Pattern Formation and Complexity in Single Cells

Abstract

In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.

Figure 1.. Cells have complex anatomy at multiple scales

(A). The anatomy of Stentor. The anterior-posterior axis is indicated by the membranellar band at the anterior end and a holdfast at the posterior end. Longitudinal stripes of cilia run the length of the body, with variable spacing between them such that a region exists where closely spaced stripes meet widely spaced stripes. This locus of stripe contrast defines a ventral surface. Together the anterior-posterior and dorsal-ventral axes define a midline, relative to which all other cellular structures have defined locations. For example the macronucleus is always to the right (from the cell’s perspective) of the midline, while the contractile vacuole pore is always to the left. (B). Detailed view of a cortical row showing pairs of basal bodies (centrioles), which link up to both longitudinal and transverse microtubule bundles (green).

Figure 2.. Multiple sources of positional information in one cell.

The left panel shows a wild-type Chlamydomonas cell with its complex internal anatomy. In the asq2 mutant (right panel), the basal bodies are displaced, and this results in some structures (nucleus and contractile vacuole) also being displaced, indicating that these structures gain positional information from the basal bodies or their associated fiber systems. Other structures (eyespot, chloroplast, pyrenoid) retain their normal position, indicating a positional information system that is not dependent on basal body position.