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X Chromosome Inactivation
X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation). The choice of which X chromosome will be inactivated in a particular embryonic cell is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism (its cell line). The result is that the choice of inactivated X chromosome in all the cells of the organism is a random distribution, often with about half the cells having the paternal X chromo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mammalian Embryogenesis
Mammalian embryogenesis is the process of cell division and cellular differentiation during early prenatal development which leads to the development of a mammalian embryo. Difference from embryogenesis of lower chordates Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates. The fact that an embryo in both placental mammals and marsupials undergoes the process of implantation, and forms the chorion with its chorionic villi, and later the placenta and umbilical cord, is also a difference from lower chordates. The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the devel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same locus (genetics), loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal Sex-determination system#Chromosomal determination, sex-determination system. If both alleles of a diploid organism are the same, the organism is #Homozygous, homozygous at that locus. If they are different, the organism is #Heterozygous, heterozygous at that locus. If one allele is missing, it is #Hemizygous, hemizygous, and, if both alleles are missing, it is #Nullizygous, nullizygous. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tortoiseshell Cat
Tortoiseshell is a cat animal coloration, coat coloring named for its similarity to tortoiseshell pattern. Like tortoiseshell-and-white or Calico cat, calico cats, tortoiseshell cats are almost exclusively female. Male tortoiseshells are rare and are usually sterile.Atkins (2003), p.105 Tortoiseshell cats, or torties, combine two colors other than white, either closely mixed or in larger patches. The colors are often described as red and black, but the "red" patches can instead be orange, yellow, or cream, and the "black" can instead be chocolate, gray, tabby cat, tabby, or blue. Tortoiseshell cats with the tabby cat, tabby pattern as one of their colors are sometimes referred to as torbies or torbie cats. "Tortoiseshell" is typically reserved for multicolored cats with relatively small or no white markings. Those that are predominantly white with tortoiseshell patches are described as tricolor, tortoiseshell-and-white, or calico cat, calico. Tortoiseshell markings appear in m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ovulation
Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and after the follicular phase. Ovulation is stimulated by an increase in luteinizing hormone (LH). The ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilized by sperm. If it is not, it will break down in less than a day. Meanwhile, the uterine lining ( endometrium) continues to thicken to be able to receive a fertilized egg. If no conception occurs, the uterine lining will eventually break down and be shed from the body via the vagina during menstruation. Some people choose to track ovulation in order to improve or aid becoming pregnant by timing intercourse with their ovulation. The signs of ovulation may include cervical muc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Primordial Germ Cells
A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants. Introduction Multicellular eukaryotes are made of two fundamental cell types: germ and somatic cells. Germ cells produce gametes and are the only cells that can undergo meiosis as well as mitosis. Somatic cells are all the other cells that form the building blocks of the body and they only divide by mitosis. The lineage of germ cells is called the germline. Germ cell specification begins during cleavage in many animals or in the epiblast during gastrulation in bi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. 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. Oogenesis results in the formation of both primary oocytes during fetal period, 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 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Germline
In biology and genetics, the germline is the population of a multicellular organism's cells that develop into germ cells. In other words, they are the cells that form gametes ( eggs and sperm), which can come together to form a zygote. They differentiate in the gonads from primordial germ cells into gametogonia, which develop into gametocytes, which develop into the final gametes. This process is known as gametogenesis. Germ cells pass on genetic material through the process of sexual reproduction. This includes fertilization, recombination and meiosis. These processes help to increase genetic diversity in offspring. Certain organisms reproduce asexually via processes such as apomixis, parthenogenesis, autogamy, and cloning. Apomixis and Parthenogenesis both refer to the development of an embryo without fertilization. The former typically occurs in plants seeds, while the latter tends to be seen in nematodes, as well as certain species of reptiles, birds, and fish. Autogamy ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Inner Cell Mass
The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of the fetus. The inner cell mass forms in the earliest stages of embryonic development, before implantation into the endometrium of the uterus. The ICM is entirely surrounded by the single layer of trophoblast cells of the trophectoderm. Further development The physical and functional separation of the inner cell mass from the trophectoderm (TE) is a special feature of mammalian development and is the first cell lineage specification in these embryos. Following fertilization in the oviduct, the mammalian embryo undergoes a relatively slow round of cleavages to produce an eight-cell morula. Each cell of the morula, called a blastomere, increases surface contact with its neighbors in a process called compaction. This results in a polari ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
X-chromosome Reactivation
X chromosome reactivation (XCR) is the process by which the inactive X chromosome (the Xi) is re-activated in the cells of eutherian female mammals. Therian female mammalian cells have two X chromosomes, while males have only one, requiring X-chromosome inactivation (XCI) for sex-chromosome dosage compensation. In eutherians, XCI is the random inactivation of one of the X chromosomes, silencing its expression. Much of the scientific knowledge currently known about XCR comes from research limited to mouse models or stem cells. Partial XCR may derepress one or more genes on the Xi, and the level of restored gene expression may not be as high as it would normally be on the active X chromosome (the Xa). Complete XCR restores the Xi to Xa and erases the epigenetic memory of XCI, meaning that inducing X-inactivation again will randomly select an X chromosome to silence, rather than deterministically silencing the original Xi. XCR is an emerging topic of interest for multiple reasons: ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity or lumen known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion (the portion of the chorion that develops villi) and the underlying uterine tissue of the mother. The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula. In humans, blastocyst formation begins about five days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells. The blastocyst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
