Integument of Mulberry Silkworm

by Elizabeth / Monday, 26 July 2021 / Published in Sericulture

The outermost tissue that covers the silkworm body is composed of dermal cells originating from the ectoderm and their secretions or derivatives. The integument has the functions of growing muscles, supporting the body, maintaining the body shape, preventing the loss of water in the body, protecting the internal organs from damage and defending the invasion of pathogens. In addition, there are many sensory organs on the integument, which are the bridge between the silkworm body and the external environment.

The structure of the integument The larval integument is composed of three layers of base membrane, dermis, and epidermis. The base membrane is the innermost layer of the integument, close to the bottom of the dermis, and is a transparent non-cellular structure film. Observed under an electron microscope, it is an amorphous particle layer. The main chemical component is neutral mucopolysaccharide. The basal membrane is the internal boundary of the integument, and has selective permeability for blood chemicals and hormones to enter the dermis. The inner surface of the base membrane is attached with nerve endings, muscles, microtrachea, etc., and the nerves and muscles pass through the base membrane and enter the epidermis.

Structure of silkworm integument

The dermis is composed of a layer of cells, each cell has a slightly spherical nucleus, and the cell membrane is not obvious. The dermal cells contain pigment particles, which are an important factor in determining body color. It also contains urate crystals, which determine the transparency of the integument. Wherever there is less or lack of urate crystals or when the larva is mature, uric acid When salt passes through the blood into the urinary organs, the integument is relatively transparent. There is also a kind of oily silkworm, because the dermal cells cannot deposit urate, the whole integument is transparent like oily paper. In the larval stage, especially when the new epidermis is formed, the dermal cells are particularly developed, and the cells divide and proliferate and enlarge to form a columnar shape. Filamentous small protrusions are formed at the upper end of dermal cells, and granular substances are secreted to form new epidermis. In addition, some dermal cells specialize into hair cells, ecdysis glands, and various sensory organs (see Silkworm Sensation).

The epidermis is the outermost part of the integument and is a non-cellular tissue secreted by dermal cells. The function and characteristics of the integument mainly lie in this layer. The epidermis is not only covering the surface of the silkworm body, but also the inner wall of all organs and tissues such as the foregut, hindgut, trachea, etc., formed by the invagination of the ectoderm. There are many small protrusions on the surface of the epidermis, which are divided into three layers: inner epidermis, outer epidermis and upper epidermis from the inside to the outside. The inner epidermis occupies about 4/5 of the epidermis and is formed by many overlapping flakes. The main components are chitin and protein, which are linked by covalent bonds to form a stable glycoprotein complex. The inner skin is generally colorless, soft, extensible and elastic. During the molting process of the larvae, most of it is dissolved and absorbed by dermal cells.

The thickness of the outer epidermis is about 1/5 of that of the epidermis, which is thinner and harder than the inner epidermis. In the internode membrane or other soft parts of the silkworm body, the outer epidermis is extremely thin or even non-existent. The outer epidermis is transformed from a part of the inner epidermis, so the basic chemical composition is the same as that of the inner epidermis. The new epidermis formed by silkworms, new pupae, and newly emerging adults has only the inner epidermis and the upper epidermis, so the texture is soft and milky white. After a certain period of time, the polyphenolase produced by the dermal cells and the epidermis accumulated under the upper epidermis The action of tyrosine produces 3 and 4 dihydroxyphenylalanine, which is then oxidized to polyphenols, and finally to o-quinone. This orthoquinone diffuses around and combines with protein molecules, so that the soft and soluble arthropod protein is tanned into harder and insoluble bone protein. As the epidermis hardens, melanin is formed, which constitutes the body color and markings of the silkworm. The upper epidermis is the outermost and thinnest layer of the epidermis. There are small protrusions on the surface. The upper epidermis does not contain chitin and is composed of a lipoprotein layer and a wax layer. The shell lipoprotein layer is on the outer epidermis and is secreted by crimson cells. It is composed of shell lipoprotein combined with lipids and proteins. It is the underlying surface of the wax layer. It has a supporting role and restricts the overgrowth of the body, which determines the body surface of the silkworm. Pattern. It is the first layer formed when new skin occurs. It separates the new epidermis from the old epidermis. The digestion products and water of the old skin can penetrate, but the molting fluid cannot pass through or be dissolved by the molting fluid. The wax layer is the body The outermost layer of the wall is secreted by dermal cells a few hours before the silkworm sheds its skin. It is formed by transportation upward through the tunnel. The wax layer of the wax layer is similar to beeswax. It is mainly an ester formed by the combination of alcohol and fatty acid, so it has lipophilic and hydrophobic properties. The absolute amount of wax contained in the unit weight and unit area of the epidermis generally has no major changes except for the slightly lower silkworms, but it increases with the day, especially when it is pupated. The wax content of the wax layer varies with the larvae’s feed. The improved rat return is more, and the lusang is less; the same mulberry variety has more mature leaves and less old and young leaves; There are more high temperatures than low temperatures in feeding. The wax content of the wax layer determines the permeability of the integument. The more wax content, the worse the permeability, which plays an important role in preventing excessive evaporation of water in the silkworm body and preventing the erosion of foreign objects. The fatty acids contained in the wax layer, especially caprylic acid and caproic acid, can prevent fungi from invading the body. Both the inner epidermis and the outer epidermis have very fine pores running up and down. They extend from the dermis to near the center of the epidermis. They branch into branches and end in small protrusions on the upper epidermis. The cytoplasm of the dermal cells extends outward. The extruded filaments are the thin ducts that are left behind after the secretion activity ends and retracts into the dermal cells. When new skin is formed, many biochemical reactions must involve the secretions of dermal cells, and these secretions are secreted and transported in stages by the pores, such as transporting substances containing chitin and protein secreted by dermal cells for tanning and coloring. The phenols and corresponding enzymes required for the reaction, forming or repairing the wax layer of the wax, etc. After the skin is formed, the pores are filled with hardened substances, which become the pillars of the skin layer.

