What is a Cell?

A couple of months ago Casey Dunn talked about animals that don't have every one of the qualities we use to define something as an individual, and how those animals make us re-evaluate our definition: A member of a colony can be a descendant of a free-living individual, but unable to live and interact with the world without the other members of the colony, making itself hard to pin down as an individual or a part of an individual. Similarly, sometimes the qualities that we associate with a cell don't always come together in a tidy package.

In the plasmodial slime mold pictured on a piece of bark (upper right), the organism's body is large and complex enough to look like it's made of many types of cells, but it is actually made up of just one long branching space. It has many nuclei within its one labyrinthine cell, with no walls or membranes separating them. The organism even has certain parts that look very different from other parts; there is a mat on the ground made of thin winding filaments, and then there are fruiting bodies that grow straight up into the air. This seems just like cellular differentiation in other creatures, but it is all coming from one cell.

Vaucheria (pictured right), the alga that is eaten by Elysia the photosynthesizing sea slug, is a similar story. Long and filamentous, the body has no cell walls between the many nuclei inside of it. One might think that the cytoplasm is simply viscous enough that the cell's insides can stay put relatively well without walls, but cytoplasm has actually been seen streaming up and down the filaments, which are long enough to be spanning different microenvironments.

The upper photograph is slime mold on a piece of bark, peeled off the trees of Providence by Nick Jourjine, and photographed by Sophia Tintori. A really wonderful comprehensive set of drawings of slime molds can be found here. The bottom photograph is Vaucheria, photographed with a confocal microscope by Asya Rahlin and Harmony Lu. The sample was unstained, and so the green and red represent artifically colored autofluorescence. Nick, Asya, and Harmony are ungraduates at Brown, and these photographs resulted from some of their work for Casey Dunn and Gary Wessel's seminar on the origins of multicelularity and the evolution of the germ line. All photographs are published under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license.

--Sophia Tintori