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Acrosome reaction: The chemical and physical changes in the membrane on the head of the sperm that allow it to penetrate an egg.

Ampulla: The wide distal end of the fallopian tube where the sperm must meet the egg in order for fertilization to take place.

Anovulatory cycle: A menstrual cycle that takes place even though ovulation has not occurred - common in perimenopausal women.

Aspiration: A procedure for collecting eggs from the ovaries done by passing a hollow needle into each fluid-like follicle. The fluid is sucked out ("aspirated") and the egg separated from the fluid under the microscope in the laboratory. An aspiration can be performed laparoscopically or, most commonly, under ultrasound guidance by the transvaginal route.

Assisted hatching: A technique done in the laboratory whereby a small opening is made in the zona pellucida of the embryo in order to facilitate implantation.

Assisted reproductive technology (ART): Any treatment that involves the manipulation of human gametes in order to assist a pregnancy. Types of ART include IVF, GIFT, ZIFT, and TET, as well as egg or embryo donation, cryopreservation, and surrogate birth.

Blastocele: The fluid-filled cavity in the middle of the blastocyst.

Blastocyst: The stage of embryonic development that is reached about one hundred and twenty hours after fertilization. At this stage, a fluid-filled cavity (the blastocele) develops in the center of the embryo and cells begin to specialize. Some of them will go on to become the fetus, some will become the placenta.

Blastomere: One of several dividing cells that make up the pre-embryo - a new mitotic division takes place approximately every twelve hours after fertilization.

Capacitation: A change in the sperm cell, causing hyperactivity and faster swimming, that enables the acrosome reaction and penetration of the egg. It takes place after the sperm has passed through the cervical mucus of the female reproductive tract or when prepared for insemination in the laboratory.

Centrosome: A hollow structure within the cytoplasm of the cell near the nucleus, closed on one end and open on the other, that contains two centrioles which organize the mitotic spindle, then separate during mitosis, each becoming part of the two new daughter cells.

Cervical mucus: The secretion produced by glands in the cervical canal which changes throughout the cycle from watery and slippery at the time of ovulation to crumbly and dry right afterward.

Cervix: The opening of the uterus at its lowest end; essentially, it is a narrow canal that connects the vaginal canal with the uterine corpus where implantation takes place.

Chromosomes: The structure within a cell's nucleus that holds the DNA. Each cell has 46 chromosomes, except for the gametes, which have 23.

Cloning: Growing cells in a laboratory that are exact replications of one another.

Corpus luteum: The yellow body that is formed from the collapsed follicle after it releases its egg. The corpus luteum produces progesterone as well as some estrogen after ovulation, in order to prepare the lining of the uterus for implantation of an embryo.

Cortical granules: Tiny vesticles which line the outside of the oolemma of the mature egg. When sperm successfully binds to the oolemma, indicating that fertilization has taken place, chemicals from these granules are released into the space beneath the zona pellucida, hardening it so that no other sperm can enter the egg.

Cryopreservation: Storing tissue or cells by freezing them in liquid nitrogen.

Culture medium: The fluid solution used in the laboratory to grow cells in vitro.

Cumulus: The sticky white cloud-like clump of cells that develops around the egg just prior to ovulation and which accompanies it during and after ovulation or aspiration.

Cytoplasm: The protein-rich gelatinous fluid surrounding the nucleus of a cell that contains microscopic structures that guide cell development and functions.

Deoxyribonucleic acid (DNA): The long strand like molecule in each cell's nucleus that contains the genetic code.

Dominant follicle: The largest follicle that develops in one female cycle containing the egg that will ovulate.

Donor egg: An egg retrieved from one woman which will be inseminated in vitro. The resulting embryo will then be transferred to another woman.

Donor embryo: An embryo formed from a dual donation of both egg and sperm which will be transferred, in a process called embryo transfer, to a recipient in order to achieve a pregnancy.

Donor Insemination: Insemination with sperm other than that of the recipient's partner.

Downregulation: The process by which the natural female cycle is halted with a GnRH agonist, like Lupron, in order to stop the pituitary from producing the gonadotropins FSH and LH and the ovary from developing follicles and producing estrogen.

Ectopic pregnancy: A pregnancy that implants outside the uterine cavity, most commonly in the fallopian tube.

Egg retrieval: Retrieval of eggs from the ovary. See also Aspiration.

Ejaculate: The seminal fluid that emanates from the penis during sexual climax and carries the sperm cells out of male reproductive tract.

