Eutely
TL;DR: Fixed somatic cell count within a species; growth by hypertrophy (cells enlarge) rather than continued proliferation.
Overview
Eutely is a developmental pattern seen in several small metazoans where the adult reaches a near-constant number of somatic cells. After a species-typical cell count is achieved, further growth occurs mainly by cells getting larger and differentiating, with little or no somatic mitosis. The canonical example is Caenorhabditis elegans: adult hermaphrodites have 959 somatic nuclei (cells) and males have 1031, with the full lineage mapped cell-by-cell. Eutely is also reported across rotifers, gastrotrichs, many nematodes, and some tardigrades, though limited, tissue-specific exceptions can occur.
Why It Matters
- Precision: Enables complete cell-lineage maps and highly deterministic development.
- Tradeoffs: Limits post-embryonic remodeling and repair compared to hyperplastic (cell-adding) growth.
- Contrast: Highlights how vertebrates rely more on post-embryonic proliferation for growth and repair.
- Control: Offers a clean case of organism-level coordination when you can’t add more agents, only govern existing ones.
Examples
- Nematodes (e.g., C. elegans): 959 somatic cells in adult hermaphrodite; 1031 in males.
- Rotifers, gastrotrichs: Species-typical counts; little somatic division after hatching.
- Tardigrades: Largely eutelic, with some clade-specific variation.
Caveats
- “Fixed” means tightly constrained rather than identical; small patterned differences and polyploidy can appear.
- Some somatic lineages may divide in injury contexts even in otherwise eutelic taxa.
- Literature sometimes counts nuclei, not whole cells; terminology and scope vary by group.