Structure And Functional Relations Of The Salivary Glands Of The Tsetse-fly
The salivary glands of insects have recently become the subject of much study not only because of their important role in the digestion of food but also because they have yielded much knowledge related to cytogenetics, nucleo-cytoplasmic relations, cell-to-cell junctions and basic secretory processes occurring within the cell. The salivary glands of tsetse-flies are of additional interest in that they serve as the ideal site of development for trypanosomes, belonging to the brucei group, towards the metacylic phase. It is at this stage that these trypanosomes become potentially infective to mammalian vertebrates, causing the fatal 'sleepingsickness' in Man, and 'nagana' in domestic animals and wild game. Very little is known of the critical factors that are provided by the glands for the attainment of this developmental stage. Such a study would require a biochemical and physiological approach. There is, however, very little information regarding the detailed structure of the tsetse.salivary glands; although some biochemical analysis of the saliva has been made, and its anti-coagulating mechanism studied recently. This investigation was undertaken to study the structure of tsetse salivary glands using histological and ultra-structural techniques; and to relate these observations to their function. The salivary glands of teneral unfed G. austeni, and the effect of a bloodmeal on the salivary glands of a 24 hr.-old tsetse are described. The epithelium is composed of a single layer of cells and surrounds a central tubular lumen which may be divided into two distinct morphological and functional regions - a distal, muscle-bound secretory region, and a proximal non-muscle-bound absorptive region. The secretory region is typified by abundant rough endoplasmic reticulum and vesicular Golgi complexes. Very few secretory vesicles are observed in the cytoplasm, while septate desmosomes are observed in the intercellular gap. The absorptive epithelium is characterised by ext~sive development of the apical cell membrane into microvilli. The endoplasmic reticulum is poorly developed while Golgi complexes are few. Of added interest is the occurence of deposits of glycogen granules in the cytoplasm. The functional significance of this requires further investigation. The central lumen serves as a reservoir for the synthesised secretion or saliva. Its ultra-structure indicates the presence of two main components - an electron-dense filamentous component and a moderately dense finely granular component. Most of the secretion is stored in the region of the secretory epithelium. The lumen of the absorptive region is small and may contain small amounts of the fine granular type of secretion also occurring in the secretory region. Discharge of the luminal secretion is brought about by the powerful contractions of a muscle layer which envelopes the secretory epithelium. It is composed of a single layer of many longitudinal and parallel fibrils orientated in a gentle spiral. Their ultra-structure closely resembles that of other insect visceral muscles. The contraction of this posteriorly situated muscle layer~results in the propulsion of the saliva along the fine channels of the ducts and hypopharnyx. The absence of antagonistic muscles is compensated for by the occurrence of a structurally modified basement membrane which underlies the muscle layer. Circularly orientated ;unbanded fibres occur in bundles apparently , " .., forming annular or ,spiral bands. It seems very likely that this basement membrane serves as a mechanical support for the contractile activities of the muscle layer. Control of the secretory and muscular activities' of the tsetse salivary glands appears to be both nervous and hormonal. Dense neuro-secretory granules have been observed in the terminal regions of the axons supplying these glands, and apparently originating from cell bodies in the fused ganglionic mass in the thorax. This direct mode of hormone transfer to the target organs differs from the well-known method whereby hormones generally reach their target organs by being first discharged into the haemolymph via the neurohaemal organ. The functional significance of this dual system of control is discussed The thin cuticle-lined salivary ducts serve:'mainly for conduction of saliva, while the salivary valve (a modification of the posterior region of the common salivary duet) controls the outflow of saliva by the intermittent contractions of the salivary valve muscles, innervated from the tritocerebrum. Two lobed ~asses of cells, referred to as pyriform bodies, .'sit$ted just anterior to the salivary valve, are also aescribed., Their ultra-structural features and lack of lumen and duot are suggestive of endocrine glands-. The nature of their seoretion seems to be lipoid and proteinaoeous. Knowledge of their funotional role in this inseot requires further investigation. There appears to be no signifioant ohange in the ultrastructure of the glands even two hours following a bloodmeal. The secretory epithelium shows a slight increase in the number of secretion vesicles, while the luminal seoretion appears only slightly altered. This is probably related to the faot that only a small quantity of saliva is 'lost' per feeding session - the secretion probably occurring in a concentrated form. It is hoped that this study of the teneral tsetse salivary glands will serve as a baseline for future work involving the reaction of these glands to varying physiological conditions, including that of trypanosome invasion. Such a study would therefore reveal the critical factors determining growth and development of trypanosomes in tsetse salivary glands. The knowledge'gained would thus serve several uses in pathological studies such as growing trypanosome cultures in the laboratory and developing new trypanocidal drugs.
University of Nairobi,