Early Developmental Stages
High-Yield Summary
- Order: fertilization → cleavage → blastulation → implantation → gastrulation → neurulation.
- Fertilization occurs in the ampulla; cortical reaction (Ca²⁺ release) blocks polyspermy and triggers completion of Meiosis II.
- Cleavage divides cytoplasm without growth; indeterminate cleavage (any cell = any fate) enables monozygotic twinning, determinate cleavage does not.
- Blastocyst = trophoblast (future placenta/chorion) + inner cell mass (future organism); implants via chorionic villi into the endometrium.
- Gastrulation forms 3 germ layers via invagination: ectoderm (skin, nervous system), mesoderm (muscle, circulatory, excretory), endoderm (gut/respiratory linings).
- Neurulation: notochord signals ectoderm → neural plate → neural folds/groove → neural tube (future CNS); neural crest cells migrate to form PNS + other tissues.
Key Terms
- Cortical reaction
- Ca²⁺ release at fertilization that depolarizes the egg membrane, blocking polyspermy and forming the fertilization membrane.
- Morula → blastula/blastocyst
- Solid ball of cells (morula) becomes hollow (blastula/blastocyst) once fluid fills the blastocoel.
- Trophoblast
- Outer layer of the blastocyst; forms chorion and placenta.
- Inner cell mass
- Cluster of blastocyst cells that becomes the organism itself.
- Blastopore
- Opening formed during gastrulation; becomes the anus in deuterostomes (humans), the mouth in protostomes.
- Notochord
- Mesodermal rod that signals overlying ectoderm to form the neural plate; never becomes part of the nervous system itself.
- Neural crest
- Cells breaking away at the neural fold edges; migrate to form the PNS, pigment cells, adrenal medulla parts, head/neck connective tissue.
Early Development Sequence
- 1Fertilization — sperm crosses corona radiata + zona pellucida via acrosomal enzymes; cortical reaction blocks polyspermy; Meiosis II completes.
- 2Cleavage — rapid mitotic divisions split existing cytoplasm (no growth); first division marks zygote → embryo.
- 3Blastulation — morula (solid) → blastocyst (hollow): trophoblast (outer) + inner cell mass (future organism).
- 4Implantation — blastocyst attaches to endometrium; trophoblast forms chorionic villi; yolk sac/allantois/amnion/chorion support the embryo before the placenta functions.
- 5Gastrulation — invagination forms the archenteron and 3 germ layers (ectoderm, mesoderm, endoderm) via the blastopore.
- 6Neurulation — notochord signals ectoderm → neural plate → neural folds/groove → neural tube; neural crest cells migrate outward.
Twin Type by Split Timing (Monozygotic)
| Split Timing | Shared Structures |
|---|---|
| Days 1–3 | Dichorionic, diamniotic — separate placenta, chorion, amnion |
| Days 4–8 | Monochorionic, diamniotic — shared placenta/chorion, separate amnion |
| Days 8–13 | Monochorionic, monoamniotic — everything shared (cord entanglement risk) |
| After day 13 | Incomplete split — conjoined twins |
Common MCAT Trap
- Dizygotic (fraternal) twins are just as genetically similar as regular siblings — always 2 separate eggs + 2 separate sperm, each with own placenta/chorion/amnion, regardless of timing.
- More shared structures in monozygotic twins = later split = higher complication risk — direction is easy to reverse by mistake.
- Blastopore fate flips between deuterostomes (anus — humans) and protostomes (mouth) — don't assume one direction universally.
Quick Recall
What structure does the notochord signal to form the neural plate?
What are the two components of the blastocyst?
Which twinning process makes monozygotic (identical) twinning possible?
What does the blastopore become in deuterostomes like humans?