MOOC Advertisement

Learn More:
Get An Introduction to Reproduction

Learn more about the the science of reproduction. Dr. Teresa Woodruff, creator of Repropedia, is offering a free online course exploring the world of reproduction.

Get Started!

A B C D E F G H I K L M N O P R S T U V W Y Z

Abortion is the termination of a human pregnancy by removal of the fetus or embryo from the womb before it has acquired the ability to survive on its own. There are two types of abortion: spontaneous and induced. A spontaneous abortion, often called a miscarriage, occurs naturally as the result of genetic, developmental, or physiological problems developed during pregnancy. An induced abortion is the termination of a pregnancy by medical or surgical methods. An induced abortion can be either therapeutic or elective. Therapeutic abortions are performed when continued pregnancy threatens the life of the mother. Elective abortions are performed at the mother’s request for other reasons.

Abstinence, in relation to reproduction, is a voluntary act or practice of refraining from sex. Often times, people are abstinent in order to prevent pregnancy, as abstinence is the only form of birth control that is 100% effective. However, abstinence may be practiced for personal, religious, or other medical reasons. 

The acrosome is a vesicle or membrane enclosed organelle that covers the anterior portion of the head of a spermatozoon until the acrosome reaction occurs.

As a spermatozoon approaches the zona pellucida (outer glycoprotein membrane) of the oocyte, the membrane of the acrosome will fuse with the plasma membrane of the oocyte, allowing the contents of the acrosome (including enzymes and antigens) to break through the egg’s tough coating. This reaction is necessary for fertilization of the egg to occur.

Activin is a powerful protein that regulates and participates in many cellular functions, including the menstrual cycle, and more specifically, the production and release of a reproductive hormone called follicle stimulating hormone (FSH). Activin is made by most organs and controls cellular proliferation and differentiation. Loss of activin can result in profound early developmental defects. Activin is regulated by a closely related protein called inhibin. Both activin and inhibin control male and female reproduction, and an imbalance in either of these hormones can result in infertility in both men and women.

The adenohypophysis is the anterior lobe of the pituitary gland and consists of the distal, intermediate, and infundibular parts. The adenohypophysis develops from a group of cells, Rathke’s pouch, that migrate toward the center of the base of the brain from the roof of the embryonic oral cavity. The post-embryonic anterior lobe contains five cell types that produce six hormones in response to the presence of releasing hormones produced by the hypothalamus: corticotropes make corticotropin (adrenocorticotropichormone, ACTH), gonadotropes make follitropin (follicle stimulating hormone, FSH) and luteotropin (luteinizing hormone, LH), lactotropes (mammotropes) make prolactin (PRL), somatotropes make somatotropin (growth hormone, GH), and thyrotropes make thyrotropin (thyroid stimulating hormone, TSH). These hormones are central to maintaining either metabolic balance in essential physiological systems (ACTH, GH, TSH) or are central to successful reproduction (FSH, LH, PRL).

The adrenal cortex is the outer tissue later of the adrenal gland and is the production site of corticosteroid and androgen hormones. It is sub-divided into three layers (zones), each structurally unique and producing a separate class of hormones. The zona glomerulosa is the most superficial layer and produces mineralcorticoids, such as aldosterone, which is involved in the regulation of blood pressure. The zona fasciculata is the middle layer and produces glucocorticoids such as cortisol, which is involved in regulating metabolism. The innermost layer is the zona reticularis and produces androgens, including androstenedione, which is the precursor to testosterone. Because the gonadal tissues normally produce more androgens than the adrenal cortex, the latter is often considered a secondary site of androgen synthesis.

The adrenal glands are endocrine glands situated directly on top of the kidneys. They are an important part of the endocrine system, producing many different types of hormones involved in biological functions, such as the stress response.

The adrenal medulla is the central, reddish-brown portion of the adrenal gland and is surrounded by the adrenal cortex. It consists of irregularly shaped cells grouped around blood vessels, which are intimately connected by the sympathetic division of the autonomic nervous system. The cells of the adrenal medulla produce, store, and release epinephrine (adrenaline), norepinephrine (noradrenaline), and a small amount of dopamine. These are the hormones involved in the fight-or-flight response, where the production of epinephrine increases heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, smooth muscle dilation, and metabolism; norepinephrine has the opposite effect.

