THE PLACENTAIt originates from the trophoblastic layer. The placenta begins to develop around the fourteenth day after fertilisation, and is fully formed by the sixteenth week of pregnancy. Its fully developed and functional by the 10th week after fertilisation. Between 12 20 weeks, placenta weighs more than fetus. The fetal organs e.g. liver, later develop and are functional so cytotrophoblast and syncytiotrophoblast degenrate.Chorionic villi The trophoblast consists of projections which proliferate and branch forming chorionic villi. They are more pronounced in the decidua basalis and this part is called chorion frondosum. Villi erode the walls of maternal blood vessels as they penetrate the decidua, opening them up to form a lake of maternal blood in which they float. The opened blood vessels are called sinuses & the areas surrounding the villus, blood spaces.

Maternal blood circulates slowly, enabling the villi to absorb nutrients, oxygen and excrete waste. These are called nutritive villi. A few villi more deeply attached to decidua are called anchoring villi. Each chorionic villus is a branching structure arising from one stem. Its Centre is composed of mesoderm, branches of umbilical vein and artery and fetal blood vessels. They are covered by a single layer of cytotrophoblastic cells and external layer of syncytiotrophoblast. Between maternal blood supply and fetal are 4 layers of tissue thus blood from the 2 circulations cant mix.Appearance of the mature placenta The fully formed placenta is a spongy disc, circular in shape, about 20 cm in diameter, and 2.5 cm thick at the centre. It Weighs one-sixth of babys weight at term and has a maternal and a fetal surface.Maternal surfaceIts the Surface attached to the uterine wall. Maternal blood gives it a dark red colour. Arranged into about 20 lobes separated by sulci (furrows) into which decidua dips down to form septa(walls). Lobes are made of lobules, each containing a single villus. Sometimes deposits of lime salts giving it gritty appearance. 2. Fetal surfaceFound adjacent to the foetus and is covered with amnion, which gives it a white, shiny appearance. Branches of the umbilical vein and arteries are visible spreading out from the insertion of the umbilical cord, which is normally in the centre. Amnion can be peeled off the surface, leaving the underlying chorionic plate except at the insertion of the cord.Functions of the placenta Respiratory function oxygen from the mother's haemoglobin passes to the foetus and the foetus gives off carbon dioxide to the maternal circulation by simple diffusion. Storage, that is, the placenta stores glycogen and re-converts it back to glucose for use as required and also stores iron and fat soluble vitamins. Nutritive - the foetus obtains nutrients from the mother's diet through the placenta. Amino acids are required body building; glucose for energy and growth; calcium & phosphorus for bones and teeth; iron & other minerals for blood formation. This nutrients from mothers diet has already been broken down into simpler forms before reaching the placenta. Placenta selects those substances required by the fetus. It can also break down complex nutrients into compounds that can be used by fetus. Excretory - waste products of metabolism and carbon dioxide are excreted into maternal blood by diffusion. Main substance excreted is carbon dioxide. Protective - the placenta acts as a barrier against transfer of infection from the mother to the foetus. However, a few organisms such as the rubella virus and the triponema syphilis are able to pass through. It also prevents most drugs from crossing to the foetus. Some drugs however cross the placenta e.g. heparin, and can cause damage to foetus. Towards end of pregnancy some immunoglobulins cross over to the foetus and confers immunity for the foetus for the first 3 months. Endocrine, where the placenta acts as an endocrine gland and produces a number of hormones as follows, which include Human Chorionic Gonadotrophin (HCG), oestrogens, progesterone and Human Placental Lactogen (HPL). HPL has a role in glucose metabolism in pregnancy.