Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 47.1 Fertilization and Cleavage Initiate Embryonic Development
Embryonic development begins with fertilization—the union of haploid sperm and egg into a diploid zygote—followed by cleavage, a rapid series of cell divisions. These early events are conserved across many species and set the stage for later morphogenesis.
1. Fertilization Overview
- Fertilization can occur externally (e.g., sea urchins) or internally (e.g., mammals)
- Triggers include the acrosomal reaction, cortical reaction, and egg activation
2. Acrosomal Reaction
- Sperm releases enzymes from its acrosome to digest the egg’s jelly coat
- Acrosomal process binds to species-specific receptors on the egg
- Fusion of membranes allows sperm nucleus entry
- Fast block to polyspermy: membrane depolarization prevents additional sperm entry
3. Cortical Reaction and Fertilization Envelope
- Cortical granules release enzymes that modify the egg membrane
- Forms a fertilization envelope and hardens the vitelline layer
- Slow block to polyspermy ensures only one sperm fertilizes the egg
- Triggered by a Ca²⁺ wave from the egg’s ER
4. Egg Activation
- Ca²⁺ rise activates metabolism—respiration and protein synthesis increase
- Nuclei fuse ~20 min after fertilization (in sea urchins)
- First division ~90 min later in sea urchins; 12–36 hrs in mammals
- Mammals lack fast block but undergo cortical reaction and zona pellucida changes
5. Cleavage
- Rapid S and M phases; G1 and G2 skipped
- Zygote divides into small blastomeres
- Forms a blastula with a central blastocoel
- Driven by maternal mRNA and proteins stored in the egg
6. Cleavage Patterns Across Species
- Holoblastic cleavage: complete division (echinoderms, mammals, frogs)
- Meroblastic cleavage: partial division in yolk-rich eggs (birds, reptiles, insects)
- In frogs, yolk causes uneven cleavage
- In insects, nuclei divide without membranes initially
7. What Ends Cleavage?
- After ~12 divisions in frogs, G1 and G2 phases reappear
- Nucleus-to-cytoplasm ratio or division count may signal the end of cleavage
- Polyspermic embryos end cleavage early, supporting the ratio hypothesis
In a Nutshell
Fertilization initiates embryonic development by merging sperm and egg, blocking polyspermy, and activating metabolic pathways. Cleavage partitions the zygote into blastomeres, forming a blastula. Despite variations across species, these early events are highly conserved and essential for setting up the body plan.