The hagfish slime gland thread cell. I. A unique cellular system for the study of intermediate filaments and intermediate filament-microtubule interactions.

Abstract

Thread cell differentiation the slime gland of the Pacific hagfish E. stouti was studied using light microscopy and scanning and transmission electron microscopy. Thread cell differentiation is remarkable in that the life history of the cells is largely dedicated to the production of a single, tapered, cylindrical, highly coiled and precisely packaged cytoplasmic thread that may attain lengths of 60 cm and diameters approaching 1.5 .mu.m. Each tapered thread, in turn, is comprised almost entirely of large numbers of intermediate filaments (IF) bundled in parallel. During differentiation of the thread, the IF become progressively more tightly packed. Various numbers of microtubules (MT) are found among the bundled IF during differentiation of the thread but disappear during the latter stages of thread differentiation. Observations of regularly spaced dots in longitudinal bisections of developing threads, diagonal striations in tangenital sections of developing threads, and circumferentially oriented, filament-like structures observed at the periphery of developing threads cut in cross section have led us to postulate a helically oriented component(s) wrapped around the periphery of the developing thread. The enormous size of the fully differentiated thread cell, its apparent singular dedication to the production of IF, the ease of isolating and purifying the threads and IF subunits, and the unique position of the hagfish in the phylogenetic scheme of vertebrate evolution all contribute to the attractiveness of the hagfish slime gland thread cell as a potential model system for studying IF subunit synthesis, IF formation from IF subunits, aggregation of IF into IF bundles and the interaction(s) of IF and MT.