2 - Cells and Development

Summary

A bias deriving from developmental biology’s roots in descriptive embryology suggests that development is restricted to multicellular organisms, but in fact remarkable developmental processes are found among unicells, e.g. trypanosomes and ciliates. Spermatogenesis and oogenesis, the processes by which sperm cells and eggs are produced, also deserve to be regarded as segments of development. In the giant unicellular green alga Acetabularia, conspicuous morphogenesis may occur even in a cell from which the nucleus has been removed months before. In some organisms (examples exist in animals, plants and other organisms), more or less extended phases of development occur in syncytia, i.e. in systems where multiple nuclei are included in an undivided mass of cytoplasm. In plants, cells are permanently connected through thin cytoplasmic threads, the plasmodesmata. In multicellular systems, the relationship between cellular-level processes (e.g., mitosis) and overall developmental effects varies and cannot be simply summarized in terms of ‘cell sociology’. Programmed cell death also contributes to several morphogenetic processes.