The developmental biology of heterocyst and akinete formation in cyanobacteria.

We will be concerned with the two major differentiated cell types of filamentous cyanobacteria--the heterocyst and the akinete. The former is generally accepted to be the site of aerobic nitrogen fixation in heterocystous cyanobacteria. The latter is a spore-like cell capable of withstanding certain environmental extremes and of germination. A short general introduction to cyanobacteria and their cell types will be included. The remainder of the review will fall into four main sections. The first will deal with the metabolism of the heterocyst and akinete, with particular reference to nitrogen fixation in the former. The next will be concerned with a description of the metabolic and ultrastructural changes associated with heterocyst and a kinete development. A third section will describe the special arrangement of the heterocyst and akinete (one of the features which makes this group of prokaryotes unique), the relationship between the cell types, and methods of altering this normal regular spatial pattern. The final section will describe in detail present theories of pattern control in cyanobacteria and the mechanisms by which the process of differentiation itself is regulated.