Ultrastructure of epithallial cells and trichocyte initials in Amphiroa (Corallinales, Rhodophyta), and possible systematic value of trichocyte development in corallines

Abstract

ABSTRACT To better understand the dynamics of the coralline algal thallus surface, scanning and transmission electron microscopy were used to examine the structure and development of epithallial cells and trichocyte initials in three species of Amphiroa. Epithallial cells possessed several distinctive organic wall layers. A densely fibrillar, moderately electron-dense layer completely surrounded each epithallial cell. Inward of this layer on the distal cell face, irregular deposits of wall material projected into the cell lumen. Outward on the distal cell surface, an electron-dense layer tapered from a central conical projection. This cone of distal wall material originated before the epithallial cell that ultimately bore it, having been deposited by an initial cell to sever its connection with a senescing epithallial cell in advance of epithallial cell replacement. This wall feature provided an indication of epithallial cell regeneration. Trichocyte initials were present beneath the layer of epithallial cells. The calcified sidewall of the epithallial cell overlaying the trichocyte chamber contained a channel that terminated in a pore through which the shaft of a trichocyte could pass. Lateral to the trichocyte initial, uncalcified walls still bearing pit plugs once shared by the initial cell and multiple generations of lost and regenerated trichocytes were compressed against the side of the trichocyte chamber. A single section of one trichocyte chamber showed 17 remnant pit plugs, indicating a high frequency of trichocyte turnover. Comparisons drawn from the published literature suggest that trichocyte development has the potential to serve as a source of systematic characters for coralline algae.