Sperm Formation of Silkworm

The formation of silkworm sperm through four stages of proliferation, growth, maturation, and deformation, all proceed in one pair of testes of male silkworm. After the embryonic development of the silkworm is completed, there is a large terminal cell at the top of each testicular cell. The function of this cell is to absorb, process and supply nutrients. There are many primitive germ cells around it, and these primitive germ cells gradually develop into primary spermatogonia. With the growth of silkworm and the progress of mitosis, the primary spermatogonia gradually leave the terminal cells and begin to develop into secondary spermatogonia about 3 days after incubation. At this time, the cytoplasmic protrusions released by the terminal cells surround the secondary spermatogonia to form seminal vesicles. The secondary spermatogonia in the seminal vesicles undergo 6 mitosis, forming 64 primary spermatocytes. This splitting process continued until the early 3rd instar. The primary spermatocytes go through a short growth period, at which time the cells are slightly enlarged, and the seminal vesicles also grow up at the same time. When the silkworms reach the end of the third instar and the beginning of the fourth instar, they enter the mature stage, and the primary spermatocytes begin to mature and divide. The first mature division is meiosis. The prophase of this meiosis is very long, and it lasts until the fourth dormant or early 5th year before entering the middle and late stages to complete the division. At the end of the first meiosis, one primary spermatocyte (2n=56) produces 2 secondary spermatocytes (n=28) with the number of chromosomes halved. After a very short interval, the second division begins, and 2 sperm cells are produced by the division of 1 secondary spermatocyte. This division is an equal division, which means that the number of chromosomes of the sperm cells formed remains half (n = 28). In this way, in each seminal vesicle, the sperm cells undergo mitosis and maturation division, resulting in a total of 256 sperm cells.

After completing the above division, the sperm cells in the seminal vesicles enter the metamorphic phase. In this process, the sperm cells first form a nuclear membrane, and then the chromatin fibers are gradually concentrated. As the sperm cells elongate as a whole, the nucleus becomes longer, forming a dense needle-like sperm head. During the elongation of the cell, the flagella, the matrix formed by the centrioles, stretches to become axons. The two accessory nuclei formed by the accumulation of mitochondria surround the axons to form the tail of the sperm. At the same time, part of the cytoplasm fell along the axons and overflowed outside the sperm cells. In addition, at the tip of the sperm’s head, there is a sharp piercing body formed by the Golgi apparatus, which is called the acrosome. In this way, after a complicated deformation process, a sperm with a length of about 400 μm and a width of only 1 μm is formed. At this time, the sperm is still surrounded by the membrane of the spermatogenic vesicle and develops into a sperm sac. The sperm in the sac is bundled, and all the sperm heads are arranged in the sperm bundle in one direction. In theory, there should be 256 sperm in a sperm bundle, but because some sperm degenerate and disappear during the development process, the actual survey number is 211 ~ 251. The number of sperm bundles in a testis is 3,000 to 4,000, and a male moth can produce up to 1.4 million to 2 million nucleated sperm.

Correspondence between sperm formation and development period: there are only primary spermatogonia in the ant silkworm testis;

• At the first instar, secondary spermatogonia increase;
• At the second instar, part of the secondary spermatogonia develops into spermatocytes;
• At the third instar, most of them are in the spermatocyte stage;
• At the 4th instar, the primary spermatocytes begin to undergo meiosis;
• At the end of 5th instar, most of them have developed into sperm cells, and they develop into sperm after clustering.