Oogenesis and Oocyte Maturation

Oogenesis Overview

Oogenesis is the complete process of the production of ova (mature eggs), where the singular term for ovum refers to a single mature egg.
In Oogenesis, there are four primary goals:
  - Increase genetic variation: Achieved through recombination.
  - Produce haploid cells: Accomplished through reduction division.
  - Synthesize macromolecules and organelles necessary for fertilization.
  - Produce stores of materials for embryo development.
Related reading for further details: Chapter 12

Phases of Oogenesis

Oogenesis consists of four key phases which validate that animal development is fundamentally rooted in oogenesis:
  1. Mitotic divisions of primordial germ cells (PGC).
  2. Initiation of meiosis and nuclear arrest: At this stage, the oocyte enters meiosis and experiences a halt in further development.
  3. Oocyte growth: Occurs mainly cytoplasmically without nuclear changes, where the oocyte prepares for embryogenesis.
  4. Resumption of meiosis and ovulation: This occurs in adulthood, culminating in oocytes completing meiosis at fertilization.

Migration and Division of Primordial Germ Cells (PGC)

Primordial germ cells experience migration during development.
  - The Genital Ridge, which is the developing gonad, produces an attractant known as Stem Cell Factor.
  - PGCs enter the gonad at Day 11.5 in mouse development.
  - The PGCs multiply to approximately 30,000 by Day 14 during a mouse's typical 21-day gestation period.

Migration and Mitosis of Primordial Germ Cells

A small number of PGCs (10-100) migrate from the posterior primitive streak to the hindgut.
These cells traverse the hindgut into the genital ridge (primordial gonad) along a pathway created by extracellular matrix proteins.
The Genital Ridge produces Stem Cell Factor as an attractant.
The PGCs undergo mitotic divisions and form over 5,000 cells, retaining their stem cell characteristics.
As they cluster with cells in the genital ridge, they form primitive sex chords.
This process is similar in both males and females, where, in females, some PGCs continue mitotic divisions to become oogonia, eventually stopping mitosis prenatally to enter the first meiotic prophase, becoming primary oocytes.

Mouse Oogenesis Timeline

Color-coded cell types in timeline:
- Yellow: Germ cells
- Red: Somatic cells (granulosa)
Oocyte clusters (cysts) maintain cytoplasmic bridges during this process.
The process of meiosis occurs in cysts, and the breakdown of cysts is influenced by steroids.
Key time points indicated in days post-coitus (dpc) and post-natal day (PND).

Oocyte Meiosis

Oogonia, after completing mitosis, enter Prophase I of meiosis, becoming primary oocytes. This transition is activated by local retinoic acid.
Genetic recombination occurs during Prophase I, particularly during the late diplotene phase of meiosis I (also known as the dictyate stage) where the oocyte remains arrested until puberty.
At puberty, some oocytes respond to LH (luteinizing hormone) and ovulate.
Overview of Arrested Stages:
  - Arrest #1: Oocytes are arrested before birth at the dictyate stage.
  - LH reactivation leads to the advancement of the oocyte to metaphase II while causing the cell to remain arrested until fertilization.
  - During meiosis, oocytes produce a polar body rather than another oocyte.

Timing of Gamete Production in Males and Females

Key points in timing:
- 10.5 dpc: PGCs are specified.
- 13.5 dpc: Proliferation and migration of PGCs into the gonads.
Female gametogenesis involves the development of the ovary, leading to periodic ovulation, while male gametogenesis is directed at spermatogonia entering meiosis I.
- The phases of meiosis for both male and female cells are detailed, showing development through the stages of Leptonema, Zygonema, Pachynema, and Diplonema, with arrests outlined.

Oocyte Growth Phase

After birth and before puberty, meiotically arrested oocytes increase in size.
Larger oocytes develop an egg coat (zona pellucida): This is a glycoprotein product of the oocyte.
Oocytes synthesize cortical granules (vesicles released at fertilization).
Oocytes become fully grown before the follicle antrum is formed, where 99.9% of oocytes become atretic.
Only those fully grown oocytes can resume meiosis in response to FSH (follicle-stimulating hormone) and LH.
High levels of cAMP maintain meiotic arrest until the LH surge occurs.

