be seen as a convoluted white tube connecting the testis to the cloaca. It runs alongside the straight ureter. Near the posterior end, the vas deferens becomes the seminal vesicle. 90 In female pigeons, the ovary can be found at the anterior end of the left kidney, as only the left oviduct and ovary are functional. The ovary is an irregular-shaped mass, containing follicles at different stages. Like the testes in the male, the ovary in younger females may be harder to discern. An ovum released by a follicle into the funnel-shaped ostium then travels down the oviduct from the ovary to the cloaca. As the ova travels along the oviduct, albumen, membranes, and shell are applied to produce a complete egg. Figure 6.5. Urogenital system of Columba livia. Image courtesy of UBC Vancouver. 91 Lab 6 assessment: Verbal quiz 9 marks; groups of 2-3 To be completed during next week’s lab period For this week’s lab, you will be assessed on the structures that you have identified in the dissections completed by your lab group (both the dogfish and the mink) during next week’s lab period. 1. You and your lab partner(s) will be asked to identify the following: a. ONE (1) structure in each organism’s digestive system (1 mark each; 3 marks total) b. ONE (1) structure in the male reproductive system of each organism (1 mark each; 3 marks total); and c. ONE (1) structure in the female reproductive system of each organism (1 mark each; 3 marks total). Discussion questions to help you prepare for the lab exam: Digestive system:  Compare and contrast the structures of the digestive system in each organism. Provide at least TWO (2) similarities and TWO (2) differences. o Discuss how TWO (2) of the differences that you noted in the digestive system of dogfish, mink, and pigeon are well adapted for those organisms and/or the environments that they live in. (6 marks) Urogenital system:  Compare and contrast the structures of the male urogenital systems of dogfish, mink, and pigeon. Provide at least TWO (2) similarities and TWO (2) differences. (6 marks) o Discuss why TWO (2) of the differences that you noted in the male urogenital system of dogfish, pigeon, and mink are well adapted for those organisms and/or the environments that they live in. (6 marks)  Compare and contrast the structures of the female urogenital systems of each organism. Provide at least TWO (2) similarities and TWO (2) differences. (6 marks) o Discuss why TWO (2) of the differences that you noted in the female urogenital system of dogfish, pigeon, and mink are well adapted for those organisms and/or the environments that they live in. (6 marks) 92