VERTEBRATE FORM AND FUNCTION – RAT DISSECTION

Purpose:

      1.  To study the structural anatomy of a vertebrate (rat).  
      2.  To locate and identify the principal organs of the digestive, respiratory, circulatory and urogenital systems of  the rat.  To demonstrate differences between male and female rats.  
      3.  To explain the general pattern of circulation in the rat and other mammals.  

Introduction:

      The white laboratory rat is often studied as a representative mammal because it exhibits most of the organs and organ systems typical of mammals.  Among the distinguishing characteristics of mammals are a body surface covered with hair, an integument (skin) with mammary glands, seven cervical (neck) vertebrae, teeth on bony jaws, movable eyelids, fleshy external ears (pinnae), a four chambered heart and a muscular diaphragm separating the thoracic and abdominal cavities.  Mammals are endothermic (use metabolic heat to maintain a constant body temperature).  The young (except monotremes) develop within the uterus of the female, have a placental attachment for nourishment and are enveloped by special fetal membranes (amnion, chorion and allantois).  Milk to nourish the young after birth is produced by mammary glands.  

            Classification

                        Phylum        Chordata

                        Subphylum   Vertebrata

                        Class            Mammalia

                        Order            Rodentia

                        Family            Muridae

                        Genus            Rattus

                        species          norvegicus

      Many instructions for dissection use anatomical terms to indicate direction and spatial relationships when the animal is in normal orientation.  You should know the meaning of such terms as:   

            anterior – situated near the head

            posterior – toward the animal’s hind end (opposite of anterior)

            distal – located away from the center of the body

            proximal – located near the center of the body (opposite of distal)

            dorsal – pertaining to the back

            ventral – pertaining to the belly or lower surface (opposite of dorsal)

            left – animal’s left

            right – animal’s right

External Anatomy   (Figure 1)

      Select a preserved specimen and place it in a dissecting tray.  Observe the two principal features that distinguish the rat as a mammal:  hair covering most of the body surface and paired mammary glands.  Locate the nipples or teats on the ventral surface of the trunk between the forelimbs and hindlimbs.  How many pairs are present?  Are they present on both males and females?

      The body of the rat is divided into an anterior head connected to a cylindrical trunk by a short thick neck.  A long tail extends posteriorly from the trunk.  Interiorly, the trunk is divided by a muscular diaphragm into an anterior thorax and a posterior abdomen.  The diaphragm is another distinguishing feature of mammals; it is a muscular sheet that divides and separates the coelom into the paired anterior pleural cavity and the posterior abdominal cavity.  The diaphragm is lacking is birds, reptiles and other lower vertebrates.

      Observe the long tail – How does its external surface differ from that of the remainder of the body?

      The head is cone-shaped with an elongate face.  On the head, find the two eyes and two ears.  The eyes have upper and lower eyelids and a reduced third eyelid or nictitating membrane, which can be found on the medial portion of the eye opening beneath the two outer lids.  Note that the ear has an external fold of tissue called the pinna.  The pinna aids in directing sound waves to the opening of the ear (external auditory meatus).  Other important sense organs located on the head are the vibrissae – long, sensory hairs that provide the rat with a very effective sense of touch.  The vibrissae can tell a rat in an instant if a hole is large enough for it to crawl into.  Note that most of the vibrissae are attached to the upper lip.

      Observe the well-developed upper and lower lips surrounding the mouth.  Locate the two external nares above the upper lip.  Inside the mouth find the two, long sharp incisor teeth that are characteristic of rodents.  These two incisors grow continuously, and the rat wears them down by gnawing on various objects and food.  Molars (grinding teeth) are found farther back in the jaw.

      Observe one of the forelimbs and note that the limb consists of an upper portion, a lower portion, and a hand-like portion much like the structure of your own arm.  Most of the bones in the rat forelimb are homologous with those of a human.  Locate the horny claws at the tip of each digit and the walking pads in the palm area.  These pads are less well-developed in the rat than they are in the cat and cat-like animals because rats have evolved a tendency to walk on the digits rather than on the entire palm or sole.

