title: How the pilidium larva grows.
abstract: BACKGROUND
  For animal cells, ciliation and mitosis appear to be mutually exclusive. While uniciliated cells can resorb their cilium to undergo mitosis, multiciliated cells apparently can never divide again. Nevertheless, many multiciliated epithelia in animals must grow or undergo renewal. The larval epidermis in a number of marine invertebrate larvae, such as those of annelids, mollusks and nemerteans, consists wholly or in part of multiciliated epithelial cells, generally organized into a swimming and feeding apparatus. Many of these larvae must grow substantially to reach metamorphosis. Do individual epithelial cells simply expand to accommodate an increase in body size, or are there dividing cells amongst them? If some cells divide, where are they located?
  
  
  RESULTS
  We show that the nemertean pilidium larva, which is almost entirely composed of multiciliated cells, retains pockets of proliferative cells in certain regions of the body. Most of these are found near the larval ciliated band in the recesses between the larval lobes and lappets, which we refer to as axils. Cells in the axils contribute both to the growing larval body and to the imaginal discs that form the juvenile worm inside the pilidium.
  
  
  CONCLUSIONS
  Our findings not only explain how the almost-entirely multiciliated pilidium can grow, but also demonstrate direct coupling of larval and juvenile growth in a maximally-indirect life history.
authors.affiliation: Oregon Institute of Marine Biology, PO Box 5389, Charleston, OR 97420, USA.
authors.forename: April M
authors.initials: AM
authors.lastname: Bird
authors.affiliation: Oregon Institute of Marine Biology, PO Box 5389, Charleston, OR 97420, USA.
authors.forename: George
authors.initials: G
authors.lastname: von Dassow
authors.affiliation: Oregon Institute of Marine Biology, PO Box 5389, Charleston, OR 97420, USA.
authors.forename: Svetlana A
authors.initials: SA
authors.lastname: Maslakova
keywords: Ciliated epithelia; Larval growth; Stem cells