ULTRASTRUCTURE OF PROTHORACIC GLANDS DURING LARVAL-PUPAL DEVELOPMENT OF THE TOBACCO HORNWORM, MANDUCA-SEXTA - A REAPPRAISAL

The structure of Manduca sexta prothoracic glands was investigated usi ng a protocol that preserves membranes. During the last larval stadium , prothoracic gland cells increase in diameter, volume, protein conten t, and perhaps number, enhancing their capacity to produce ecdysteroid s. The glands' strand-of-cells morphology, their in situ location, the presence of gap junctions between cells, and junctional foot-like str uctures within cells support previous findings that prothoracicotropic hormone stimulates ecdysteroidogenesis via Ca2+ -induced Ca2+ release . A different method of tissue fixation from that previously used to i nvestigate the ultrastructure of Manduca sexta prothoracic glands has revealed a significantly different ultrastructure. These new findings begin to define roles for endoplasmic reticulum and mitochondria in ec dysteroid synthesis and support the hypothesis that the glands secrete the steroid hormone via exocytosis. The structural dynamics of the gl ands are discussed in the context of the glands' function during Mandu ca sexta larval-pupal development.