PpiMHCIIa expression outside the tentacle root. (A) PpiMHCIIa expression in polar fields. Eight patches of PpiMHCIIa are identified. The black arrowheads point to the four internal-most patches (thought to correspond to stem cells) and the white arrowheads indicate the 4 external-most patches (thought to correspond to proliferating cells but not stem cells). (A’) Summary of the PpiMHCIIa expression pattern (blue patches) in polar fields. (A”) EdU labelling in the aboral sensory complex after a 12 h pulse and no chase. (B) Expression of PpiMHCIIa in the gonads (male on the left and female on the right). Spermatozoids (Spz) are not stained. (B’) DAPI counter-staining of panel (B). (B”) Oocyte (Oo) surrounded by its nurse cells, both expressing PpiMHCIIa. (C) PpiMHCIIa expression at the aboral extremity of a comb row (the black arrow points to an aboral baby comb). (C’) DAPI counterstaining of panel (C). The white arrow points to the aboral baby comb. (D) PpiMHCIIa expression in the youngest baby comb at the oral extremity of a comb row. (D’) DAPI counterstaining of panel (D). (E) PpiMHCIIa expression in the six youngest combs located at the oral extremity of a comb row of another specimen. (F) PpiMHCIIa expression in a mature comb. (G) Detailed view of the lateral extremity of a mature comb showing the localised expression of PpiMHCIIa. (G’) DAPI counter-staining of panel (G). ao: apical organ; Cpf: Central zone of polar field; Mpf: Marginal zones of polar field; Ps: Polster cells; Spg: Spermatogenesis stages; Spz: Spermatozoids. Scale bars: A, A”, C, C’: 50 μm; B, B’: 100 μm; B”: 10 μm; D, D’: 5 μm; E, F: 25 μm; G, G’: 20 μm.