Topic 14- Development 1

Outcomes:

• sequence animal development

• compare and contrast fertilization in different animals

• sequence the process of fertilization

• sequence the process of cleavage

• compare and contrast cleavage patterns

• hypothesize and diagnose the impact of variability on cleavage patterns

I. Introduction

A. Development

• Development refers to all changes that occur during an organism’s lifetime.

B. Model Organism

• Deuterostome development: A developmental mode in animals where the blastopore becomes the anus.

C. Ongoing Process

• The egg has a plasma membrane and outer coverings that:

  • Protect it from environmental factors.

  • Facilitate the exchange of nutrients and waste during early development.

• Mechanisms like gametic isolation and prevention of polyspermy ensure successful fertilization.

II. Fertilization

A. Introduction

• Fertilization = n (sperm) + n (egg) → 2n (zygote)

• Key outcomes of fertilization:

  • Restores diploid (2n) ploidy.

  • Sometimes determines sex.

  • Activates the egg and begins embryonic development.

• Begins with sperm contacting the egg:

  • Acrosome reaction: Enzymes released from the sperm’s acrosome break down the zona pellucida (ZP).

  • Plasmogamy: Fusion of egg and sperm plasma membranes.

  • Cortical reaction and slow block to polyspermy prevent additional sperm from entering.

  • Egg activation begins.

  • Karyogamy: Fusion of egg and sperm nuclei → zygote.

  • Ends with first cleavage → embryo now exists.

B. Fertilization in Mammals

1. Structures Around Egg

• Plasma membrane (PM)

• Zona pellucida (ZP)

• Follicle cells

2. Vocabulary

• Acrosome: Packet of enzymes in sperm head.

• Cortical granules: Vesicles in egg that release enzymes during cortical reaction.

• Polyspermy: Fertilization by more than one sperm.

3. Process Details

• Sperm binding to ZP triggers acrosomal reaction.

• Enzymes from the acrosome dissolve outer layers of the egg.

• Sperm fuses with egg plasma membrane (plasmogamy).

• Cortical reaction: Calcium ions are released from the smooth ER into the cytoplasm.

  • This triggers cortical granules to release enzymes.

  • These enzymes modify the ZP to prevent more sperm entry (slow block to polyspermy).

C. Egg Activation

• Triggered by rise in intracellular Ca²⁺ levels.

• Leads to:

  • Increased cellular respiration.

  • Activation of maternal enzymes and proteins.

  • Protein synthesis using existing maternal mRNA.

• After a few hours:

  • Sperm nucleus is guided to egg nucleus via microtubules.

  • Meiosis II completes in egg (oogenesis).

  • Karyogamy results in a diploid, totipotent zygote.

III. Cleavage

A. Introduction

• Single-cell zygote undergoes rapid mitotic divisions to form the embryo.

• Characterized by:

  • No G₁ or G₂ phases.

  • No cell growth or protein synthesis.

B. Stages of Cleavage

1. Zygote (2n) – single cell.

2. Embryo (2n) – 2 or more cells.

3. Blastula (2n) – hollow ball of ~128 blastomeres.

   • Cells produced are identical via mitosis (indeterminate cleavage).

   • Cleavage furrow: Indentation during cytokinesis in the animal hemisphere.

   • Animal pole: Develops into the embryo.

   • Vegetal pole: Contains yolk.

C. Cleavage Patterns

1. Equal Holoblastic

• Cleavage furrow passes entirely through the egg.

• Equal division of cytoplasm → equal blastomeres.

• Occurs in eggs with little yolk (e.g., sea urchins, humans).

2. Unequal Holoblastic

• Cleavage furrow passes entirely through egg but off-center.

• Unequal division of cytoplasm → unequal blastomeres.

• Occurs with moderate yolk (e.g., frogs).

3. Meroblastic

• Cleavage furrow does not fully pass through egg.

• Cleavage only occurs in animal pole.

• Occurs with large yolk (e.g., birds, reptiles, some fish).