Peroxisomes start their life in the endoplasmic reticulum.

Peroxisomes belong to the ubiquitous organelle repertoire of eukaryotic cells. They contribute to cellular metabolism in various ways depending on species, but a consistent feature is the presence of enzymes to degrade fatty acids. Due to the pioneering work of DeDuve and coworkers, peroxisomes were in the limelight of cell biology in the sixties with a focus on their metabolic role. During the last decade, interest in peroxisomes has been growing again, this time with focus on their origin and maintenance. This has resulted in our understanding how peroxisomal proteins are targeted to the organelle and imported into the organellar matrix or recruited into the single membrane surrounding it. With respect to the formation of peroxisomes, the field is divided. The long-held view formulated in 1985 by Lazarow and Fujiki (Lazarow PB, Fujiki Y. Biogenesis of peroxisomes. Annu Rev Cell Biol 1985; 1: 489-530) is that we are dealing with autonomous organelles multiplying by growth and division. This view is being challenged by various observations that call attention to a more active contribution of the ER to peroxisome formation. Our contribution to this debate consists of recent observations using immuno-electronmicroscopy and electron tomography in mouse dendritic cells that show the peroxisomal membrane to be derived from the ER.