The integuments of pupae and adults are roughly similar in structure to larvae, and are also composed of three layers of basal membrane, dermis and epidermis. The difference is: the dermis of the adult is very thin; in the epidermis of the pupae and the adult, there is an extra layer of the middle epidermis between the outer and inner epidermis than the larva (the internode membrane of the pupae and the adult, and the lack of outer epidermis in the lateral epidermis of the adult And the middle epidermis); the degree of tanning of the epidermis is also different. The pupae’s epidermis is highly tanned, and the outer epidermis is dark and hard. The degree of hardening and colouring of the adult epidermis is lighter than that of the pupal stage, and is limited to the back and ventral surfaces of the head, chest and abdomen, so the adult abdomen is soft and flexible.

Derivatives of the integument are integument protrusions and ecdysis glands. integument protrusions can be divided into two types: non-cellular and cellular. Acellular protrusions exist on the surface of the larvae’s epidermis, and are many small protrusions that are needle-like, thorn-like or even rod-like. The shape, size, and density of the protrusions vary with silkworm species and markings. Cellular protrusions can be divided into single-cell protrusions and multi-cellular protrusions. Single-cell protrusions are specialized by a dermal cell and become hair gamma cells. In the larval stage, the trichogonia cells form bristles toward the surface of the integument. The bristles are articulated with the epidermis. The hairs fall off from the base during molting, and new hairs are regenerated by the hair primitive cells. The bristles have a tactile effect. When they are adults, the hair primitive cells generate white scales on the body surface. There are several nicks on the integument. The shape and distribution of the scales are different depending on the location where they are implanted. The scales on the wings are wide and short, with many nicks; the body is longer and longer, with fewer nicks; the front of the link has more scales than the rear, and the back has more scales than the side. There are no scales on the internode membrane. The scales are arranged neatly on the body surface in a complex tile shape, which has the effect of protecting the body. The multicellular protrusions are formed by a part of the epidermis and the dermis that protrude outward, and are present in the tail horn in the center of the back of the 8th abdominal segment.

The dragon-horned silkworm species unique to China, due to abnormal division of some dermal cells present on the dorsal line of the 4th, 3rd, 5th, 8th and so on during the 3rd and 4th sleep, the cell hypertrophy bulges toward the body surface and forms tumors. In the shape body, a total of 4 pairs are formed in the 4 links, and at most, 1 pair is born on the lateral backline of the 2nd to 11th somites. Moulting glands are white cystic glands that are specialized by dermal cells. There are 15 pairs in total, of which 13 are located in the inner dermis layer on the back of the first to 12th somites, and the other 3 are located at the base of each thoracic segment. The ecdysis is composed of 3 cells, one of which is large with a dendritic nucleus and is a secretory cell, and the other 2 are smaller, nucleated and spherical, and contain ducts, which are ductal cells that open on the surface of the epidermis. The ecdysis glands in each instar become cyclical activities. During sleep, the secretory cells swell and fill with ecdysis fluid. During sleep, the ducts open and release ecdysis fluid between the new upper epidermis and the old inner epidermis. By virtue of the protease and chitinase in the ecdysis fluid, It dissolves the old inner epidermis and separates the old and new epidermis so that the old outer epidermis can be removed. After molting, the ecdysis glands enter a static state and will not start to move until the next time they sleep.