Biology 30 · Development
From one cell to a baby
Development is a sequence, so learn it in order: fertilization, cleavage, implantation, germ layers, organs, birth. Most questions ask what happens at one step, or which hormone runs it.
The first week
The idea: the zygote divides again and again without growing (cleavage), becomes a hollow ball called a blastocyst, and implants in the endometrium about a week after fertilization.
- Fertilization in the oviduct makes a diploid zygote.
- Cleavage: rapid mitosis with no overall growth, giving a solid ball, the morula.
- The blastocyst is hollow. Its inner cell mass becomes the embryo; its outer layer, the trophoblast, helps form the placenta.
- Implantation: the blastocyst embeds in the endometrium about a week after fertilization.
Twins
Identical twins come from one early embryo splitting in two, so they share the same DNA. Fraternal twins come from two separate eggs fertilized by two sperm.
Germ layers and organs
The idea: during gastrulation the embryo forms three germ layers, and every organ in the body comes from one of them.
| Germ layer | Becomes | Memory hook |
|---|---|---|
| Ectoderm | Nervous system, skin, hair, nails, eye lens | ecto = outer |
| Mesoderm | Muscle, bone, blood, heart, kidneys, reproductive organs | meso = middle |
| Endoderm | Lining of the digestive and respiratory tracts, liver, pancreas | endo = inner |
Organogenesis, the forming of organs, happens mostly in the first trimester. After about eight weeks, the embryo is called a fetus, and development shifts mainly to growth.
Teratogens
A teratogen is anything that can cause birth defects: alcohol, some medications, and some infections such as rubella. They are most harmful in the first trimester, because that is when organs are forming. The neural tube, which becomes the brain and spinal cord, closes within the first month — often before a pregnancy is known — which is why folic acid is recommended early.
Watch out: mixing up the layers. The nervous system comes from the outer layer, ectoderm, even though it ends up deep inside the body.
The placenta and pregnancy hormones
The idea: the endometrium must be maintained for the whole pregnancy. First hCG keeps the corpus luteum making progesterone; later the placenta makes the hormones itself.
| Membrane | Job |
|---|---|
| Amnion | Fluid-filled sac that cushions and protects the embryo |
| Chorion | Forms the fetal side of the placenta |
| Yolk sac | Makes the first blood cells |
| Allantois | Forms blood vessels of the umbilical cord |
How the placenta works
Maternal and fetal blood run very close together but do not mix. Oxygen and nutrients diffuse across the thin membranes to the fetus; carbon dioxide and wastes diffuse back. Harmful substances can cross the same way.
The hormone handoff
- Normally the corpus luteum breaks down about two weeks after ovulation.
- After implantation, the embryo's tissue releases hCG, which keeps the corpus luteum making progesterone. hCG is what pregnancy tests detect.
- By about three months, the placenta makes enough progesterone and estrogen itself, so the corpus luteum is no longer needed.
Birth and breastfeeding
The idea: labour is driven by oxytocin in a positive feedback loop; after birth, prolactin makes milk and oxytocin releases it.
Labour: positive feedback
The baby presses against the cervix, which signals the pituitary to release oxytocin. Oxytocin makes the uterus contract harder, which presses the baby against the cervix even more, which releases more oxytocin. The loop keeps building until birth ends it. Unlike negative feedback, it amplifies the change.
Breastfeeding
- Prolactin (anterior pituitary) tells the mammary glands to produce milk.
- Oxytocin (posterior pituitary) squeezes the milk out — the let-down reflex.
- Suckling stimulates the release of both.
Watch out: prolactin makes milk; oxytocin releases it. And oxytocin shows up twice in this unit — contractions and let-down.