External view of a fetal pig

Fetal pigs are unborn pigs used in elementary as well as advanced biology classes as objects for dissection. Pigs, as a mammalian species, provide a good specimen for the study of physiological systems and processes.

[edit] Use in Biology Labs

Along with frogs and earthworms, fetal pigs are among the most common animals used in classroom dissection. There are several reasons for this, the biggest being that pigs, like humans, are mammals. Shared traits include common hair, mammary glands, live birth, similar organ systems, metabolic levels, and basic body form. They also allow for the study of fetal circulation, which differs from that of an adult. Secondly, fetal pigs are easy to obtain because they are by-products of the pork industry. Fetal pigs are the unborn piglets of sows that were killed by the meat packing industry. These pigs are not bred and killed for this purpose, but are extracted from the deceased sow’s uterus. Fetal pigs not used in classroom dissections are often used in fertilizer or simply discarded^[1]. Thirdly, fetal pigs are cheap, which is an essential component for dissection use by schools. They can be ordered for a little more than $25 at biological product companies. Fourthly, fetal pigs are easy to dissect because of their soft tissue and incompletely developed bones that are still made of cartilage^[2]. In addition, they are relatively large with well-developed organs that are easily visible. As long as the pork industry exists, fetal pigs will be relatively abundant, making them the prime choice for classroom dissections.

[edit] Development

Dissected fetal pig, showing the thoracic cavity, including the rib cage, the heart in the pericardium, the lungs, the diaphragm, part of the liver, and all those organs under the neck.

The size of the fetal pig depends on the time allowed for the mother to gestate:

[edit] Nutrition

No studies have found significant data regarding the mother swine’s diet and fetal pig survival rate. However, there is a correlation between a mother pig having a nutritious diet containing proteins, vitamins and minerals during gestation period and the survival rate of piglets. The correlation, however, is not statistically different. Weight is also not a factor of survival rate due to the fact that a healthier diet does not lead to a heavier child or a greater chance of live birth. .^[3].

[edit] Placental Development

The placenta is used as a means of transferring nutrients from the mother to the fetus. The efficiency at which nutrients are transferred dictates the health and growth of the fetus. FRP, or fetal weight: placental weight ratio, was commonly used to determine placental efficiency. However, increasing FPR does not prove to increase litter size. Instead, a more accurate way of determining fetus growth is through certain characteristics of the placental lining. The placenta is made of a folded trophoblast/endometrial epithelial bilayer. The width and length of the placenta folds are positively related and increase as gestation progresses.

The width of the placental folds decreases until day 85 of gestation. From here, the width increases with gestation and is at its largest around day 105. The rate at which these folds increase is negatively related to fetus size. Thus, greater fold widths will be seen in smaller fetuses. Although increasing placental fold width does increase the interaction between fetus and mother, nutrient exchange is not most efficient in smaller fetal pigs, as would be expected. Many other factors, including depth of placental folds, are also responsible for these interactions. ^[4]

[edit] Prenatal Development

The prenatal development of the fetus includes all the tissue and organ development. Within hours of mating, the sperm and egg undergo fertilization in the oviduct and three days later the egg moves into the uterus. The cells begin to specialize by day six, and attach themselves to the uterus lining by day eleven. From fertilization to day 18, the endoderm, ectoderm and mesoderm have been forming inside the embryo, and are completely formed by day 18, the same day the placenta forms. The endoderm transforms into the lungs, trachea, thyroid gland, and digestive tract of the fetus. The ectoderm has a greater role in the development of the fetus. It forms into the skin, nervous system, enamel of the teeth, lining of the intestine, mammary and sweat glands, hoofs, and hair. The mesoderm forms the major organ components that help keep the fetus alive. It forms the muscles and connective tissues of the body, blood vessels and cells, the skeleton, kidneys, adrenal glands, heart, and the reproductive organs. By day 20, most of the major organs are visible, and the last half of gestation focuses greatly on increasing the size of the fetuses.^[5].

[edit] Development of Lymphoid and Haematopoiesis tissues

The development of the lymphatic system and the formation of blood circulation occur at different stages of fetal pig development. The first lymphatic organ to become present is the thymus. Lymphocyte builds up in the spleen on the 70th day. By day 77, the thymus is already completely developed and is distinguishable from other organs. Also, follicles are present on the tongue and intestines on day 77. On the 84th day, Periarteriolar lymphoid sheaths appear in the fetal pig. By this time, the liver and bone marrow are active and functional.^[6].

[edit] Environmental Effects on Swine Reproductive Performance

Studies have shown that litter size, the amount of floor space during the growing period, and the number of pigs the gilt, or female pig, is placed with while growing affect the reproduction rates of the gilts. Data from a study in 1976 by Nelson and Robinson showed that gilts from a small litter size ovulated more than the gilts from the larger litters. The study suggests stress plays a role in impacting the reproduction. The amount of floor space has been shown to impact the time it takes gilts to reach puberty. An adequate amount of floor space allowed the higher percentage of gilts to reach puberty sooner than those gilts who had less floor space. The gilts placed in smaller groups bore one more pig per litter than gilts in larger groups. Still, the environment in which the fetal gilt develops is significant to the reproductive and physiological development.^[7].

[edit] Preservation in formaldehyde

Fetal pigs are often preserved in formaldehyde, a carcinogenic substance. A 1980 study found that exposure to formaldehyde could possibly cause nasal cancer in rats, leading to research on whether this was possible in humans or not. ^[8]. In 1995 it was concluded by the International Agency for Research on Cancer (IARC) that formaldehyde could be a probable carcinogen for humans. ^[9].

