An oocyte (, oöcyte, or ovocyte) 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 a female fetus in the ovary during Oogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonium. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.
Formation
The formation of an oocyte is called oocytogenesis, which is a part of oogenesis.
[ answers.com] Oogenesis results in the formation of both
primary oocytes during fetal period, and of
secondary oocytes after it as part of
ovulation.
|
| | third trimester |
| | [[Dictyate]] in prophase I for up to 50 years |
| | Halted in metaphase II until fertilization |
| | Minutes after fertilization |
| | |
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.
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 within an ovum or immature ovum is located is the cell-nest.
Cumulus-oocyte complex
The cumulus-oocyte complex contains layers of tightly packed cumulus cells surrounding the oocyte in the Graafian follicle. The oocyte is arrested in Meiosis II at the stage of metaphase II at the diplotene stage and is considered a secondary oocyte. Before ovulation, the
Cumulus oophorus goes through a structural change known as cumulus expansion. The granulosa cells transform from tightly compacted to an expanded mucoid matrix. Many studies show that cumulus expansion is critical for the maturation of the oocyte because the cumulus complex is the oocyte's direct communication with the developing follicle environment. It also plays a significant role in fertilization, though the mechanisms are not entirely known and are species specific.
Maternal contributions
In order for an oocyte to become fertilized and ultimately grow into a fully functioning organism, it must be able to regulate multiple cellular and developmental processes. The oocyte, a large and complex cell, must be able to 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.
Avoidance of damage to germ-line DNA
The DNA of a cell is vulnerable to the damaging effect of oxidative free radicals produced as byproducts of cellular metabolism. DNA damage occurring in oocytes, if not repaired, can be lethal and result in reduced fecundity and loss of potential progeny. Oocytes are substantially larger than the average somatic cell, and thus considerable metabolic activity is necessary for their provisioning. If this metabolic activity were carried out by the oocyte's metabolic machinery, the oocyte genome would be exposed to the reactive oxidative by-products generated. Thus it appears that a process evolved to avoid this vulnerability of germline DNA. It was proposed that, in order to avoid damage to the DNA genome of the oocytes, the metabolism contributing to the synthesis of much of the oocyte's constituents was shifted to other maternal cells that then transferred these constituents to oocytes.
Thus, oocytes of many organisms are protected from oxidative DNA damage while storing up a large mass of substances to nurture the zygote in its initial embryonic growth.
mRNAs and proteins
During the growth of the oocyte, a variety of maternally transcribed messenger RNAs, or
Messenger RNA, 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
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
fertilisation, activation of zygotic transcription, and formation of body axes.
Below are some examples of maternally inherited mRNAs and proteins found in the oocytes of the African clawed frog.
|
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 binding protein |
fatvg | mRNA | Vegetal hemisphere | Germ cell formation and cortical rotation |
Mitochondria
The oocyte receives
Mitochondrion 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
that 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 (mtDNA).
Nucleolus
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.
Ribosomes
Maternal cells also synthesize and contribute a store of
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
and mutation in them have been linked to infertility.
Prophase I arrest
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of
meiosis.
In humans, as an example, oocytes are formed between three and four months of
gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (
dictyate), which may last for many years, four copies of the
genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage appears to provide the informational redundancy needed to
DNA repair of the
germline.
[ The repair process used likely involves homologous recombinational repair.][ In particular, DNA double-strand breaks can be repaired during the period of prophase arrest by homologous recombinational repair and by non-homologous end joining.][Leem J, Lee C, Choi DY, Oh JS. Distinct characteristics of the DNA damage response in mammalian oocytes. Exp Mol Med. 2024 Feb 14. . Epub ahead of print. PMID 38355825] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.[
]
Paternal contributions
The spermatozoon that 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.
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
-
Nondisjunction—a failure of proper homolog separation in meiosis I, or sister chromatid separation in meiosis II can lead to aneuploidy, in which the oocyte has the wrong number of chromosomes, for example 22,X or 24,X. This is the cause of conditions like Down syndrome and Edwards syndrome in humans. It is more likely with advanced maternal age.
-
Some oocytes have multiple cell nucleus, although it is thought they never mature.
See also
-
Cortical granule
-
Cryoconservation of animal genetic resources
-
Folliculogenesis
-
Oocyte maturation inhibitor
-
Polar body
-
Symmetry breaking and cortical rotation
-
Oocyte abnormalities
-
List of distinct cell types in the adult human body
Sources
External links