Embryo: A fertilized egg that has gone through one or more cell divisions. Also, the stage of human development that is marked from the time of fertilization through the eighth week of pregnancy. See also Zygote.

Embryo transfer: The replacement of one or several embryos back into the female reproductive tract; usually understood to mean transcervical embryo transfer, in which the embryos are placed directly into the uterine cavity approximately 72 hours after fertilization. See also Tubal embryo transfer.

Endometrial biopsy: A sample of tissue taken from the endometrium. In egg donation, it is performed on the recipient prior to the donation attempt during a practice cycle. The purpose is to examine the condition of the endometrium and determine its receptivity to an embryo.

Endometriosis: A condition in which nests of cells from the endometrium grow outside of the uterine cavity, most commonly in the abdomen and pelvis, behind the uterus and in the ovaries. It is associated with infertility and may lead to tubal blockage and pelvic pain.

Endometrium: The tissue that lines the interior of the uterus. It grows during estrogen stimulation, becomes receptive to implantation when stimulated by progesterone, and sloughs off during menses.

Epididymis: The location in the male reproductive tract where sperm collect after leaving the testicles.

Estradiol (E2): The most potent of a class of hormones called estrogens, produced by the follicle, the corpus luteum, and the placenta.

Estrogen: The female sex hormones that predominates during the first half or follicular phase of the menstrual cycle. This hormone, in addition to assuring female sex characteristics, also interacts with over three hundred tissues in the body, including the bones and the heart. Its levels are greatly diminished after menopause.

Fallopian tube: One of two narrow tubes that carries the egg from the ovary to the uterus. Fertilization of the egg by the sperm takes place at the distal end of this tube. See also Oviduct.

Fertilization: Penetration of the egg by the sperm.

Fetus: The stage of human development that follows the embryo; from about 7 to 8 weeks after fertilization until birth.

Fimbriae: The finger-like projections of the fallopian tubes that pick up the egg as it falls from its follicle after ovulation.

First polar body: A small satellite portion of the egg located within the zona pellucida which is pinched off from the main part of the egg to fertilization. It contains half the chromosomes as a result of the first meiotic division. See also Second polar body.

Follicle: The fluid-filled, estrogen-producing sac that encloses the egg inside the ovary. During ovulation, the follicle bursts open, releasing the egg into the peritoneal cavity.

Follicle-stimulating hormones (FSH): The gonadal hormone released by the pituitary gland that acts as a messenger of the ovary or testicles, telling them to ripen one or more follicles in the female and to produce sperm in the male.

Follicular phase: The first half of the female menstrual cycle, driven by the production of FSH by the pituitary and characterized by the release of estrogen from the follicle.

Gamete: The male or female reproductive cell, e.g. the sperm or the egg.

Gamete intrafallopian transfer (GIFT): A laparoscopic procedure where the sperm and egg are placed in the fallopian tube together.

Germ cells: The precursors of gametes.

Gestational surrogacy: A pregnancy where a woman other than the mother carries the fetus. This usually occurs in cases where the mother has had a hysterectomy and has no uterus, but may also occur when the mother has an illness that would make labor and delivery too dangerous for her.

Gonadotropin-releasing hormone (GnRH): The hormone released by the hypothalamus that stimulates the pituitary to produce and release the gonadotropins FSH and LH.

Gonadotropins: The hormones FSH and LH which stimulate ovarian function in the female and testicular function in the male.

Gonads: The gamete-producing organs, e.g. the ovary in the female and the testis in the male.

Granulosa cells: The helper cells in the follicle that make estrogen and progesterone.

Hatching: The activity of the embryo breaking out of the zona pellucida so that it can implant in the endometrium.

Human Chorionic gonadotropins (hCG): A hormone produced by the placenta during pregnancy, which is also akin to LH. During an ovulation stimulation cycle, this hormone is given 36 hours before aspiration to prepare the follicle to release their eggs.

Human menopausal gonadotropin (hMG): A preparation of the gonadotropins FSH and LH manufactured from the collected urine of postmenopausal women. After menopause, the ovaries send no feedback signals to the pituitary, which consequently produces large quantities of gonadotropins that are then excreted in the urine. See Repronex, Menopur.

Implantation: The attachment of the embryo to the uterine wall.

In vitro fertilization (IVF): An ART procedure that involves aspirating eggs from a woman's ovary and combining them in a petri dish with sperm that has been ejaculated into a container and prepared in the laboratory. The resulting embryos are then transferred back into the woman's body, either through her cervix or through the fallopian tube.

