Abortion is the termination of human pregnancy. There are two different types of abortion: spontaneous and induced. A spontaneous abortion, often called a miscarriage, occurs naturally as the result of genetic, developmental, or physiological problems in pregnancy. An induced abortion is the termination of 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.
The acrosome is vesicle or membrane enclosed compartment that covers the head of a spermatozoon until the acrosome reaction occurs. The acrosome contains enzymes that help sperm get through the egg coat (zona pellucida) to get to the egg.
The acrosome reaction is an event that occurs when sperm bind to the egg coat (or zona pellucida), causing the chemicals contained in the acrosome to be released. The acrosome reaction helps the sperm get through the egg’s covering to reach the egg’s plasma membrane and is necessary for fertilization.
Activin is a powerful protein that regulates many cellular functions including the production and release of a reproductive hormone called follicle stimulating hormone. Activin is made by most organs and controls cellular proliferation and differentiation. Loss of activin results in profound early developmental defects. Activin is regulated by a closely related protein called inhibin. Activin and inhibin control male and female reproduction and an imbalance in either of these hormones results in infertility in men and women.
The adenohypophysis is the anterior lobe of the pituitary gland. Also called the pars distalis, it 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: corticotropes make corticotropin (adrenocorticotropic hormone, 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 stmulating 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. It is sub-divided into three layers, each producing a different class of hormones. The zona glomerulosa is the most superficial layer and produces mineralcorticoids such as aldosterone, which is involved in the control of blood pressure. The zona fasciculata is the middle layer and produces glucocorticoids such as cortisol, which is involved in controlling metabolism. The deepest layer is the zona reticularis and produces androgens. Because the gonadal tissues normally produce much more androgens than the adrenal cortex, the latter is often considered a secondary site of androgen synthesis.
The adrenal glands are glands located above 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 hormones increase heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, smooth muscle dilation, and metabolism.
Afterbirth refers to the placenta and fetal membranes that are expelled from the uterus after a baby is born.
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.
Alginate is a polysaccharide found in the cell walls of algae. When mixed with calcium, alginate will form a solid-like gel.
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 the embryo and to exchange gases used by the embryo. The allantois expands to form a large sac. It fuses with the chorion to make up a major part of the fetal placenta. The fetal bladder is connected to the allantois. With advancing embryonic development, the size of human allantois is decreased and becomes an elongated sac and part of the umbilical cord.
Amenorrhea is the absence of menstrual bleeding for three months in a woman who previously had normal menstrual cycles, no menses by age fifteen in an adolescent who has normal sexual development or no menses by age thirteen in an adolescent without sexual development.
Amniocentesis is a prenatal test that allows your healthcare practitioner to diagnose health concerns, genetic diseases and chromosomal abnormalities in the womb using a sample of amniotic fluid. This is the fluid that surrounds your baby in the uterus. Amniocentesis is typically performed when an expecting mother is between 16 and 22 weeks pregnant. An amniocentesis cannot detect structural birth defects.
Amnion is the pouch-like tissue 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 surrounding the embryo at birth. During later development, the amnion serves as a reservoir for urine, which is mixed with fluid which derives from maternal serum; this mixture cushions the developing embryo and provides a buoyant environment that allows symmetrical growth of the embryo and prevents embryonic adherence and growth onto the placental tissues. The amnion remains present at birth in most species. The amnion is absent in fish and amphibians.
The amniotic sac is a thin membrane filled with fluid that surrounds the growing fetus. It is a tough yet pliable membrane that is crucial to a successful pregnancy outcome.
The fallopian tube is comprised of three parts. The ampulla is the main section of the tube and is the site where fertilization commonly occurs.
Anaphase is the stage of cell division during which chromosomes separate and are pulled toward opposite poles of the cell by the spindle. In meiosis 1, homologous pairs of chromosomes separate, resulting in two haploid sets of chromosomes. In mitosis and meiosis II, sister chromatids separate.
Androgens are a class of steroid hormones, produced in the testes, adrenal cortex, and ovaries. 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.
Aneuploid refers to a state in which a cell or cells has too many or too few chromosomes. This is often seen in the form of trisomy, such as Trisomy 21 or 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 (absence of egg release from the ovary). Anovulation may be physiological before puberty, during pregnancy and lactation or after menopause or pathological due to disorders affecting mainly 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).
The anterior pituitary gland is the front portion of the pituitary gland. It secretes hormones that control many physiological processes, including growth, reproduction and stress.
An antral (or Graafian follicle) is the most mature ovarian follicle stage. It is characterized by its large diameter and the presence of a liquid-filled space, otherwise know as an antrum.
The anus is the end of the gastrointestinal tract opposite from the mouth. It controls the removal of feces from the body. The anus is the posterior most opening on the perineum.
Apoptosis is programmed cell death. It is a natural mechanism by which a cell intentionally destroys itself without causing damage to the organism.
Aromatase 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.
Artificial insemination (AI) consists in introducing sperm directly into the female genital tract, without sexual intercourse. This technique is used for fertility purpose in human (assisted reproductive technique or sperm donation program) and in animals (genetic selection for productivity, sperm bank of endangered species). Fresh or frozen washed sperm is placed in the vagina using conception device or in the uterus by means of a catheter at the time of ovulation to improve the chance of fertilization of the released oocyte(s).
Assisted hatching is a special technique in 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 promote implantation.
Assisted reproductive technologies (ART) are clinical procedures developed to help women conceive. Examples of ART include in vitro fertilization, intracytoplasmic sperm injection (ICSI), preimplantation genetic diagnosis (PGD), cryopreservation, and intrauterine insemination (IUI). Currently, it is estimated that 4 million children worldwide have been born though 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 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 technologies such as intracytoplasmic sperm injection can be used.
Atherosclerosis is a condition that results in the hardening and narrowing of the arteries. Cholesterol, fat and other substances accumulate along the wall of blood vessels and form hard structures called plaques. These plaques build up over time, begin to block the ovaries, and can lead to cardiovascular disease.
Atresia, with regard to ovarian follicles, is the death of follicles after they have formed, through any cell death pathway. Atresia may occur at any follicle stage, but seems to be more frequent at the primordial and late secondary stages before puberty, and at the late secondary stages 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.
A programmed cell death in which the cell uses its own lysosomal compartment to degrade organelles. Autophagy and apoptosis can be differentiated by several morphological and biochemical markers.
Autosomes are non-sex determining chromosomes common to males and females. Humans have 22 pairs of autosomes and one pair of sex chromosomes.
Azoospermia is the medical term for a condition in which no sperm are 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. However, depending on the type of azoospermia, certain assisted reproductive technologies, such as TESE and ICSI, can be used to help a male with this condition conceive.