Embryonic Development and Placenta Formation in Humans

After implantation, the implanted embryo develops rapidly from a ball of cells into a recognisable human form with a beating heart within a few weeks. The placenta, a temporary organ formed partly from embryonic and partly from maternal tissue, connects the growing embryo to the mother's uterus.

This topic matters because the placenta is the embryo's lifeline — it supplies oxygen and food and removes wastes. Problems with the placenta can restrict fetal growth, and the sequence of development explains why certain weeks of pregnancy are most sensitive to infections, alcohol and medicines.

Key Concepts

• Gastrulation: the process in the third week by which the embryo forms three germ layers — ectoderm, mesoderm and endoderm.
• Ectoderm: gives rise to skin and the nervous system. Mesoderm: forms muscles, bones, blood and kidneys. Endoderm: forms the lining of the gut and lungs.
• Placenta: disc-shaped organ formed from the fetal chorion and maternal endometrium, connected to the fetus by the umbilical cord.
• Chorionic villi: finger-like projections containing fetal blood vessels that dip into maternal blood spaces, allowing exchange.
• hCG, hPL, oestrogen and progesterone: hormones connected with the placenta; hCG maintains early pregnancy, and the placenta later takes over progesterone and oestrogen production.
• Amnion and amniotic fluid: the membrane and fluid cushioning the embryo against shocks.

How It Works

1. Week 3: gastrulation lays down the three germ layers; the foundation of the nervous system appears.
2. Week 4: the heart begins to beat; limb buds appear.
3. Weeks 5 to 8: all major organs begin forming; by the end of week 8 the embryo is called a fetus.
4. Meanwhile the trophoblast develops chorionic villi that grow into the endometrium, forming the placenta, which is fully functional by about week 12.
5. Exchange: oxygen and nutrients diffuse from maternal blood across the villi into fetal blood; carbon dioxide and wastes diffuse the opposite way. The two bloods normally do not mix.
6. The placenta also acts as an endocrine organ, secreting hCG, hPL, oestrogens and progestogens.

Worked Example

Track where three organs come from using the germ layers. The brain develops from ectoderm; the heart develops from mesoderm; the inner lining of the intestine develops from endoderm. Now follow size: at week 4 the embryo is about 4 to 5 mm long, roughly a grain of rice; by week 8 it is about 30 mm; by week 12 about 9 cm. So between weeks 4 and 12 its length grows from about 5 mm to about 90 mm, an increase of 90 / 5 = 18 times.

Common Misconceptions

• "The mother's blood flows directly into the baby." Wrong. Maternal and fetal blood stay separate; substances cross by diffusion across the villi.
• "The placenta filters out everything harmful." It blocks many substances, but alcohol, nicotine, some viruses and drugs can cross and harm the fetus.
• "The embryo is fully formed from the start." Organs develop in a fixed sequence over weeks; the first 8 weeks are the most delicate period.

Real-World Applications

Ultrasound examinations use developmental milestones to estimate the age and health of the fetus. Doctors advise folic acid before and during early pregnancy because it supports neural tube (ectoderm) development. Placental hormone levels such as hCG are monitored in blood tests to check pregnancy progress.

Embryonic development builds the body from three germ layers week by week, while the placenta grows alongside as the exchange surface and hormone producer that sustains pregnancy.