Intracytoplasmic sperm injection (ICSI): A procedure to correct male infertility in which a single sperm is injected into a mature egg so that fertilization can take place in the laboratory.

Intrauterine insemination (IUI): The simplest type of ART, where ejaculated sperm is prepared in the laboratory and then placed into the uterine cavity at an appropriate time in the woman's cycle.

Isthmus: The part of the fallopian tube closest to the uterus where the embryo must travel before implantation.

Laparoscopy: A surgical procedure in which a tiny incision is made near the navel and a telescope-like instrument is inserted in order to examine the internal organs.

Lupron: A brand of leuprolide acetate, the medication given to stop the natural menstrual cycle. It suppresses the pituitary, which suppresses ovarian function, putting the body into a state similar to menopause.

Luteal phase: The second half of the female menstrual cycle, dominated by the LH surge and subsequent production of progesterone by the corpus luteum. It begins with ovulation and ends with menses.

Luteinizing hormone (LH): The gonadal hormone released by the pituitary gland that stimulates hormonal production by the follicle and corpus luteum in the female and the testicle in the male. It is produced in large quantities during the female mid-cycle surge.

LH surge: The release of great quantity of the hormone LH from the pituitary, which triggers ovulation and the resumption of meiosis in the egg.

Meiosis: The process of cell division typical only of gametes. During this process, which means "lessening" in Greek, the cell rids itself of its genetic load by throwing off half the original number of chromosomes.

Menopur: One of several brands of hMG. This medication is used to stimulate the ovaries to produce multiple, large follicles.

Menstruation: The sloughing of the lining of the uterus which marks the end of one female cycle and the beginning of another. It is characterized by low levels of estradiol and a thin endometrium.

Micromanipulation: The use of a powerful microscope and remotely controlled fine instruments to manipulate gametes and embryos, e.g. to inject the sperm inside the egg in the laboratory (ICSI). Also used for Assisted Hatching and Preimplantation Genetic Diagnosis (PGD).

Mitosis: The type of cell division typical of all cells except gametes. The genetic material is duplicated, then partitioned off into two cells. When the cell splits in half, the two daughter cells have exactly the same amount and same copies of the original DNA.

Mitotic spindle: The spindle-shaped array of microtubules which hold the 46-chrosomes (23 pairs) during mitotic division.

Motility: The designation of the quality of sperm movement.

Multiple birth: A pregnancy that results in more than one child.

Nucleus: The structure inside each cell that contains DNA.

Oocyte: The female reproductive cell; an egg.

Oogonia: The germ cell precursors of eggs within fetal ovaries.

Oolemma: The cell membrane of the egg. It is surrounded by the zona pellucida.

Ovary: The female gonad, or reproductive organ, that holds the oocytes and is responsible for the production of estrogen and progesterone.

Ovarian hyperstimulation syndrome: A potentially dangerous condition that can result after ovarian stimulation and ovulation or follicle aspiration in which vasoactive elements from the corpus luteum release fluid from blood vessels into the abdominal cavity. This can result in dehydration, clot formation, or difficulty in breathing.

Ovarian stimulation: The use of medication (as FSH-like drug) to encourage the ovary to produce multiple follicles and eggs in a cycle.

Oviduct: Fallopian tube.

Ovulation: The release of the egg from the follicle.

Pituitary gland: The gland at the base of the brain that secretes the gonadotropins and other hormones that control the endocrine system.

Pre-embryo: The term used by ethicists to denote the stage from fertilization to about two weeks after implantation when the fetal plate forms and when the implanted pregnancy can no longer split into more than one individual. (Embryologists commonly refer to all stages beyond fertilization as the "embryo.")

Pregnancy test: A blood or urine test to detect whether levels of hCG (human chorionic gonadotropin) are elevated, indicating that implantation of the embryo has occurred.

Preimplantation Genetic Diagnosis (PGD): Examination of embryonic cells prior to implantation to determine whether the embryo will be genetically normal.

Premature ovarian failure: A disorder of the female reproductive system whereby the ovaries stop functioning before the age of 40, also called "premature menopause."

Primary follicle: An early form of the follicle, containing an immature egg in which the surrounding helper cells first start to form a fluid-filled center. See also Follicle.

Primordial follicle: The earliest form of the follicle, developed in the fetal ovary, containing an immature egg and a single layer of helper cells. Most follicles in the ovary are of primordial variety at any given time. They remain in this state until they begin the process toward ovulation by becoming primary follicles. See also Follicle.

