Correlations between endocrine gland ultrastructure and hormone titers in the fifth larval instar of Manduca sexta

Abstract

Correlations were made between endocrine gland ultrastructure and circulating hormone titers of Manduca sexta to investigate the mechanisms of hormone biosynthesis and secretion. Both the prothoracic glands (PTG), which secrete ecdysone, and the corpora allata (CA), which secrete juvenile hormone (JH), were studied. In the prothoracic glands, the intracellular spaces increase in area and reach their maximum size following the major ecdysteroid peak in the fourth and fifth larval instars. Within the intercellular spaces are multivesicular sacs (MVS), structures which are clusters of vesicles bounded by another membrane. Since these sacs are larely depleted of their internal vesicles after the second cycle of tropic hormone stimulates the PTG to secrete ecdysone, the MVS probably release a gland cell product at this time. In the CA, concentric whorls of smooth endoplasmic reticulum are present in larval glands, when the JH titer is high, but are absent from pupal CA when the JH titer is low. The peak of JH at Days 6–8 of the fifth larval instar occurs after an increase in seen in the neurosecretory cell axon diameters suggesting that the CA are stimulated by a brain hormone to release JH. The number of Golgi complexes increases in pupal CA and dense bodies are present in pupal but not larval glands. These Golgi complexes may be involved in the manufacture of lysosomal enzymes which degrade JH within the gland itself.