15.4 Development of Human Foetus
Human development begins when a single sperm successfully fertilises a secondary oocyte within the Fallopian tube. Following fertilisation, the resulting diploid zygote divides repeatedly by mitosis as it travels towards the uterus, transforming into a morula and then a blastocyst before implanting into the uterine wall. To sustain and protect the developing foetus, specialised structures including the placenta and umbilical cord develop to facilitate nutrient, gas, and waste exchange, while keeping maternal and foetal circulations safely separate.
Learning Objectives
- Explain the process of fertilisation and the early developmental stages of an embryo up to implantation.
- Describe the role of human chorionic gonadotropin (HCG) hormone during early pregnancy.
- Describe the structure and functions of the placenta and umbilical cord in foetal development.
- Justify the importance of the physical separation between foetal and maternal blood circulatory systems.
Early Embryo Development, Implantation, and Placental Exchange
Trace the chronological pathway of the zygote down the Fallopian tube into the endometrium, then explore the magnified anatomical schematic of maternal-foetal exchange across the placental membrane.
Zygote
A diploid cell formed when the sperm nucleus fuses with the ovum nucleus.
Morula
A solid ball of cells produced by repeated mitotic divisions.
Blastocyst
A hollow sphere of cells whose inner cell wall develops into the embryo.
Implantation
The blastocyst embeds itself into the thickened endometrium.
- Glucose
- Amino acids
- Hormones
- Antibodies
- Oxygen
- Carbon dioxide
- Nitrogenous wastes such as urea
Short Explanation
Fertilisation
Fertilisation occurs when one sperm penetrates the secondary oocyte in the Fallopian tube. Upon penetration, changes occur to the secondary oocyte's membrane to prevent the entry of other sperms. Fertilisation is complete when the sperm nucleus fuses with the ovum nucleus to form a diploid zygote.
Early development
While travelling down the Fallopian tube toward the uterus, the zygote undergoes repeated mitotic divisions. The zygote first forms a two-cell embryo, then continues dividing to form a solid ball of cells called a morula. The morula transforms into a hollow sphere of cells known as a blastocyst.
Implantation
The blastocyst embeds itself into the thickened wall of the endometrium. This process is called implantation. The thickened inner cell wall of the blastocyst develops into the embryo, and the blastocyst continues to grow throughout pregnancy.
Role of HCG
Human chorionic gonadotropin (HCG) is produced by the placenta during pregnancy. It ensures that the corpus luteum does not degenerate and continues to secrete progesterone and oestrogen in the early stages of pregnancy. HCG is excreted and can be detected in the urine of pregnant mothers using home pregnancy test kits.
Placenta and umbilical cord
The placenta is formed from both maternal endometrial tissue and embryonic tissue. It is connected to the foetus via the umbilical cord, which contains one umbilical vein and two umbilical arteries.
Endocrine role of the placenta
By the fourth month of pregnancy, the corpus luteum degenerates and stops secreting progesterone. The placenta takes over the production of progesterone and oestrogen required to maintain the thickness and vascularisation of the endometrium.
Separation of circulations
Maternal blood and foetal blood do not mix; they are separated by a thin membrane. This separation protects the foetus from toxins and bacteria, prevents fragile foetal capillaries from bursting under high maternal blood pressure, and prevents lethal blood agglutination.
Try to Answer First
Answer in your mind, then press "Check Answer".
What happens to the secondary oocyte membrane immediately after a sperm enters?
What is the main function of the HCG hormone during the early stages of pregnancy?
Why must maternal blood and foetal blood be kept separate by a thin membrane?
Common Mistakes
- Confusing the roles of the umbilical vein and umbilical arteries: the umbilical vein carries oxygenated, nutrient-rich blood, while the umbilical arteries carry deoxygenated, waste-rich blood.
- Assuming the placenta is composed solely of maternal tissue: it is actually formed from both maternal endometrial tissue and embryonic tissue.
- Believing that the placenta acts as a complete, impenetrable barrier against all harmful substances: it cannot block drugs, alcohol, cigarette chemicals, or viruses like HIV and rubella.
Concept Misunderstandings
Maternal blood flows directly into the foetus's blood vessels to supply oxygen and nutrients.
Maternal and foetal blood never mix directly; all substances diffuse across a thin separating membrane between maternal blood spaces and foetal blood capillaries.
The corpus luteum secretes progesterone throughout the entire nine months of pregnancy.
The corpus luteum degenerates around the fourth month; the placenta takes over the role of secreting progesterone and oestrogen for the remainder of the pregnancy.
Summary
- Fertilisation: Fusion of sperm and ovum nuclei in the Fallopian tube forms a diploid zygote.
- Development & Implantation: Zygote → 2 cells → Morula → Blastocyst → Implantation into the endometrium.
- HCG: Placental hormone that maintains the corpus luteum during early pregnancy; detectable in urine.
- Umbilical Cord: Contains one umbilical vein (oxygen/nutrients to foetus) and umbilical arteries (carbon dioxide/urea to placenta).
- Placenta: Exchange site for gases, nutrients, and wastes; endocrine producer of progesterone and oestrogen from month 4 onwards.
- Separation of Circulations: Prevents capillary rupture, stops blood agglutination, and protects against certain pathogens/toxins.
Short Activity
Objective: test your understanding of foetal development, HCG, placenta and umbilical cord functions.
A. Objective Quiz
Which blood vessel carries blood rich in oxygen and nutrients from the placenta to the foetus?
What is the primary function of the hormone human chorionic gonadotropin (HCG) in early pregnancy?
B. Fill in the Blanks
During embryonic development, repeated mitotic divisions convert the two-cell embryo into a solid ball of cells called a .
By the fourth month of pregnancy, the corpus luteum degenerates and the takes over the role of producing progesterone and oestrogen.
C. Matching / Drag and Drop
Match each reproductive structure or hormone to its correct function or description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Structure or Hormone to Its Function
Write the correct structure or hormone next to each description.
| Description / Function | Structure / Hormone |
|---|---|
| 1. Carries oxygen- and nutrient-rich blood from the placenta to the foetus | |
| 2. Carry deoxygenated blood and metabolic waste like urea from the foetus to the placenta | |
| 3. Maintains corpus luteum activity during the first trimester and is detectable in urine tests | |
| 4. Site of substance exchange formed from endometrial and embryonic tissues |
Answer choices: Umbilical vein • Umbilical arteries • HCG • Placenta
