Hormones In Mulberry Silkworm

A special chemical substance with physiological activity produced by the special endocrine organs or cells of the silkworm under certain conditions. Silkworm hormones are directly secreted into body fluids and have the effect of controlling growth and development of other organs or tissues.

Ecdysone

The silkworm larvae period undergoes periodic dormancy and molting, then pupation, moth formation, and egg diapause are all controlled by hormones. The central part of the silkworm’s post-embryo development is molting, that is, periodically removing the epidermis to facilitate continued growth. The necessary condition for molting is that the tissue is stimulated by an endocrine called ecdysone or its active relatives. The source of this ecdysone is the prothoracic gland, and the secretory activity of the prothoracic gland is controlled by a group of neurosecretory cells in the brain, that is, these neurosecretory cells can secrete after being stimulated by external temperature changes and photoperiods. Thymus hormone is a brain hormone that transmits information to the prothymus.

Whether it is larval molting or metamorphosis molting, it depends on ecdysone, but which type of molting depends on the third hormone, the juvenile hormone secreted by the pharyngeal body. While secreting ecdysone, larvae molting when there is juvenile hormone, and pupal molting and moth molting when there is no juvenile hormone. The secretory activity of the pharyngeal body is also controlled by the brain.

In the growth and development of silkworm, diapause occurs in the egg stage. Its characteristic is that the metabolic activity is reduced to a minimum under the control of hormones. This hormone is called diapause hormone, which is caused by the hypopharyngeal nerve of the silkworm. Section secreted.

The prothoracic glands that secrete ecdysone are distributed on the tracheal plexus of the larvae’s first stoma, with a total length of about 2 mm, paired with left and right sides. It can be divided into 4 parts: stem, front branch, middle branch and back branch.

The stem moves along the longitudinal trachea from the first tracheal plexus towards the head to the vicinity of the head. The front branch is divided into two branches, the front dorsal branch and the front abdominal branch. The front dorsal branch is slightly shorter and attached to the back edge of the head. In the concave part of the head, the anterior abdominal branches are attached to the posterior edge of the triangular orifice on the posterior ventral surface of the head. The middle branch is located on the dorsal side of the center of the shaft. It is a short lateral branch with the tip attached to the side of the esophagus. The posterior branch is located on the medial posterior edge of the first tracheal plexus, divided into the posterior dorsal branch and the posterior abdominal branch. The posterior branch attaches to the body wall at the back boundary of the first and second thoracic segments, sometimes attached to the digestive tube; the posterior abdominal branch and The dorsal branches are opposite and attached to the body wall at the ventral boundary of the first and second thoracic segments toward the abdomen. The morphology of the prothymus gland varies with silkworm species and individuals, and it elongates and branches with the increase of age. . There are many subdivided trachea and nerve distribution from the hypopharyngeal ganglia and thoracic ganglia on the glands. There is a thicker common membrane on the outside, and the inside is filled with polygonal or oval secretory cells. The nucleus is large, the small silkworm stage is spherical, and the large silkworm stage becomes a branch with irregular outlines. The secretion activity changes periodically depending on the instar of the silkworm. In the later stages of the incubation period, fine granular secretions appear in the cytoplasm near the nucleus, and soon the granules become larger and move to the edge of the cell to form a secretion layer. After the 4th instar silkworm passes through the period of 2/3 of this instar, the secretion layer in the cell thickens, and then becomes thinner due to the secretion into the blood; the 5th instar silkworm is prevalent in the mature silkworm stage to produce secretions, and it can be seen that its secretion into the blood In the image, the highest titrated concentration is reached after pupation, and it drops significantly in the first 1-2 days after pupation, and then gradually rises thereafter. About 5 mg of ecdysone crystals can be obtained from 100 kg of silkworm pupae, and about 1.3 mg can be obtained from 100 kg of female moths.

The chemical structure of ecdysone is similar to cholesterol. It is a steroid hormone, referred to as ecdysone. Alpha-ecdysone and β-ecdysone have been found in silkworms. The content of α-ecdysone is much higher than that of β-ecdysone. There are many hormones, and the hormonal activity of β-ecdysone is 3 to 5 times stronger than α-ecdysone.

Insects cannot synthesize steroids. The silkworm produces ecdysone from sitosterol in mulberry leaves through cholesterol, that is, sitosterol in mulberry leaves is absorbed by the digestive tract and cuts the hydrocarbon group at the C-24 position of sitosterol in the body. Conversion to cholesterol. Prothoracic glands use cholesterol present in the blood as raw materials to synthesize α-ecdysone in the glands, then secrete them out of the glands and convert them into β-ecdysone. The target tissue of ecdysone is the dermis, and its basic role is to promote protein metabolism and cause molting.

