Microtubular dynamics in granulosa cells of periovulatory follicles and granulosa-derived (large) lutein cells of sheep: relationships to the steroidogenic folliculo-luteal shift and functional luteolysis.

Abstract

Indirect immunofluorescence light microscopy was used to monitor temporal perturbations in the microtubular (tubulin) system of granulosa/lutein cells in paraffin-embedded sections of periovulatory follicles and corpora lutea of sheep. Estrogen-active granulosa cells of preovulatory follicles not yet exposed to the gonadotropin surge immunostained intensely for tubulin. Immunostaining of the microtubular matrix diminished after the onset of the surge and coincident with an abrupt fall in follicular estradiol production. A transient period of microtubular retraction was characterized by low-level steroid hormone output. Microtubules reappeared with the approach of ovulation and increase in follicular progesterone biosynthesis (luteinization). Treatment of animals during the preovulatory period with colchicine, a drug that binds specifically with tubulin and interferes with microtubular assembly, obstructed the follicular shift toward progesterone. Microtubular dynamics (polymerization<->depolymerization) evidently underpin fundamental mechanisms of follicular steroidogenesis. Finally, corpora lutea were isolated from ewes on Day 10 of the estrous cycle before (0 h) and after administration of prostaglandin (PG) F2 alpha. There was a small augmentation in luteal concentrations of progesterone at 2 h, followed by a sharp decrease from 4 to 16 h. Luteal weights were reduced (structural regression) at 24 h. Sections of large (PG-sensitive) steroidogenic cells of control corpora lutea typically displayed a radiating microtubular network. After administration of PGF2 alpha, tubular matrices of large cells were scant; mitochondrial clustering was evident in transmission electron micrographs. Affixed disassembly of the cytoskeleton of large luteal cells may be a heretofore unrecognized event in the biomechanics of functional luteolysis--perhaps uncoupling cholesterol translocation to mitochondrial cytochrome P450 side-chain cleavage.