SN4 SHORT NOTE 5M ★ High-yield

Describe the structure of the placenta and the placental barrier.

Anatomy · 1st Professional — Pre-Clinical

Definition and Development

The placenta is a temporary organ formed from fetal (chorion frondosum) and maternal (decidua basalis) tissue, which mediates exchange between mother and fetus.

It is haemochorial — maternal blood bathes the chorionic villi directly, with no intervening maternal vessel wall
At term it is a discoid organ, 15–20 cm across, 500–600 g, about a sixth of fetal weight

Structure of a Chorionic Villus

Layer	Early pregnancy	At term
Syncytiotrophoblast	Continuous multinucleate layer; no cell boundaries	Thinned, with syncytial knots
Cytotrophoblast (Langhans cells)	Complete inner layer	Discontinuous or absent
Villous mesenchyme	Loose, with Hofbauer cells (macrophages)	Reduced
Fetal capillary	Centrally placed	Dilated and pushed to the periphery

Functions

Respiratory — exchange of oxygen and carbon dioxide; fetal haemoglobin has a higher affinity for oxygen
Nutritive — glucose by facilitated diffusion, amino acids by active transport, and iron and calcium against a gradient
Excretory — urea, creatinine, bilirubin
Endocrine — hCG (maintains the corpus luteum), human placental lactogen, oestrogen and progesterone
Barrier — against some organisms and drugs, though far from complete
Immunological — transfers maternal IgG, giving passive immunity for the first months

What Crosses and What Does Not

Crosses the placenta	Does not cross
Oxygen, carbon dioxide, water, electrolytes	Maternal red cells (normally)
Glucose, amino acids, free fatty acids	IgM, IgA, IgE
IgG	Heparin, insulin
Most drugs; alcohol; nicotine	Large protein hormones
Torch organisms — Toxoplasma, Rubella, Cytomegalovirus, Herpes, Syphilis, HIV	Most bacteria

Applied Aspects

Rh incompatibility — fetal red cells entering the maternal circulation at delivery sensitise an Rh-negative mother; her IgG crosses in a later pregnancy and causes haemolytic disease of the newborn. Prevented by anti-D immunoglobulin
Placenta praevia — implantation in the lower segment; painless bright red bleeding in late pregnancy
Abruptio placentae — premature separation; painful bleeding with a tense uterus
Placenta accreta — villi invade the myometrium where the decidua is deficient, usually over a caesarean scar; the placenta cannot separate and causes torrential haemorrhage
Hydatidiform mole — hydropic villi with trophoblastic proliferation and no fetus; hCG is very high, and it may progress to choriocarcinoma
Almost every drug crosses, so prescribing in pregnancy must assume fetal exposure unless proved otherwise

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