The natural silk synthesized by the Bombyx mori and spun in the form of cocoon is originally synthesized in the silk gland.
The silk gland of Bombyx mori is an exocrine gland which secretes a large amount of silk protein. At the completion of larval maturation during the 5th instar developmental stage, the silk glands are very large and reach a length of about 25 cm and make up about 40% of the body weight (Tashiro et al., 1968a). The silk gland is a paired organ which consists of modified labial/salivary glands located at the two lateral sides under the alimentary canal (Kataoka and Akai, 1979). Each gland is a tube comprising of glandular epithelium with two rows of cells surrounding the lumen (Lucas et al., 1958). Silk glands have three distinct regions- posterior region, middle region and the anterior region (Fig 3). The blunt tubular posterior region is highly folded and remains attached to the tracheal bushes of the silkworm.
It is about 15cm in length. This part is mainly made up of 500 secretary cells whose main function is to synthesize silk fibroin (Adachi-Yamashita et al., 1980). The most prominent and widest part of the silk gland is the middle region which is about 7cm long, containing about 300 secretory cells (Dong et al.
, 2016). It remains folded in a W-shaped structure and comprises of 3 limbs- posterior, middle and anterior limbs. The posterior limb secretes sericin-I which gets surrounded by sericin-II secreted by the middle limb. Sericin-II is again surrounded by sericin-III secreted from the anterior limb (Takasu et al., 2002). The middle region of the silk gland functions as a reservoir of fibroin where the fibroin gets matured during the storage period. The anterior part about 2cm long is a thin duct composed of about 250 cells which have no secretory function. Its only function is to transport the assembled silk to the spinneret (Takasu et al.
, 2002). The spinneret is a projection of the median part of the labium, which draws the silk out in the form of a fine filament. The Filippi’s gland situated in the head region of the larvae secretes some waxy material to the silk thread, lubricating the passage of silk while coming out.