[edit] Anatomy of the fetal pig

Fetal pig brain situated in the cranium.

The anatomy of a fetal pig is similar to that of the adult pig in various aspects. Systems that are similar include the nervous, skeletal, respiratory (neglecting the under developed diaphragm), and muscular. Other important body systems have significant differences from the adult pig.

[edit] Circulatory System

Pigs, like all mammals, have a four-chambered heart. Blood enters the right atrium via the superior and inferior vena cava. The blood is then pumped into the right ventricle from where it is pumped to the lungs to be oxygenated via the pulmonary arteries. Oxygen-rich blood is then pumped through the left atrium and into the left ventricle. The walls of the left ventricle are thicker than those of the other chambers, due to fact that the muscle of the left ventricle must be strong enough to pump oxygen-rich blood throughout the body.

The aortic arch of a fetal pig has two arteries attached to it, the brachiocephalic artery and the subclavian artery. As the aorta descends, it splits into two large iliac arteries. An umbilical artery branches near the base of each iliac artery. The umbilical arteries run through the umbilical cord, carrying blood to the maternal placenta where it becomes oxygenated, nutrient-rich, and free of waste. This oxygenated, nutrient-rich blood is then returned to the liver of the fetus via the umbilical vein.

There are only a few differences between the circulatory system of an adult pig and a fetal pig, besides from the umbilical arteries and vein. There is a shunt between the wall of the right and left atrium called the foramen ovale. This allows blood to pass directly from the right to left atrium. There is also the ductus arterius which allows blood from the right atrium to be diverted to the aortic arch. Both of these shunts close a few minutes after birth.

[edit] Digestive

The monogastric digestive system of the fetal pig harbors many similarities with many other mammals. The fetal pig's digestive organs are well developed before birth, although it does not ingest food. These organs include the esophagus, stomach, small and large intestines. Mesenteries serve to connect the organs of the fetal pig together. In order for digestion to occur, the fetal pig would have to ingest food. Instead, it gains much needed nutrition from the mother pig via the umbilical cord. In the adult pig, food will follow the general flow through the esophagus, which can be located behind the tracheae. From the oral cavity, the esophagus leads to the stomach, small intestine, and large intestine. Other organs developing during fetal pig development such as the gallbladder, pancreas and spleen are all critical in contributing to the overall flow of the digestive system. After being digested and absorbed, the food follows through the large intestine and is excreted through the rectum and anus. In the fetal pig however, the metabolic wastes are sent back to the mother through the umbilical cord where the mother excretes the wastes. Other remaining wastes remain in the fetal pig until birth.

The oral cavity of the fetal pig begins developing before birth. The tongue's taste buds, located in the enlarged papillae, facilitate food handling after birth. These taste buds develop during fetal development. Adult pigs have up to 15,000 taste buds, a much larger number than the average human tongue, which has 9,000.^[10]. The dental anatomy of the fetal pig shows differences from adult pigs. The fetal pig develops primary teeth (which are later replaced with permanent teeth). Some may erupt during fetal stage, which is why some of the pigs that are/will be dissected show evidence of teeth. Depending on the age of the fetal pig, it is natural to see eruptions of third incisor and canine in the fetal pig^[11]. Because the fetal pigs were still in the mother’s uterus, teeth will still form which supports reasons for hollow unerupted teeth that may be seen. Similar to human dental anatomy, the overall dental anatomy of the pig consists of incisors, canines, pre-molars, and molars. Exploring the dental anatomy even further, piglets can have 28th teeth total and adult pigs can have 44 teeth total.^[12] Urogenital

Reproductive system of a female fetal pig

The fetal pig's urogenital system is similar to the adult pig's system with the exception of the reproductive organs. The fetal pig urinary tract is relatively developed and easy to locate during dissection. The kidneys are located behind the abdominal organs and are partially embedded into the dorsal body wall by the spine. The ureters carry the urine to the urinary bladder, the large sack-like organ by the umbilical artery and vein, to the urethra. From there, the urine can be excreted.

Female

If the fetal pig is a female, there will be a fleshy protrusion ventral near the anus called the genital papilla ^[13]. The female's internal reproductive system is located below the kidneys. The two sac-like organs attached to the coil-like fallopian tubes are the ovaries.^[14]. The uterus, which becomes the vagina, is located where the fallopian tubes meet. This system can be difficult to find as it is small as well as extremely dorsal and posterior to the other systems.

Male

To externally determine if the fetal pig is male, look for the urogenital opening located behind the umbilical cord. Also note the swelling behind the hind legs of the fetal pig ^[15]. This will be the scrotum. The male's internal reproductive system has two scrotal sacs, which depending on the age of the fetal pig may or may not have developed testes.^[16]. The epididymis coil on the testes connects to the vas deferens. The vas deferens crosses over the ureter and enters the urethra, which then connects to the penis located just posterior to the skin. Similar to the female system, it may be difficult to identify all parts.

[edit] References

[edit] External links

Look up fetal pig in Wiktionary, the free dictionary.

• Fetal Pig Dissection
• Standard Fetal Pig Dissection Lab
• Where to Buy Fetal Pigs
• Alternatives to Dissecting Fetal Pigs
• Ethical Questions and Concerns About Dissecting Fetal Pigs