THE PROTHORACIC GLANDS OF MANDUCA-SEXTA - A MICROSCOPIC ANALYSIS OF GAP-JUNCTIONS AND INTERCELLULAR BRIDGES

The insect prothoracic gland is an ecdysteroidogenic (molting hormone) structure that under the control of a brain neuropeptide is responsib le for the surges in ecdysteroid titer required for cellular reprogram ming, molting and metamorphosis. The glands are stimulated by the brai n neurohormone via a signal transduction cascade that involves cAMP, C a2+/calmodulin and protein kinases. Since the precision of the ecdyste roid surge is vital for the coordination of those biochemical processe s encompassing molting, an investigation was initiated to provide basi c information for a future analysis of how the 200-250 cells of the M. sexta prothoracic gland communicate to assure a rapid, uniform and te mporally correct surge of ecdysteroids. An analysis of gap junctions b etween cells of the prothoracic glands was undertaken using scanning ( SEM) and transmission electron microscopy (TEM) as well as freeze frac ture. The SEM data revealed that during the fifth larval instar the ce ll size, shape and surface morphology underwent dramatic changes, e.g. , the cell size increased from 30-40 mu m in diameter on day 1 to abou t 65 mu m on day 7. In addition, there was the development of cell-to- cell major connecting areas and intercellular bridges. TEM analysis de monstrated that both the major connecting areas and intercellular brid ges contain interdigitating cellular processes from neighboring cells. Typical gap junctions are distributed in an organized manner, occurri ng on limited portions of the cell membrane, with preferential localiz ation close to the distal ends of these processes. They are septilamin ar structures with a central cleft 2-3 nm wide and electron-dense stra ta that appear to be symmetrical. Numerous multilamellar bodies are of ten seen in these areas. The size and frequency of both gap junctions and multilamellar bodies appear to be correlated with the developmenta l stage and physiological state of the insect. Freeze fracture images of the region of contact between interdigitating processes reveals clu sters of particles on the E face. These clusters represent gap junctio ns and are similar to gap junction clusters reported for other insect tissues. The clusters are often irregular in shape, and the particles are always randomly arranged. Many of the particles are elongated, sug gestive of dimers or the bridging of adjacent particles, and these elo ngated particles are matched by a corresponding pattern of pits found on the P face. These gap junctions may promote the synchronization of secretory activity of the cell comprising the prothoracic glands.