Download Fetal Pig dissection lab

Names: ______________________________________________
Case#________
______________________________________________
Tray#________
FETAL PIG DISSECTION
Background:
Pigs are mammals in the order Artiodactyla, along with animals such as cows and deer. The fetal
pigs we use are byproducts of the pork meat-processing industry and would have been thrown away if not
used for labs such as ours. Our choice of study animal is of particular interest because it retains some fetal
attributes, such as 1) modifications to the circulatory system to facilitate gas and nutrient exchange with the
mother and 2) immature stages of the reproductive anatomy.
Safety and Clean up:
Wear gloves, apron, and goggles throughout lab and clean up. It is advisable to wear closed toed
shoes, too. Scalpels are sharp!! Be extra careful when cleaning the scalpel.
You and your partner are responsible for keeping the dissection tools and tray clean! You are to
use your assigned tools and tray and keep them clean. I will check tools and trays randomly and if they
are not clean you will lose points. The dissecting tools must be washed and dried so they don’t rust. The
scalpel blade might rust anyway. Obtain a new one if necessary.
Make sure you store your fetal pig in its plastic bag with the top of the bag twisted and rubber
banded. This will help keep the smell down and prevent your pig from drying out. Keep your pig in the
assigned box each time.
You must do a very careful dissection to see the structures and organs in this lab. It is
easy to crush or accidently remove important structures before you realize what they are. Avoid
this by reading instructions completely before proceeding, looking at diagrams provided, and not
cutting any more than necessary- use the blunt probe, forceps, and needle whenever you can,
then scissors. Use the scalpel only as a last resort.
External Features, Planes, and directions:
Directional Terms:
left and right
anterior (cranial)
posterior (caudal)
superior
inferior
dorsal
ventral
medial
lateral
proximal
distal
stated relative to the subject, not relative to people viewing it
towards the front (head) of the animal
towards the back (tail) of the animal
higher on the subject- typically used on bipeds such humans
lower on subject- also used on subjects such as humans
towards the back
towards the belly
towards the mid-sagittal section
away from the mid-sagittal section
towards the mid-sagittal section along a limb
away from the mid-sagittal section along a limb
External features: Observe your pig carefully and be able to identify structures. Know which part
is dorsal, caudal and so on because this will help you find structures and dissect properly.
____Pinna: external ear
____Nares: nostrils
____Eyes: closed, upper and lower lids
____Anus: exterior, posterior, opening to the digestive tract
____Umbilical cord: connected fetus to placenta while in uterus
____Mammary papillae: nipples, on ventral surface on either side of umbilical cord
____Urogenital opening: where urine exits body
Measurement, age, sex:
Estimate gestational age (time from conception) of pig by measuring length from tip of snout to
base of tail.
__________________________ cm
10 weeks
14 weeks
15 weeks
16 weeks
17 weeks
=
=
=
=
=
~18cm
~20cm
~22cm
~28cm
~30cm
________________________ approximate gestational age
The sex of the pig can be determined from external structures. Both males and females have
mammary papillae or nipples on the ventral surface so that cannot be used to determine sex. In both
sexes the anus is located just beneath the tail. However, the urogenital opening varies between the sexes
and that is how sex is determined.
Urogenital opening:
Males: posterior to where the umbilical cord attaches to the belly. This is the orifice from which the male will
urinate and is also where the penis (normally held internally under the skin posterior to the opening) protrudes
when erect. The male also has scrotal sacs anteroventral to the anus. The loose flaps of skin will contain the
testes from just before birth through adulthood.
Females: urogenital papilla just anteroventral to the anus, from which she would urinate. This is also the
opening to her reproductive tract.
Sex of your fetal pig is _______________________
*****Examine a pig of the opposite sex!!*******
Be sure you can identify a male or female pig from external structures!