Adrenocorticotropic hormone (ACTH), also known as corticotropin, is a polypeptide hormone produced in the anterior pituitary gland. Its release often results in greater production and release of corticosteroids.

Also called placental expulsion, afterbirth refers to the placenta and fetal membranes that are expelled from the uterus via the birth canal following the birth of the baby.

In endocrinology, an agonist is a chemical or hormone that is capable of stimulating a cell in a manner akin to another hormone. It usually acts as a hormone mimic and binds to the same receptor used by the mimicked hormone in order to induce a biological response.

AI

Alginate is a polysaccharide found in the cell walls of algae. When mixed with calcium, alginate will form a solid-like gel used to encapsulate cells.

The allantois is a fetal membrane that develops from a small vesicle at the hindgut of the very early embryo. Its function is to collect liquid waste from and to exchange gases used by the embryo. The fetal bladder is connected to the allantois. With advancing embryonic development, the size of the human allantois is decreased and becomes an elongated sac and part of the umbilical cord. The allantois is vestigial in humans.

An allosome differs from a typical autosome in that it determines the sex of an individual (e.g. the X and Y chromosomes are allosomes). In humans, an ovum contains the X allosome, and a sperm contains either the X or Y allosome.

Amenorrhea is the absence of menstrual bleeding or suppression of the normal menstrual cycle for three or more consecutive months in a woman who previously had normal menstrual cycles (secondary amenorrhea). Amenorrhea also refers to the absence of menses by age 14 in an adolescent with no development of secondary sexual characteristics, or by age 16 in an adolescent who has had normal sexual development (primary amenorrhea). Common causes of amenorrhea include pregnancy, lactation during the months following pregnancy, underlying issues with one’s reproductive organs or functions, contraceptive medication or other drugs/therapies (like chemotherapy or radiation), menopause, low levels of energy stores or malnutrition, stress or anxiety disorders, and excessive weight loss or exercise.

Amniocentesis is a prenatal test that allows one’s healthcare practitioner to diagnose health concerns, genetic diseases, and chromosomal abnormalities in the womb by extracting a sample of amniotic fluid. This is the fluid that surrounds the baby in the uterus during development, where fetal cells that contain important health information are present. Amniocentesis is typically performed when an expectant mother is between 15 and 18 weeks pregnant and, due to the small level of risk, is often times only advised for women who have an increased risk of giving birth to a baby with genetic defects. Amniocentesis cannot detect structural birth defects. It is also the most accurate way to determine the sex of the baby before birth.

The amnion is the pouch-like tissue present in the early embryo that lies over the dorsal surface of the epiblast portion of the developing inner cell mass. It is formed from cells that detach from the cytotrophoblast layer of the trophoblast stage of the embryo. As gestation progresses, the amnion grows outward and eventually surrounds the entire embryo and, in live-bearing animals, the umbilical cord, for which it forms an epithelial covering. In the latest stages of gestation, the amnion adheres to the inner cell layer of the chorion, forming the inner of the two membranes (the two together serving as the amniotic sac) surrounding the fetus. During later development, the amnion serves as a reservoir for urine, which is mixed with fluid that derives from maternal serum; this mixture cushions the developing fetus and provides a buoyant environment that allows for symmetrical growth and prevents embryonic adherence and growth onto the placental tissues. The amnion remains present at birth in most species but is absent in fish and amphibians altogether.

The amniotic cavity is the fluid-filled space within the amniotic sac where the developing fetus resides.

Amniotic fluid is the protective liquid that surrounds the developing fetus within the amniotic sac. Most of the amniotic fluid consists of fetal urine, so low amounts can indicate a lack of placental flow.

The amniotic sac is a thin but tough pair of membranes (namely, the chorion and amnion) filled with fluid. The amniotic sac surrounds the growing embryo, and later, the fetus. It is tough, yet pliable and transparent and is crucial for a successful pregnancy outcome, as it provides necessary support and cushioning.

