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An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid)
gametangium A gametangium (plural: gametangia) is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants. In contrast to gametogenesis in animals, a gametangium is a haploid ...
of certain thallophytes.


In the mammalian fetus

Oogonia are formed in large numbers by mitosis early in fetal development from
primordial germ cell Primordial may refer to: * Primordial era, an era after the Big Bang. See Chronology of the universe * Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis * Primordial nuclide, nuclides, a few radioactive, that formed before t ...
s. In humans they start to develop between weeks 4 and 8 and are present in the fetus between weeks 5 and 30.


Structure

Normal oogonia in human
ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
are spherical or ovoid in shape and are found amongst neighboring
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syst ...
cells and
oocyte 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 female gametogenesis. The female g ...
s at different phases of development. Oogonia can be distinguished from neighboring somatic cells, under an
electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a h ...
, by observing their nuclei. Oogonial nuclei contain randomly dispersed fibrillar and granular material whereas the somatic cells have a more condensed nucleus that creates a darker outline under the microscope. Oogonial nuclei also contain dense prominent
nucleoli The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of s ...
. The chromosomal material in the nucleus of mitotically dividing oogonia shows as a dense mass surrounded by
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...
s or double membranes. The
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
of oogonia appears similar to that of the surrounding somatic cells and similarly contains large round mitochondria with lateral
cristae A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...
. The Endoplasmic Reticulum (E.R.) of oogonia, however, is very underdeveloped and is made up of several small vesicles. Some of these small vesicles contain
cisternae A cisterna (plural cisternae) is a flattened membrane vesicle found in the endoplasmic reticulum and Golgi apparatus. Cisternae are an integral part of the packaging and modification processes of proteins occurring in the Golgi. Function Protei ...
with ribosomes and are found located near the golgi apparatus. Oogonia that are undergoing degeneration appear slightly different under the electron microscope. In these oogonia, the chromosomes clump together into an indistinguishable mass within the nucleus and the mitochondria and E.R. appear to be swollen and disrupted. Degenerating oogonia are usually found partially or wholly engulfed in neighboring somatic cells, identifying phagocytosis as the mode of elimination.


Development and differentiation

In the
blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...
of the mammalian embryo, primordial germ cells arise from proximal epiblasts under the influence of extra-embryonic signals. These germ cells then travel, via amoeboid movement, to the genital ridge and eventually into the undifferentiated gonads of the fetus. During the 4th or 5th week of development, the gonads begin to differentiate. In the absence of the Y chromosome, the gonads will differentiate into ovaries. As the ovaries differentiate, ingrowths called cortical cords develop. This is where the primordial germ cells collect. During the 6th to 8th week of female (XX) embryonic development, the primordial germ cells grow and begin to differentiate into oogonia. Oogonia proliferate via mitosis during the 9th to 22nd week of embryonic development. There can be up to 600,000 oogonia by the 8th week of development and up to 7,000,000 by the 5th month. Eventually, the oogonia will either degenerate or further differentiate into primary oocytes through asymmetric division. Asymmetric division is a process of mitosis in which one oogonium divides unequally to produce one daughter cell that will eventually become an oocyte through the process of
oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...
, and one daughter cell that is an identical oogonium to the parent cell. This occurs during the 15th week to the 7th month of embryonic development. Most oogonia have either degenerated or differentiated into primary oocytes by birth. Primary oocytes will undergo oogenesis in which they enter meiosis. However, primary oocytes are arrested in prophase 1 of the first meiosis and remain in that arrested stage until puberty begins in the female adult. This is in contrast to male primordial germ cells which are arrested in the spermatogonial stage at birth and do not enter into spermatogenesis and meiosis to produce primary spermatocytes until puberty in the adult male.


Regulation of oogonia differentiation and entry into oogenesis

The regulation and differentiation of germ cells into primary gametocytes ultimately depends on the sex of the embryo and the differentiation of the gonads. In female mice, the protein RSPO1 is responsible for the differentiation of female (XX) gonads into
ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
. RSPO1 activates the β-catenin signaling pathway by up-regulating ''Wnt4'' which is an essential step in ovary differentiation. Research has shown that ovaries lacking ''Rspo1'' or ''Wnt4'' will exhibit sex reversal of the gonads, the formation of ovotestes and the differentiation of somatic
sertoli cells Sertoli cells are a type of sustentacular "nurse" cell found in human testes which contribute to the process of spermatogenesis (the production of sperm) as a structural component of the seminiferous tubules. They are activated by follicle-stimula ...
, which aid in the development of
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, wh ...
. After female (XX) germ cells collect in the undifferentiated gonads, the up-regulation of ''Stra8'' is required for germ cell differentiation into an oogonium and eventually enter meiosis. One major factor that contributes to the up-regulation of ''Stra8'', is the initiation of the β-Catenin signaling pathway via RSPO1, which is also responsible for ovary differentiation. Since RSPO1 is produced in somatic cells, this protein acts on germ cells in a
paracrine Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse ove ...
mode. ''Rspo1'', however, is not the only factor in ''Stra8'' regulation. Many other factors are under scrutiny and this process is still being evaluated.


