Silk Gland of Mulberry Silkworm

The organ that secretes silk material in the silkworm is deformed from the lower lip gland formed by the ectoderm. Silk glands are a pair of translucent and multi-flexed large-shaped tubular organs arranged on both sides of the ventral surface of the digestive tract. At the end of the 5th instar, its volume is second only to the digestive tract. From the position and function, it can be divided into spinning tube, front silk gland, middle silk gland and posterior silk gland.

Silk gland of larva

The spinning tube is a thin tube in the head. The front part is called the front zone or the spinning zone; the central part is called the middle zone or the squeezing zone; the back is called the back zone or the common tube. The spinning area is a thin tube, opening at the spinning hole on the back of the spinner at the front end of the lower lip. The squeezing area is slightly enlarged, the tube wall is thick, and there is a pressing rod formed by black sword-shaped chitin pieces on the back, and half of the tubular black chitin pieces on the ventral surface opposite to the pressing rod. There are 3 pairs of muscle bundles on the back of the pressing area, which are fixed on the body wall upward, sideward, and downward respectively. When these muscles contract, the inner cavity of the silk pressing area expands, and when it is relaxed, it shrinks due to the elasticity of the tube wall, so it has the effect of adjusting the thickness and cross-sectional shape of the silk. The common tube has a small lumen, with a pair of botryoid glands on both sides of the back end, called Philip’s glands.

The front silk gland is connected to the spinning tube, which is two symmetrical long tubes of the same thickness. It extends from the head to the third abdominal segment. It is relatively straight when it is an ant silkworm, and then grows as it grows. There are many bends at the 5th instar. The length of the silkworm is up to 45 mm when it is mature, and the pipe diameter is 0.2 to 0.3 mm. The anterior silk gland has no muscle or tracheal distribution, and has no function of synthesizing and secreting silk material. It is a duct through which silk material flows.

The middle silk gland is the thickest part of the whole silk gland. It is 85 mm long when the silkworm is mature, and the tube diameter is 4 to 5 mm. The total length is divided into three sections with two flexions, namely the anterior area (the third abdominal section to the sixth abdominal section), the middle area (the sixth abdominal section to the second thoracic section) and the posterior area (the second thoracic section to the second thoracic section). 4 abdominal section). The central silk gland secretes sericin, which has various colors due to different silkworm species.

The posterior silk gland starts from the 4th abdominal segment with many flexures, and extends back to the 6th abdominal segment (even to the front end of the 8th abdominal segment after the 5-year-old full food period) and ends with a blind tube. When the silkworm is mature, it is 170-200 mm long, with a tube diameter of 1-1.4 mm, which is the longest part of the silk gland. The posterior silk gland secretes silk fibroin without coloring.

There are muscles and trachea on the middle and posterior silk glands. Muscles are used to fix the position of the silk glands. In addition to supplying oxygen, the trachea also has a fixed position.

The tissue structure of silk gland The silk gland wall is composed of 3 layers of outer membrane, silk gland cells, and inner membrane. The inner cavity is filled with silk material. The outer membrane is a cell-free thin film with uniform thickness and elasticity, which protects the cell layer. The inner membrane is mainly composed of chitin. There are many micropores in the inner membrane of the middle and posterior silk glands, which are the pores for the secretion of silk material. The intima varies in thickness due to the location of the silk gland. The intima of the front silk gland is thicker, and the inner membrane of the middle and posterior silk glands is thinner, which is renewed every sleep. The silk gland cells are in the shape of a long hexagon, the left and right rows of cells are surrounded by a tube. The nucleus is rod-shaped at 1st instar and gradually branched after 2nd instar, and the nucleus is complex dendritic at 5th instar. Silk gland cells have already divided in the embryonic stage, and only increase in size in the larval stage, without dividing and proliferating. The number of silk gland cells and the proportion of each silk gland varies with species. The number of cells is the most in European varieties, followed by Japanese varieties and Chinese varieties. The number of silk gland cells is related to the amount of silk production, and the number of cells in the multi-filament system is larger.

There are developed rough endoplasmic reticulum, Golgi apparatus and mitochondria in the cytoplasm of the silk gland cells of the 5th instar silkworm. The cell membrane near the outer membrane has many folds, and there are microvilli near the glandular cavity. The thin silk fibroin synthesized on the ribosome is transferred to the Golgi apparatus through the endoplasmic reticulum, where it is concentrated into silk fibroin pellets, and then leaves the Golgi apparatus to accumulate on the inside of the cell, and is secreted into the glandular cavity through the microvilli. When a large amount of silk fibroin accumulates in the lumen of the posterior silk gland, it flows to the middle silk gland. The fine structure of the gland cells in each area of ​​the middle silk gland is roughly the same as that of the posterior silk gland, except that the silk protein synthesized from the middle silk gland is sericin. Silk glands grow rapidly after the 3rd day of 5th instar age, and the secretion of silk material also increases rapidly. The raw material of silk material comes from mulberry leaves. At this time, the quality and quantity of mulberry leaves directly affect the production of silk material. During the entire 5th instar period, about 70% of silkworm white matter digested and absorbed by silkworms from mulberry leaves becomes silk material.

1. Bottom membrane  2. Vacuoles of endoplasmic reticulum   3. Rough endoplasmic reticulum   4. Secretory globules   5. Silk fibroin in glandular cavity   6. Golgi apparatus   7. Silk fibroin globules 8. Lipids  9. Mitochondria  10. Microvilli  11. Nucleus  12. Nucleolus  13. Nuclear membrane  14. Nucleolus  15. Nuclear pore  16. Plasma membrane  17. Ribosome  18. Silk material layer  19. Trachea  20. Microtrachea

Posterior silk cell structure