Oocyte Development

Introduction

Historic drawing comparing the size of the mature human oocyte with a spermatozoa

Prior to release from the ovary oocytes (eggs, ova) are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following puberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation. Early oocytes are also classified as immature (germinal vesicle (GV) or metaphase I (MI) stage). The breakdown of the germinal vesicle indicates a resumption of meiosis and the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division in human oocytes.

In an adult human female the development of a primordial follicle containing an oocyte to a preovulatory follicle takes in excess of 120 days.

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Historic Embryology: 1921 Urogenital Development | 1921 External Genital Development | Historic Disclaimer

Some Recent Findings

Oocyte formation by mitotically active germ cells purified from ovaries of reproductive-age women "Germline stem cells that produce oocytes in vitro and fertilization-competent eggs in vivo have been identified in and isolated from adult mouse ovaries. Here we describe and validate a fluorescence-activated cell sorting-based protocol that can be used with adult mouse ovaries and human ovarian cortical tissue to purify rare mitotically active cells that have a gene expression profile that is consistent with primitive germ cells. Once established in vitro, these cells can be expanded for months and can spontaneously generate 35- to 50-μm oocytes, as determined by morphology, gene expression and haploid (1n) status. Injection of the human germline cells, engineered to stably express GFP, into human ovarian cortical biopsies leads to formation of follicles containing GFP-positive oocytes 1-2 weeks after xenotransplantation into immunodeficient female mice. Thus, ovaries of reproductive-age women, similar to adult mice, possess rare mitotically active germ cells that can be propagated in vitro as well as generate oocytes in vitro and in vivo." (these cells described as oogonial stem cells (OSCs), are very rare—only about 1 out of 10,000 ovarian cells)

More recent papers

This table shows an automated computer PubMed search using the listed sub-heading term. Therefore the list of references do not reflect any editorial selection of material based on content or relevance. References appear in this list based upon the date of the actual page viewing. References listed on the rest of the content page and the associated discussion page (listed under the publication year sub-headings) do include some editorial selection based upon both relevance and availability. Links: References | Discussion Page | Pubmed Most Recent Search term: Oocyte Development Amanda Charlesworth, Hedda A Meijer, Cornelia H de Moor Specificity factors in cytoplasmic polyadenylation. Wiley Interdiscip Rev RNA: 2013, 4(4);437-61 PMID:23776146 Bo Zhao, Yi-Xian Cun, Xie-Chao He, Ping Zheng [Maternal-effect Floped gene is essential for the derivation of embryonic stem cells in mice]. Zool. Res.: 2013, 34(3);E82-6 PMID:23776005 Alicia R Timme-Laragy, Jared V Goldstone, Barry R Imhoff, John J Stegeman, Mark E Hahn, Jason M Hansen Glutathione redox dynamics and expression of glutathione-related genes in the developing embryo. Free Radic. Biol. Med.: 2013; PMID:23770340 Jordi Roca, Maria J Martinez-Alborcia, Maria A Gil, Inmaculada Parrilla, Emilio A Martinez Dead spermatozoa in raw semen samples impair in vitro fertilization outcomes of frozen-thawed spermatozoa. Fertil. Steril.: 2013; PMID:23768987 Daniela Paes Almeida Ferreira Braga, Amanda S Setti, Rita de Cássia S Figueira, Rogério Bonassi Machado, Assumpto Iaconelli, Edson Borges Influence of oocyte dysmorphisms on blastocyst formation and quality. Fertil. Steril.: 2013; PMID:23768986

Movies

‎‎Ovulation Page | Play

‎‎Fertilization Page | Play

‎‎Fertilisation Page | Play

‎‎Pronuclear Fusion Page | Play

Movies

Oogenesis

A human infant ovary histology, showing the large number of oocytes occupying the ovary cortical region. Compare this with a mature ovary and note the absence of any follicle development in the infant. These early oocytes remain at the diplotene stage of the meiosis I during development from fetal life and postnatal childhood, until puberty when the lutenizing hormone (LH) surges stimulate the resumption of meiosis.

The graph below shows the changes in human germ cell numbers in the ovary with age, peaking at about 7 million (occuring in early fetal development) and then decreasing by apopotic cell death. At puberty there remain only about 400,000 and only about 10% of these will be released through reproductive life. (More? Menstrual Cycle)

Human ovary non-growing follicle model

Meiosis

Secondary follicle with oocyte.

In females, the total number of eggs ever to be produced are present in the newborn female initially arrested at the diplotene stage of the meiosis I from fetal life through childhood until puberty, when the lutenizing hormone (LH) surges stimulate the resumption of meiosis.

1. All eggs are arrested at an early stage (prophase I) of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
2. In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
3. The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
4. At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.

Links: Cell Division - Meiosis

Polar Body

Human oocyte at metaphase II showing polar body at top

The breakdown of the germinal vesicle indicates a resumption of meiosis and the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division in human oocytes. The polar body is a small cytoplasmic exclusion body formed to enclose the excess DNA formed during the oocyte (egg) meiosis and following sperm fertilization. There are 2-3 polar bodies derived from the oocyte present in the zygote, the number is dependent upon whether polar body 1 (the first polar body formed during meiosis 1) divides during meiosis 2. This exclusion body contains the excess DNA from the reductive division (the second and third polar bodies are formed from meiosis 2 at fertilization). These polar bodies do not contribute to the future genetic complement of the zygote, embryo or fetus.

Recent research in some species suggest that the space formed by the peripheral polar body (between the oocyte and the zona pellucia) can influence the site of spermatozoa fertilization.

