Tissue Dissolution & Tissue Formation In Silkworm Metamorphosis

Insect metamorphosis can be divided into three types: arnetabola, heterometabola, and holometabola. Mulberry silkworm (Bombyx mori) goes through four developmental stages: egg – larva – pupa – moth, pupation and moth metamorphosis twice, and finally develop into adult worms. Adults are completely different from larvae in terms of external morphology, internal structure, physiological function, and lifestyle. The silkworm is a holometabolous insect.

From the perspective of physiology, the life circle of silkworm can be divided into nutritional physiological period and reproductive physiological period.

The larvae are the physiological period of nutrition, and the system of the body tissue and organ system of the silkworm is all around the efficiency of food intake and food digestion and absorption. The nutritional physiology of this period not only takes in the energy materials needed for life activities at that time, but also reserves the nutrients during the non-feeding period. The construction of the adult body is all conducive to egg formation, mating behavior and egg laying activities. Therefore, with the metamorphosis of larvae to adults, the body structure undergoes a fundamental change. The larva’s mouthparts, digestive tract, and almost all tissues and organs’ cells collapse or change during the metamorphosis stage, that is, enter the histolysis process. Correspondingly, some of the newly formed adult tissues are modified to use larval tissues, while organs that are not in the larval stage are developed from the imginal disc.

Morphological changes of various tissues and organs of silkworm during metamorphosis

There are two types of tissue dissociation: programmed cell death and cell remodeling. The former are like thoracic foot, silk glands, muscles, etc., and the latter are like Martens’ tubes, fat bodies, etc. When the tissue begins to dissociate depends on the physiological development of the cell itself. The direct cause is hormones, especially ecdysone. The process of cell death first is cell autolysis, that is, the lysosomes in the cell fuse and decompose other organelles in turn, and the cells gradually become smaller. The smaller cells are ingested by neighboring cells or blood cells. The swallowed cells are eventually digested. When this process spreads to the whole tissue, it causes the dissociation of tissues and organs, and the final digestion products are released into the blood.

In the process of cell reconstruction, autolysis of organelles also occurs, but in this case, the nucleus is not decomposed, and the decomposition is limited to specific organelles. It is still unclear how to distinguish between decomposed organelles and non-decomposed organelles. In the fat body cells of the pupae, only a very small amount of mitochondria, peroxisomes, endoplasmic reticulum and Golgi apparatus are seen. However, glycogen, lipids and proteins accumulate in large amounts in the cytoplasm. When stimulated by adult differentiation, these organelles re-differentiate and develop various organelles. The newly developed organelles are different in morphology from those seen in the larval stage.

Tissues and organs such as wings, thoracic feet, compound eyes, and genitals that are not seen at the larval stage develop from the primordia of the adult. The differentiation of the primordium of the adult has already occurred in the early stage of embryogenesis. At that time, unlike the larval cell line, there is no tissue differentiation and functional differentiation, but repeated mitosis. Therefore, the number of cells in the adult primordium increased logarithmically throughout the larval stage. After receiving the information of abnormal development, the adult primordium took different actions. For example, the cells of the wing disc and the leg disc of the thoracic foot are hypertrophied to increase their volume and swell out of the body, covering the body surface with two flat layers of epithelial cells. During pupation, all the adult primordia are placed in their proper positions. As the adult develops, the cells continue to proliferate and differentiate. While secreting the adult epidermis and completing the external changes of various sensory organs, the internal muscles and tracheal branches are extended, and the establishment of the neural network completes the entire adult tissue system.

The formation of adult organs from imginal disc is a special phenomenon of holometabolous insects.