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).
Formation
in the process of maturation of the
ovum.]]
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.
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 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.
Maternal Contributions to the Oocyte
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.
Maternal 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. 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.
Maternal Mitochondria
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.
Maternal 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.
Maternal 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.
Paternal Contributions to the 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.
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. It is more likely with advanced maternal age.
Some oocytes have multiple nuclei, although it is thought they never mature.
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
Folliculogenesis
polar body
External links
Slide
Category:Germ cells