Umbilical Cord: contains blood vessels that connected fetus to the placenta of the mother. Examine the cut
end of the cord. You should be able to see two arteries and a vein. Sometimes the vein is collapsed and
difficult to see. Use your scissors to make a fresh cut though the cord about 1 cm from the body and see if the
vein is more visible. Draw a cross section of the umbilical cord below. . The umbilical vein travels directly
down into the body cavity, while the umbilical arteries travel posteriorly, staying associated with the
body wall. The lungs do not function in the embryonic mammal. Instead, all gas and nutrient
exchange occurs at the placenta. Although it has been removed from your fetal pig, the placenta is a
part of the embryo (not the mother), and is a large, pancake-shaped structure at the terminus of the
umbilical cord. The blood of the mother and embryo do not normally mix, but are very close together
and diffusion is rapid. The umbilical vein carries nutrient-laden, oxygenated blood back to the
embryo, and the umbilical arteries bring deoxygenated, nutrient-depleted blood to the placenta. Note
that usually this is the other way around: most arteries carry oxygenated blood, while veins carry
deoxygenated.
Feet: Examine the feet. How many toes does the pig have? _______________.
To keep the pig positioned properly in the tray tie a different piece of string around the “wrist” or “ankle” of
each leg of the pig. Position the pig dorsal side down and ventral side up. Wrap each string around the
plastic pegs on the dissecting trays. Pull the cord fairly tight so that the legs are spread apart but not so
tightly that the skin is damaged. When you remove your pig from the tray each day, just unwrap the string
from the pegs. Don’t untie the string!
Beginning the Dissection
Make a small incision with the scalpel through the skin just anterior (cranial) to the umbilical cord, just
big enough that you can insert a pair of scissors (rounded blade beneath the skin). See Figure 2, the large
dot near umbilicus.
Now cut the skin midsagittally and anteriorly to a point just behind the small, hairy papilla on the upper
part of the throat. This is the “hair on its chinny, chin, chin”. Note that the tough skin is relatively thin so If you
are trying to cut into something stiff and hard (bone) or brown and fibrous (muscle), you are cutting too deeply.
Continue your incision in the posterior direction by going around the umbilical cord, missing it by a cm
or so to either side, making a pair of incisions, about two cm apart, back to either side of the anus, making sure
to always stay at least two cm apart. The bladder is under here. If this is done right, you will have created a
flap that includes the umbilical cord and a swath of skin and body wall running back towards the anus.
Before you deepen the incision, consider the structure of what’s inside. The internal organs are only
loosely held within the body cavities inside the body wall. There are three such body cavities: the abdominal
cavity, the largest, containing most of the organs; the pleural cavity, which houses the lungs; and the
pericardial cavity, which contains the heart. Each cavity is lined with tissue called serous membrane. The
abdominal cavity is lined with parietal peritoneum. Each organ is covered with visceral peritoneum. A small
amount of fluid between the layers allows them to slip against one another so that they don’t bind together. At
times there are bands of tissue linking the organs to one another, or to the body wall. These extensions of the
peritoneum are called mesenteries.
Deepen the incision from the initial large dot up to where we will make the incision at line 2. The
diaphragm is just under here and we do NOT want to cut through it just yet. Put your finger into the deepened
incision and feel for the diaphragm or end of the ribs, then make the incision indicated by line 2.
Finally make the cuts indicated by line 3.
Slowly deepen the cuts in the abdominal area. If you are very careful, you will cut through muscle
and then see the peritoneum.
Our goal is to be able to pull back the skin flaps of the body wall along the long incision between the
front and hind legs. Do NOT lift the flap with the umbilical cord!! As you separate the flaps under the front legs
you may need to use scissors or a scalpel to carefully cut the edges of the diaphragm at the body wall so the
flap can be pulled back.
Drain or sponge any fluid inside with paper towels. Be careful!! It might be easier to proceed if you
use dissecting pins to keep these flaps open.
