|"Stripped" human oocyte; granulosa cells that had surrounded this oocyte have been removed.|
|Gray's||subject #3 38|
An oocyte, 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 a mitotic division to form an oogonium. During oogenesis the oogonium becomes a primary oocyte (pronounced oh'a-site).
The formation of an oocyte is called oocytogenesis, which is a part of oogenesis. 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||1N||Oocytogenesis (mitosis)||third trimester|
|primary Oocyte||diploid/46||2N||Ootidogenesis (meiosis 1) (Folliculogenesis)||Dictyate in prophase I until ovulation|
|secondary Oocyte||haploid/23||2N||Ootidogenesis (meiosis 2)||Halted in metaphase II until fertilization|
|Ootid||haploid/23||1N||?||Minutes after fertilization|
Oocytes are rich in cytoplasm which contains yolk granules to nourish the cell early in development.
During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle
The only normal human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can can have 23,X (female-determining) or 23,Y (male-determining).
The space wherein an ovum or immature ovum is located is the cell-nest.
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.
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  . 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 . Below are some examples of maternally inherited mRNAs and proteins found in "Xenopus laevis" oocytes.
|Name||Type of Maternal Molecule||Localization||Function|
|VegT ||mRNA||Vegetal Hemisphere||Transcription Factor|
|Vg1 ||mRNA||Vegetal Hemisphere||Transcription Factor|
|XXBP-1 ||mRNA||Not Known||Transcription Factor|
|CREB ||Protein||Ubiquitous||Transcription Factor|
|FoxH1 ||mRNA||Ubiquitous||Transcription Factor|
|p53 ||Protein||Ubiquitous||Transcription Factor|
|Lef/Tcf ||mRNA||Ubiquitous||Transcription Factor|
|FGF2 ||Protein||Nucleus||Not Known|
|FGF2, 4, 9 FGFR1 ||mRNA||Not Known||FGF Signaling|
|Ectodermin ||Protein||Animal Hemisphere||Ubiquitin Ligase|
|PACE4 ||mRNA||Vegetal Hemisphere||Proprotein Convertase|
|Coco ||Protein||Not Known||BMP inhibitor|
|Twisted Gastrulation ||Protein||Not Known||BMP/Chordin Bindng Protein|
|fatvg ||mRNA||Vegetal Hemisphere||Germ Cell Formation and Cortical Rotation|
The oocyte receives mitochondria from maternal cells, which will go on to control embryonic metabolism and apoptotic events . 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 .
In mammals, the nucleolus of the oocyte is derived solely from maternal cells  . 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  .
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 
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 . Currently under investigation is the possibility of other cytoplasmic contributions made to the embryo by the spermatozoon
William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig Heller (2003). Life: The Science of Biology(7th ed.), pp. 823–824
|Stages of human development