      Near the base of the tail locate the anus.  You can determine the gender of your specimen by studying the adjacent urogenital structures.  Male rats have a scrotum ventral to the anus that holds the two testes during reproductive season.  (At other times the testes are suspended within the abdominal cavity, and the scrotum is an empty sac.)  Anterior to the scrotum is the penis with the opening of the male urogenital system at its end.  The penis is usually withdrawn into a sheath of skin, the prepuce.  If you have a female rat, find the vaginal opening ventral to the anus and the separate opening of the urethra.

Internal Anatomy

Mouth and Pharynx    (Figure 2)

      To expose the oral cavity, cut through the muscles and tissue suspending the jaw on one side.  Use heavy scissors (or bone cutters) cut the bones at the corner of the jaw on the same side.  Pry it open and locate the following structures.

      The anterior part of the tongue is attached ventrally by a thin sheet of tissue (frenulum).  Rats have two types of teeth – incisors (4 cutting teeth) and molars (12 grinding teeth).  How many of each type to you find?  Find the hard palate covering the anterior part of the roof of the oral cavity and the soft palate covering the posterior portion.  The oral cavity ends and the pharynx begins at the base of the tongue.  The pharynx is a common passageway for the digestive and respiratory tracts.  On the floor of the pharynx find the glottis, a slit-like opening into the trachea (wind-pipe), which is covered by a small flap of tissue, the epiglottis.  Opening into the pharynx are the internal nares (nasal chamber) and the eustachian tubes (connect with the middle ears).

Other internal organs

     To study other organs of internal anatomy cut through the ventral muscles to expose the organs within the abdominal cavity.  Make an incision through the muscles just to the right of the ventral midline.  Start just anterior to the anus, and continue anteriorly to the neck region.  Cut through the pectoral girdle and pull apart the ribs in the chest region to expose the heart and lungs.  Make two transverse cuts through the skin and superficial muscles on each side of the pectoral girdle and another pair of cuts in front of the pelvic girdle.  Take care to not damage the organs within the abdominal cavity beneath the muscles.  Free the diaphragm from the body wall and pin aside the skin and body wall to expose the abdominal organs. 

Respiratory System   (Figure 3)

      During ventilation air enters the external nares and passes through the nasal passages downward through the glottis into the larynx.  Air is filtered and warmed as it passes through the nasal passages.  Vocal cords in the larynx allow rats to make audible sounds.

      From the larynx, air passes to the trachea.  The trachea is a hollow tube supported by a series of cartilaginous rings.  Trace the path of the trachea from the larynx to the primary bronchi and into the lungs.  The primary bronchi enter the lungs and branch to many bronchioles and terminate in the many alveoli where gas exchange occurs.

Digestive System   (Figure 4)

      The digestive system consists of the mouth, oral cavity, pharynx, esophagus, stomach, small intestine, large intestine and rectum, and terminates at the anus.  The esophagus is a collapsible tube leading from the pharynx to the stomach (dorsal to the trachea).  It enters the stomach on the lesser curvature (concave surface).  Trace the path of the esophagus to its entrance into the stomach.  The stomach lies beneath the liver on the left side of the abdomen.  The stomach consists of a storage area, a glandular portion that secretes mucus, hydrochloric acid and pepsin, and a posterior region (pyloric region) which controls passage of food into the small intestine.  Food leaves the stomach as a fluid suspension, chyme.  It enters the duodenum, the first part of the small intestine.

     Remove the stomach by cutting the esophagus and duodenum.  Slit the stomach lengthwise, cutting through the cardiac and the pyloric sphincters, muscles that regulate passage of material into and out of the stomach.  The internal surface of the stomach is covered by gastric mucosal cells which secrete mucus that prevents the stomach from digesting itself.  When the protection fails, a peptic ulcer forms.

