Gigantic animal cells suggest organellar scaling mechanisms across a 50-fold range in cell volume.

Abstract

The largest cells are orders of magnitude bigger than the smallest cells. Organelle content scales to maintain cell function, with different organelles increasing in volume, length, or number as cells increase in size. Scaling may also reflect functional demands placed on organelles by increased cell size. Amphibians exhibit exceptional diversity in cell size. Using transmission electron microscopy, we analyzed 3 species whose enterocyte cell volumes range from 228 to 10,593 μm3. We show that nuclear volume increases by an increase in radius while mitochondrial volume increases by an increase in total network length; the endoplasmic reticulum and Golgi apparatus, with their complex shapes, are intermediate. Notably, all 4 organelle types increase in total volume proportional to cell volume, despite variation in functional (i.e., metabolic, transport) demands. This pattern suggests that organellar building blocks are incorporated into more or larger organelles following the same rules across species that vary ~50-fold in cell sizes, consistent with a "limited precursor" model for organellar scaling that, in turn, assumes equivalent cytoplasmic concentrations of organellar building block proteins. Taken together, our results lead us to hypothesize that salamanders have evolved increased biosynthetic capacity to maintain functional protein concentrations despite huge cell volumes.