The form of the oocyte, or egg, produced from the primary oocyte late in the life of the maturing follicle, just before ovulation. The egg stays at this stage until fertilisation by a sperm cell (spermatozoon).

Originally a botanical term meaning asexual reproduction, or reproduction by "budding" rather than by genetic recombination involving an egg and a sperm. Animals can be cloned either by splitting an embryo during cleavage (nature does this in producing monozygotic twinning) or by inserting an adult cell nucleus into a suitably primed secondary oocyte, a process called somatic cell nuclear transfer (and generally still very inefficient, resulting in a new developing embryo in at most a few percent of attempts). See also therapeutic cloning and reproductive cloning.

A collection of specialised granulosa cells, surrounding the ovulating egg (or secondary oocyte) in a sticky, mucus-like matrix. Sticks to the fallopian tube's fimbrial end after ovulation by a specific interaction with the tube's cilia.

The part of a cell that is not the nucleus (the nucleus contains the chromosomes). The cytoplasm is contained by the cell's plasma membrane and contains all the other cellular structures, including the mitochondria. Genetic inheritance is mostly by way of the nucleus (with a contribution from mother and father); a small part is by way of the cytoplasm (with a contribution only from the mother). It is the cytoplasm of the egg (as a secondary oocyte) into which a sperm cell (spermatozoon) is injected in the process of intracytoplasmic sperm insertion. See also mtDNA.

The state of a cell with 23 chromosomes (half the normal diploid chromosome state), found normally only in the spermatozoon and the egg (as the secondary oocyte). The noun form of this adjective is haploidy (the state of being haploid).

IVF, or fertilisation of the egg (the secondary oocyte) by a sperm (a spermatozoon) in vitro, i.e. in the laboratory. Necessary if the fallopian tubes are diseased or missing; useful if sperm fertilising capacity is doubtful, because evidence of fertilisation can be seen before the egg is transferred as an embryo.

Maturation in the laboratory of the egg (as a primary oocyte) obtained from an immature tertiary follicle until it becomes a secondary oocyte competent to be fertilised by sperm using in vitro fertilisation. The smaller the follicle, the lower the proportion of eggs that mature successfully. ICSI can be used to increase the proportion of eggs that will fertilise, but the embryos on average do no better, and so no advantage is conferred by ICSI in this situation.

A process similar to mitosis in which two successive divisions of a diploid cell's nucleus result in four 'daughter' cells, each with a haploid number of chromosomes. Unlike mitosis, each chromosome therefore duplicates just once (before the beginning of meiosis). Meiosis in humans (and other higher animals) takes place only among the germ cells (oogonia and spermatogonia, which will have been multiplying by mitosis. By differentiating into, respectively, primary oocytes or primary spermatocytes, each with 92 chromatids, meiosis commences. With completion of the first meiotic division the products (including secondary oocytes and spermatocytes) each contain 46 chromosomes. With completion of the second meiotic division the haploid number (23) of chromosomes, suitable for fertilisation, is reached. In the testis, meiosis and the production of new sperm cells (spermatozoa) can continue throughout life, but in the ovary all egg cells that survive commence meiosis about 20 weeks before birth, spending the remaining time (up to 50 years or more) locked up in primordial follicles as primary oocytes. Whereas a primary spermatocyte gives rise to four haploid sperm cells, a primary oocyte produces just one secondary oocyte (the spare 46 chromosomes are dumped into the first polar body just before ovulation), and then one egg cell (the spare 23 chromosomes are dumped into the second polar body after fertilisation).

Natural process by which a mature follicle in the ovary opens to release the secondary oocyte, or egg, enclosed in a sticky blob of mucus-like material, the cumulus mass.

Can occur when an egg (as a secondary oocyte) undergoes activation (by itself, with some non-specific stimulus, or through fertilisation by a sperm) and starts to divide (it undergoes cleavage), but the male chromosomes are not incorporated and the egg remains haploid; its further development will soon stop, probably well before implantation. Can be the cause of apparent (but false) late fertilisation in the IVF lab.

A tiny, compact packet of excess chromosomes discarded first by the primary oocyte as it becomes a secondary oocyte just before ovulation -- the first polar body, with 46 chromosomes; and second by the secondary oocyte immediately after fertilisation (or after activation by other means) -- the second polar body, with 23 chromosomes. The polar bodies lie in the perivitelline space. The chromosome content of each polar body can be tested as part of a preimplantation diagnosis maneuver to infer whether the corresponding oocyte has an aneuploidy: an extra chromosome in the polar body means a chromosome not enough in the oocyte, and vice versa; this is called polar body analysis.

fertilisation of an egg (a secondary oocyte) by more than one sperm (spermatozoon). In in vitro fertilisation, more common if eggs are recovered that are either immature or overly mature. Evident later, with the appearance of more than two pronuclei. In natural conditions, a cause of a polyploid state in the embryo. Another name for it is polyspermic fertilisation.

The form of the ovum, or egg, produced in the ovaries of fetuses by oogonia that have begun the first part of the cell division known as meiosis (by which the chromosomes will eventually halve in number). Persists into childhood and adult life by containment in follicles. Gives rise to a secondary oocyte and the first polar body just before ovulation.

(ROSNI) Experimental form of testicular sperm extraction (TESE) followed by intracytoplasmic sperm insertion (ICSI) in which the nucleus of a round (very immature) spermatid is isolated for injection into the egg (or secondary oocyte). Intended to be used in treating non-obstructive azoospermia with severe maturation arrest, when more spermatozoa are not obtainable. Animal studies show higher pregnancy rates than with round spermatid injection (ROSI), but limited studies in humans still indicate very high rates of embryopathy. Not recommended clinically.

(SCNT) Transfer of the nucleus of a somatic cell into a secondary oocyte or zygote from which the egg or zygote's chromosomes have been removed, in such a way that cleavage still occurs, in effect causing the cell's nucleus to 'bud', or reproduce without genetic recombination, and hence a form of asexual reproduction or cloning now commonly used for reproductive cloning in animals and, it is hoped, for therapeutic cloning to create embryonic stem cells for treatment of serious degenerative disease and cancers.

The result of a reaction of the immune system against sperm cells (spermatozoa) and, by limiting the ability of sperm to show motility, a contributory (or occasionally the only) cause of infertility. Can be present in serum, in cervical mucus or in semen. Can be agglutinating (which make sperm stick in clumps), immobilising (which cripple sperm particularly effectively and kill them), or coating, which interfere with sperm attachment to the zona pellucida of the egg (the secondary oocyte), preventing fertilisation. The screening test for sperm antibodies involves immunobeads.

(SUZI) An obsolete IVF technique involving sperm microinjection, in which one or more sperm are injected through the zona pellucida into the perivitelline space of the egg (the secondary oocyte. Not very effective, because its efficiency is limited by sperm having to have undergone the acrosome reaction). Nowadays completely replaced by intracytoplasmic sperm insertion (ICSI), where the acrosome reaction is not a requirement.