Follicle Cell Activity in the Ovarioles of Habrophlebia eldae (Ephemeroptera: Leptophlebiidae)

Abstract

Ultrastructural analysis carried out on the panoistic ovarioles of Habrophlebia eldae (Ephemeroptera: Leptophlebiidae) demonstrated that the organization and activity of the follicular epithelium change according to secretory function performed during oogenesis. In previtellogenic ovarian follicles, the cuboidal follicle cells are packed into a columnar epithelium and are connected by gap junctional plaques, forming an oocytefollicle cell epithelium complex tightly interlocked by way of microvilli. The transition to vitellogenic growth is marked by the appearance of a complex endocytic apparatus in the cortical ooplasm. The plasma membrane folds to form coated pits and vesicles, both being adorned with a clathrin lattice-work. Upon initiation of vitelline envelope deposition, follicle cells flatten, retract their microvilli, and interconnect by means of septate junctions. Chorionic envelopes are laid down in sequence, creating a three-layered structure whose organization varies according to the chorionic pattern of longitudinal ridges. Each ridge is composed of several columnar projections.. In the final secretory phase, follicle cell membranes become folded and release closely packed microgranules: (1) that adhere to both sides of the chorionic columnar projections; and (2) that form a sheet of amorphous material enveloping the egg surface prior to ovulation. The synthesis of these complex egg coverings and their role in this primitive insect group also are discussed. Ephemeropteran ovaries consist of several ovarioles arranged side by side to form parallel rows. Each ovariole is made up of follicles and exhibits a panoistic organization (Brinck, 1957; Sold'an, 1979). In a recent ultrastructural study, follicle cells were shown to be involved in vitelline and chorionic membrane deposition (Mazzini & Gaino, 1988). The follicular epithelium is a cellular system highly specialized for a secretory function. As presumed for the panoistic ovarioles of the stick insect Bacillus rossius, egg pinocytosis appears to be stimulated by extracellular protein secretion by follicle cells following interaction with vitellogenin (Mazzini & Giorgi, 1984). The follicular epithelium thus represents the primary target for juvenile hormone, and may have metabolic control over early vitellogenesis (Giorgi & Mazzini, 1984; Mazzini & Giorgi, 1984, 1986). Nevertheless, follicle cells are important for synthesis of precursor material, which in turn, is released for egg envelope organization (Mathew & Rai, 1975; Norton & Vinson, 1982). Chorionic layers are complex; their formation in a well-defined sequence requires energy expenditure by follicle cells. Organization of the chorion is requisite among insects laying eggs in water (e.g., Ephemeroptera), in which case species survival is enhanced by specialized egg envelopes (Koss & Edmunds, 1974). Follicle cells, then, constitute a primary cell system for insect egg growth and protection. I This study was supported by funds from the Ministry for Public Education (M.P.I., ROMA). TRANS. AM. MICROSC. SOC., 109(3): 300-310. 1990. ? Copyright, 1990, by the American Microscopical Society, Inc. This content downloaded from 40.77.167.104 on Sun, 24 Jul 2016 05:30:35 UTC All use subject to http://about.jstor.org/terms VOL. 109, NO. 3, JULY 1990 A detailed ultrastructural analysis carried out on the ovarioles of Habrophlebia eldae Jacob & Sartori, 1984 provided information on those events occurring in follicle cells during oogenesis. MATERIALS AND METHODS For transmission electron microscopy (TEM), ovarian follicles dissected from larval stages of Habrophlebia eldae Jacob & Sartori, 1984 were fixed in Karnovsky's medium (1965), rinsed in 0.1 M cacodylate buffer (pH 7.2) at 4?C, and postfixed in 1% osmium tetroxide in cacodylate buffer. Selected material was rinsed, dehydrated in a graded ethanol series, and embedded in Epon 812 or in mixture of Epon-Araldite. Sections were cut with a Reichert ultramicrotome, mounted on Formvar-coated copper grids, and stained with conventional uranyl acetate and lead citrate. For freeze-fracturing, ovarioles were fixed as described above and infiltrated gradually with glycerol to a final concentration of 30%. They were frozen in Freon 22 cooled to the temperature of liquid nitrogen. Fracture and carbon coating were carried out in a Balzer BAF 301 freeze-etching apparatus set at -115?C. Replicas were digested with Chlorox and mounted on Formvar-coated copper grids. Thin sections and freeze-fracturing preparations were examined with Philips 300 and 400 transmission electron microscopes.