      The small intestine consists of three parts.  The duodenum is most anterior; it receives ducts from the liver and pancreas, and is where most digestion occurs.  The jejunum is much shorter and connects to the posterior portion of the small intestine, the ileum.  The ileum empties into the large intestine (colon).  Find the ascending, transverse and descending colon.  The colon is important in the resorption of ions and water from the contents of the gut.  The terminal portion of the colon is the muscular rectum that leads to the anus.

     Cut out a 2 cm section of the small intestine, slit it open and place it under water in a dish.  Use the dissecting microscope to observe the velvety internal lining made up of numerous finger-like projections called villi.  The villi are highly vascularized, containing capillaries that transport the products of digestion to the liver for processing.

      The liver has four lobes.  Bile ducts from each lobe connect to the duodenum.  A gall bladder is typically absent or reduced in rats.  Anterior to the liver is the muscular diaphragm, which separates the abdominal and thoracic cavities.  The pancreas is not a discrete organ in rats and consists of patches of tan or pinkish tissue embedded in the mesentery between the duodenum and stomach.

      Two accessory organs are the spleen and the thymus.  The spleen is a dark, elongated structure attached to the mesentery along the greater curvature of the stomach.  The spleen stores red blood cells and is important in the immune system.  The thymus (if present) is adult rats consists of a small mass of tissue on the ventral surface of the trachea, anterior to the heart.  It is most well developed in young rats where is serves to produce lymphocytes.  It decreases in size with age.  Its main function is in immunity and defense against infections.    

Circulatory System

      The circulatory system of the rat is typical of all mammals, including humans.  There are two main features:  a four-chambered heart and two separate division for circulation – a systemic circuit and a pulmonary circuit.

      The systemic circulation carries oxygenated blood from the heart  (Figure 5) through the branching arteries to most organs and tissues in the body, collects deoxygenated blood from these organs and tissues via a system of capillaries and veins and returns the blood to the heart.  The pulmonary circulation carries deoxygenated blood from the heart to the lungs to be replenished with oxygen at the alveoli and brings oxygenated blood to the heart.  In mammals, deoxygenated blood from the body enters the right atrium by the large vena cavae.  From the right atrium blood enters the right ventricle and is pumped to the lungs via the pulmonary arteries.  Oxygenated blood from the lungs travels through the pulmonary veins to the left atrium of the heart.  From the left atrium, the blood goes to the left ventricle and is pumped through the aorta and its arterial branches to all parts of the body.  We will look more closely at the anatomy of the heart next week.

     Carefully remove the pericardial membrane surrounding the heart.  Locate the two small auricles (auricles are external ear-like flaps) (atria = internal chambers) lying on the surface of the ventricles.  Note that the ventricles are not equal in size – which is larger?  Why?  Extending anteriorly from the heart locate the large aorta.  The aorta exits from the left ventricle, curves to the left and passes dorsally to the heart.  This large curvature forms the aortic arch.  The coronary, brachiocephalic, left common carotid and left subclavian arteries branch from the aortic arch.  Posterior to the heart the aortic arch continues as the dorsal aorta, which runs along the dorsal wall of the coelom.  Along its course to the posterior end of the animal the dorsal aorta branches to several organs and tissues.  Blood in the systemic circulation returns to the heart via the vena cavae.  The cranial (anterior) vena cava drains the head, neck, forelimbs and thoracic regions.  The caudal (posterior) vena cava is a single vein extending from the posterior end of the abdominal cavity to the heart.  Locate the attachment of the large veins to the heart.  After passing into the right ventricle, blood is pumped to the lungs through the pulmonary trunk, which divides into the left and right pulmonary arteries.  The trunk is visible passing from bottom right to upper left over the front of the heart and passing between the two atria.  Following gas exchange in the lungs, blood returns to the heart via the pulmonary veins and enter the left atrium.

