Wim van Egmond
The plasma membrane of a protist cell demarcates the outer boundary of cellular control, but cellular influence often extends beyond this physical limit. Many cells, for instance, are capable of producing secretions that contribute to their structural soundness or that provide increased security from outside stimuli.
Volvox is a genus of colonial organisms that secrete a gelatinous substance called mucilage that binds them together into a hollow sphere. In addition to this extracellular matrix, many Volvox species also exhibit cytoplasmic connections between cells. These cellular bridges are generally considered a sign of greater cellular integration than can be found in most other planktonic colonies, especially since breaking them leads to the death of not only the individuals directly involved, but to the entire group. In fact, some scientists believe that Volvox has crossed the evolutionary gap from colonial to multicellular.
Various types of evidence have been cited that support the notion of the evolutionary importance of Volvox, including the reproductive habits of its species and the differentiation that can be found among the cells of its colonies. Perhaps the most interesting indication of its significant relation to multicellular life forms is the structural similarity of its colonies to the early developmental stages of higher organisms. More specifically, the fertilized nuclei of the zygotes of almost all animals go through a process of repeated divisions to produce an embryo called the blastula. Interestingly, the blastulae of organisms in the phylum Chordata appear strikingly similar to a Volvox colony, consisting of a hollow sphere composed of a single layer of cells.
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Photomicrographs are © 2000-2013 by Wim van Egmond.