Ultrastructure of trichocyte (hair cell) complexes in Jania adhaerens (Corallinales, Rhodophyta)

Trichocyte complexes of Jania adhaerens (Corallinales, Rhodophyuta) were Studied by scanning and transmission electron microscopy. The trichocyte complex is composed of trichocytes (hair cells), their associated initial cells, and a previously unknown ephemeral cell, here termed the crown cell. The crown cell, which is part of the epithallial layer, is the original apical cell of each filament forming a trichocyte complex. The trichocyte initial divides to produce both a trichocyte devoid of chloroplasts and a subtending cell that serves as the initial cell for the next trichocyte generation (secondary trichocyte initial). A hair growing from the apical end of the trichocyte passes through a collar of distinctive wall material deposited originally around the apical end of the primary trichocyte initial and the base of the crown cell. The first hair passing through this channel destroys the crown cell before emerging from the thallus surface. The cytoplasm of hair-bearing trichocytes is particularly rich in Golgi vesicles. After the hair is lost and as the trichocyte base degenerates, the secondary trichocyte initial enlarges, filling the chamber that formerly housed both the initial and the trichocyte. The pit plug and crosswall that had been shared by the initial cell and the senescent trichocyte are pushed aside and compressed by the enlarging initial. These wall structures persist between the initial cell and the surrounding calcium carbonate wall. The enlarged secondary initial cell either becomes quiescent, persisting in the undivided state and being equivalent to a megacell or heterocyst, or it divides, producing a new trichocyte and initial. Remnants of more than 20 generations of walls and pit plugs were observed within the chambers of some trichocyte complexes. These wall remnants potentially provide a record of every cycle of regeneration of each trichocyte produced over the life of the alga.