Urogenital System

      The excretory and reproductive organs of the rat are closely related and are considered together as components of a single system – the urogenital system.  The primary components of this system are the kidneys, the ureters, the urinary bladder and the urethra.  The kidneys are located on the dorsal wall of the abdominal cavity.  A thin layer of tissue separates them from the abdominal cavity.  Attached to the surface of each kidney is a thin reddish tissue, the adrenal gland.  These glands produce several important hormones.  The kidneys primarily serve to concentrate nitrogenous wastes of metabolism and produce urine.  Urea is actually produced in the liver and transported to the kidney via the blood.  The concentration of wastes and the production of a concentrated urine are important water conservation adaptations of rats and many terrestrial animals.

     Cut through the peritoneum covering one kidney, remove the fat deposits around the kidney and make a longitudinal section with a razor blade or scalpel.  Use the dissecting microscope to observe that the kidney tissue consists of an outer cortex, an inner medula (darker in color) and a central area (the pelvis).  Notice the kidneys’ proximity to the dorsal aorta.  The blood pressure drops very little as blood passes from the aorta into the kidneys via the renal artery.  A high pressure is essential to force-filter the blood through the walls of the glomerular capillaries in the renal cortex.

      The inner concave surface of the kidney is where the ureter and blood vessels attach.  Try to identify the renal artery and renal vein adjacent to the large ureter.  The ureter carries urine to the urinary bladder.  From the urinary bladder, urine is carried in the urethra out of the body.  Trace the urethra from the urinary bladder to its external opening adjacent to the clitoris (female) or to the penis (male).

      Reproductive organs.

     To study the reproductive systems, probe through the muscle between the legs to locate the pubic symphysis, the portion of the pelvic girdle that fuses in a position ventral to many of the reproductive structures and the rectum.  Be careful not to go too deep – use scissors to cut through the muscles and the symphysis.  Press the hind limbs apart and trim the ends of the symphysis.  Use a probe to remove connective tissue and expose the urethra.

      MALE  (Figure 6)  Sperm are produced in the testes.  During breeding season, the testes are located in the scrotum (a large sac located ventral to the anus).  At other times, the testes are usually retracted into the posterior part of the abdominal cavity.  Locate the testes in the scrotum or the abdominal cavity.  Carefully cut through the connective tissue surrounding one testis to find the epididymis (a tightly coiled mass of tubules attached to the surface of the testis where sperm mature).  A smaller tube, the vas deferens, carries sperm from the epididymis to the urethra.  Follow the vas deferens from one testis and try to find where it joins with the vas deferens from the other testis to empty into the urethra.  The urethra carries sperm from the two vas deferentia to the penis, from which it is deposited in the vagina of the female during copulation.  The sperm are suspended in the seminal fluid which is produced from secretions from associated glands.  The paired seminal vesicles and single prostate gland are located near the junction of the vas deferens and the urethra.  The large bulbourethral glands lie posteriorly.  These three glands secrete additional (seminal) fluids that carry sperm during an ejaculation.

      FEMALE  (Figure 7)      Eggs (ova) are produced in paired ovaries found just posterior to the kidneys.  The ovary are suspended by two sheets of connective tissue.  The oviducts in the rat are small, short tubes that lead from the funnel-shaped ostia where ova are released from the ovaries.  The oviducts connect with the two branches of the uterus.  The uterus consists of two large branches (or uterine horns) that open separately into the vagina (or birth canal).  The uterine horns represent a morphological adaptation that allows the simultaneous development of several embryos and multiple births.  The vagina leads from the body of the uterus to the exterior and receives sperm during copulation.  Mature ova descending through the oviducts are fertilized in the uteri of the rat.  The zygotes (developing embryos) are subsequently implanted into the uterine wall.  The gestation period of the white rat is about 21 days.  A litter averages between 6-8 pups.

      Since you are physically dissecting the reproductive system of only one gender, you should observe the dissection of a rat of the opposite sex and become familiar the various structures of both.