Developmental Biology Study Guide

Overview of Oogenesis in Drosophila melanogaster

• Oogonium:
  • Definition: A diploid stem cell located in the ovaries responsible for female gamete development.
  • Function: Undergoes mitosis to produce more oogonia or differentiate into primary oocytes.
  • Mitosis leads to the primary oocyte entering meiosis upon commitment to become an egg.

Formation of the 16-Cell Cyst

• After committing to oocyte development, the oogonium undergoes four incomplete mitotic divisions, resulting in 16 interconnected cells.
  • Division Process:
  • Sequence: 1 → 2 → 4 → 8 → 16 (each division doubles the number of cells).
  • Incomplete divisions mean cells stay connected via ring canals (cytoplasmic bridges).

Differentiation of the 16 Cells

• Out of the 16 cells formed:
  • 1 cell becomes the oocyte.
  • 15 cells become nurse cells.
• Factors influencing oocyte selection:
  • Position within the cyst.
  • Microtubule polarity and specific protein localization (e.g., Bicaudal D).

Role of Nurse Cells

• Nurse cells support the oocyte by:
  • Synthesizing and transferring RNAs, proteins, and organelles through ring canals.
  • Ensuring the oocyte is equipped to develop into an embryo post-fertilization.

Transition to Embryogenesis

• Once matured, the oocyte is:
  • Ovulated and potentially fertilized.
  • Completes meiosis upon fertilization.
  • Results in a zygote that starts embryonic development.

Zygotic Expression

• Definition: Point when the embryo starts utilizing its own genome for RNA and protein production.

1. General Overview:
   • Early development is governed by maternal gene products.
   • Zygotic genome activation (ZGA) signifies the switch to the embryo's own DNA transcription.
2. In Drosophila:
   • Zygotic expression begins during the mid-blastula transition (MBT) around nuclear division cycle 14.
   • During MBT, cell cycles slow, transcription begins, and cellularization occurs.
   • Gene Zelda acts as a key transcription factor activating zygotic gene expression.
3. In Mammals:
   • ZGA is gradual, significantly occurring at different stages:
     • Mouse: Minor at 1-cell stage, major at 2-cell stage.
     • Human: Major at the 8-cell stage.

• Summary of ZGA:
  • Pre-ZGA: Embryo relies on maternal mRNAs/proteins.
  • At ZGA: Embryo starts expressing its own genes.

Structure of Drosophila Ovary

• Each egg chamber (follicle) includes:
  • 1 Oocyte: The egg cell selected from a 16-cell cyst.
  • 15 Nurse Cells: Sister cells that supply the oocyte with necessary materials.
  • Follicle Cells: Somatic cells surrounding the egg chamber that assist in signaling and development.

Organization of Insect Ovaries

1. Polytrophic Ovarioles (e.g., Drosophila):
   • Act as mini conveyor belts for egg production with stepwise maturation.
2. Other Types (Telotrophic & Panoistic):
   • Variation in nurse cell arrangement.

• The ovarian structure enables continuous egg production, aiding research into oocyte development.

Microtubule Organization in Oocyte Development

• Stage 1–6: Microtubules centered around the nucleus, positioned for material transport by nurse cells using motor proteins (Dynein and Kinesin).
• Stage 7–9: Microtubule reorganization marked by:
  • Posterior to anterior relocation of the microtubule organizing center (MTOC).
  • Establishes the polarity necessary for embryo axial development (anterior vs. posterior).

Communication Mechanisms During Oogenesis

1. Ring Canals: Connect germline cells, facilitating nutrient and mRNA transport.
2. Gurken Signaling: Involves interactions between germline (oocyte) and somatic (follicle) cells, crucial for microtubule polarity and development.
3. Coordinated Timing: Synchronization of cell cycles and morphogenetic changes ensures proper development timing.

Gurken mRNA and Axis Specification

• Gurken: A TGF-alpha-like protein secreted by the oocyte, activates EGFR on follicle cells to determine dorsal-ventral polarity.
• Localization shifts from posterior to anterodorsal as the oocyte nucleus migrates.
• Essential for establishing the anterior-posterior and dorsal-ventral axes of the embryo.

Key Proteins in Oocyte Development

1. Nanos:
   • Specified posterior cell fates and inhibits translation of maternal hunchback mRNA.
2. Cup:
   • Prevents premature translation of oskar mRNA, ensuring correct timing and localization.
3. Smaug:
   • Targets maternal mRNAs for degradation during the maternal-to-zygotic transition.
4. Caudal:
   • Activates posterior fate genes, with its mRNA uniformly distributed but protein translated only posteriorly.

Summary of Significance

• Proper localization of mRNAs (Bicoid, Oskar, Nanos, and others) and the establishment of microtubule polarity are critical for developing body axes in the Drosophila embryo. Disruptions in these processes can lead to improper embryo development.