Progesterone (P4): The hormone produced by the corpus luteum that readies the endometrium to receive an embryo for implantation.

Prolactin: The hormone produced by the pituitary during pregnancy to prepare the mammary glands for lactation.

Pronuclear stage: The stage of embryonic development just after fertilization when the chromosomes from the egg and sperm are bundled separately and before both sets of genes has co-mingled.

Pronucleus, female and male: A nucleus like structure that holds either the male or female half of the chromosomes in a fertilized zygote before the genes have commingled. The pronuclei are visible with a microscope 14 to 18 hours after insemination.

Pulse oximeter: The instrument that measures respiration in the patient during follicle aspiration. This device can detect the oxygenation of the blood that flows through a finger. It tells us that the patient is not only breathing normally, but that she is oxygenating well and her circulation is carrying oxygenated blood to all parts of her body.

Repronex: One of several brands of hMG. This medication is used to stimulate the ovaries to produce multiple, large follicles.

Retrograde ejaculation: A disorder of the male reproductive system in which the ejaculate flows backwards into the bladder rather than out through the prostatic urethra.

Retrograde menstruation: A condition of the female reproductive system whereby menstrual blood and debris flows back into the pelvic cavity rather than out through the cervix and into the vagina during the menstrual period. A small amount of retrograde menstruation is common and probably normal; in combination with other factors or if extreme, however, it is thought to be responsible for the disease endometriosis.

Scrotal sac: The sac-like structure that holds the testes.

Second polar body: The second satellite of the egg which results from the second meiotic division. It contains half the DNA remaining in the egg and a tiny portion of cytoplasm. Since second meiosis is completed only after successful penetration of the egg by the sperm, the presence of a second polar body is evidence of successful fertilization. See also First polar body.

Semen: The mixture of sperm and seminal fluid released during ejaculation.

Seminal fluid: The liquid that carries the sperm out of the male reproductive tract.

Sperm: The male reproductive cell.

Sperm aster: The star-like structure that surrounds the decondensed sperm head after fertilization. The formation of the aster is thought to be the sperm's method of moving and organizing the chromosomes prior to and during the 2-pronucleaur stage.

Sperm washing: A laboratory technique using a culture medium, a centrifuge or a density gradient in order to separate sperm cells from seminal fluid.

Spermatocyte: An immature sperm cell.

Superovulation: The use of medication (an FSH-like drug) to encourage the ovary to produce multiple follicles and eggs in a cycle. Also, known as "controlled ovarian hyperstimulation." See also Ovarian stimulation.

Testicle: The male gonad or reproductive organ that holds the sperm and produces the hormone testosterone.

Testosterone: The male sex hormone produced in large quantities in the male and in lesser quantities by the female. It is the chemical precursor of estradiol.

Trophectoderm: The outer layer of cells of the blastocyst.

Trophoblast: The leading cells of the implanting embryo that produce hCG and are destined to become the placenta.

Tubal embryo transfer (TET): An ART technique whereby the embryo, grown in the laboratory, is placed laparoscopically into the fallopian tube. This procedure is often used for women who have had prior uterine surgery or who have abnormally shaped uteri because they were exposed to DES in utero. These conditions would make transcervical embryo transfer difficult or impossible.

Ultrasound: High-frequency sound waves that bounce off tissues and organs in the body in order to make an image on the monitor.

Ultrasound-directed aspiration: The ART technique that uses a vaginal probe and ultrasound to guide a hollow needle to retrieve eggs from follicles in the ovary.

Uterus: The muscular, pear-shaped organ in the female reproductive system that carries the developing fetus. It is lined with a layer of cells called the "endometrium."

Vagina: The portion of the female reproductive system that connects the external sex organs (vulva) with the internal (cervix and uterus).

Vas deferens: The tube in the male reproductive system that connects the epididymis where the sperm are stored, to the prostate, where they pick up the seminal fluids on their way to the penis.

Vasectomy: A method of sterilizing the male by surgically creating an obstruction of the vas deferens. This blocks the passage of sperm so that it cannot leave the testicular area.

Zona pellucida: The human eggshell; the hard protein covering that protects the egg and subsequently the embryo up to the point of implantation.

Zygote: A fertilized egg. After this one-celled entity has gone through several cell divisions; it is usually called the "embryo."

Zygote intrafallopian transfer (ZIFT): The ART technique whereby a fertilized egg is placed into the fallopian tube.