Non-fiction: Pattern Formation

Pattern Formation: Ciliate studies and models
Joseph Frankel
1989

I finally finished reading this book, about pattern formation in ciliate systems. The ciliates are a phylum of protozoa (single celled life forms that are organized more like animal cells than like bacteria), but the evolutionary distance between ciliates and animals and ciliates and plants are very great. For unicellular organisms ciliates tend to be quite large with a highly organized cell cortex, which make them very amenable to studies on a microscope. The beginning of the book does a very nice job of presenting a comparative cell biology of different ciliate species and outlining the characteristics that set them apart from other cells.

Image of the ciliate Tetrahymena thermophila from
a fluorescence microscope. The rows of cilia that
run the length of the cell are highly regular, and the
asymmetrical oral apparatus is in red at the top. Image © 
2014 Galati et al. from the cover of J. Cell Biol. 27(6).

Ciliates are so named for their high numbers of cilia, which are sometimes described as "hair-like projections" that the cell beats in order to move and sense its environment. On the cell cortex, all ciliates possess several rows of cilia, as well as an oral apparatus (also made up largely of cilia) and a contractile vacuole pore, which they can use to regulate the amount of water inside the cell. The ciliate body plan also tends to be asymmetrical, and possess handedness - in normal cells, you always see these structures organized in one way and not the mirror image.

Many experiments are described in detail, involving random mutation and micro-surgical dissections, where biologists have been able to perturb the structure of the cell cortex in ciliates and even to generate cell doublets and mirror-image versions of cells. We know that in these cases, the molecular building blocks that comprise these cellular structures possess the same handedness and asymmetry as normal cells, and that it is only the larger scale assembly of these structures that is mirrored compared to normal. Close microscope studies of how cells grow and adjust their structure after manual manipulation with needles, and how they duplicate and either divide. revert to normal, or die in cases of doublets and mirror image cells, all give us valuable insights into the rules that govern pattern formation in biological systems, and how cells know where to place specific structures.

Mutants that are "doubled" versions of cells can help us 
figure out how cells decide where specific structures are
supposed to go. Image from J. Frankel (2008) Euk Cell 7(10).

Most of these observations are still not out-dated today, partly because ciliates have not received the
vast amount of attention that model organisms like mice, flies, and yeast have. But it's particularly exciting to read this book in the age of genomics, when we are much more easily able to sequence genomes and silence genes to study their phenotypes. Genetic studies on these organisms should be able to shed much more light into the precise molecular mechanisms that govern pattern formation at the cellular level.

Additionally, we tend to think of the cell's DNA as the hereditary component. But in cells like these its important to consider the structural organization of the cell and its cortex, and to ask whether all of the positional information needed to create and maintain a normal cell is all hard-coded into the genome, or whether the already-established structure of the cell contributes to heredity as well. If we were able to remove the cell's entire cortex, would the genetic material be sufficient to faithfully reproduce the exact same pattern as the
original cortex? Or is the pattern present on the cortex required to guide new structures to the correct place?