- 3. Composition of brochosomes
-
- Brochosomes are extraordinarily stable structures. They
are insoluble in water and organic solvents, cold or boiling. They are
resistant to heat. Dry masses of brochosomes are highly hydrophobic.
Previous studies have indicated that brochosomes contain
both protein and lipid components. This conclusion has been based on positive
staining by certain histochemical dyes observed using the light microsope
(Smith & Littau 1960; Gouranton & Maillet 1968) and electron microscopic
observations of brochosomes digested with various enzymes (Gouranton &
Maillet 1968). Preliminary results of our ongoing study indicate that ca.
60% of the brochosome skeletons comprise glycine-rich proteins. Some known
glycine-rich structural proteins of insects account for the strength of
silk (fibroin), egg cases, egg chorion, and flexible cuticle (resilin).
- Current project
- 4. Production of brochosomes
- 4.1. Malpighian tubules of leafhoppers
- 4.2. Secretory cells
- 4.3. Development of brochosomes
- 4.4. Changes of secretory programs
in ontogenesis
- 4.5. Secretory products
other than brochosomes
Brochosomes are produced in special modified segments
of the Malpighian tubules of leafhoppers (Cicadellidae). The Malpighian
tubules are the main excretory organs of insects, somewhat analagous to
kidneys of vertebrates, their primary functions being excretion and ion
exchange. However, in some insects the Malpighian tubules or parts of them
are modified to produce various materials for external use (cocoons, covers,
"spittle", defense). Production of brochosomes is one of the
most remarkable cases of acquiring by these organs a secondary function.
- 4.1. Malpighian tubules
of leafhoppers
- Leafhoppers have four Malpighian tubules. Each tubule
has an inflated middle segment comprising secretory cells which produce brochosomes. Other tubule parts serve excretory
function. They display the same structure as the regular Malpighian tubules
found in most insects.
-
- The drawing shows a typical alimentary canal and the
Malpighian tubules of a leafhopper. Brochosomes produced in secretory segments
of the tubules are released through the hindgut (arrows).
4.2. Secretory
cells
-
- The micrograph below shows a cross-section of a brochosome-secreting
Malpighian tubule cell of the 1st instar nymph of Cicadella viridis
(L.). The cytoplasm displays numerous secretory vacuoles containing developing
or mature brochosomes. Vacuoles with mature products release their content
into the tubule lumen. Numerous Golgi complexes,
where brochosomes originate, are seen in the basal (outer) parts of the
cell. Each secretory cells produces a vast number of brochosomes.
- Development of brochosomes
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