Fetal Pig Anatomy

In this lab you'll dissect a fetal pig to get a look at the anatomy of a mammal. In addition, you should study the two pre-dissected specimens available in lab.

Objectives

Recognize the structures labeled on the pictures on this page or listed in bold in the text.

Specimens

• Fetal pig that you can dissect with your group
• Pre-dissected pig in plastic case
• Sagittally sliced pig sealed in plastic

Fetal pig, dissected

We have a dissected specimen in lab, preserved in a plastic case. This would be a good one for the lab exam. On the photo below, I've labeled some structures that I might potentially ask you about on the lab exam.

Fetal pig sagitttal section

Here's another specimen available in lab.

Dissect your own pig

The best online guide to fetal pig dissection is probably the Virtual Fetal Pig Dissection at Whitman College. I recommend using that as your guide.

Here are some features you should look for:

Determine the sex of your pig

Before you start dissecting, examine the outside of the pig and determine its sex. Look for these features:

• Males: Urogenital opening is located near the umbilicus; the penis is hidden inside. The scrotal sac may be visible as a swelling just ventral to the anus, depending on the age of the fetus.The testes are still deep inside the body cavity; they don't descend into the scrotal sac until later.
• Females: Look for the urogenital papilla, located just below the anus.

Both males and females have nipples, just as in humans.

Reference: Sexing your pig.

Neck region

Begin your dissection in the neck region. Try to cut as little as possible. Once you open the body cavity, you will generally be able to separate the different organs by simply pulling them apart with your fingers, forceps, or a probe. The more you cut things up, the harder it will be to figure out what you’re looking at. Cut midline on the ventral surface of the neck to expose the underlying muscles. Carefully separate the muscles to observe the underlying structures. Locate and understand the functions of the following structures:

• Larynx: an enlarged structure on the trachea. If you cut it open, you can see the vocal cords inside.
• Thymus gland: an endocrine (hormone-secreting) gland that helps regulate the immune system. It’s a large, spongy structure covering the ventral surface of the trachea and often extending into the thoracic cavity adjacent to the heart.
• Thyroid gland: another endocrine gland; it’s a small bilobed structure just posterior to the larynx. The thyroid secretes hormones that help regulate metabolism.
• Trachea: the airway; it's reinforced with rings of cartilage so it doesn't collapse.
• Esophagus: carries food from mouth to stomach; soft and muscular so it can move a food bolus by peristalsis.

Thoracic cavity

Vertebrates have true coeloms. In mammals, the coelom is divided into two main cavities: the thoracic cavity, which contains the lungs, and the abdominal cavity, which contains the digestive system. The thoracic cavity and the abdominal cavity are separated by the diaphragm. Note the many membranes lining the coelom and holding the organs in place.

Look for these structures in the thoracic cavity:

• Lungs: they have several lobes. Note the how spongy the tissue is.
• Heart: muscular and easy to find. The heart is surrounded by a pericardial sac. Note the aorta, where high-pressure blood leaves the heart on its way to the systemic circulation. You may also see the right and left carotid arteries, which supply blood to the head. See this diagram for the fetal pig heart, and the Wikipedia Heart article for some good diagrams of human heart anatomy.
• Diaphragm: a sheet of muscle and connective tissue that helps in breathing.

Abdominal cavity: digestion & absorption

Locate and understand the functions of the following structures:

• Liver: very large and dark. It has several lobes. You’ll need to lift it out of the way to see the organs beneath. The gall bladder is a small organ attached to the underside of the liver; it's usually greenish because it contains bile.
• Stomach:
• Small & large intestine
• Rectum
• Stomach: may be hidden beneath the liver. Note the esophagus leading into the stomach and the duodenum (first part of the small intestine) leading out. After passing through the large intestine, digested chyme goes to the rectum before being eliminated from the body.
• Mesenteries: thin, transparent sheets of connective tissue containing blood vessels connecting the intestine and other organs. The mesenteries are folds of the peritoneum, which is the smooth, shiny layer that lines the abdominal cavity.
• Pancreas: white and looks a little bit like cauliflower. It secretes digestive enzymes and buffers.

