Fertilization and Early Embryogenesis

Fertilization marks the beginning of embryogenesis.
- Two gametes (sperm and oocyte) unite to initiate the developmental process.
- The interaction involves changes in membrane potential and cellular responses, preventing polyspermy.

Cortical granules, located just beneath the membrane, release contents in response to membrane potential change.
- Contents include digestive enzymes, polysaccharides, glycoproteins, and hymen.
- Enzymes like serine proteases cleave connections between the egg membrane and the extracellular matrix, allowing membrane separation.
- These polysaccharides cause osmotic imbalance, leading to water influx and membrane inflation.
- Peroxidase released cross-links tyrosines, solidifying the egg's protective matrix and hindering access for additional sperm.

A positive change in membrane potential stimulates calcium release.
- The calcium wave initiates the cortical granule fusion.
- Calcium stored in the endoplasmic reticulum facilitates oocyte maturation.
- Fluorophores indicate calcium release; their fluorescence indicates increased calcium presence.

Upon fertilization, metabolic activities, including lipid biosynthesis, are activated, crucial for forming membranes during cleavage.
- Following calcium wave, DNA and protein synthesis commence, with histones, tubulin, and actin produced, essential for subsequent divisions.
- NaF kinase activates the cell cycle, leading cells through phases until mitosis.

After fertilization, sperm nucleus separates from the centriole necessary for microtubule organization during cleavage.
- Centrioles, brought by sperm, organize microtubules required for chromosome separation in mitosis.
- Fertilization does not contribute mitochondrial DNA from the sperm; all mitochondria are maternally inherited.

Sperm chromatin undergoes processing post-fusion.
- Chromatin proteins are substituted with egg-derived proteins.
- Histone and lamina phosphorylation decondenses sperm chromatin, preparing for DNA replication.
- Zygotic nucleus formation occurs as nuclei migrate and fuse after initial cell cycles.

Cleavage refers to rapid cell divisions occurring post-fertilization, producing many blastomeres from a single zygote.
- Division rate varies across species (e.g., slower in mammals vs. quick in sea urchins).
- The cleavage stage lacks significant cell growth, focusing on cell division and genetic material distribution.
- Cyclins and cyclin-dependent kinases regulate the cell cycle through phosphorylation at specific phases.
- The regulatory mechanism drives transitions between cell cycle phases.

The cell cycle is accelerated during cleavage, allowing continued mitotic divisions with limited resting periods.
- At the blastula transition, synchronicity in cell divisions diminishes, with G1 and G2 phases introduced.
- Continued RNA synthesis from the developing embryo occurs near the end of cleavage.

Cleavage patterns depend on yolk distribution:
- Holoblastic cleavage: Entire embryo divides into cells containing yolk (e.g., amphibians).
- Isolecithal: Even division leads to cells of similar sizes.
- Mesolecithal: Dense yolk leads to unequal cell sizes.
- Meroblastic cleavage: Yolk remains undivided, typically seen in species with significant yolk reserves (e.g., birds).

Initial axes (anterior-posterior, dorsal-ventral, left-right) develop early in embryonic development.
- Molecular mechanisms and signals coordinate organ placement across both sides.

Utilization of model organisms, such as Drosophila melanogaster (fruit fly), to study developmental pathways.
- Drosophila serves as a primary genetic and embryological research organism, aiding in the understanding of signaling pathways and embryogenesis principles.