Lecture 4: Gametogenesis and Fertilization

15.1.2026

Meiotic arrest

Generally in oogenesis

In mammals, oocytes are arrested in meiotic prophase I until ovulation, then meiotic metaphase II until fertilization

Asymmetric meiosis

Only 1 mature 1n egg per meiosis

3n worth of DNA discarded in first and second polar bodies

• Possibly to conserve volume

• Doesn’t disrupt oocyte patterning

• Might also help maintain ploidy (univalents tend to move to the polar bodies)

Maintenance of large oocytes

Some species use “nurse cells” to produce and transfer proteins and RNAs

Others ramp up transcription

Most species transfer proteins (e.g. yolk proteins) from other somatic tissues

Symmetry in oocytes

Tunicates, frogs, zebrafish have animal-vegetal axis

In Drosophila, both dorsal-ventral and anterior-posterior are set up in oocyte

In some species (C. elegans) oocytes are symmetric

Oocyte layers

Vitelline envelope in invertebrates

Zona pellucida and cumulus cells in mammals

Features of mature chordate sperm

• Flagella

• Mitochondria at the base

• Golgi converted into acrosome

• Compact nucleus

But sperm often vary across species

Requirements for internal fertilization

Sperm must:

• Reach site of fertilization (fallopian tube in mammals, oviduct in vertebrates) or storage (spermathecae in insects and nematodes)

• Survive the reproductive tract and be selected/ activated (“capacitation” in mammals)

• Bind to and fuse with oocyte

Requirements for external fertilization

Sperm must:

• Find eggs in environment

• Get “activated”

• Bind to and fuse with oocyte

• Fertilize egg from the right species

Chemoattractant and chemotaxis

Ex: Resact in sea urching

• Resact binds a chemoreceptor on sperm and activates a calcium channel

• Transient calcium spike causes an asymmetrical flagellar beat, creating a “biased random walk”

• Also increases mitochondrial respiration to cause faster swimming

In mammals:

• Sperm are carried most of the way to the egg

• Chemotaxis occurs close to the egg (progesterone released by cumulus cells stimulates sperm swimming)

How to sperm stick to the egg?

Acrosome releases enzymes and exposes a new section of the sperm cell membrane

Help sperm penetrate ECM layers

Bindin

Sperm-egg binding factor

Acrosomal reactions across species

• In sea urchins, interaction with egg jelly triggers the acrosome reaction (which also forms the acrosomal process)

• In mammals, the acrosomal reaction occurs after binding with the zona pellucida. Interactions with the zona pellucida are species-limited and mediated by sacral proteins

Fast block to polyspermy

• 1-3 seconds

• Transient

• In external fertilization

Slow block to polyspermy

• within 30 seconds

• Permanent

• In most animals

• Has to do with membrane potential

• Resting potential of oocytes is ~ -70 mV; goes to ~20mV