Ultrastructure of Lyonet's gland in the silkworm (Bombyx mori L.)

Abstract

The gland cells of Lyonet's gland, which is accessory to the silk gland in the silkworm larva, is characterized by the presence of complicated canaliculi bearing microvilli on their inner surface, large numbers of mitochondria and remarkably convoluted basal plasma membrane. On the other hand, the cell lacks the well-developed cytoplasmic membrane system such as rough-and smooth-surfaced endoplasmic reticula and Golgi bodies. though free ribosomes are numerous. Secretory vesicles are absent, and the canaliculi contain no dense material. From such ultrastructural observations, i t was suggested that a possible role of the gland may be the exchange of the small molecules such as water and ions, rather than the hitherto supposed secretory role of a cementing sunstance of silk proteins. The lumen of the proximal part of the glandular duct contains a kind of proteinaceous substance which can be demonstrated histochemically and is regarded as similar to one of the silk proteins in the silk gland, not to the real product of the Lyonet's gland.

The Ultrastructure and Functions of the Silk Gland Cells of Bombyx mori

“…Machida (1965) found that the secretion of Filippi's gland contained proteins of completely different amino acid composition from fibroin, and he concluded that the gland could not be considered to be a secretory silk gland. Waku and Sumimoto (1974) have suggested that the gland may function in the exchange of small molecules (such as water) and ions, rather than having a secretory role.…”

14. Labial glands, silk and saliva

“…14. 6) is marked by digestion and casting of the duct cuticle into the lumen, loss of differentiation and withdrawal of nuclear lobes into the center of the cell (Hakim 1976 c, Waku andSumimoto 1974). This is followed by elaboration of new cytoplasm containing many free ribosomes, and by a resculpting of the nucleus into an even more branched form (Fig.…”

“…Waku and Sumimoto (1974) suggested that the gland may function in the exchange of small molecules and ions, rather than having a secretory role. A cell from Filippi's gland is characterized by a ramified nucleus, the presence of complicated canaliculi bearing microvilli on their inner surface, large numbers of mitochondria, and a remarkable convoluted basal plasma membrane (Akai, 1984).…”

“…It is notable that they are strongly developed in one trifine noctuid (Heliothis, a heliothine) and reportedly completely absent in another (Spodoptera, a 'hadenine') (Chi et al 1975). In a TEM study Waku and Sumimoto (1974) demonstrated that in Bombyx the accessory glands possess deep canaliculi lined with microvilli, short basal infoldings and many mitochondria. There is little endoplasmic reticulum.…”

“…It is usually markedly shorter than the secretory compartment but in a few examined taxa it is equal/ subequal to the latter (Dauberschmidt 1933: Geometridae: Hypomecis [as Boarmia]; Arctiidae: Lithosia [as Oeonistis]). The cell number is highly variable, within the Bombycoidea ranging from 200 (Manduca) to a thousand (some saturniids, Waku & Sumimoto 1974). The lumen of the duct, including that of the unpaired anterior portion, is lined with a rigid cuticle, which usually bears prominent annular strengthening ridges that are functionally analogous to tracheal taenidia.…”