To Be or Not to Be a Flatworm: The Acoel Controversy
Localization of BrdU-containing cells in the rhabditophorans Prosthiostomum siphunculus, Pseudostylochus intermedius, Planocera reticulata (A–C, Polycladida) and Macrostomum spirale (D, Macrostomorpha). (A) Anterior part of an adult, (B–D) juveniles. Insets: details of the epidermis, encompassed by dotted lines. Anterior to the left. (A–B, D) Note the lack of S-phase cells (brown or green nuclei) in the epidermis after 30 min or 12 h (in B) BrdU pulse. (A) Lower inset shows the protruding pharynx also lacking proliferating cells. (C) BrdU-labeled cells (green nuclei) migrated from the mesodermal space to the epidermis and differentiated into epidermal cells during the 7 days chase period. Scale bar is 50 µm for (D), 100 µm for (B), 400 µm for (C) and 1 mm for (A).