Polar Body Extrusion Model

The following cartoon model from mouse oocyte study of polar body extrusion, involving cortical cap protrusion and spindle midzone-induced membrane furrowing.

(A) Chromosomes induce formation of a cortical actomyosin cap/ring prior to polar body extrusion.	(B) Egg activation induces the cortical cap protrusion.	(C) The anaphase spindle midzone induces unilateral furrowing.	(D) Spindle rotation.	(E) Spindle midzone induces bilateral furrowing and abscission of polar body.
The squared region of the cortical cap/ring is shown on the top, an actin cap (red) surrounded by a myosin II ring (green).

Assisted reproductive techniques involving intracytoplasmic sperm injection (ICSI) have looked at the "quality" of the polar body and found that the morphology is related to mature oocyte viability and has the potential to predict oocyte fertilization rates and pregnancy achievement.

Links: Meiosis

Calcium Release

Oocyte calcium ion (Ca²⁺) release occurs after spermatozoa fusion and is part of the reactivation of meiosis (arrested at metaphase II) and the primary block to polyspermy. Earlier in oocyte meiosis, between prophase I (germinal vesicle stage) and MII, this release mechanism is developed within the cell.

Oocyte cytoplasmic changes include:

1. endoplasmatic reticulum reorganization.
2. IP3 receptor increase in both number and sensitivity.
3. increase in calcium ion concentration.

Cortical Granules

Human oocyte (MII) showing cortical granules (green)

Mouse oocyte cortical granules (red)

The release of cortical granules by exocytosis, the "cortical reaction", occurs following spermatozoa fertilisation and is the main block to polyspermy by modifying the zone pellucida. These granules develop from the golgi apparatus initially forming smaller vesicles that coalesce to form mature membrane bound cortical granules (0.2 to 0.6 microns in diameter) located in the cortex of unfertilized oocytes. In mammals, cortical granule production in the developing follicular oocyte is an ongoing and continuous process, with newly synthesized granules translocating to the cortex until the time of ovulation.

Cortical granules:

• vary in time of initial development between species.
  • primordial follicle stage - rat and mouse.
  • primary follicle stage - human, monkey, hamsters, and rabbit.
• vary in type formed in the same species.
• migration requires the microfilaments of the cell cytoskeleton.
• are evenly distributed in the cortex of unfertilised oocytes.
• contain carbohydrates, proteinases, ovoperoxidase, calreticulin, N-acetylglucosaminidase

Oocyte-Follicle Cell Interaction

The oocyte and the surrounding granulosa cells have a complex paracrine interactions during follicle growth and development. Oocyte maturation has been shown to depend on secretory products of both the granulosa and cumulus cells.

Oocyte Factors

• promotes granulosa cell proliferation in preantral and antral follicles (GDF-9, BMP15)
• cumulus expansion and granulosa cell differentiation are dependent upon oocyte-derived factors
• BMP15 inhibits FSH-stimulated progesterone production

Oocyte Different Species

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  Monkey

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  Mouse

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  Mouse oocyte and zona pellucida EM

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  Mouse

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  Canine (dog)

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  Canine (dog)

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  Salamander

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  Sheep

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  Sheep

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  Sheep

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  Sheep

Oocyte Protein Expression

The table above shows the pattern of protein expression (as percentages of total) in the mouse germinal vesicle and MII oocyte according to 14 molecular function categories.

Links: Germinal vesicle oocyte protein expression | MII oocyte protein expression | Zygote Protein Expression | Mouse Development | Oocyte Development | Zygote

Oocyte Telomerase Reverse Transcriptase

There is a redistribution of the enzyme that regulates telomere length during oocyte development. The following oocyte images are from a recent study of sheep in vitro follicle development.

preantral	early antral
early antral	preovulatory follicle

• TERT - Red (Cy3-conjugated secondary antibody) (telomerase reverse transcriptase, TERT)
• DNA - Green (SYBR Green 14/I)

Sheep Oocyte TERT: preantral | early antral | early antral | preovulatory follicle | Oocyte Development | Sheep Development

Abnormalities

Trisomy 21 female karyotype

Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.

• Autosomal chromosome aneuploidy
  • trisomy 21 - Down syndrome
  • trisomy 18 - Edwards syndrome
  • trisomy 13 - Patau syndrome

• Sex chromosome aneuploidy
  • monosomy X - Turner's Syndrome
  • trisomy X - Triple-X syndrome
  • 47 XXY - Klinefelter's Syndrome

Links: Trisomy 21 | Abnormal Development - Genetic | Genital System - Abnormalities

Additional Images

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  Human oocyte

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  Human oocyte metaphase I

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  Human oocyte metaphase II

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  Mouse oocyte cortical granules

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  MII Oocyte incomplete cytoplasmic maturation

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  MII Oocyte complete cytoplasmic maturation

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  Mouse oocyte balbini body EM

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  Historic drawing human

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  Historic photo human

References

Reviews

Rong Li, David F Albertini The road to maturation: somatic cell interaction and self-organization of the mammalian oocyte. Nat. Rev. Mol. Cell Biol.: 2013, 14(3);141-52 PMID:23429793

Min Liu The biology and dynamics of mammalian cortical granules. Reprod. Biol. Endocrinol.: 2011, 9();149 PMID:22088197

Search

• NCBI Bookshelf oocyte | oogenesis | oocyte development

• Pubmed oocyte | oogenesis | oocyte development

Terms

• oolemma - (zona pellucida, vitelline membrane)

Glossary Links

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What Links Here?

Cite this page:

Hill, M.A. (2013) Oocyte Development. Retrieved June 19, 2013, from http://php.med.unsw.edu.au/embryology/index.php?title=Oocyte_Development

Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G