Structure of Silk Gland in Silkworms

 

Structure of Silk Gland in Silkworms

Silk production in silkworms (Bombyx mori) is a remarkable biological process carried out by highly specialized structures known as silk glands. These glands are modified salivary glands that are enormously developed to produce and secrete silk proteins used during cocoon formation.

Overview of Silk Glands

Number: There are two umbers of silk gland, one on each side of the body.

Location: It is extend along the entire length of the larva’s body (especially the last 4–5 instars).

Function: Secrete two main proteins—fibroin and sericin—which combine to form the silk thread.

Length: Can measure up to 4–6 cm in length when uncoiled, even though the larva itself is only about 3–5 cm long.

Time of Activity: Most active during the final larval stage, just before the spinning of the cocoon.

 

Three Distinct Regions of the Silk Gland

Silk glands are structurally and functionally divided into three regions, each responsible for different stages of silk synthesis:

1. Anterior Silk Gland (ASG)

Length: Smallest segment

Location: Front portion of the silk gland

Function:  Anterior silk gland does not secrete silk proteins. It plays a critical role in the transport of silk proteins toward the spinneret (mouth part).  It also assists in spinning the silk fiber into a continuous thread during cocoon formation.

2. Middle Silk Gland (MSG)

Length: Intermediate in size

Function: Responsible for secreting Fibroin, the core structural protein of the silk thread. Fibroin is a fibrous protein that forms the central filament of silk, providing strength and flexibility. It is produced as a thick viscous liquid and then solidifies upon exposure to air during spinning.

 

3.  Posterior Silk Gland (PSG)

Length: Largest and most active segment

Function: It synthesizes Sericin, a glue-like protein that coats the fibroin fibers. Sericin helps bind the fibroin fibers together, giving the silk its unique tensile strength and cohesion. It also contributes to the stickiness and elasticity of raw silk.

 

 Spinneret and Silk Spinning

1. Once synthesized, the silk proteins are transported through the anterior silk gland to the spinneret.
2. The spinneret is a narrow duct at the mouth end of the silkworm, through which liquid silk is extruded.
3. As the proteins pass through, they solidify into silk threads due to air exposure and mechanical drawing action by the larva’s movement.

Biochemical Composition of Silk

Component	Protein Type	Produced by	Role in Silk
Fibroin	Fibrous	Middle Silk Gland	Core structure of silk thread
Sericin	Globular	Posterior Silk Gland	Acts as glue binding fibroin fibers

 

Key Points to Remember

1. Silk glands are specialized exocrine glands.
2. Each region plays a distinct role in silk production.
3. Coordination between all three regions is crucial for the formation of high-quality silk.
4. The entire process is genetically regulated, with hormonal control influencing protein synthesis.

Nature of Silk

Silk is a natural protein fiber produced by silkworms, primarily Bombyx mori, during the formation of their cocoons. It has been widely used for centuries due to its luxurious texture, luster, and strength. The fiber is secreted by the silk glands of the larva and spun into a cocoon that protects the pupating worm.

Composition of Silk

Silk consists mainly of two types of proteins: Fibroin and Sericin.

1. Fibroin (75–80%): 

Primary Component: Fibroin forms the core structural protein of the silk fiber.

Structure: It is a fibrous protein, largely made up of repeating amino acid sequences, especially:

• • Glycine (Gly)

  • Alanine (Ala)
  • Serine (Ser)

  These small amino acids allow the formation of beta-pleated sheets, which pack tightly and give silk its unique properties.

• Properties Conferred:

• • Strength: Due to the tight packing of beta-sheets.

  • Luster: The smooth surface of fibroin reflects light at various angles, giving silk a natural shine.

  • Elasticity: The hydrogen bonds between protein chains provide elasticity and flexibility.

• Insoluble Nature: Fibroin is insoluble in water, making the silk durable and resistant to degradation.

2. Sericin (20–25%)

• Role: Sericin is a gum-like protein that acts as a binding agent, coating the fibroin filaments and holding them together in the cocoon.

• Functions:

  • Helps in cocoon formation by acting as a cementing substance.

  • Provides protection to the fibroin core against microbial attacks and environmental damage.

• Removal (Degumming):

  • For textile use, sericin is removed in a process known as degumming, which involves boiling the silk in soap or alkaline solutions.

  • Removal of sericin enhances the softness and sheen of silk fabric.

• Hydrophilic Nature: Sericin is water-soluble and rich in serine and other polar amino acids.

References

• Ganga, G., & Sulochana, C. R. (1997). An Introduction to Sericulture. Oxford & IBH Publishing.
• Akai, H. (1983). Structure and function of the silk gland. Experientia, 39(5), 443–449.
• NCBI Bookshelf: Biology of the Silkworm [https://www.ncbi.nlm.nih.gov/books/]