The structure and development of dolipore septa and associated clamp c
onnections are described for the ectomycorrhizal holobasidiomycete Pis
olithus tinctorius, following freeze-substitution of growing hyphae. S
epta in the main hypha and clamp are formed synchronously and are comp
leted within a few minutes. They are produced by a furrowing of the pl
asma membrane and concurrent wall deposition. Fine filaments occur in
a ring adjacent to the deposited septum. Radial and parallel filaments
, that occur in a complex arrangement around the apex of the membrane
infolding, are likely to be instrumental in bringing about cytokinesis
. The pore opening is reduced to about 140 nm and there is still no pa
renthesome capping it, indicating that this is organised late in septa
l pore development. At maturity, the pore is surrounded by a dome-shap
ed, perforate parenthesome on each side and is filled with filamentous
electron-opaque material which spreads laterally over the adjacent se
ptal membrane. Filaments radiate from this material to contact the par
enthesome. The entire structure is interpreted as a co-ordinated whole
, with the radiating filaments anchoring and supporting the parentheso
me, so that its shape, position and orientation in relation to the por
e entrance are maintained. Similarly the rough endoplasmic reticulum (
ER) parallel to the septum also appears anchored to the plasma membran
e by short fine filaments. Continuities between the lumen of this ER a
nd the parenthesome could not be found, and the evidence indicates tha
t the rim of the parenthesome is anchored to the plasma membrane rathe
r than to the ER. Septal structure and development are discussed in re
lation to symplastic continuity in the hyphae and cell-to-cell transpo
rt across the dolipore.