lecture 3 fertilization

Fertilization

The process involving the fusion of sperm and egg

Egg

A non-motile gamete

Sperm

A motile gamete that moves toward the egg

Fertilization sequence

A common sequence across species despite differences in fertilization modes

Contact and recognition

The first major event between sperm and egg

Species-specific fertilization

Ensures sperm fertilizes only an egg from the same species

Polyspermy

Entry of multiple sperm leading to abnormal development

Monospermy

Normal condition where only one sperm enters the egg

Block to polyspermy

Mechanisms that prevent additional sperm from entering

Fusion of genetic material

Combination of sperm and egg nuclei

Egg activation

Initiation of embryonic development triggered by sperm nucleus

Sperm structure

Composed of head, midpiece, and tail

Head (sperm)

Contains haploid nucleus and acrosome

Acrosome

Membrane-bound structure with enzymes for egg penetration

Midpiece

Contains organelles and centriole for structural support

Tail

Contains axoneme responsible for movement

Axoneme

Microtubule-based structure composed of tubulin

Dynein

Motor protein driving sperm tail movement

Capacitation

Functional change making sperm capable of fertilization

Egg structure

Typically spherical and surrounded by protective layers

Vitelline envelope

Layer with proteins and glycoproteins for sperm binding

Jelly coat

Additional outer layer in some species like fish and amphibians

Cortex

Gel-like outer cytoplasmic region beneath egg membrane

Microvilli

Finger-like projections aiding sperm binding and entry

Cortical granules

Vesicles releasing proteins to block polyspermy

Ooplasm

Cytoplasm of the egg rich in nutrients and regulatory materials

Maternal contribution

Most cytoplasmic materials and organelles come from egg

Nutritive proteins

Provide energy in the form of yolk

Maternal mRNA

Pre-formed transcripts used immediately after fertilization

Morphogenetic factors

Regulate cell differentiation and body plan formation

Protective molecules

Include UV-protective pigments and DNA repair enzymes

Metaphase II arrest

Stage where oocyte development is paused in many species

Completion of meiosis II

Triggered by fertilization

Polar bodies

Byproducts of meiosis resulting in one haploid egg

Chemoattraction

Egg releases chemicals guiding sperm movement

Chemotaxis

Movement of sperm toward chemical signals

Resact

Chemoattractant that binds sperm receptors

Ca2+ influx

Enhances sperm motility and direction

Acrosomal reaction

Release of enzymes from acrosomal vesicle

Acrosomal vesicle

Structure at sperm tip containing enzymes

Extracellular matrix binding

Sperm attaches after penetrating egg layers

Sperm penetration

Enzymatic digestion of egg coverings

Bindin

Species-specific glycoprotein on sperm tip

Bindin receptors

Located on egg vitelline envelope

Fast block to polyspermy

Rapid electrical change preventing sperm entry

Depolarization

Change from negative to positive membrane potential

Na+ influx

Causes depolarization of egg membrane

Slow block to polyspermy

Permanent structural block to sperm entry

Fertilization envelope

Hardened layer preventing additional sperm entry

Cortical reaction

Release of cortical granule contents after fertilization

CGSPs

Enzymes that cleave proteins linking vitelline envelope

Osmotic gradient

Draws water causing envelope elevation

Udx1

Enzyme producing hydrogen peroxide (H2O2)

Ovoperoxidase

Uses H2O2 to cross-link proteins in envelope

Cross-linking

Strengthens fertilization envelope

Ca2+ release

Triggers cortical granule reaction

IP3

Signaling molecule that releases Ca2+ from ER

Phospholipase C

Enzyme that produces IP3 from PIP2

DAG

Product of PIP2 hydrolysis alongside IP3

Endoplasmic Reticulum

Stores and releases Ca2+ during fertilization