Now carefully pull up slightly on the flap with the umbilical cord. You will see the umbilical vein
extending from inside the umbilicus up through the liver and toward the head. Pull up gently and cut the
umbilical vein. After cutting the vein, leave the flap extending back between the hind legs. See Figure 3 below.
Abdomen and digestive organs:
The organs of the abdominal cavity are covered by the peritoneum, a serous membrane. Chances
are you pulled the peritoneum off the abdominal cavity when you pulled the flaps of the body wall to the side.
*****Find the peritoneum._________
Much of the upper part of the abdomen is covered by the liver, while the lower part is filled by the
intestines. The liver processes blood as it leaves the digestive system, and also produces bile. Bile contains
emulsifiers, a chemical that reduces the surface tension of fats, making them more digestible. Bile is stored in
the gall bladder prior to use. You can see the gall bladder attached to the underside of the right lobe of the
liver. You should also note that the umbilical vein ties into the vascular system at the liver.
*****Find the liver. _________
How many lobes does it have? ____________ The liver is attached to the
diaphragm by a thin cordlike structure called the round ligament_______. Attached to the round ligament is a
thin sheet of tissue called the falciform ligament. ___________
*****Find the gall bladder. ________
Describe the appearance, color, shape_______________________
____________________________________________________
Using forceps and a needle carefully remove the visceral peritoneum from the gall bladder and trace
the duct from the gall bladder. Where does it enter the digestive tract? _____________________________
Lift up the liver to find the stomach. You might have to cut some portions of the liver to reveal the
stomach. Cut off only one small lobe of the liver if possible. The long, flat pinkish organ that lies along the
outer curve of the stomach is the spleen; it is for erythrocyte storage and processing and is part of the immune
system..
*****Find the spleen ____________
*****Find the stomach ____________
Examine the stomach thoroughly. *****Find the junction of the stomach and the esophagus. ______
This is the lower esophageal sphincter. Be able to identify: greater curvature of the stomach__________,
lesser curvature of the stomach_______, fundus______, body_______, pylorus______. *****Find the
junction of the stomach and the small intestine.______. This is the pyloric sphincter.
Cut open the stomach along the greater curvature to expose the sphincters and the inside of the
stomach. Use a scalpel to make the initial opening and then use scissors. Be careful not to damage organs
attached to stomach.
The stomach is filled with a thick liquid which you must clean out with paper towels. Do you think this
is food? ___________ Why?_________________________________________________________
(The greenish, brown debris found in the stomach and else where in the digestive tract is meconium. It consists of bile,
mucus, and sloughed off skin and epithelial cells of digestive tract. During fetal life this material is discharged into the
amniotic fluid that surrounds the embryo. It is swallowed by the fetus via the mouth to practice eating and digesting. The
first bowel movement of a new born is called the meconium.)
Once you have opened and cleaned the stomach you should be able to see the lower esophageal
sphincter_______, the pyloric sphincter_______, and the rugae______, folds which allow for expansion of
the stomach.
Carefully lift up the stomach and in the mesentery between the lesser curvature of the stomach and the
small intestine and you should see the pancreas__________. It is a whitish, granular organ, very flat, that
has two lobes. It is involved with digestive enzymes and hormone production. The greater part of the gland is
located behind the stomach. The pancreatic duct goes from the pancreas to the small intestine. It is very
difficult to see, but try to find it.
Now we will examine the intestines. There are two major portions to the intestine, the small intestine
___________and the large intestine_____________, or colon. They are named based on their diameter
differences, not on length. The small intestine is a long, coiled tube, divided into three (3) regions: the
duodenum, jejunum, and ileum. The anterior, curved, and relatively short, portion of the small intestine leaving
the stomach is the duodenum. *****Find the duodenum ____________
The two remaining, approximately equal portions of the small intestine, the jejunum and the ileum, have no
readily distinguishable boundary. The jejunum is the middle region of the small intestine, and the ileum is the
part that attaches to the large intestine.
