Morphogenesis

Morphogenesis (from the Greek morphê shape and genesis creation) describes the process of cellular differentiation that takes place during the embryonic development of an organism. The change from a cluster of unitary cells to structured tissues, specialized cells and organs is controlled by the genetic "program" and can be modified by environmental factors. The morphogenes[?] (proteins that control morphogenesis) that determine the fate of cells are proteins that interact with DNA. They can either activate or deactivate genes that, in turn, can activate other genes (Fig. 1). The localized expression (production) of a protein results in a protein gradient. Above a threshold of concentration, the protein is active and works as a transcription factor. (A transcription factor regulates the amount of protein that is produced from a gene.)

The remainder of the article explains the morphogenesis of the species Drosophila melanogaster (the fruit fly). The development of Drosophila is particularly well studied, and it is typical for insects. Other multicellular organisms use similar mechanisms, although the details of the information transfer between the cells of the developing organism can differ from the one described here.

Figure 1 : Schematic drawing of transcription factor (or morphogene) gradients. The morphogenes (or transcription factors) displayed are the proteins bicoid[?] (bicoid is a maternally transcribed gene that organizes the anterior development in Drosophila) and hunchback (hunchback is the 'partner' of bicoid in anterior/posterior development in Drosophila). These proteins play an important role in determining the anterior part of the body (head and thorax[?]) from the posterior part (abdomen). (DNA/mRNA are striped, proteins are filled, respectively.) (a) The bicoid mRNA (messenger RNA) is an intermediate copy of a piece of the DNA. It is transported to the ribosomes, where protein biosynthesis takes place) is located on the left side of the embryo. All cells of the embryo have inactive hunchback DNA. (b) The bicoid mRNA is expressed as bicoid protein, resulting in a protein gradient with the highest concentration of protein on the left side of the embryo. (c) The hunchback DNA is activated once the amount of bicoid protein passes a certain threshold. This results in a sharp borderline, which divides the part where hunchback is expressed from the part where hunchback is not expressed.

In the early stages of morphogenesis in an insect embryo, four types of differentiation can be distinguished:

• Anterior (head and thorax)
• Posterior (abdomen)
• Dorso-ventral (front- and backside)
• Terminal (special structures at the unsegmented ends of the embryo)

In the later stages of development, these basic compartments become more and more divided. The same transcription factor can be used several times, having different meanings in the different body parts. Several morphogenes have been studied in Drosophila melanogaster, the fruit fly. In 1995, the Nobel Prize for Physiology and Medicine was awarded for studies concerning the genetic control of early embryonic development to Christiane Nüsslein-Volhard, Edward B. Lewis[?] and Eric Wieschaus[?]. Their researches identified genetic screens and exemplified the role played in early embryological development by Hox genes. An example is the so-called antennapedia mutation. In Drosophila, antennae and legs are created by the same "program", they only differ in a single transcription factor. If this transcription factor is damaged, the fly grows legs instead of antennae. See images of this "antennapedia" mutant and others, at FlyBase (http://flybase.bio.indiana.edu).

The term morphogenesis can also be used to describe the development of unicellular life forms that do not have an embryonic stage in their life cycle, or to refer to the evolution of a body structure within a taxonomic group. Morphogenetic responses may be induced in organisms by hormones, or by environmental chemicals ranging from substances produced by other organisms to toxic chemicals or radionuclides released as pollutants.

See also : embryo -- model organism

Back to : biology -- developmental biology