Ferilization and Diferentation

1. Fertilization (Mechanism)

Sequence of Events

  1. Sperm contacts egg

    • Acrosome releases hydrolytic enzymes

    • Enzymes digest egg coverings (jelly coat)

  2. Penetration

    • Sperm enters egg membrane

  3. Cortical Reaction

    • Cortical vesicles release cortical granules

    • Forms fertilization membrane

    • Blocks additional sperm → prevents polyspermy

Activation

  • Sperm entry triggers:

    • ↑ metabolic rate

    • ↑ oxygen consumption

  • Prepares zygote for cleavage

Syngamy

  • Male + female pronuclei fuse

  • Restores diploid (2n)

  • Forms zygote

2. Types of Fertilization

Feature

Internal

External

Location

Inside body

Outside body

Gametes

Fewer

Many

Examples

Humans

Fish, sea urchins

3. Differentiation (Core Concept)

  • All cells:

    • Same DNA

  • Different cell types:

    • Due to selective gene expression

Key Terms

  • Differentiation

    • Cells become specialized

  • Determination

    • Cell fate becomes fixed (irreversible)

  • Committed cell

    • Cannot change developmental path

4. Experimental Evidence

Hans Driesch (Sea Urchins)

  • Early blastomeres (2–4 cells):

    • Each → complete organism

  • Indicates:

    • Cells initially totipotent

Later Stages

  • After ~3rd cleavage:

    • Cells produce only parts

  • Indicates:

    • Differentiation has begun

5. Cytoplasmic Determinants

  • Egg cytoplasm contains:

    • Inducers + repressors

  • Distributed during cleavage

Cleavage Pattern

  • First cleavages:

    • Equal distribution

  • Third cleavage:

    • Unequal distribution

    • Cells become different

Outcome

  • Different cytoplasmic environments → different gene activation

6. Indeterminate vs Determinate Development

Indeterminate (Regulative)

  • Early cells:

    • Can form entire organism

  • Example:

    • Humans

Determinate (Mosaic)

  • Cell fate fixed early

  • Example:

    • Tunicates

7. Control of Early Development

  • Early stages:

    • Controlled by cytoplasm (maternal factors)

  • Evidence:

    • Cells lack functional nucleolus → minimal transcription

Transition

  • Later:

    • Embryo genome activates

    • Cells control own development

8. Environmental Effects on Differentiation

External Factors

  • Gravity, light, pH, pressure

Example:

  • Fucus algae

    • “Down” side → holdfast formation

Internal Factors (Induction)

  • Cells influence nearby cells

Examples:

  • Eyecup experiment

    • Induces lens formation

  • Spemann organizer

    • Dorsal lip → induces nervous system

    • Can create duplicate body axes

9. Spemann’s Differentiation Model

Core Principles

  1. All cells have same genome

  2. Cytoplasmic differences → initial gene activation

  3. Environment influences development

  4. Cells release signals (inducers/repressors)

  5. Differentiation spreads (cascade effect)

  6. Once determined → irreversible

Conceptual Model

  • Development = branching pathways

  • Cell fate becomes progressively restricted

10. Evidence for Differentiation Progression

Tissue Transplants

  • Early stage:

    • Cells adapt to new location

  • Later stage:

    • Cells retain original fate

Cell Reaggregation

  • Gastrula cells:

    • Reform germ layers

  • Neurula cells:

    • Reform specific organs

Nuclear Transplant (Briggs & King)

  • Early nucleus → full organism

  • Later nucleus → reduced success

  • Advanced nucleus → no development

11. Key Conclusion

  • Differentiation involves:

    • Irreversible changes in gene expression

  • Early development:

    • Cytoplasm-driven

  • Later development:

    • Genome-driven

12. High-Yield Summary

  • Acrosome → enzyme release

  • Cortical reaction → blocks polyspermy

  • Syngamy → diploid zygote

  • Early cells = totipotent

  • Cytoplasmic determinants → cell fate

  • Determination = irreversible commitment

  • Induction = cells influence neighbors

  • Differentiation = gene expression changes

  • Early control = cytoplasm

  • Later control = nucleus/genome