Cellular and molecular processes leading to embryo formation in sponges: evidences for high conservation of processes throughout animal evolution.

The emergence of multicellularity is regarded as one of the major evolutionary events of life. This transition unicellularity/pluricellularity was acquired independently several times (King 2004). The acquisition of multicellularity implies the emergence of cellular cohesion and means of communication, as well as molecular mechanisms enabling the control of morphogenesis and body plan patterning. Some of these molecular tools seem to have predated the acquisition of multicellularity while others are regarded as the acquisition of specific lineages. Morphogenesis consists in the spatial migration of cells or cell layers during embryonic development, metamorphosis, asexual reproduction, growth, and regeneration, resulting in the formation and patterning of a body. In this paper, our aim is to review what is currently known concerning basal metazoans--sponges' morphogenesis from the tissular, cellular, and molecular points of view--and what remains to elucidate. Our review attempts to show that morphogenetic processes found in sponges are as diverse and complex as those found in other animals. In true epithelial sponges (Homoscleromorpha), as well as in others, we find similar cell/layer movements, cellular shape changes involved in major morphogenetic processes such as embryogenesis or larval metamorphosis. Thus, sponges can provide information enabling us to better understand early animal evolution at the molecular level but also at the cell/cell layer level. Indeed, comparison of molecular tools will only be of value if accompanied by functional data and expression studies during morphogenetic processes.