Oocytes

Oocyte
File:Oocyte.jpg
"Stripped" human oocyte; granulosa cells that had surrounded this oocyte have been removed.
Gray's subject #3 38
MeSH Oocytes

An oocyte (UK /ˈəsʌɪt/, US /ˈ..st/), oöcyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC) which undergoes mitosis to form an oogonium. During oogenesis the oogonium becomes a primary oocyte.

Formation

Main article: Oogenesis

The formation of an oocyte is called oocytogenesis, which is a part of oogenesis.[1] Oogenesis results in the formation of both primary oocytes before birth, and of secondary oocytes after it as part of ovulation.

Cell type ploidy/chromosomes chromatids Process Time of completion
Oogonium diploid/46(2N) 4C Oocytogenesis (mitosis) third trimester
primary Oocyte diploid/46(2N) 4C Ootidogenesis (meiosis 1) (Folliculogenesis) Dictyate in prophase I for up to 50 years
secondary Oocyte haploid/23(1N) 2C Ootidogenesis (meiosis 2) Halted in metaphase II until fertilization
Ootid haploid/23(1N) 1C Ootidogenesis (meiosis 2) Minutes after fertilization
Ovum haploid/23(1N) 1C

Characteristics


Cytoplasm

Oocytes are rich in cytoplasm which contains yolk granules to nourish the cell early in development.

Nucleus

During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle.[2]

The only normal human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can have 23,X (female-determining) or 23,Y (male-determining).

Nest

The space wherein an ovum or immature ovum is located is the cell-nest.[3]

Maternal Contributions


Because the fate of an oocyte is to become fertilized and ultimately grow into a fully functioning organism, it must be ready to regulate multiple cellular and developmental processes. The oocyte, a large and complex cell, must be supplied with numerous molecules that will direct the growth of the embryo and control cellular activities. As the oocyte is a product of female gametogenesis, the maternal contribution to the oocyte and consequently the newly fertilized egg is enormous. There are many types of molecules that are maternally supplied to the oocyte which will direct various activities within the growing zygote.

mRNAs and Proteins

During the growth of the oocyte, a variety of maternally transcribed messenger RNAs, or mRNAs, are supplied by maternal cells. These mRNAs can be stored in mRNP (message ribonucleoprotein) complexes and be translated at specific time points, they can be localized within a specific region of the cytoplasm, or they can be homogeneously dispersed within the cytoplasm of the entire oocyte.[4] Maternally loaded proteins can also be localized or ubiquitous throughout the cytoplasm. The translated products of the mRNAs and the loaded proteins have multiple functions; from regulation of cellular "house-keeping" such as cell cycle progression and cellular metabolism, to regulation of developmental processes such as fertilization, activation of zygotic transcription, and formation of body axes.[4] Below are some examples of maternally inherited mRNAs and proteins found in Xenopus laevis oocytes.

Name Type of Maternal Molecule Localization Function
VegT[5] mRNA Vegetal Hemisphere Transcription Factor
Vg1[6] mRNA Vegetal Hemisphere Transcription Factor
XXBP-1[7] mRNA Not Known Transcription Factor
CREB[8] Protein Ubiquitous Transcription Factor
FoxH1[9] mRNA Ubiquitous Transcription Factor
p53[10] Protein Ubiquitous Transcription Factor
Lef/Tcf[11] mRNA Ubiquitous Transcription Factor
FGF2[12] Protein Nucleus Not Known
FGF2, 4, 9 FGFR1[11] mRNA Not Known FGF Signaling
Ectodermin[13] Protein Animal Hemisphere Ubiquitin Ligase
PACE4[14] mRNA Vegetal Hemisphere Proprotein Convertase
Coco[15] Protein Not Known BMP inhibitor
Twisted Gastrulation[11] Protein Not Known BMP/Chordin Bindng Protein
fatvg[16] mRNA Vegetal Hemisphere Germ Cell Formation and Cortical Rotation


Mitochondria

The oocyte receives mitochondria from maternal cells, which will go on to control embryonic metabolism and apoptotic events.[4] The partitioning of mitochondria is carried out by a system of microtubules which will localize mitochondria throughout the oocyte. In certain organisms, such as mammals, paternal mitochondria brought to the oocyte by the spermatozoon are degraded through the attachment of ubiquitinated proteins. The destruction of paternal mitochondria ensures the strictly maternal inheritance of mitochondria and mitochondrial DNA or mtDNA.[4]

Nucleolus

In mammals, the nucleolus of the oocyte is derived solely from maternal cells.[17] The nucleolus, a structure found within the nucleus, is the location where rRNA is transcribed and assembled into ribosomes. While the nucleolus is dense and inactive in a mature oocyte, it is required for proper development of the embryo.[17]

Ribosomes

Maternal cells also synthesize and contribute a store of ribosomes that are required for the translation of proteins before the zygotic genome is activated. In mammalian oocytes, maternally derived ribosomes and some mRNAs are stored in a structure called cytoplasmic lattices. These cytoplasmic lattices, a network of fibrils, protein, and RNAs, have been observed to increase in density as the number of ribosomes decrease within a growing oocyte.[18]

Paternal Contributions

The spermatozoon which fertilizes an oocyte will contribute its pronucleus, the other half of the zygotic genome. In some species, the spermatozoon will also contribute a centriole which will help make up the zygotic centrosome required for the first division. However, in some species, such as in the mouse, the entire centrosome is acquired maternally.[19] Currently under investigation is the possibility of other cytoplasmic contributions made to the embryo by the spermatozoon.

During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centrosome, which is responsible for maintaining the microtubule system. See anatomy of sperm

Abnormalities

See also

  • Oocyte maturation inhibitor

References

Resources

William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig Heller (2003). Life: The Science of Biology(7th ed.), pp. 823–824

See also

External links

  • Micrograph of a primary oocyte and follicle of a monkey
Preceded by
none
Stages of human development
Sperm + Oocyte
Succeeded by
Zygote
de:Eizelle
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