Abdominal cavity: other organs

Locate and understand the functions of the following structures:

• Kidneys. The two kidneys are not actually located in the abdominal cavity; they occupy another coelomic compartment dorsal to the abdominal cavity. (They are outside of the peritoneum.) You won't see them until you move the intestines aside. Urine from the kidneys goes into the urinary bladder, and then through the urethra as it is eliminated from the body. You can also dissect the kidneys themselves; see the page on kidneys at Whitman College for more information.
• Spleen. The spleen is a flat organ located near the stomach. It performs several functions related to producing and maturing new blood cells and eliminating old ones. Blood passes through open sinuses in the spleen, rather than being confined to narrow blood vessels.
• Urinary bladder & urethra. The urethra is the tube that carries urine from the urinary bladder to the urinary opening.
• Ovaries, uterus (females) or Testes (males). See the photos online to find these. The size of the testes varies significantly, depending on the age of the fetal pig. The testes of males and the ovaries of females both arise from the same embryonic structures; however, the testes migrate during fetal development until they descend into the scrotal sac.

Circulatory system

Mammalian hearts have four chambers (see Campbell, p. 42.4). Each side of the heart has an atrium that receives blood from elsewhere in the body and a ventricle that pumps the blood out of the heart. The right atrium receives blood from the systemic circulation and passes it to the right ventricle, which pumps the blood to the pulmonary circuit. After the blood passes through the lungs it goes to the left atrium and then into the left ventricle, which pumps the blood into the systemic circuit. The first part of the systemic arterial circuit is the aorta, which soon branches out to supply various regions of the body.

Fetal circulation is different from adult circulation. In the fetus, blood doesn’t get oxygenated in the lungs; it gets oxygenated at the placenta. The umbilical arteries carry blood from the fetus to the placenta. The umbilical vein carries blood from the placenta back to the fetus. (Remember that in the placenta substances are exchanged between fetal and maternal blood, but the blood itself does not mix.) Therefore, the most highly oxygenated blood in the fetus is in the umbilical vein. Blood from the umbilical vein gets mixed with the rest of the systemic circulation and returns to the right atrium. The blood entering the right atrium is the most oxygenated blood in the fetal heart, but it’s the least oxygenated blood in the adult heart. The fetus has two key tricks to adapt to this fact:

First, some of the blood that leaves the right ventricle bypasses the lungs. In an adult, this blood needs to go to the lungs to get oxygenated, but the fetus has a ductus arteriosus that short-circuits this blood flow, allowing some blood to go directly into the aorta and then into the systemic circulation.

Second, in the fetal heart, there is an opening between the right atrium and the left atrium. This opening is called the foramen ovale. The foramen ovale is helpful in the fetus because it lets the oxygenated blood from the placenta get circulated faster. The circulation pattern is somewhat similar to that of a frog (Campbell, p. 874). The foramen ovale normally closes up at birth, keeping blood flow of the two sides of the heart completely separate. In some people, the foramen ovale does not close up. This condition, called patent foramen ovale, can result in serious health problems.

Here's a diagram of circulation in a fetal pig:

Image courtesy of OpenWetWare.

And here's a diagram of human fetal circulation:

Image by OpenStax College [CC BY 3.0], via Wikimedia Commons.

Sample test questions

You should be prepared to identify any of the structures labeled on the diagrams on this page or listed in bold type in the text of this page. You might see either of the two specimens shown on this page, or a dissected specimen like the one you examined in lab.

Also, you should be aware of the main differences between fetal and adult circulation.

References & further reading

Fetal pig dissection

Virtual Fetal Pig Dissection at Whitman College. Click through all the links for an excellent step-by-step dissection guide.

Fetal Pig Dissection Pictures at APU.

Fetal circulation

Assessment of flow distribution in the mouse fetal circulation at late gestation by high-frequency Doppler ultrasound. Zhou et al., 2014 Physiological Genomics. This research article presents some amazingly detailed ultrasound images of fetal circulation in a mouse. Enjoy it for the pictures.

Fetal Circulation Right Before Birth. This Khan Academy video gives a clear explanation of fetal circulation.

Fetal Circulation, a YouTube video from AK lectures.