| dc.description.abstract | 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. | en |
