An ultrastructural study of septal development in hyphae of Polyporus biennis

A method was developed which allowed the ultrastructural study of septal formation in the basidiomycete Polyporus biennis. The method involved fixing and embedding single clamp connections. Clamp connections with septa at desired developmental stages were located by light microscopy. The septum grew by the incorporation of vesicles of wall material precursors. The rim of the developing septum was drawn centripetally inwards by a contracting collar of microfilaments within a darkly staining matrix. The inflation of the central pore swelling was governed by realigned microfilaments. The parenthesomes were formed, starting at the apex, by the utilization of the microfilament/matrix material lying along the septum. On completion of the parenthesomes a transient striated structure, governed by microfilaments, was formed in the pore channel and the areas enclosed by the parenthesomes. The maturation of the septum involved the laying down of ER along the septum and the occlusion of each end of the pore channel.