– “Caps” (made of repeated DNA sequences) found at the ends of chromosomes that protect the ends of the chromosomes from degradation. The telomeres on a chromosome shorten with each round of cell replication. Telomere shortening has been suggested to be a “clock” that regulates how many times an individual cell can divide (that is, when the telomeres of the chromosomes in a cell shorten past a particular point, the cell can no longer divide).

Tissue culture

– See Culture

Totipotent cells

Stem cells that have unlimited developmental capability. The totipotent cells of the very early embryo (an embryo prior to the blastocyst stage) have the capacity to differentiate into extraembryonic tissues, membranes, the embryo, and all postembryonic tissues and organs.


– Making an RNA copy from a gene or other DNA sequence. Transcription is the first step in gene expression.


– A genetic process resulting in a heritable alteration of the properties of a cell. In the case of cultured cells, transformation often refers to the acquisition of new properties, such as unlimited culture lifespan.


– The process of forming a protein molecule from information contained in messenger RNA.


– The outer layer of the developing blastocyst that will ultimately form the embryonic side of the placenta.


– The extraembryonic tissue arising from the outer layer of the blastocyst, involved in implantation and later in development of the placenta and chorion.


– Commonly called “ultrasound.” An imaging technique that uses high-frequency sound waves to create an image. During pregnancy, ultrasonography can be used to provide an image of the developing fetus, including the entire body, organs and surrounding tissue.


– Not having developed into a specialized cell or tissue type.

Unipotent stem cell

– A stem cell that both divides and gives rise to a single mature cell type, such as a spermatogenic stem cell, which only gives rise to sperm.

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