The septum of the lateral axon of the earthworm: a thin section and freeze-fracture study.

Septa occur between the axonal segments in the lateral giant septate axon of the nerve cord of the earthworm. This septum is demonstrated here to be permeable to fluorescein and to exhibit a negligible time delay for impulse transmission. Periodic anastomoses between the two lateral axons of the nerve cord are revealed by fluorescein. The permeability of the septum is correlated with the demonstration that nexuses occur along the septum. In thin sections, the nexuses may appear as long septilaminar or pentalaminar membrane appositions, but most frequently appear as a series of short or punctate membrane appositions. In freeze-fracture replicas, the nexuses appear as particles 10-12 nm in diameter on the PF face and as pits on the EF face. The particles and pits are arranged in plaques, in anastomosing strands, or most frequently in small plaques with strands of particles or pits emerging from the periphery. In addition to the nexuses, a junction characterized by the presence of 31 nm diameter hemispherical densities on the cytoplasmic surfaces of the septal membranes is revealed in thin sections. The densities are paired on the adjacent septal membranes, and most frequently are shown by optical diffraction to be arranged on the membrane surfaces in hexagonal or rhomboidal lattices with a centre-to-centre spacing of 34.8 nm. In freeze-fracture replicas, an array of particles and pits with a similar lattice symmetry and spacing to the arrays of hemispherical densities is demonstrated.