Juvenile hormone

The white globular body of the pharyngeal system secreting juvenile hormone is located on the ventral surface of the larval head-thoracic junction, close to the two sides of the digestive tube, left and right in pairs, and is connected to the nerves branching from both sides of the brain through the side of the heart. The larval pharyngeal lateral body increases with the growth of the silkworm, and the diameter in the fifth instar is about 1 mm. The pharyngeal body is composed of 20-30 globular secretory cells, and the outer bread has a common membrane. The nucleus of secretory cells is nearly round, and granular or liquid cell secretions can be seen in the cytoplasm. This secretion (juvenile hormone) is periodically secreted into the blood. The larvae have been secreting juvenile hormones to maintain their larval morphology from the beginning of their development to the third day of the 5th instar. After that, they stop secretion and begin to secrete moths in the second half of the pupae.

Juvenile hormone is a kind of terpene substance. Juvenile hormone Ⅰ (C 18 ), juvenile hormone Ⅱ (C 17 ) and juvenile hormone Ⅲ (C 16 ) have been extracted from insects. Silkworm has an active substance. The basic role of juvenile hormone is to inhibit protein metabolism. Thousands of substances that are extracted from plants and have juvenile hormone activity in synthetic compounds are known, which provides favorable conditions for the study of the mechanism of juvenile hormone and its application.

Brain hormones

Brain hormone is a peptide hormone with a molecular weight of 4400±400. Brain hormones are also called prothymogenic hormones. It is a neurosecretory substance secreted by neurosecretory cells in the brain, which has the function of activating the prothoracic gland and promoting the secretion of ecdysone. The neurosecretory cells in the mulberry silkworm brain can be divided into several groups according to their distribution positions, such as the back center, both sides, rear and ventral sides of the brain. Brain hormones are secreted by the cells in the central and both sides of the neurosecretory cell group and enter through the axons Pharyngeal side body, and then released in the silkworm body. The secretion activity of brain nerve secretion cells is affected by the temperature and light during the invigorating period and feeding period of silkworm eggs. The brain of the silkworm that is invigorated under low temperature and dark conditions is better than that of the silkworm brain under the condition of high temperature and bright invigoration. Less hormones. In the large silkworm stage, the silkworms with high breeding temperature secrete more brain hormones. Those with more brain hormone secretion have an inhibitory effect on the secretion activity of the pharyngeal body itself. Therefore, the secretion activity of the brain has a significant effect on the secretion of juvenile hormone. It has a regulatory role and at the same time controls the secretory activity of the prothoracic glands.

Diapause Hormone

Diapause hormone is a neurosecretory substance secreted by the hypopharyngeal ganglia. Some large-shaped nerve cells in the hypopharyngeal ganglion are neurosecretory cells, which are distributed on both the dorsal and ventral sides of the hypopharyngeal ganglion and the part connecting the nerve cord. The number and size vary with silkworm species and nature. On both sides of the mid-ventral ganglion of pupae and moths, about 2/3 from front to back, a pair of cells closely related to diapause of silkworm eggs were found, especially called diapause hormone secreting cells (Fukuda et al., 1967 ), in addition there is a group (more than 4) of cells with larger cytoplasmic granules. One of them is located between diapause hormone secreting cells, called diapause regulator cells. Among the diapause regulator cells that produce diapause eggs, there are concentrated lysosomes (Park, K, E.1973, 1975). Diapause hormone-secreting cells have been seen to have secretions in the 5-year-old food period, and secretion activities prevail after the 3rd day of pupation, and they are directly secreted into the blood. Its secretory activity is controlled by the brain, that is, the brain promotes the secretory activity of the hypopharyngeal ganglion, and the primary chemical system is the strongest, the secondary chemical is the second, and the polymorphic is the weakest.

The diapause hormone active substance extracted from the head of the silkworm moth has two active parts after purification. They are called diapause hormone A and diapause hormone B. The molecular weight of A is 3300 and B is 2000, both of which are peptides. substance. It is known that the target tissue of diapause hormone is the ovary. When the egg cell in the ovary develops to a certain period, the egg cell receives the influence of the diapause hormone to determine its diapause. Because the degree of development of each egg cell in the ovary is different, During the entire period of ovarian development, the secretion of diapause hormone is required. Diapause hormone acts on the ovary and can promote the activity of ovarian trehalase, thereby increasing the glycogen content in the egg and making it develop into a diapause egg. On the second day after this kind of diapause egg is laid, due to the presence of diapause hormones in the egg, glycogen is converted into sorbitol and glycerol, which cannot continue to oxidize and supply energy, hindering the development of the embryo in the egg, thus forming a stagnation. The physiological state of fertility.

Eclosion Hormone

Bombyx mori emergence hormone is secreted by neurosecretory cells in the brain, especially the central neurosecretory cells in the brain. There are already emergence active substances in the brain during pupation, and its activity gradually increases after pupation, and the brain activity is the highest on the 5th day after pupation. This emergence hormone, produced by the secretory cells of the brain nerves, is transported to the pharyngeal side at any time and stored in the pharyngeal side, and is secreted into the blood before emergence. Adult latent worms under 16 hours of light and 8 hours of darkness will have emergence hormones in their blood 10-20 minutes after light, and they will emerge after 55-75 minutes. The hormone activity reached the highest level 30 minutes before emergence (30 units per milliliter of blood), and then rapidly declined, and the time for the emergence hormone to exist in the blood was only 15-20 minutes.