*****Find the jejunum ____________ ileum ______________
Spread apart some of the coils of the small intestine and note the mesentery____________. It is a
double layered membrane that holds the intestines in place. Blood vessels and nerves run through the
mesentery. These blood vessels are part of the hepatic portal circulation. All the veins from the stomach,
pancreas, and intestines drain into the hepatic portal vein_________ which empties into the liver.
View portions of the interior of the small intestine. Using a scalpel, make a small longitudinal cut
through part of the duodenum and jejunum. Clean the meconium carefully. Notice that the interior has a
velvety texture. This is the villi________.
Trace the ileum to its point of attachment with the large intestine (colon). There you can find a blind pouch
called the cecum____________. In many animals, the cecum assists with digestion. In humans, the remnant
of the cecum is called the appendix. The first part of the large intestine in the pig is called the spiral
colon________. It is visible as a compact, coiled mass on the left side of the abdominal cavity. The posterior
portion of the large intestine is the rectum __________. Locate this structure passing from the spiral colon as
a straight tube into the pelvic cavity. Recall that the external opening of the rectum is the anus_______
Cut the cecum where it joins the ileum in a manner similar to the way you cut the small intestine.
Clean the cecum. You should be able to now see the ileocecal valve________.
Urogenital System
The urogenital system is a combination of the urinary system, devoted to filtration of the blood and
excretion of the filtrate (the urine), and the reproductive system, which is devoted to sperm and egg
production, as well as carrying embryos within the body. Since the paths of these systems often share
common tracts, they are often discussed together.
The paired kidneys_________ are attached to the dorsal wall of the abdominal cavity. The kidneys
filter the blood, removing excess water and undesirable solutes and produce the liquid waste we know as
urine. You must move aside the intestines to find them. The kidneys are retroperitoneal, that is, “behind”
(outside) the peritoneum.
If you tease off a little of the tissue covering the ventral surface of one kidney, you can find the renal
artery_______ and renal vein_______, which carry blood to and from the kidney, and the ureter_________,
the narrow, white, convoluted tube which drains the urine from each kidney. Each of these attach to the medial
side of the kidney. While you are there, locate the adrenal gland______, a narrow band of tissue immediately
above and medial to each kidney.
Trace the ureter from the kidney to the urinary bladder,______ which is in the flap of tissue containing
the umbilical cord. Just move aside structures in the way, don’t destroy them.
What blood vessels run on either side of the urinary bladder? ____________________________
___________________________________________________________________________________
Describe the pig’s urinary bladder. __________________________________________________
____________________________________________________________________________________
*****Find the urethra ________ which carries urine from the bladder to outside of the body.
Use a sharp scalpel to cut the kidney lengthwise from the front to the back. See if you can see three
different layers, cortex, medulla, and pelvis.
Female Reproductive System
The ovaries______________ are a pair of light-colored oval bodies located posterior to the kidneys.
They are much smaller than the kidneys. This is where eggs are first made. The uterine tubes (fallopian
tubes or oviducts) are very small, highly convoluted tubes lying on the dorsal surface of the ovaries. The
expanded end of the fallopian tube, called the ostium , partially covers the ovary and picks up the eggs from
the ovary. The fallopian tubes deliver eggs to the larger, uterine horns_________, which are the beginning of
the uterus_____________. The two horns unite in the midline to form the body of the uterus which lies dorsal
to the urethra. The broad ligament can be seen running laterally from the body of the uterus to the uterine
horns.
To dissect the rest of the female reproductive system, the pelvic cavity must be exposed. Remove the
skin from the ventral pelvis and cut through the pelvic muscles and the pubic symphysis bone in the midventral line. We have bone cutters. Cut with care since the urethra lies immediately beneath the pubis. Locate
the urethra, the tube carrying urine from the urinary bladder. Dorsal to the urethra, identify the
vagina________, the tube leading from the posterior end of the uterus.
