An oocyte, 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, which then undergoes mitosis,
forming oogonia. During oogenesis, the oogonia become primary oocytes.
Formation 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 chromosome as 23,X,
whereas sperm can have 23,X or 23,Y. = Nest =
The space within an ovum or immature ovum is located is the cell-nest.
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.
= 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 own 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 germ line 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 mRNAs, are supplied
by maternal cells. These mRNAs can be stored in mRNP 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. = 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 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 or 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 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 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. 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: a
signalling or activating factor, which causes the metabolically dormant oocyte
to activate; the haploid paternal genome; 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 nuclei,
although it is thought they never mature.
See also Cortical granule
Folliculogenesis Oocyte maturation inhibitor
Polar body Symmetry breaking and cortical rotation
References Sources
William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig
Heller. Life: The Science of Biology, pp. 823–824.
External links Micrograph of a primary oocyte and
follicle of a monkey