Eutely is a consistent number of cells in an animal’s adult form, found in small animals such as rotifers, nematodes, tardigrades, gastrotrichans, and dicemids. It occurs when embryonic cells have a limit for total divisions, making an organism only grow through cell enlargement. Eutely can occur at the organ level and can be useful for scientists studying an organism’s developmental history or genome. The smallest eutelic organisms are simple worms such as dicihemids, and some colonies of algae show eutely.
Eutely, or cell consistently, refers to a specific number of cells in an animal’s adult form, consistent across species. Only very small animals are eutelic, generally less than 1 mm in size and with fewer than 100,000 total cells. Some examples of animals that display eutely include rotifers, nematodes, tardigrades, gastrotrichans, and dicemids. Although all eutelic organisms have cell constancy in common, their cell size can differ by many orders of magnitude, and a 1000-cell organism can range in size from 0.1 mm to over 10 mm.
Eutely occurs when the initial embryonic cells each have a limit for total divisions. After reaching this limit, the organism can only get bigger due to cell enlargement rather than cell division. Eutely can be contrasted with most other animals having an indefinite number of total cells. Eutely can also occur at the organ level, as in the eutelic nematode Ascaris, which always has 162 neurons. Eutely can be useful for scientists seeking to analyze the specific developmental history of any specific cell in an entire organism, or see how an organism’s genome (genotype) relates to its biological form (phenotype). It makes a given organism easier to understand, the most notable example being in the nematode C. elegans, one of the most widely used and best understood model organisms in science, along with mice and fruit flies.
The smallest eutelic organisms are simple worms such as dicihemids, parasites that live in the renal appendages of cephalopods, ranging in length from 0.1 to 9.0 mm. Dicyemids can be fixed to as few as 20-30 cells. Some colonies of algae such as Eudorina, evolutionary intermediates between unicellular and multicellular organisms, show eutely, with colony sizes always of 32, 64 or 128 cells. These cells communicate with each other chemically in a process called quorum sensing to ensure that the total number of cells in the colony remains constant over time.
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