ABSTRACT
Among prokaryotes and lower eukaryotes, the formation of spores is an example of such cellular differentiation (see Topic L3). Among complex multicellular eukaryotes, embryonic cells differentiate into highly specialized cells, for example muscle, nerve, liver and kidney. In all but a few exceptional cases, the DNA content remains the same, but the genes which are transcribed have changed. Differentiation is regulated by developmental control genes (see Topic N2). Mutations in these genes result in abnormal body plans, such as legs in the place of antennae in the fruit fly Drosophila. Studying such gene mutations allows the process of embryonic development to be understood. In multicellular organisms, co-ordination of the activities of the various tissues and organs is controlled by communication between them. This involves signaling molecules such as neurotransmitters, hormones and growth factors which are secreted by one tissue and act upon another through specific cell-surface receptors.
