male oogenesis

Why do females have limited germ cell production?

• oogonia stop mitotic division and stem cell renewal prenatally

• Males have a self-renewing population of spermatogonia that lasts throughout their adult life, while all female germ cells enter meiosis during the fetal stage

• Female is born with a finite “bank” of oocytes that declines over time through cell death and ovulation

4 phases of oogenesis

1. Mitotic division of primordial germ cells: PGCs migrate to the gonad and multiply rapidly via mitosis to increase the initial population

2. Start of meiosis and nuclear arrest: Oogonia enter meiosis I and stop at the dictyate stage (very last) of prophase I before birth

3. Oocyte growth: after birth, the oocyte increases in size and accumulates cytoplasmic materials (RNA, protein, organelles) needed for an embryo, though the nucleus remains arrested

4. Resumption of meiosis and ovulation: triggered by the LH surge in the adult, the oocyte completes meiosis I and proceeds to meiosis II (arresting again) until fertilization

Migration of primordial germ cells

Migrate from the posterior primitive streak through the hindgut into the genital ridge (primordial gonad)

• direction movement: directed by extracellular matrix proteins that act as a pathway and chemical attractant stem cell factor, which is produced by the genital ridge

Embryo/fetus

Female germ cells: mitosis of PGCs; entry into Meiosis I (Prophase I)

Male germ cells: mitosis of PGCs; mitotic arrest

Birth to puberty

Female germ cells: arrested in meiosis I (dictyate stage)

Male germ cells: mitotic arrest or slow mitosis

Puberty/adult

Female germ cells: resumption of meiosis I; arrest in meiosis II

Male germ cells: continuous mitosis and meiosis

Fertilization

Female germ cells: completion of meiosis II

Male germ cells: No answer

Meiotic arrest #1

• Dictyate stage (late diplotene) of prophase I

• Occurs prenatally

• Is restarted by the LH surge during the estrous/menstrual cycle

Meiotic arrest #2

• metaphase II

• This occurs after the completion of meiosis I

• Is restarted by fertilization (sperm entry)

Asymmetric cell division in oogenesis

• Is asymmetric to ensure the ovum retains almost all of the cytoplasm, organelles, and stored nutrients (RNA/proteins) required to support the early embryo

• Products: this division produces one mature oocyte and small, non-functional cells called polar bodies (the first polar body after meiosis I and the second after meiosis II)

Polar bodies

• small cells during meiotic divisions containing “discarded” extra sets of chromosomes

• They have very little cytoplasm and eventually degenerate

Pronuclei

• The haploid nuclei of the sperm and the egg within the fertilized ovum before they fuse

• Typically one male pronucleus and one female pronucleus present just before syngamy

What is produced as oocytes grow?

• RNA and proteins: stored for later use during embryonic development

• Zona pellucida: glycoprotein “egg coat”

• Cortical granules: vesicles containing enzymes released at fertilization to prevent polyspermy

Paracrine signaling

The oocyte secretes growth factor (like GDF9 or BMP15) that diffuse to the surrounding granulosa cells

• example: in booroola sheep, a mutation in the growth factor receptor causes the sheep to ovulate double the normal amount of eggs

Gap junctions

Direct contact via cumulus cell process that penetrate the zona pellucida to touch the oocyte

• importance: these allow the exchange of ions, nutrients, and signaling molecules (like cAMP) which help maintain meiotic arrest

Why do males produce more germ cells?

• Their germ cell mitosis (stem cell renewal) continues throughout their entire life

• Female stem renewal ends in the fetal stage, males maintain a population of spermatogonial stem cells that can divide indefinitely

Outcomes of aging on oogenesis

1. Decrease in quantity: a dramatic decline in the total number of follicles/oocytes due to continuous atresia (reaching menopause when the pool is exhausted)

2. Decrease in quality: an increase in oocyte defects, such as chromosomal abnormalities or spindle failures, leading to lower fertility or higher miscarriage rates

Stimulus that controls final oocyte maturation

LH surge

Outcome 1 of final oocyte maturation

Nuclear maturation

• resumption of meiosis (completion of meiosis I and advancement to metaphase II)

Outcome 2 of final oocyte maturation

Cytoplasmic/cumulus maturation

• the expansion of the cumulus cell mass (via hyaluronic acid) and reorganization of the cytosol to prepare for fertilization