Ovum

After insertion of an lUD, polymorphonuclear leukocytes and macrophages accumulate in the uterine cavity. These cells appear to phagocytize sperm and Hberate a blastotoxic toxin (92,93). Intrauterine devices also may create a hostile environment, perhaps because antibodies are produced that interfere with implantation of the fertilized ovum (93). 

The primary difficulty with periodic abstinence is the month-to-month variation in the time of ovulation. Whereas the ovum can only be fertilized during the first 12 to 24 hours after its release from the ovary, sperm remain viable longer in the female reproductive tract, able to fertilize an ovum for 5—7 days and perhaps longer. Thus, intercourse several days prior to ovulation can result in pregnancy. 

Ur zeile, /. primitive ceil, ovum, -zeugung, /. abiogenesls. -zustand, m. original (or primitive) state. 

Other actions of estrogen include fluid retention, protein anabolism, thinning of the cervical mucus, and the inhibition or facilitation of ovulation. Estrogens contribute to the conservation of calcium and phosphorus, the growth of pubic and axillary hair, and pigmentation of the breast nipples and genitals. Estrogens also stimulate contraction of the fallopian tubes (which promotes movement of the ovum), modify the physical and chemical properties of the cervical mucus, and restore the endometrium after menstruation. 

The metabolic myopathies are exceptionally complex. Mitochondrial disorders are usually multisystem disorders, in which metabolic dysfunction affects muscle, liver, CNS, and special senses (especially vision) in almost any combination. There is evidence that some forms of mitochondrial disease are inherited, and the preponderance of maternal rather than paternal inheritance is consistent with an abnormality in the mitochondrial genome because almost all (and perhaps all) mitochondria are derived from the ovum. 

An important feamre of human mitochondrial mtDNA is that—because aU mitochondria are contributed by the ovum during zygote formation—it is transmitted by maternal nonmendefian inheritance. 

Transgenic Describing the introduction of new DNA into germ cells by its injection into the nucleus of the ovum. 

Conception is most likely to occur when viable sperm are present in the upper region of the reproductive tract at the time of ovulation. Fertilization occurs when a spermatozoan penetrates an ovum. Approximately 6 to 8 days after ovulation,... 

Ectopic pregnancy Presence of a fertilized ovum outside the uterine cavity. 

Ovulation Periodic ripening and rupture of a mature follicle and the discharge of an ovum from the cortex of the ovary. 

Self-organisation in ontogenesis, i.e., the development of an individual organism from an ovum to the complete organism. 

In the formation of the zygote, all mitochondria are contributed by the ovum. Therefore, mtDNA is transmitted by maternal inheritance in a vertical, nonmendelian fashion. Strictly maternal transmission of mtDNA has been documented in humans by studies of restriction fragment length polymorphisms (RFLPs) in DNA from platelets. As exemplified by the disorders outlined above,... 

Contraception is the prevention of pregnancy following sexual intercourse by inhibiting sperm from reaching a mature ovum (i.e., methods that act as barriers or prevent ovulation) or by preventing a fertilized ovum from implanting in the endometrium (i.e., mechanisms that create an unfavorable uterine environment). 

IUDs cause low-grade, intrauterine inflammation and increased prostaglandin formation. In addition, endometrial suppression is caused by the progestin-releasing IUD. They are spermicidal and also interfere with implantation of the fertilized ovum. Efficacy rates are greater than 99% with both perfect use and typical use. 

With the exception of certain viruses, the blueprint for all organisms is contained in code by deoxyribonucleic acid (DNA), a giant macro-molecule whose structure allows a vast amount of information to be stored accurately. We have all arisen from a single cell, the fertilized ovum containing two sets of DNA (packaged with protein to form chromatin), one set from our mother, resident in the nucleus of the unfertilized ovum, the second set from our father via the successful sperm. Every cell in the adult has arisen from this one cell and (with the exception of the germ cell and specialized liver cells) contains one copy of these original chromosome sets. 

Womb A hollow, thick-walled, muscular organ in which the impregnated ovum is developed into a child. [NIH]... 

Transgenic animals are produced by transferring cDNA into the pronucleus of a fertilized ovum. The resultant transgenic animal has the new gene (transgene) in all of its cells including its reproductive tissues. Transgenic animals are now widely used as experimental models in which to study human diseases. 

Triploidy refers to cells that contain three copies of each chromosome (69 total). THploidy, which usually occurs as a result of the fertilization of an ovum by two sperm cells, is common at conception, but the vast majority of these conceptions are lost prenatally. However, about 1 in 10,000 live births is a triploid. These babies have multiple defects of the heart and central nervous system, and they do not survive. 

A particularly important type of developmental toxicity is called teratogenicity. After fertilization the ovum - a single cell - begins to proliferate, making more of its own kind by a series of divisions. In humans, at about the ninth day the remarkable process of cell... 

Progestins, of which progesterone is a prime example, induce endometrial maturation following ovulation, setting the stage for implantation of a fertilized ovum and pregnancy ... 

The reproductive system includes the ovaries in the female and testes in the male, together with the ducts and tubes in which the ovum or spermatozoa travel and meet in the fallopian tube, and the environment in which the foetus develops. Since both ovary and testis produce hormones, there is overlap with the endocrine system (Chapters 12 and 19). 

The essential feature of sexual reproduction is that genetic information from two individuals is combined to determine the characteristics of their offspring. In fertilisation, one cell (the ovum), which contains only one half of the normal number of chromosomes, receives chromosomes from another cell, the sperm, that also contains half the normal number of chromosomes. The result is a cell that then contains the normal number of chromosomes. The process that makes this possible is meiosis (Chapter 20) which not only results in a halving of the number of chromosomes, but almost randomly determines which characteristics are passed on to the offspring. 

Much of the behaviour of humans and society as a whole is designed to provide an opportunity for a sperm and an ovum to meet in the uterus. However, sexual reproduction is not the only means of reproduction asexual reproduction occurs frequently in the animal kingdom and in many tissues in order to increase the number of cells during growth or to maintain the number due to cell death. The question arises, therefore, why sexual reproduction has arisen at all since it is very expensive in the use of resources for example, in the process of selecting a suitable mate, and in the requirement for males, whose only function in some species is fertilisation of the females, yet they use resources that could be used by the females and by the offspring. Perhaps because of this, a few species of vertebrates (e.g. some lizards) have retained the option for asexual reproduction. This process is known as parthenogenesis. Mammals, however, have lost this option. This... 

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