11.4.5 The Placenta

• Mammals can be split into different categories on the basis of their mechanism for foetal nourishment
  • Placental mammals, such as humans
  • Monotremes, or egg laying mammals, such as a platypus
  • Marsupials, such as kangaroos, whose offspring develop in a pouch
• Placental mammals rely on the complex system of blood vessels specially designed to maximise exchange of substances between mother and foetus without any direct connection between them
• The placenta is also responsible for production of key pregnancy hormones oestrogen and progesterone
• The foetus is connected to the placenta via the umbilical cord and is contained within the amniotic sac filled with amniotic fluid which protects the foetus

Structure of the placenta

• The placenta is an organ primarily made up of a complex arrangement of blood vessels arranged into placental, or chorionic, villi with maternal blood flow distributed around the villi
  • Throughout the course of the pregnancy, the number of villi increases to meet the demands of the growing foetus
• Maternal blood and foetal blood never mix directly, but flow either side of a layer of cells that make up the placental barrier; there is a very short distance between the maternal and foetal blood to allow exchange of nutrients and gases
  • The mother's blood flows out of the mother's blood vessels and forms pools in the spaces surrounding the placental villi; these spaces are known as the inter-villous spaces
• The placental membrane, or barrier, provides a selectively permeable barrier which restricts the exchange of substances between mother and baby
• Substances that move across the barrier from mother to foetus include
  • Oxygen
  • Antibodies
    • Antibodies cross the placenta using a mechanism called endocytosis
  • Water
  • Glucose
  • Unwanted or harmful substances may also cross the placental barrier, including alcohol, drugs or small pathogens such as viruses
    • Bacterial pathogens are too large to cross the barrier
• Substances that move across the barrier from foetus to mother include
  • Carbon dioxide
  • Water
  • Urea
• The placenta is connected to the growing foetus by the umbilical cord

The placenta brings the maternal and foetal blood systems close but without directly mixing

• Hormones play a fundamental role in promoting foetal development and maintaining the pregnancy
• The levels of hormones change throughout the stages of pregnancy and development of pregnancy organs, such as the placenta
• Initially, the degenerating corpus luteum (a group of cells that form in the ovary following ovulation) releases hormones that support the initial stages of the pregnancy, up to about week 9 
• The placenta then takes over in releasing steroid hormones, oestrogen and progesterone
  • The role of oestrogen is to stimulate growth of the uterus and mammary glands (commonly called breasts in humans)
  • The role of progesterone is to support the lining of the uterus to maintain the pregnancy
• If the placenta fails to initiate hormone production, a miscarriage may occur

Pregnancy hormones change throughout gestation and with placental growth