Embryonic Development & Organ Tissue Formation of Silkworm

After silkworm eggs are fertilized, embryos occur in the eggs and even larvae are formed.

Embryogenesis

Cleavage and blastoderm formation. About 2 hours after laying eggs, the male nucleus and the female nucleus combine to form a zygotic nucleus, which begins embryonic development. The zygotic nucleus repeatedly divides into many daughter nuclei in a mitotic manner. This process is called cleavage. . The daughter nucleus that splits from the fracture is also called the cleavage nucleus, and then continues to divide and multiply. Together with the surrounding cytoplasm, it is scattered in the yolk and gradually moves to the periplasm of the egg surface, starting from under the yolk membrane at one end of the egg hole. Connected to each other gradually downwards, arranged into a single layer of uniform thickness, surrounding the inner egg yolk, this layer of cells is called the blastoderm, the formation of the blastoderm is about 15 hours after laying eggs (2 chemical species ). The blastoderm is formed. After the embryo is formed, part of the cleavage nucleus remaining in the yolk will produce yolk cells with itself as the center and surrounding yolk granules. The function of the yolk cells is to supply nutrients required for embryonic development and to swallow degenerated cells. , And gradually degenerate and disappear as the embryo develops.

Germ formation. After the blastoderm is formed, the cells of the blastoderm on the side of the egg hole gradually abut each other and thicken. This is the primordium of the embryo, called the blastocyst. The formation of germ band is about 18-25 hours after laying eggs. The blastoderm cells outside the blastoderm gradually become thinner.

The formation of embryo, amniotic membrane and serosal membrane. After the embryonic zone is formed, the peripheral part of the embryonic zone begins to sink into the inside of the egg, and the cells of the blastoderm that connect the periphery of the embryonic zone gradually become thinner and become a thin film, and go outside the embryonic zone. The square forms a protrusion, and there are folds that are folded into two layers of film inside and outside, called amniotic folds. The amniotic membrane folds extend to the center of the blastoderm and finally join each other. The folds disappear to form two separate inner and outer membranes. The outer layer is called the serosa, which is closely attached to the vitelline membrane. The inner layer is called the amniotic membrane and covers the blastoderm. Ventral. At this point, the blastocyst and serosal membrane are completely separated and sink into the yolk to form an independent embryo. Embryo formation is about 24 to 30 hours after laying eggs. With the formation of serosa, the serosal cells of the aging eggs gradually deposit pigment particles to form the inherent colors of different species of silkworm eggs. The cavity between the amniotic membrane and the embryo is called the amniotic cavity. The cavity is filled with liquid, called amniotic fluid. The amniotic cavity and the amniotic fluid have the function of protecting the embryo and preventing embryonic hardening.

Germ layer differentiation. The silkworm embryo has ectoderm, mesoderm and endoderm. When the embryo is just formed, there is only a single cell layer. Later, when the embryo body elongates, the middle part becomes narrow, and when the two ends expand, a longitudinal groove appears along the midline of the embryo’s ventral surface, which is called the original groove. (The germ layer begins to differentiate) The primordial sulcus is formed by the proliferation of embryonic ventral midline cells and gradually invades. As the invaginated cells continue to proliferate, they stretch out on both sides of the inner surface to form a layer of cells. At the same time, the cells on both sides of the primordial sulcus are opposed to each other. Extend, and eventually meet, make the original groove disappear, at this time two layers of cells have formed inside and outside. The outer layer is called the ectoderm, and the inner layer is called the mesoderm. The mesoderm appears earliest in the middle part of the abdomen of the embryo, and gradually expands to the head and tail. The endoderm appears later. After the embryo develops to the formation of the front and hindguts, some special cells differentiate to form the endoderm on the ventral surface of the blind end of each trapped part.

Embryo body segmentation and formation of appendages. At the same time as the mesoderm is formed, the embryo body begins to segment, and transverse grooves appear on the surface of the embryo body, which can be clearly divided into 18 embryonic nodes. The first section is particularly large, called the head leaf (head fold), the second, third, and fourth embryonic section are called the mouth section, the 5th, 6th, and 7th section are called the thoracic section, the remaining 11 sections are called the abdominal section, and the last section is also called Caudal lobe (tail fold), after the embryonic body is segmented, there is a small depression in the middle of the head lobe called mouth depression (orifice), the center of the caudal lobe is also indented to form anal depression (anal canal), and then in each embryonic node A pair of small sac-like protrusions appeared on both sides of the ventral surface, and various appendages were formed later. Two round protrusions appearing at the top of the head lobe form the upper lip, and the protrusions on both sides form antennae. The protrusions on both sides of the ventral surface of the second to fourth embryonic segments (mouth segments) successively form the upper palate, lower palate and lower lip. Later in the process of shortening the embryo body, the mouth segments gradually merge with the head lobe into the head. The 3 pairs of protuberances from the 5th to 7th embryonic nodes form the 3 pairs of thoracic larvae. The protuberances of the 10th to 13th embryonic nodes and caudal lobe continue to develop to form the gastropods of the larvae. The protuberances of the 8th, 9th and 14th to 17th embryonic nodes gradually The degradation disappeared. The 17th embryonic section and caudal lobe merge.

The formation of major organs

Body wall formation. The body wall is formed by the ectoderm. When the embryo is stratified, a part of the body wall on the ventral surface is formed, and then stretches from the ventral surface to the back. First, the back of the embryo’s tail is sealed into a cylindrical shape, and then gradually stretched forward; at this time, the back end of the head is also sealed into a cylindrical shape, and it gradually stretches backwards to connect back and forth.

