Model Answer
0 min readIntroduction
The placenta is a temporary physiological connection between the developing fetus and the mother, facilitating nutrient exchange, waste removal, and hormone production. Eutherian mammals, also known as placental mammals, are characterized by a longer gestation period and a well-developed placenta. The structure of the placenta varies considerably across different mammalian species, and classification based on the arrangement of villi – the finger-like projections of the chorion – provides a useful framework for understanding these differences. This answer will detail the various types of placenta found in eutherians, alongside a discussion of the crucial hormonal roles it plays in maintaining pregnancy and initiating birth.
Types of Placenta in Eutherian Mammals (Based on Villi)
The placenta is classified into four main types based on the distribution of villi on the chorionic membrane:
1. Diffuse Placenta
In this type, the villi are scattered uniformly over the entire surface of the chorion. There are no distinct cotyledons or discs. This is found in equines (horses), ruminants (cows, sheep), and pigs. The maternal and fetal surfaces are roughly equal in area. The chorionic villi are short and branching.
2. Cotyledonary Placenta
The chorion bears a number of rounded or oval projections called cotyledons. Each cotyledon represents a localized area of villous attachment to the uterine wall. This type is characteristic of ruminants like sheep and cattle. The maternal surface has caruncles, which are corresponding areas for cotyledon attachment. There is no complete fusion of the cotyledons.
3. Discoid Placenta
The placenta is a single, disc-shaped structure. The villi are concentrated in a circular area. This is found in primates (including humans) and rodents. The maternal surface is relatively smooth. The discoid placenta allows for efficient exchange due to its large surface area and close apposition of maternal and fetal tissues.
4. Chorioallantoic Placenta
This is the most advanced type of placenta, found in carnivores (dogs, cats) and rodents. The chorion and allantois fuse to form the chorioallantoic membrane, which comes into close contact with the uterine wall. The villi are numerous and branching, forming a complex network. This type allows for highly efficient exchange of nutrients and gases.
The following table summarizes the key differences between these placental types:
| Placental Type | Villi Distribution | Examples | Maternal Surface |
|---|---|---|---|
| Diffuse | Scattered uniformly | Horses, Pigs | Roughly equal to fetal surface |
| Cotyledonary | Cotyledons (rounded projections) | Cattle, Sheep | Caruncles |
| Discoid | Single disc-shaped structure | Humans, Rodents | Relatively smooth |
| Chorioallantoic | Numerous branching villi (chorion & allantois fused) | Dogs, Cats | Variable |
Placental Hormones and Their Roles
The placenta functions as an endocrine organ, producing several hormones crucial for maintaining pregnancy and facilitating parturition:
- Human Chorionic Gonadotropin (hCG): Produced by the syncytiotrophoblast, hCG maintains the corpus luteum during early pregnancy, ensuring continued production of progesterone and estrogen. It is the hormone detected in pregnancy tests.
- Progesterone: Initially produced by the corpus luteum (maintained by hCG), progesterone production shifts to the placenta after the first trimester. It maintains the uterine lining, prevents uterine contractions, and suppresses maternal immune response to the fetus.
- Estrogen: Also produced by the placenta, estrogen promotes uterine growth, increases blood flow to the uterus, and prepares the mammary glands for lactation.
- Human Placental Lactogen (hPL): Also known as human chorionic somatomammotropin, hPL alters maternal metabolism to provide nutrients to the fetus. It increases maternal insulin resistance, making more glucose available to the fetus.
- Relaxin: Produced in smaller amounts, relaxin helps to relax the pelvic ligaments and soften the cervix in preparation for labor.
Towards the end of pregnancy, a decline in progesterone and an increase in estrogen levels, along with the release of prostaglandins, trigger uterine contractions and initiate labor. The placenta also plays a role in the expulsion of the fetus and placenta after birth.
Conclusion
In conclusion, the placenta exhibits diverse structural adaptations in eutherian mammals, categorized primarily by the arrangement of villi. These variations reflect the differing physiological demands of each species. The placenta’s endocrine function, mediated by hormones like hCG, progesterone, estrogen, and hPL, is paramount for maintaining pregnancy, supporting fetal development, and orchestrating the process of parturition. Understanding these intricacies is crucial for comprehending reproductive biology and addressing potential complications during gestation and childbirth.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.