Developmental Biology Terms

  • Oogonium (Oogonia)

    • Definition: Female germ cell involved in egg development.
    • Characteristics:
    • Diploid stem cell found in ovaries.
    • Undergoes mitosis to produce more oogonia or differentiate into a primary oocyte.
    • Commits to becoming an oocyte and enters meiosis.

  • Formation of the 16-Cell Cyst (Drosophila)

    • Development Process: Four incomplete mitotic divisions from the oogonium lead to a cyst of 16 interconnected cells:
    • Cell division progression: 1 → 2 → 4 → 8 → 16.
    • Cells remain connected by cytoplasmic bridges (ring canals).

  • Differentiation of the 16 Cells

    • Outcome: 1 cell becomes the oocyte, and 15 cells become nurse cells.
    • Influencing Factors:
    • Position within the cyst.
    • Microtubule polarity and specific protein localization (e.g., Bicaudal D).

  • Role of Nurse Cells

    • Function:
    • Synthesize and transfer RNAs, proteins, organelles to the oocyte.
    • Ensures oocyte is prepared for embryo development post-fertilization.

  • Oocyte to Embryo Transition

    • Process:
    • Mature oocyte is ovulated and fertilization triggers completion of meiosis.
    • Results in a zygote, commencing embryonic development.

  • Zygotic Expression

    • Definition: Embryo utilizes its genome to produce RNAs and proteins rather than relying on maternal contributions.
    • Key Concepts:
    • Early development managed by maternal gene products.
    • Zygotic genome activation (ZGA) is the transition to embryo expressing its own genes.

  • Zygotic Expression Timeline (Drosophila)

    • Occurs during mid-blastula transition (MBT) around nuclear division cycle 14.
    • Events at MBT:
    • Cell cycles slow down, zygotic genome transcription starts, and cellularization of syncytial blastoderm begins.
    • Key gene: Zelda (transcription factor).

  • Mammal ZGA Comparison

    • Mouse: Minor ZGA at 1-cell stage, major at 2-cell stage.
    • Human: Major ZGA at 8-cell stage.

  • Ovary Structure (Drosophila)

    • Components of each egg chamber (follicle):
    • 1 Oocyte (result from 16-cell cyst).
    • 15 Nurse Cells (support oocyte development).
    • Follicle Cells (form epithelial layer around germline cells).
      • Roles in signaling and patterning (anterior/posterior and dorsal/ventral axes).

  • Insect Ovary Types

    • Polytrophic Ovarioles (e.g., Drosophila):
    • Each acts as a mini conveyor belt, continually surrounding oocytes with follicle cells as they mature.
    • Telotrophic & Panoistic Ovarioles (other insects):
    • Variations exist in nurse cell proximity to germarium,
    • Maintain continuous egg production.

  • Microtubule Organization (Stages 1-6)

    • Initial setup: Microtubules centered around oocyte nucleus at posterior end.
    • Motor proteins (Dynein and Kinesin) assist mRNA and protein transport.

  • Major Change (Stage 7-9):

    • Posterior follicle cells signal oocyte via EGF receptor pathway, causing the MTOC to relocate and microtubule polarity to flip.
    • New polarity enables transport of essential mRNAs (Oskar and Bicoid) to respective ends.

  • Microscopic Communication via Ring Canals

    • Connection between oocyte and nurse cells allows direct transport of materials without membrane barriers.

  • Significant Factors in MTOC Dynamics

    • Gurken mRNA: Encodes TGF-alpha-like signaling protein, plays key role in patterning axes.
    • PKA Localization: Asymmetric PKA localization is crucial for new MTOC establishment and microtubule polarity.

  • Plasma Membrane Role in Microtubule Organization

    • Non-centrosomal microtubule organization crucial for embryo axis formation.
    • Proteins at plasma membrane anchor and stabilize microtubules, allowing for directional transport of mRNAs.

  • Important mRNA/Proteins

    • Nanos: Preserves posterior development through inhibition of hunchback mRNA.
    • Cup: Prevents premature translation of Oskar during transport to posterior.
    • Smaug: Targets maternal mRNAs for degradation, aiding zygotic transition.

  • Caudal Transcription Factor

    • Distributed uniformly in oocyte but translated only at the posterior, coordinating with Bicoid for anterior-posterior axis development.

  • Dorsal-Ventral Polarity:

    • To be discussed further in detail.