An amniotomy, or manual puncture of the amniotic sac, is usually performed at the time of labor to induce delivery of the baby.

The fallopian tube is comprised of three parts. The ampulla is the secondary, dilated section of the tube and is the main site where fertilization commonly occurs in both mice and humans.

Anabolic steroids (the proper scientific term being ‘anabolic-androgenic steroids’, abbreviated AAS) are synthetic testosterones prescribed to treat a medical condition where hormonal deficiencies are present, or used illegally with the desire of increasing muscle mass and/or the appearance of masculine sex characteristics. Use is most common among young men. Anabolic steroids can be taken orally, injected into the muscles, or rubbed on the skin in the form of a cream. There are many effects of the unregulated use of anabolic steroids by young men, including testicle shrinkage, reduced sperm count or infertility, baldness, development of breasts (gynecomastia), and an increased risk for prostate cancer.

Anaphase is the stage of cell division (mitosis or meiosis) during which chromosomes separate and are pulled toward opposite poles of the cell by the spindle in preparation for cell division. In meiosis I, homologous pairs of chromosomes separate, resulting in two haploid sets of chromosomes, a reduction from the diploid set of chromosomes present at the start. In mitosis and meiosis II, sister chromatids separate.

Androgens are a class of steroid hormones produced in the testes, adrenal cortex, ovaries, and to some extent, fat cells. In males, androgens are essential for the growth and maintenance of male sex organs and secondary sex characteristics. In females, androgens serve as the precursor to estrogen. Testosterone is an example of an androgen.

Androstenedione is an intermediate steroid horome produced in the adrenal glands and gonads (the testes and ovaries) as a precursor to the androgen, testosterone, and the estrogens, estrone and estradiol. It is, in part, produced in response to the release of ACTH by the anterior pituitary gland.

Anencephaly is a neural tube defect in which a fetus never develops parts of its brain and skull. Almost all babies born with this disorder will die shortly after birth, however, a mother may choose to terminate her pregnancy beforehand.

Aneuploidy traces back to the point of cell division when chromosomes are not separated properly and refers to a state in which a cell or cells has/have too many or too few chromosomes. This is often seen in the form of trisomy, such as Trisomy 21 (Down syndrome), or monosomy, such as Turners Syndrome. In female gametes, aneuploidy can lead to miscarriages, infertility, and birth defects.

Anovulation is the absence of ovulation, or oocyte release from the ovary, in the menstrual cycle. Anovulation may be physiological before puberty, during pregnancy and lactation, or after menopause. It may also be pathological due to disorders affecting the ovaries, pituitary gland, or hypothalamus. Chronic anovulation accounts for about 30% of all cases of female infertility. The most common cause of chronic anovulation is polycystic ovarian syndrome (PCOS).

In endocrinology, an antagonist is a chemical or hormone that is capable of blocking the binding of an agonist to its receptor but does not induce a biological response upon binding itself.

The anterior pituitary gland is the front portion of the pituitary gland. It secretes hormones that regulate many physiological processes, including growth, reproduction, lactation, and stress.

Anti-mullerian hormone (AMH) is a hormone secreted by the Sertoli cells in the embryonic developing testes of males. AMH represses the development of the Mullerian Duct (also called paramesonephric ducts, developing female reproductive tract). AMH is also secreted by granulos cells of preantral and smaller antral follicels in the adult ovary. During male embryonic development, AMH insures the regression of female structures (fallopina tubes, uterus, cervix, and upper vagina), enabling the proper development of the wolffian duct (male reproductive tract, vas deferens, seminal vesicles). In the female, AMH secretion occurs during the reproductive lifespan until menopause; because of this, AMH can be used as a marker of the ovarian reserve. 

An antral follicle (or Graafian follicle) has reached the most mature ovarian follicle stage of folliculogenesis. An antral follicle is characterized by its large diameter and the presence of a liquid-filled space, otherwise know as an antrum.

