Science and Technology Glossary
Activation
The process through which a secondary oocyte is stimulated to resume meiosis. This may be due to penetrating spermatozoon or an artificial substitute.
Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. Aneuploidy occurs during cell division when the chromosomes do not separate properly between the two cells.
Assisted Hatching (AH)
A procedure undertaken in in vitro fertilization (IVF) laboratories wherein a small hole is made in the zona pellucida of an embryo to facilitate hatching.
Blastocyst
The mammalian conceptus in the postmorula stage with the cells of the blastocyst forming a spherical shell that encloses the blastocele and the inner cell mass from which the embryo forms. The outer cell layer is the trophectoderm.
Blastomere
One of the cells produced by cleavage of a fertilized oocyte; a cleavage cell.
Conceptus
The derivatives of a fertilized oocyte at any stage of development from fertilization to birth; includes extraembryonic membranes as well as the embryo and fetus. The products of conception; all structures that develop from the zygote, both embryonic and extraembryonic.
Compaction
A process through which a cleaving embryo changes from a collection of individual cells into a solid mass with indistinguishable cell membranes. Compaction is the result of the formation of tight intercellular junctions, which cause blastomeres to become closely apposed.
Crossing-over
A process occurring during synapsis in which pairs of homologous chromosomes bearing linked genes mutually exchange corresponding parts.
Cytokinesis
A phase in mitosis or meiosis, which involves division of the cytoplasm.
Cytoplasmic fragmentation
One or more spontaneous cytoplasmic divisions, which may often superficially resemble cleavage; disorganized breakdown of the cytoplasm. Fragments may contain DNA, but are more likely to contain no nuclear material.
Diandry
Triploidy in which the extra haploid set is of paternal origin.
Digyny
Triploid in which the extra haploid set is of maternal origin.
Embryo
The stage of the organism after fertilization and prior to fetal development.
Fertilization
The union of male and female gametes leading to the formation of a unique zygote.
Fetus
The developing conceptus after the embryonic stage; the fetal period begins at the end of the eighth postovulatory week when >90% of the >4500 named structures of the adult human body have appeared. The fetal period persists until birth.
Gamete
The oocyte or the spermatozoon; a mature haploid reproductive cell; any cell, which, upon union with another gametic cell, results in the development of a new individual.
Haploid
Possessing half the diploid or somatic number of chromosomes.
Immature oocyte
An oocyte with chromosomes at prophase I (PI). A germinal vesicle-bearing oocyte.
Intracytoplasmic Sperm Injection (ICSI)
A procedure performed in IVF laboratories wherein a single sperm is injected into the cytoplasm of an oocyte to assist the fertilization process.
Metaphase I (MI) oocyte
An oocyte with chromosomes at metaphase I of maturation, characterized by the absence of both a first polar body and germinal vesicle. An oocyte at an intermediate stage of maturation.
Metaphase II (MII) oocyte
An oocyte with chromosomes at metaphase II of maturation, characterized by the presence of a first polar body. A fully mature oocyte. A secondary oocyte.
Monosomy
A condition characterized by one less than the normal diploid number of chromosomes.
Mosaicism
A condition in which a conceptus or individual possesses two or more separate and distinct cell lines.
Nondisjunction
The failure of a pair of homologous chromosomes to separate during cell division.
Nucleus
The spheroid mass enclosed in a thin membrane, which is the center for the synthesis of specific cellular proteins and transmission of hereditary traits; contains a nucleolus or several nucleoli and a diffuse nucleoplasm.
Nucleolus
A rounded, retractile structure present in the nucleus of most cells which is the site of the synthesis of ribosomal RNA; the nucleolus becomes enlarged during periods of synthesis and atrophies during quiescent periods.
Oocyte
The female gamete from inception of the first meiotic division until fertilization. In oogenesis, a cell which develops from an oogonium.
Oogonium
The cell that gives rise to the primary oocyte during oogenesis; oogonia proliferate by mitotic division during early fetal life.
Ovum
A female gamete or germ cell. An oocyte. The term is often used to refer to such disparate structures as an oocyte and a 3 week-old embryo and, thus, has little or no scientific value.
Parthenogenesis
Activation and subsequent development of an oocyte without fertilization; may occur naturally or through artificial stimulation.
Penetrated oocyte
An oocyte that has been penetrated by a spermatozoon; strictly speaking, an oocyte in which gamete plasma membranes have become confluent. The stage before pronuclei are formed. Penetration of the oocyte usually occurs within 3h of sperm/oocyte exposure.
Perivitelline space
Space surrounding the vitellus (in fact, the 'yolk' but in this sense, the oocyte). This space may contain the first and second polar bodies or extracellular fragments.
Preimplantation Genetic Diagnosis (PGD)
The genetic testing of embryos created through IVF; A multi-step process involving a cell biopsy, genetic analysis and embryo transfer. The genetic material for testing can be obtained from a cell or cells (blastomeres) removed from an early embryo or from polar body cells. Genetic analysis can be done by chromosomal analysis to assess the number and/or structure of chromosomes, or by DNA analysis to detect specific gene mutations. Some IVF clinics offer PGS, which refers to preimplantation genetic screening and includes detection of aneuploidy only.
Polar body
First polar body: The structure extruded into the perivitelline space at the end of telophase I. Chromosomes are divided between the oocyte and first polar body (23 chromosomes, 46 chromatids, 2n DNA in each); those in the oocyte are attached to spindle microtubules. For a while after its formation, the first polar body remains connected to the oocyte by the meiotic spindle, forming a cytoplasmic bridge. Chromosomes within the first polar body may remain clumped together, may undergo a second meiotic division, or may scatter within the cytoplasm. Usually, a nucleus is not formed. The first polar body contains cortical granules because of its extrusion before sperm penetration and cortical granule release.
Second polar body: The structure extruded into the perivitelline space after sperm penetration. The second polar body may be nucleated.
Polyploidy
A condition in which a conceptus or individual possesses one or more sets of homologous chromosomes in excess of the normal diploid set, as in triploidy (3 n), tetraploidy (4 n), hexaploidy (6 n) or octoploidy (8 n).
Primary oocyte
The oocyte formed in the ovary before birth. Primary ooctyes begin the first meiotic division before birth, but completion of prophase I does not occur until puberty.
Pronuclei
Structures formed during fertilization from sperm and oocyte chromatin. See Prezygote. An oocyte with chromosomes at prophase I of maturation, characterized by a germinal vesicle.
Spermatozoon
A mature male germ cell, the specific output of the testes. The generative element of the semen, which serves to fertilize the oocyte, it consists of a head, neck, midpiece, and tail.
Syngamy
The active union of two gametes in fertilization to form a zygote. The process of reorganization and pairing of maternal and paternal chromosomes in the zygote after pronuclear membrane breakdown.
Triploidy
A condition in which a conceptus or individual possesses three times the haploid number of chromosomes (3 n).
Trisomy
A condition characterized by having one more than the diploid set of chromosomes (2n + 1.)
Zygote
The one cell stage after pronuclear membrane breakdown and before the first cleavage. This stage is characterized by maternal and paternal chromosomes assuming positions on the first cleavage spindle and, thus, lacks a nucleus. Usually seen 18-24 h after insemination or injection.