Oocyte RNA Synthesis

As oocytes grow, they also synthesize RNA, primarily for use in later embryonic development.
Some of this RNA is stored undegraded and remains untranslated until it is needed after fertilization.

Oocyte and Follicle Growth Comparison (Oogenesis vs. Folliculogenesis)

Secondary/Antral Follicles: Overview of the interactions between oocytes and follicular development is delineated.

Oocyte's Role in Follicle Development

The oocyte, though a small part of the follicle, plays a crucial role in controlling follicle development through:
- Interplay of the oocyte, granulosa cells, and the hypothalamus/pituitary interactions, regulating follicle development through two types of communication.

First Type of Communication

Oocytes secrete growth factors that diffuse to and influence granulosa cells within the follicle wall and antrum.

Evidence of Growth Factors Influence

The Booroola sheep study shows that these sheep ovulate two X oocytes due to a mutated gene that encodes a receptor for a growth factor produced solely by oocytes.

Second Type of Communication

Direct Contact Using Cumulus Cell Processes:
    - Cumulus cells extend processes through the zona pellucida, establishing gap junctions with the oocyte.
    - These junctions facilitate the exchange of ions and nutrients between the oocyte and cumulus cells.
    - At the LH surge, these junctions break down, and cAMP levels decrease, impacting oocyte maturation.

Decline of Oocyte Number

Oocyte number reaches maximum during the fetal stage:
- Appearances of the oocyte number are charted across stages: Early Fetus, Late Fetus, Birth, Puberty, and Menopause, indicating ongoing atresia and naturally declining numbers.

Stem Cell Renewal in Females

Female stem cell renewal ceases in the fetal stage as all oogonia enter meiosis, with explicit numbers of germ cells highlighted:
- Peak at 7 million at 20 weeks gestation, declining to 1 million at birth, and 300,000 at puberty, which is less than 5% of the original number due to loss via peritoneal cavity, meiosis, and atresia.

Follicle Number Decline

A graphical representation illustrates that the number of follicles in women declines exponentially over years, shown as follows:
- Number of follicles: Starts above 10^6 and declines toward 0 after several decades.

Oocyte Defects and Maternal Age

Data indicates that as maternal age increases, the risk for chromosomal defects, particularly Trisomy 21 (Down Syndrome), escalates as illustrated by the percentage of risk per live birth correlated with the mother's age.
- The proportion rises from <0.1% at age 20 to 3.75% at age 45.

LH Activation Changes in Oocytes Near/At Ovulation

The fourth phase includes LH activating significant changes in oocytes around and at ovulation, termed “maturation.”
Key changes facilitated include:
    - Resumption of meiosis (nuclear maturation).
    - Expansion of the cumulus mass.
    - Development in the cytosol to re-organize sperm nuclei at fertilization.

Cumulus Cell Maturation

In immature oocytes, cumulus cells maintain tight associations.
Upon LH signaling, cumulus cells expand their mass by producing hyaluronic acid, effectively pushing the cells apart.
This expansion can occur in vitro through the combination of FSH/LH.

Oocyte Meiotic Resumption (Nuclear Maturation)

At puberty, oocytes within certain large follicles are activated by the LH surge, which could take 40-50 years since meiosis began in females.
Key steps include:
  - Oocytes complete meiosis I and progress into meiosis II, where they are arrested at metaphase II.
  - The excess chromosomes are disposed of into the polar body.
  - The LH surge facilitates the release of the oocyte from the ovary (ovulation).
  - If fertilization occurs, it triggers the completion of meiosis II and the formation of a second polar body, resulting in 1 primary oocyte forming 1 secondary oocyte and 1 mature ovum.

Summary of Mitosis/Meiosis During Oogenesis

Mitosis and a portion of meiosis are completed prior to birth, marking the end of stem cell proliferation at this stage.
Critical aspects of oogenesis occur during the fetal stage, especially genetic recombination.
Oocytes remain arrested in the dictyate stage of meiosis I until ovulation when the follicle grows around them.
Oocytes assist in controlling follicle development and continue without complete meiosis until ovulation or fertilization.
Ultimately, 1 primary oocyte produces 1 mature oocyte (ovum) upon completion.