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Gametes, Oocyte Structure, Oogenesis, Spermatogenesis, Fertilization,…
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Oocyte Structure
Size and Composition
Large cell (~100 µm in diameter, much larger than typical somatic cells).
Contains reserves of lipids, proteins, and polysaccharides.
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Cytoplasmic Structures
Balbiani body: Aggregates of organelles (Golgi, mitochondria).
Organelles (Golgi, mitochondria, ER) increase as the oocyte matures.
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Oogenesis
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Meiosis and Arrest
Oogonia undergo mitosis, differentiate into primary oocytes, and arrest in prophase I.
Maturation
Hormonal signals resume meiosis I during ovulation, forming a secondary oocyte.
Final Stages
Meiosis II is completed upon fertilization, producing a mature egg and polar bodies.
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Spermatogenesis
Phases
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Primary spermatocytes divide by meiosis I, forming secondary spermatocytes.
Secondary spermatocytes complete meiosis II, producing spermatids.
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Fertilization, Gastrulation, and Types of Placenta
Fertilization
Steps
Sperm Capacitation
Membrane becomes more fluid, enabling penetration.
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Fusion and Activation
Sperm and egg membranes fuse, triggering the egg to complete meiosis II.
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Gastrulation
Definition
Reorganization of the blastula into a three-layered structure (ectoderm, mesoderm, endoderm).
Germ Layers
Ectoderm: Nervous system, skin, sensory organs.
Mesoderm: Muscles, bones, blood vessels, kidneys.
Endoderm: Gastrointestinal tract, liver, pancreas, respiratory lining.
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