Horizontal Boundary Cells, a Special Group of Somitic Cells, Play Crucial Roles in the Formation of Dorsoventral Compartments in Teleost Somite

Abstract

Summary: Establishment of robust gene expression boundary is crucial for creating elaborate morphology during development. However, mechanisms underlying boundary formation have been extensively studied only in a few model systems. We examined the establishment of zic1/zic4-expression boundary demarcating dorsoventral boundary of the entire trunk of medaka fish (Oryzias latipes) and identified a subgroup of dermomyotomal cells called horizontal boundary cells (HBCs) as crucial players for the boundary formation. Embryological and genetic analyses demonstrated that HBCs play crucial roles in the two major events of the process, i.e., refinement and maintenance. In the refinement, HBCs could serve as a chemical barrier against Wnts from the neural tube by expressing Hhip. At later stages, HBCs participate in the maintenance of the boundary by differentiating into the horizontal myoseptum physically inhibiting cell mixing across the boundary. These findings reveal the mechanisms underlying the dorsoventral boundary in the teleost trunk by specialized boundary cells. : Abe et al. find horizontal boundary cells (HBCs) crucial players for the formation of the dorsoventral compartments of the teleost myotome. HBCs express Hhip, which is necessary for the refinement of the boundary, and they contribute to the horizontal myoseptum, an anatomical structure dividing the dorsoventral compartments. Keywords: boundary formation, boundary maintenance, boundary cell, dorsoventral patterning, somite, dermomyotome, canonical Wnt signaling, hedgehog signaling, SDF1/CXCR4 signaling, Hhip