HUBS3511 · Human Reproduction And Pregnancy
The Placenta
The placenta is a fetomaternal organ that does three jobs at once: it exchanges (gases, nutrients, waste), it protects (immune barrier plus passive antibody transfer), and it acts as a transient endocrine gland that drives the whole hormonal course of pregnancy. Its functional unit is the chorionic villus: fetal tissue dipped into a lake of maternal blood. The single most-examined fact is that maternal and fetal blood never mix — everything crosses a thin barrier. HUBS3511 asks you to label the villus and the layers of the placental barrier; match each substrate to its transport mechanism (O₂/CO₂ diffusion, glucose facilitated, amino acids active, IgG endocytosis); explain hCG (its source, its job of rescuing the corpus luteum, and that it is the basis of pregnancy tests); place the luteal–placental shift (~wk 6–9); and link estriol to the fetal adrenal (the fetoplacental unit).
What this chapter covers
- 015.0 The chorionic villus: the haemochorial exchange unit
- 02The placental barrier: syncytio + cyto + lamina + fetal endothelium
- 03Spiral-artery remodelling by EVT (and its failure → preeclampsia)
- 04Umbilical vessel directions (vein = oxygenated to fetus)
- 055.1 Placental transport: substrate → mechanism map
- 06The amino-acid (active) and IgG-vs-IgM traps
- 075.2 The placenta as an endocrine organ: hCG, progesterone, estriol, hPL
- 08The luteal–placental shift and the fetoplacental unit
Worked example: match each substrate to its placental transport mechanism
- +1(a) Oxygen — simple diffusion down its partial-pressure gradient, no ATP. Fetal HbF (higher O₂ affinity) pulls it off maternal HbA.
- +1(b) Glucose — facilitated diffusion via the GLUT-1 carrier, down its gradient, no ATP.
- +1(c) Amino acids — active transport against the gradient (the only uphill one), which is why fetal amino-acid levels can EXCEED maternal.
- +1(d) Maternal IgG — receptor-mediated endocytosis (FcRn), giving the newborn passive immunity.
- +1State the IgG/IgM trap: IgG crosses but IgM does NOT — so IgM in fetal/cord blood implies the fetus made it (fetal infection).
Key terms
- Chorionic villus
- The functional unit of the placenta: a finger of fetal tissue (a fetal capillary in a mesenchymal core, sheathed in trophoblast) projecting into the maternal intervillous space. The branching villous tree maximises exchange surface area.
- Placental barrier
- The thin layers separating maternal and fetal blood (which never mix): syncytiotrophoblast → cytotrophoblast → basal lamina → fetal capillary endothelium. It thins with gestation as fetal demand rises.
- Spiral-artery remodelling
- Extravillous trophoblast invade and convert maternal spiral arteries into wide, low-resistance, high-flow vessels. Failed remodelling leaves the placenta ischaemic and is the root cause of preeclampsia and IUGR.
- hCG
- Human chorionic gonadotropin, secreted by the syncytiotrophoblast. It mimics LH to rescue the corpus luteum (so it keeps making progesterone in early pregnancy) and is the target of pregnancy tests; it peaks at ~wk 8–10.
- Luteal–placental shift
- The hand-over (~wk 6–9) at which the placenta matures enough to make its own progesterone and takes over from the corpus luteum, which is no longer essential thereafter.
The Placenta FAQ
Do maternal and fetal blood mix in the placenta?
No — this is the single most-examined fact. The placenta is haemochorial: maternal blood bathes the villi in the intervillous space, but the fetal blood stays inside the villus capillaries, separated by the thin placental barrier. Substances cross the barrier; the bloods never touch.
Why can fetal amino-acid levels be higher than maternal?
Because amino acids cross by active transport, the only mechanism that moves a substrate against its gradient. Glucose, by contrast, only crosses by facilitated diffusion (GLUT-1), so it cannot exceed the maternal level.
Does hCG keep the placenta alive?
No — hCG rescues the corpus luteum (so the corpus luteum keeps making progesterone in early pregnancy). It does not directly maintain the placenta. After the luteal–placental shift (~wk 6–9) the placenta makes its own progesterone.
Why is the umbilical vein oxygenated?
Because the reference organ is the placenta, not the lungs. The umbilical vein carries oxygenated blood TO the fetus, and the two umbilical arteries carry deoxygenated blood back to the placenta — the reverse of the usual systemic convention.
Exam move
Build the chapter around two reliable mark-earners: the layered placental barrier (out→in) and the substrate→mechanism map. Memorise the map as three buckets — no-ATP/down-gradient (O₂, CO₂, glucose, fatty acids), ATP/against-gradient (amino acids, so fetal can exceed maternal), and endocytosis (IgG via FcRn) — and pre-load the two traps (amino-acid sign; IgG crosses but IgM does not). For the endocrine half, lock the hCG facts (source = syncytiotrophoblast; job = rescue the corpus luteum; basis of pregnancy tests; peaks wk 8–10) and the timeline (luteal→placental shift wk 6–9; estriol needs the fetal adrenal; hPL drives insulin resistance). Keep the cross-link ready: failed spiral-artery remodelling → preeclampsia.