The formation of the nervous system. The nervous system is formed by the recessed ectoderm. When the cranial lobe protrudes, the nerve groove appears in the original sulcus. The ectoderm on both sides of the nerve groove differentiates into several large neuroblasts. The neuroblasts divide and proliferate to form a pair of cell groups in each node. The front and back are connected into two columns, and later separated from the ectoderm to form an independent nerve tissue. In the middle of each blastoderm, it expands and develops into a pair of ganglia, and shrinks between the two pairs of ganglia to form a linear nerve cord. At the beginning, in the 18 embryonic ganglia, except for the terminal ganglion, each ganglion has a pair. The cephalic lobe is the largest and develops into a cranial ganglion. The second to fourth pairs merge into one to form a hypopharyngeal ganglion. The fifth to fourteenth sections of the embryo body still retain a pair of each, and the fifteenth and sixteenth pairs form a pair. The 17th embryonic section degenerates and disappears, so the larva has 13 pairs of ganglia. Each pair of ganglia is connected by a nerve cord, which is the formation of the central nervous system. The central nervous system separates nerve branches to form peripheral nerves and sympathetic nerves. Sensory organs are also specialized by ectoderm cells, and sensory nerves are produced by specialized sensory neurons, grow inward, and enter the ganglia. Motor nerves are generated by motor neurons in the ganglion, extending outward and distributed in the muscle tissue.

The formation of the respiratory system. The respiratory system is formed by the invagination of the ectoderm. After the embryonic body is segmented, when the body wall stretches from the ventral to the back, there is a pair of depressions on both sides of the 5th to 17th embryonic nodes. During the development process, the depressions of the 5th, 8th and 15th blastoderm develop into the air valve. The depressions of the remaining embryonic nodes degenerate and disappear. These depressions form the air valve and continue to sink into a tube. After extending to a certain extent, they are divided into two branches to extend forward and backward respectively. The nodes extend relative to each other and connect to each other to form a longitudinal trachea. . At the same time, it branches into the tracheal plexus and crosses the trachea. Subsequently, many bronchus and microtrachea were separated and distributed to various organs and tissues.

Formation of the digestive tract. The front and hind intestines of the digestive tract are formed by ectoderm. When the embryonic body begins to be segmented, the head lobe and caudal lobe appear orifice depression and anal depression respectively, and they extend into a blind tube to form the foregut and hindgut respectively. Some endoderm cells are differentiated on both sides of the front and hind intestines. These cells divide and proliferate, develop into two cell bands on both sides of the ventral surface, and gradually divide and widen, and then connect into a semi-tubular tissue. It then extends from both sides to the back, and is sealed on the back to form a cylindrical midgut at the later stage of embryonic development. Before hatching, the partition between the front, middle, and back intestines disappears and communicates with each other.

Formation of silk gland. The silk gland is formed by the ectoderm. When the air valve is invaded, it is recessed into two small holes at the base of the lower lip protrusion of the fourth embryonic node, and gradually deepens into two thin tubes. When the two lower lip protrusions heal into the lower lip, the front ends of the two small tubes synthesize a spinning tube. After the embryo reverses (embryonic movement), the silk glands continue to extend and begin to bend, and gradually form the silk glands of the larvae.

The formation of the Markov tube. The Markov tube is formed by the ectoderm. When midgut tissue begins to appear at the blind end of the hindgut, a pair of tubular protrusions will appear on the left and right sides of the near blind end, each branching into 3, while the midgut is formed, they respectively elongate forward and attach to the wall of the midgut. Up, then flex down again, into the rectal wall.

The formation of dorsal blood vessels. The dorsal blood vessels are formed by the mesoderm. When the embryonic body stretches from the ventral to the back, some macronucleated cells appear at the ends of the mesoderm. This is the primitive tissue that develops into the dorsal tube, called cardiomyocytes. . The cardiomyocytes divide and proliferate and become one piece. One end is connected to the body wall, and the other end is connected to the midgut wall. As the body wall and midgut grow to the back, the cardioblasts on both sides gradually approach. When the midgut completes the dorsal fusion, it is also formed into a tube under the dorsal midline. In addition, blood cells are also formed by mesoderm differentiation.

The formation of muscle and fat body. Muscle is formed by the differentiation of the mesoderm. When the adnexal protrusion occurs in the embryo, the mesoderm is gradually divided into two layers. The outer layer differentiates into body wall muscles, and the inner layer differentiates to form the muscle layers of various organs in the body. During the development of the mesoderm, the fat body differentiates into some cell masses, which are dispersed in the body cavity, then divide and proliferate, and gradually connect into strips, masses or nets of fat bodies, which fill in between various organs and tissues. It is also believed that the fat body is formed by the differentiation of small yolk cells in the egg.

The formation of gonads. Primordial germ cells are differentiated from the ventral surface at about 30% of the posterior pole of the egg when the blastoderm is formed, and the blastocyst is slightly larger when it forms. The number varies depending on the species, about 12-30. As the embryo develops, it gradually migrates to the sides of the 12th germline and is surrounded by cells differentiated from the mesoderm to form a pair of gonadal glands (testes or ovaries). The lower end extends into a duct, which is later connected to the reproductive bud formed by the invagination of the ventral ectoderm.

In addition to the above-mentioned main organs and tissues, salivary glands, prothoracic glands, pharyngeal lateral bodies, and crimson cells are all formed by the ectoderm, and the subesophageal glands are formed by the mesoderm.