Gametogenesis and Fertilization

Primordial germ cells (PGCs) form at the _____ of the ____ and gastrulate through the _____ where they associate with the ______ that will form the _____

• Form at the posterior portion of the epiblast

• Gastrulate through the posterior primitive streak where they associate with the endoderm that will form the gut

How many PGCs are there initially?

50

What eventually moves into genital ridges?

While most Oct4+ ICM cells will differentiate into Oct4- epiblasts, the few cells that retain Oct4 are called PGCs and eventually move into the genital ridges to form germ cells eventually

Starting at the hindgut, where do PGCs migrate?

Migrate anteriorly (up towards head) then dorsally (towards back) to the genital ridges, then splits to both lateral sides at around E11 for mice and week 6 for humans

What happens first when germ cells have migrated to the gonads?

They still have 2 copies of genome at this point!

1. Divide mitotically to produce millions of germ cells (we want lots!)

After germ cells have divided mitotically in the gonads, what happens next?

2. Meiosis

   1. Homologous chromosomes pair, synapse and recombine (we want diversity!)

   2. They reduce their chromosome number from diploid to haploid

After germ cells have underwent meiosis in the gonads, what happens next?

Gametes differentiate into sperm or mature oocytes

Is the timing of meiosis the same in females and males?

No! Drastically different

When does meiosis occur in females?

Starts during embryonic development, around week 12

• Meiotic arrest before birth

• Therefore, # of follicles at birth is FIXED

When does meiosis start in males?

Starts at puberty

• Mitotic arrest before rest

• Mitotic proliferation at birth again

• # of sperm precursors INCREASE following birth

How does the number of germ cells in the human ovary changes over one’s lifespan?

• MAX # of germ cells reached ~6 months (~6×10^6)

• 1/3 of max # at birth (2×10^6)

• Slowly decreases with age following birth

Where do sperm mature?

Seminiferous tubules within the testi

• More towards the center = precursor

What is the difference between Aₛ, A1, and B spermatogonia?

• Aₛ: Undifferentiated spermatogonia that include stem cells capable of self-renewal.

• A1: Differentiating spermatogonia undergoing mitotic divisions.

• B: Final spermatogonial stage that produces primary spermatocytes for meiosis.

What does a primary spermatocyte undergo?

1 primary spermatocyte → Meiosis I (2n → 1n) → 2 haploid secondary spermatocyte

What does a secondary spermatocyte undergo?

Meiosis II → 4 1n spermatids → further maturation → sper,

What is the axoneme and what is its core structure made of?

Internal structure of flagella made up of alpha and beta tubulin proteins

Why are mitochondria clustered within the midpiece?

Midpiece is like the motor for the tail; lots of ATP is required for movement

What is the acrosome?

Modified golgi apparatus containing proteolytic enzymes necessary to digest the extracellular matrix surrounding the egg, allowing the sperm to reach the egg cell membrane with which it fuses

What does the vitelline envelope do for the egg?

Provides rigidness and toughness

How much larger is the sea urchin egg volume than that of sperm

10,000x more

What does the egg contain?

1. Nutritive proteins (energy and amino acids via yolk proteins)

2. Ribosomes and tRNA (structural proteins and enzymes, zygote uses these)

3. Messenger RNAs (necessary for early development; inactive until fertilization)

4. Morphogenic factors (asymmetrical distribution of transcription and paracrine factors in the egg guides embryonic cell differentiation during early development)

5. Protective chemicals (UV filters, DNA repair enzymes, antibodies)

Does the egg have centrioles?

NO! Pronucleus and centriole enter from haploid male

What allows the centrosome/pronucleus of the male to reach toward the female pronucleus?

Microtubules