Figure 8. Model of planarian epidermal lineage progression.
Schematic representation of the distribution of neoblasts, epidermal progeny cells in the mesenchyme, and differentiated epidermal cells along the anteroposterior (top left, dorsal view) and dorsoventral (top right, cross section) axes. Gut branches are shown as reference. Below: sigma-class neoblasts give rise to zeta-class neoblasts, which are epidermal progenitors. zfp-1 is required to generate epidermal progeny cells, which begin to express different markers as cells undergo multiple transitions and intercalate into the mature epidermis. Gradient boxes represent domains and relative expression intensity of specified genes. egr-5 is most strongly expressed in AGAT-1+ cells and is required for proper differentiation of epidermal post-mitotic progeny cells. zpuf-6+epidermal cells likely represent a branching point in the epidermal cell fate decision and can generate multiple different cell types including marginal adhesive cells (NB.22.1E and laminB), multiciliated cells (rootletin), and other unknown cell types. Bottom panel: schematic of egr-5(RNAi) phenotype. Loss of egr-5 disrupts the proper spatiotemporal transition of post-mitotic epidermal cells, resulting in prog-1, AGAT-1 and zpuf-6 to be co-expressed in the same cell, and leading to a depletion of mature epidermal cells. The resulting loss of epidermal integrity is sensed by the neoblasts (induced stress response) and causes an expansion of neoblasts and multiple progenitors before animals eventually lyse due to irreparable loss of an intact epidermal barrier.