Cystidium

Abstract: Cystidia spanning the gill cavity may be ‘distant’, having other cells separating them, or ‘adjacent’, with no intervening cell; and, in either case, both cystidia may emerge from the same hymenium (described here as ‘ cis ’) or from opposite hymenia (‘ trans ’). If the distribution of cystidia is entirely randomized the frequency of adjacent pairs will depend on the population density and there will be an equal number of cis and trans in both the distant and adjacent categories. Quantitative data from serial sections of a primordium show that there is a positive inhibition of formation of neighbouring cystidia in the same hymenium such that formation of a cystidium actively lowers the probability of another being formed in the immediate vicinity. The extent of the inhibitory influence extends over a radius of about 30 μm and is strictly limited to the hymenium of origin. Cystidial density distribution on the face of the gill is fairly uniform, but at the gill edge the density of cystidia is locally increased. It is suggested that differentiation leading to cystidium formation is activated by the concentration of a component of the atmosphere, possibly water vapour, in the gill cavity immediately above the developing hymenium. The distribution pattern of cystidia is thus dependent on interplay between activating and inhibiting factors. At early stages in growth of the cystidium across the gill cavity the cell(s) with which the cystidium will come into contact in the opposing hymenium are indistinguishable from their fellow probasidia. However, when the cystidium comes firmly into contact with the opposing hymenium, the hymenial cells with which it collides develop a distinct granular and vacuolated cytoplasm, more akin to that of the cystidium itself than to the neighbouring probasidia. This suggests that a contact stimulus sets in train an alternative pathway of differentiation leading to an adhesive cell type called the cystesium.