Brent Cornell

    
The placenta functions as the life support system for the foetus, serving two key functions:

• It facilitates the exchange of materials between the mother and foetus
• It secretes hormones to maintain the pregnancy after the corpus luteum has degenerated

Structure of the Placenta

• The placenta is a disc-shaped structure that nourishes the developing foetus
• It is formed from the development of the trophoblast upon implantation and eventually invades the uterine wall
• Maternal blood pools via open ended arterioles into intervillous spaces within the placenta called lacunae
• Chorionic villi extend into these pools of blood and mediate the exchange of materials between the foetus and the mother
• Exchanged material is transported from the villi to the foetus via an umbilical cord, which connects the foetus to the placenta
• Upon birth, the placenta is expelled from the uterus with the infant – it is then separated from the infant by severing the umbilical cord (the point of separation becomes the belly button)

Overview of the Structure of the Placenta

Material Exchange

• The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus
• Chorionic villi are lined by microvilli to increase the available surface area for material exchange
• Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae
• Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
• Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels

Exchange of Materials between Foetal and Maternal Blood

Hormonal Role

• The placenta takes over the hormonal role of the ovaries (at ~12 weeks) and begins producing estrogen and progesterone
• Estrogen stimulates the growth of uterine muscles (myometrium) and the development of the mammary glands
• Progesterone maintains the endometrium, as well as reducing uterine contractions and potential maternal immune responses
• Both estrogen and progesterone levels drop near the time of birth

The Role of the Placenta in the Hormonal Control of Pregnancy