Oogonial stem cells

It is theorized that oogonia either degenerate or differentiate into
primary oocyte 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 female gametogenesis. The female ge ...
s which enter
oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...
and are halted in
prophase I Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
of the first meiosis post partum. Therefore, it is believed that adult mammalian females lack a population of germ cells that can renew or regenerate, and instead have a large population of primary oocytes that are arrested in the first meiosis until puberty. At puberty, one primary oocyte will continue meiosis each menstrual cycle. Because there is an absence of regenerating germ cells and oogonia in the human, the number of primary oocytes dwindles after each menstrual cycle until
menopause Menopause, also known as the climacteric, is the time in women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause usually occurs between the age of 47 and 54. Medical professionals often ...
, when the female no longer has a population of primary oocytes. Recent research, however, has identified that renewable oogonia may be present in the lining of the female ovaries of humans, primates and mice. It is thought that these
germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embr ...
s might be necessary for the upkeep of the reproductive follicles and oocyte development, well into adulthood. It has also been discovered that some stem cells may migrate from the bone marrow to the ovaries as a source of extra-genial germ cells. These mitotically active germ cells found in mammalian adults were identified by tracking several markers that were common in oocytes. These potential renewable germ cells were identified as positive for these essential oocyte markers. The discovery of these active germ cells and oogonia in the adult female could be very useful in the advancement of fertility research and treatment of infertility. Germ cells have been extracted, isolated and grown successfully ''in vitro''. These germ cells have been used to restore fertility in mice by promoting follicle generation and upkeep in previously infertile mice. There is also research being done on possible germ line regeneration in primates. Mitotically active human female germ cells could be very beneficial to a new method of embryonic stem cell development that involves a
nuclear transfer Nuclear transfer is a form of cloning. The step involves removing the DNA from an oocyte (unfertilised egg), and injecting the nucleus which contains the DNA to be cloned. In rare instances, the newly constructed cell will divide normally, rep ...
into a zygote. Using these functional oogonia may help to create patient-specific stem cell lines using this method.


Controversy

There is a significant controversy regarding existence of mammalian oogonial stem cells. The controversy lies in negative data that has originated from many laboratories in the United States. Multiple approaches to verify the existence of oogonial stem cells have yielded negative results, and no research group in United States has been able to reproduce initial findings.


In certain thallophytes

A '' Thalassiosira pseudonana'' diatom oogonium beginning to expand through the cell wall. Artificial coloring denotes chlorophyll (blue) and DNA (red). In phycology and mycology, oogonium refers to a female
gametangium A gametangium (plural: gametangia) is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants. In contrast to gametogenesis in animals, a gametangium is a haploid ...
if the union of the male (motile or non-motile) and the female gamete takes place within this structure.Stegenga, H. Bolton, J.J. and Anderson, R.J. 1997. ''Seaweeds of the South African West Coast.'' Bolus Herbarium, University of Cape Town. Smyth, G.M. 1955. ''Cryptogamic Botany.'' vol. 1. McGraw-Hill Book Company In '' Oomycota'' and some other organisms, the female oogonia, and the male equivalent
antheridia An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also t ...
, are a result of sexual sporulation, i.e. the development of structures within which meiosis will occur. The haploid nuclei (gametes) are formed by meiosis within the antheridia and oogonia, and when fertilization occurs, a diploid oospore is produced which will eventually germinate into the diploid somatic stage of the thallophyte life cycle. In many algae (e.g., '' Chara''), the main plant is haploid; oogonia and antheridia form and produce haploid gametes. The only diploid part of the life cycle is the spore (fertilized egg cell), which undergoes meiosis to form haploid cells that develop into new plants. This is a haplontic life cycle (with zygotic meiosis).


Structure

The oogonia of certain Thallophyte species are usually round or ovoid, with contents are divided into several
uninucleate {{Short pages monitor