Oogenesis and Follicle Development

Introduction

• Ovogenesis is the formation of oogonia, their multiplication, and differentiation into oocytes
• Female gametes have 23 chromosomes and are haploid cells
• Oocytes depend on somatic cells (cells that are not reproductive cells) in immediate contact, including follicular cells
• Follicular cells presenting 46 chromosomes, are intrinsically linked to folliculogenesis

Prepubescent Ovary

• Has a large number of reserve follicles, contrasting with the adult ovary

Adult Ovary

• Less rich in reserve follicles, presenting different maturation stages of ovarian follicles in the cortical region
• The medullary region has numerous vessels and nerves

Ovary and Follicle Development

• The ovary is the site of follicular development, progressing from the primordial reserve follicle to the pre-ovulatory follicle

Oocyte Transformations

• Occur inside the follicle
• The oocyte remains arrested in prophase of the first meiotic division for a long time
• Meiosis resumes from ovulation to become fertilizable

Fetal Life Stages

• At 8 weeks, gonocytes multiply in waves in the fetal ovary and transform into oogonia
• Oogonia exit the mitotic cycle and transform into oocyte I
• At 12 weeks, the first oocytes I are observed
• During the second trimester, the oocyte initiates multiplication of follicular cells, which originate from mesodermal genital ridges

Prepubertal Phase

• The oocyte interacts constantly with follicular cells within the follicle, isolated from the ovarian stroma and blood circulation
• The oocyte replicates DNA before entering meiosis, resulting in oocyte I with 46 chromosomes and 4N DNA
  • "N" signifies DNA quantity, not ploidy level; oocyte I is 2n chromosome and 4Q DNA

Meiosis Initiation

• Oocyte I undergoes meiosis with a long prophase I, including leptotene, zygotene, pachytene, and diplotene stages
• Mitosis of oogonia coexists until the 5th month
• Oocyte I blocks at the diplotene stage of the first meiotic division, becoming isolated in primordial follicles (reserve follicles) called the germinal vesicle

Germinal Vesicle

• A large vesicle is visible within the cytoplasm
• The oocyte remains blocked until a signal at puberty, stimulated by an activating signal involving Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) secreted by the anterior pituitary gland, stimulated by a hypothalamic hormone

Pubertal Phase

• After a resting period, the hypothalamic-pituitary axis restarts follicular growth and oocyte meiosis
• The oocyte receives signals from follicular cells stimulated by FSH and LH, enabling it to end the first prophase and block in metaphase I
• The hormonal peak of ovulation allows the completion of the first meiotic division, initiation of the second, and blockage in metaphase II

Cycle Origin

• The female cycle beings at puberty; external and internal stimuli cause the hypothalamus to secrete a hormone stimulating pituitary gonadotropin secretion, producing FSH and LH

Follicular and Luteal Phase

• Gonadotropins stimulate follicular cells, which secrete estrogens and progesterone
• The secretions organize into a 28-day cycle, with day one defined as the first day of menstruation
• At the cycle's start FSH increases, causing follicular cells to secrete estrogens with a slight delay, cells also acquire LH receptors preparing them to respond to the LH peak associated with the pre-ovulatory FSH peak

Ovulation

• FSH and LH peaks cause changes in the follicle, with the oocyte resuming meiosis to metaphase of the second meiotic division

Folliculo-Oocyte Interactions

• Essential for all stages of maturation.

Menopause

• Folliculogenesis stops, with gonadotropins stimulating nearly atretic ovaries
• Occurs when fewer than 1000 follicles remain in both ovaries

Meiosis Resumption

• Final oocyte maturation occurs within 36-40 hours between the start of the ovulatory gonadotropin discharge and follicular rupture, affecting the nucleus, cytoplasm, and membrane
• At the nuclear level, meiosis resumes after being blocked in prophase I since fetal life
• Oocytes gaining meiotic competence have a diameter of at least 100um

Gap Junction Rupture

• Caused by the LH peak, interrupting communication between the oocyte and surrounding granulosa and cumulus oophorus cells, which inhibited and blocked meiosis
• The rupture allows the oocyte to resume and complete the first meiotic division, progressing to metaphase of the second division
• In the cytoplasm the germinal vesicle shifts to an eccentric position, ruptures, and allows the oocyte to resume its meiotic course

Meiotic Divisions

• First meiotic division results in two haploid cells with 23 chromosomes of unequal size
• The oocyte II, receives all cytoplasm from mother cell
• The first polar globule is reduced to nuclear mass, receives only traces of cytoplasmic material
• The oocyte II immediately enters the second meiotic division but blocks under the effect of Cytostatic Factor (CSF)

Chromosome Management

• The completion of the second meiotic division is only possible after fertilization or another stimulation
• It ensures that the oocyte presents 23 chromosomes with a single copy of DNA
• The emission of the 2e polar globule signals the completion of the 2e meiotic division

Oocyte Maturation

• Occurs in contact with follicular cells, increasing the oocyte's size up to 120 microns
• Accompanied by massive synthesis of maternal transcripts used in early embryonic development and specific protein synthesis, including Male Pronucleus Grown Factor (MPGF) for decondensation of sperm chromatin post-fertilization

Mature Cytoplasm

• Equally distributes cytoplasmic granules containing beta hexoaminidase and other proteases
• These granules distribute along the plasma membrane and become cortical granules

Composition Of Oocyte Membrane

• The oocyte’s transparent fibrous coat (zona pellucida) is made up of four glycoproteins: ZP1, ZP2, ZP3 and ZP4, where ZP3 and ZP4, are polypeptide chains, disposed in alternation with ZP2 connected by ZP1 bridges
• The mature oocyte has spherical sites plasmic of homogenous granularity
• The diameter is about 120 microns with observation of the polar body.

Layers and Composition Of Oocyte

• Cortical granules lie beneath the plasma membrane
• The oocyte has zone pellicide as well as communicating junctions Cellular make up
• The granulosa cells (corona radiata) are joined by follicular cells that make up the cumulus oophorus
• In metaphase 2 a thick fibrous shell zone pellucidis is present Périvitelline space
• Lies between the fibrous shell & plasma membrane

Folliculo-Oocyte Interactions

• Fundamenta, with follicular cells expressing gonadotropin receptors
• Stimulated by FSH, they synthesize steroid hormones and growth factors, necessary for oocyte maturation
• Intense exchanges occur in both directions
• The oocyte sends signals to follicular cells, inducing mitoses via growth factors like GDF 9

Oocyte Reserves

• At the 7th month of gestation the fetus has 7 million follicles which deplete down to about 1-2 million at birth
• Dwindles to only < 1000 during menopause
  • This is affected by degeneration that reduces the ovarian reserve
• Female chromosomal anomalies occur from age 38 and onwards
  • Oocytes accumulate anomalies over time that are difficult to repair

Female Cycles

• The cycle extends to the maturation of the primodal follicle to become the ovulation dominant follicle
• The ovarian cycle coincides with the uterine cycle

Gametogenesis Differences

• Male spermatogonia use mitosis constantly through life which provides a continuous supply with existing cells stock
• Females completely divide during the intra-uterine stage
• The oocytes meosis is slow, and may not ever full complete
• Oocytes are asymmetrical

STAGES TO KNOW

• Oogonia
  • Present in the cortex of the ovary
  • Becomes ovocyte once its been able to transforms into mature gamete
• Beginning of Meiosis; spherical structure
  • Contains a nucleolus - an absence of polar bonds
• Oocytes (primary & secondary oocytes)
  • Primary oocyte found in the dominating follicle
• Begins 2nd meosis post puberty is spherical and contains 1 polar bond