The anus is the ring-shaped opening at the end of the gastrointestinal tract opposite from the mouth. It is also the end of the alimentary canal and the posterior most opening on the perineum. It controls the removal of feces (solid waste material) from the body.

Apoptosis is a type of programmed cell death in multicellular organisms. It is a natural mechanism by which a cell intentionally destroys itself without causing damage to the organism. However, excessive apoptosis may cause atrophy (shrinking or shriveling), while lack of apoptosis in the body can lead to cancer and uncontrolled cell proliferation.

Aromatase (also referred to as estrogen synthetase or estrogen synthase) is the key enzyme responsible for estrogen production. Aromatase enzymes act by accelerating the conversion of testosterone, an androgen, into estrogens. Estrogens have been shown to promote the growth of some breast cancers. Aromatase inhibitors can decrease estrogen levels and have been used for treating breast cancer, as well as other disorders.

ART

Artificial insemination (AI) consists of introducing sperm directly into the female genital tract, without sexual intercourse. This technique is used for fertility purposes in humans (with the use of assisted reproductive technology or a sperm donation program) and in animals (to employ genetic selection for productivity or by using a sperm bank of endangered species). Fresh or frozen washed sperm is placed in the vagina using a conception device or in the uterus by means of a catheter at the time of ovulation to improve the chances of fertilization of the released oocyte(s).

Assisted hatching is a special technique used as an assisted reproductive technology. A protective coating, called the zona pellucida, shields the developing embryo as it travels through the fallopian tube and enters the uterus. In order for the embryo to attach to the uterine wall, the zona must be dissolved and the embryo released. This is a naturally occurring process called embryo hatching. Assisted hatching was developed to overcome barriers to this process due to zona hardening or thickening. The zona can be penetrated using chemical agents, mechanical pressure, or a laser to assist in the release of the embryo and to promote implantation.

Assisted reproductive technology (ART) refers to a set of clinical procedures developed to help women conceive (become pregnant). Examples of ART include in vitro fertilization, intracytoplasmic sperm injection, preimplantation genetic diagnosis, cryopreservation, and intrauterine insemination. Currently, it is estimated that four million children worldwide have been conceived using ART procedures.

Asthenospermia is the medical term for a condition in which a male’s sperm have reduced motility (the ability to swim or move). Genetic, idiopathic, and iatrogenic factors can result in this condition. Although male infertility is most commonly attributed to a low sperm count, having sperm with poor motility can lead to infertility, as well, because, once ejaculated, the sperm must be able to travel through the vagina, cervix, and uterus before reaching the fallopian tubes where fertilization will take place. To overcome infertility due to asthenospermia, assisted reproductive technology procedures can be utilized, such as intracytoplasmic sperm injection.

Atresia, with regard to ovarian follicles, is the degeneration of follicles after they have formed (but before they have fully matured) through hormonally regulated apoptosis. Atresia may occur at any follicular stage of the menstrual cycle but seems to be more frequent at the primordial and late secondary stages before puberty and at the late secondary stage during adult cycling.

Attrition, with regard to germ cells, is the death of germ cells before follicle formation. This process most likely occurs through apoptosis but may also involve autophagy or other forms of cell death.

Autophagy is a form of programmed cell death in which lysosomes degrade unnecessary and dysfunctional cells. Autophagy can be a way to maintain energy levels in cells during periods of starvation. Autophagy and apoptosis can be differentiated by several morphological and biochemical markers.

Autosomes are non-sex-determining chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (allosomes).

AZH

Azoospermia is the medical term for a condition in which no measurable amount of sperm is present in a male’s semen. Genetic, idiopathic, and iatrogenic factors can result in this condition. There are two types of azoospermia: obstructive azoospermia, which refers to problems in sperm transport from the testis to the penis, and non-obstructive azoospermia, which refers to defects in sperm production. Because of the lack of sperm in the ejaculate, azoospermia results in infertility and sterility. However, depending on the type of azoospermia, certain assisted reproductive technology procedures, such as TESE and ICSI, can be used to help a man affected by this condition conceive a child.