Separate the urethra from the vagina. Toward the posterior end, the vagina and urethra unite to form a
common passage called the urogenital sinus (vulva) which opens to the outside at the urogenital papilla
Male Reproductive System
The testes, site of sperm and substantial hormone production, are located in the same area as the
ovaries throughout most of embryonic development. However, as birth approaches, they descend, and
eventually reside in the scrotum. Locate a testis. If both have descended, you will have to cut open the scrotal
sac to get to one. The epididymis should be located along the medial side of the testis. It begins at the cranial
end of the testis and extends to its caudal end. The vas deferens carries the sperm from the epididymis to
empty into the urethra. Trace the vas deferens and note how it loops over the ureter and enters the dorsal
surface of the urethra. This circuitous path is the result of the descent to the scrotum. The small hole in the
body wall through which the testes descended and through which the vas deferens now emerges is called the
inguinal canal.
The urethra is the common path for urine and sperm to the outside (via the urogenital opening). Along
the way the urethra passes through the penis. The penis is an intromittant organ, that is, an extensible
structure for insertion into the female’s vagina during mating. The penis lies just under the skin immediately
posterior to the umbilical cord and the urogenital opening.
Thoracic Cavity and Respiratory System
As you deepen the cut made from the chin to the diaphragm, you will have to cut through the sternum
or the ribs. Gently pull apart the body wall of the chest along the incision. You may have to crack ribs. For
ease of identification of the rest of the cavity, pin down the flaps.
The thoracic cavity is lined by membranes. The chest wall has the parietal pleura._________
Examine the lungs. They are tightly covered by the visceral pleura. Notice the number and position
of the lobes. Look in your text or another source and compare the number of lobes the pig has to the human.
Pig _______________________________________________________________________________
Human ____________________________________________________________________________
Gases are normally inhaled through the nostrils or external nares. _________ The nostrils lead to
the nasal cavity, which is separated from the oral cavity by the palate. You can see the palate as the roof
of the mouth. The oral and nasal cavities terminate posteriorly in the pharynx, and for a short distance air
and food use the same path.
To better view the structures inside the mouth, cut the mouth wider on either side. Open the mouth
fully and examine: hard palate,______ soft palate,_______ teeth,______ and tongue with papillae._____
Normally when the mouth is opened, it is NOT possible to see the epiglottis______ the cartilage flap that
covers the glottis_______ the opening of the trachea. Because of the cut we made it is possible to see these
structures. Note the difference between the tracheal opening and the esophagus in the pharynx._____
Now go back to the neck opening. Note several delicate structures which might get damaged as we work on
the larynx and trachea. The thymus___________ is the spongy mass of tissue next to the trachea,
continuing all the way down to the heart. The thymus is very large in the fetus and the young but shrinks
with age. Its function is to prepare T-lymphocytes, part of the immune system.
The thyroid gland ____is located in this area, a small "discolored" piece of bean shaped tissue
appearing to be tacked onto the trachea amongst the thymus. The thyroid is an endocrine gland important
in metabolism.
Clear away obstructing tissue from the larynx________ or voice box. Do NOT destroy the thyroid
or all of the thymus although you might have to remove some of the thymus.
Although they are quite small, you should be able to identify the thyroid cartilage,_______ the
cricoid cartilage______ and the epiglottis_______
Looking down the larynx, you should be able to see whitish folds that are the vocal cords_______ .
These are not well developed in the fetal pig, but would be in the adult. Why? _____________________
____________________________________________________________________________________
Identify the trachea by its cartilage ring structure. Use the dissecting needle to feel the rings. Notice
that these technically are NOT rings but “C” shapes. Feel the back of the trachea to see that the rings are not
continuous. If you are very careful you will be able to follow the trachea until it branches into two bronchi
which enter the lungs.
*****Find the esophagus _________ which lies under (dorsal) to the trachea. The walls of the
esophagus are collapsed except when a bolus of food is present. It really isn’t a “tube” but more flattened.
Try to follow the esophagus down toward the stomach. Notice how it pierces the diaphragm.
