for fertilization. fertilization will cause the egg to complete meiosis ii. in human females there is proliferation of the oogonia in the fetus, meiosis starts then before birth and stands by at meiotic division i up to 50 years, ovulation begins at puberty. egg growth a 10 - 20 μm large somatic cell generally needs 24 hours to double its mass for mitosis. by this way it would take a very long time for that cell to reach the size of a mammalian egg with a diameter of 100 μm (some insects have eggs of about 1,000 μm or greater). eggs have therefore special mechanisms to grow to their large size. one of these mechanisms is to have extra copies of genes: meiotic division i is paused so that the oocyte grows while it contains two diploid chromosome sets. some species produce many extra copies of genes, such as amphibians, which may have up to 1 or 2 million copies. a complementary mechanism is partly dependent on syntheses of other cells. in amphibians, birds, and insects, yolk is made by the liver (or its equivalent) and secreted into the blood. neighboring accessory cells in the ovary can also provide nutritive help of two types. in some invertebrates some oogonia become nurse cells. these cells are connected by cytoplasmic bridges with oocytes. the nurse cells of insects provide oocytes macromolecules such as proteins and mrna. follicular granulosa cells are the second type of accessory cells in the ovary in both invertebrates and vertebrates. they form a layer around the oocyte and nourish them with small molecules, no macromolecules, but eventually their smaller precursor molecules, by gap junctions. mutation and dna repair the mutation frequency of female germline cells in mice is about 5-fold lower than that of somatic cells, according to one study. the mouse oocyte in the dictyate (prolonged diplotene) stage of meiosis actively repairs dna damage, whereas dna repair was not detected in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis. the long period of meiotic arrest at the four chromatid dictyate stage of meiosis may facilitate recombinational repair of dna damages. spermatogenesis mammalian spermatogenesis is representative for most animals. in human males, spermatogenesis begins at puberty in seminiferous tubules in the testicles and go on continuously. spermatogonia are immature germ cells. they proliferate continuously by mitotic divisions around the outer edge of the seminiferous tubules, next to the basal lamina. some of these cells stop proliferation and differentiate into primary spermatocytes. after they proceed through the first meiotic division, two secondary spermatocytes are produced. the two secondary spermatocytes undergo the second meiotic division to form four haploid spermatids. these spermatids differentiate morphologically into sperm by nuclear condensation, ejection of the cytoplasm and formation of the acrosome and flagellum. the developing male germ cells do not complete cytokinesis during spermatogenesis. consequently, cytoplasmic bridges exist during interphase to ensure connection between the clones of differentiating daughter cells. these bridges are called a syncytium, and feature a tex14 and kif23 ring in their centre. in this way the haploid cells are supplied with all the products of a complete diploid genome. sperm that carry a y chromosome, for example, are supplied with essential molecules that are encoded by genes on the x chromosome. success of germ cell proliferation and differentiation is also ensured by a balance between germ cell development and programmed cell death. identification of «death triggering signals» and corresponding receptor proteins is important for the fertilization potential of males. apoptosis in germ cells can be induced by variety of naturally occurring toxicant. receptors belonging to the taste 2 family are specialized to detect bitter compounds including extremely toxic alkaloids. so taste receptors play a functional role for controlling apoptosis in male reproductive tissue. mutation and dna repair the mutation frequencies for cells throughout the different stages of spermatogenesis in mice is similar to that in female germline cells, that is 5 to 10-fold lower than the mutation frequency in somatic cells thus low mutation frequency is a feature of germline cells in both sexes. homologous recombinational repair of double-strand breaks occurs in mouse during sequential stages of spermatogenesis, but is most prominent in spermatocytes. the lower frequencies of mutation in germ cells compared to somatic cells appears to be due to more efficient removal of dna damages by repair processes including homologous recombination repair during meiosis. mutation frequency during spermatogenesis increases with age. the mutations in spermatogenic cells of old mice include an increased prevalence of transversion mutations compared to young and middle-aged mice. diseases germ cell tumor is a rare cancer that can affect people at all ages. as of 2018, germ cell tumors account for 3% of all cancers in children and adolescents 0–19 years old. germ cell tumors are generally located in the gonads but can also appear in the abdomen, pelvis, mediastinum, or brain. germ cells migrating to the gonads may not reach that intended destination and a tumor can grow wherever they end up, but the exact cause is still unknown. these tumors can be benign or malignant. on arrival at the gonad, primordial germ cells that do not properly differentiate may produce germ cell tumors of the ovary or testis in a mouse model. induced differentiation inducing differentiation of certain cells to germ cells has many applications. one implication of induced differentiation is that it may allow for the eradication of male and female factor infertility. furthermore, it would allow same-sex couples to have biological children if sperm could be produced from female cells or if eggs could be produced from male cells. efforts to create sperm and eggs from skin and embryonic stem cells were pioneered by hayashi and saitou's research group at kyoto university. these researchers produced primordial germ cell-like cells (pglcs) from embryonic stem cells (escs) and skin cells in vitro. hayashi and saitou's group was able to promote the differentiation of embryonic stem cells into pgcs with the use of precise timing and bone morphogenetic protein 4 (bmp4). upon succeeding with embryonic stem cells, the group was able to successfully promote the differentiation of induced pluripotent stem cells (ipscs) into pglcs. these primordial germ cell-like cells were then used to create spermatozoa and oocytes. efforts for human cells are less advanced due to the fact that the pgcs formed by these experiments are not always viable. in fact hayashi and saitou's method is only one third as effective as current in vitro fertilization methods, and the produced pgcs are not always functional. furthermore, not only are the induced pgcs not as effective as naturally occurring pgcs, but they are also less effective at erasing their epigenetic markers when they differentiate from ipscs or escs to pgcs. there are also other applications of induced differentiation of germ cells. another study showed that culture of human embryonic stem cells in mitotically inactivated porcine ovarian fibroblasts (pof) causes differentiation into germ cells, as evidenced by gene expression analysis.