This is as good a time as any to examine the diaphragm________. The diaphragm, a muscle,
defines the boundary between the pleural and abdominal cavities. When the diaphragm flexes(flattens and
moves downward), it expands the pleural cavity. This causes a negative pressure in that space, causing air
to be drawn into the lungs. Exhalation results from relaxation of the diaphragm. When breathing under
duress, such as during exercise, the intercostal muscles in the body wall aide in further expanding the rib
cage, allowing for increased lung volume.
Examine the lungs. How many lobes on the left lung? ___________ right lung?______ Remove part
of a lung and slice very thin sections. Look at the sections under a dissecting microscope. Describe what you
see. __________________________________________________________________________________
______________________________________________________________________________________
Lift the lungs out of the way and examine the heart_________ which is enclosed by the
pericardium._____
Remove some of the thymus and tweeze away the pericardium. Examine the heart and Identify:
Apex____ the point of the heart; the top of the heart is called the base_________ Use the diagram to help
you find: left and right atria _________, left and right ventricles _______________, interventricular
sulcus ________ the groove between the two ventricles usually filled with a coronary artery.______ a vessel
that supplies blood to the heart itself.
Lift the apex of the heart and find the inferior and superior vena cava _______ which enter the right
atrium. The pulmonary artery________ leaves the right ventricle and divides to form the two pulmonary
arteries to the lungs. Try to trace these vessels.
Now find the aorta________, an arch that comes from the left ventricle.
Coming off the aorta are three important vessels. The first is the brachiocephalic artery_______,
which will bring blood to the arms and head. The next is left common carotid artery _______ and then the
left subclavian artery__________
In fetal pigs and in fetal humans there is a vessel called the ductus arteriosus_________ that serves
as a shunt between the pulmonary artery and the aorta prior to birth. In the fetus, where the lungs are not
functioning in respiration, most of the blood bypasses the lungs. It passes from the right ventricle into the
pulmonary artery and then through the ductus arteriosus to the aorta. At birth, the ductus arteriosus normally
closes and all the blood from the right ventricle goes to the lungs. Examine the dorsal side of the heart to find
the ductus arteriosus.
The left common carotid artery is going to branch into the left and right carotid arteries ________, which
can be seen in the neck on either side of the trachea. Near the carotid artery on either side are two jugular
veins_______ the external and the internal. Dorsal to the artery and veins is the vagus nerve _______ which
appears like a white cord.
Move the abdominal viscera to the right so you can see the abdominal aorta _____ and the inferior vena
cava (caudal vena cava) _________. Notice the relationship between the abdominal aorta and the renal
artery and the inferior vena cava and the renal vein.
If time permits you may dissect some of the pig’s head. Find the parotid salivary gland. To do this, use the
scalpel to make an incision through the skin and facial muscles beginning at the base of the ear. Cut carefully
as it is easy to cut into the parotid gland.
The brain and Spinal cord. Using the scalpel, make an incision through the skin of the head as in the
diagram below. Peel off the skin and look for sutures in the skull. Using scissors and forceps carefully pull or
break off parts of the skull until you have exposed the dura mater_____ covering the brain.
The brain and spinal cord are protected by three membranes, the meninges. The dura mater is the thickest
and toughest. Cut through the dura mater to expose the pia mater, the meninge attached directly to the
brain. Identify the right and left cerebral hemispheres. Notice the longitudinal fissure. Find the
cerebellum. Under the cerebellum you may be able to identify the medulla. You might have to remove
some of the brain to see structures beneath the cerebrum.
The pig has 12 pairs of cranial nerves just like humans. These nerves mainly serve the sense organs of
the head. See if you can find them.
The spinal cord is surrounded by the vertebrae of the spinal column. To expose the spinal cord, remove
the skin from an area of the back about 8 cm long and 2-3 cm wide. Use scissors to cut through the
vertebrae. You should be able to see the spinal cord with the branching spinal nerves.