<紀要論文>
Spermatogenesis of the silkworm and its bearing on radiation induced sterility. II

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概要 The present study consists of two parts. In the first part of this paper, the normal spermatogenesis of the silkworm was described with special reference to the correlation between the developmental s...tages of the larvae and the germ cell stages and to the duration of the successive stages of spermatogenesis. In the second part, radiation responses of spermatogenic cells were investigated by means of fertility tests and histological and cytological observations on the irradiated testes (or germ cells). The main purposes of this experiment were, first, to investigate the reasons for the seeming contradiction between the findings on mice and adult Drosophila on the one hand, and those of the silkworm on the other as to the cytological causes of radiation induced sterility and, second, to establish the latest larval stage at which irradiation yields spermatozoa that were treated exclusively as spermatogonia. The results may be summarized as follows. 1. During normal spermatogenesis of the silkworm primary spermatocytes differentiate first around the middle of the second instar and increase in number with the development of the larva. In the testis of the early fourth instar larva. primary spermatocytes in synaptic and pachytene stages prevail. The testis of the early fifth instar larva contains germ cells ranging from primary spermatogonia to very early spermatids, with the spermatocytes in late meiotic prophase forming the majority. 2. The duration of each stage in spermatogenesis of the silkworm was estimated from the time table of the first appearance of cells in successive stages. It was found that meiotic prophase takes about 10 days, while late meiotic stages proceed within a short time. It takes about six days from the beginning of the spermiogenes is up to the completion of the eupyrene spermatozoa. The estimates obtained from a similar study on the development of regenerated spermatogonia after 1000 r irradiation agreed well with those obtained from unirradiated germ cells. This indicates that irradiation of primary spermatogonia with 1000 r does not affect the subsequent development of the treated germ cells. 3. Examination of the semina l vesicles of the newly emerged male moths showed that no apyrene spermatozoa were transferred from the testis to the extra-testicular organs of males. This observation led to the conclusion that both active and non-motile sperm usually observed in the bursa copulatrix of females after copulation are derived from the eupyrene spermatozoa. The cause for the difference in activity among eupyrene spermatozoa remains unexplained. 4. Results of the histological examination of irradiated testes of the s ilkworm were in good agreement with those of previous workers on other animals. The secondary spermatogonia, especially in their later stages, were shown to be the most sensitive to damage by radiation and to be easily killed even by low X-ray doses. These dead cells degenerate rapidly and are removed from the testis within a short time. Spermatocytes were shown to be more sensitive to radiation in late meiotic prophase than in the synaptic and pachytene stages. The majority of the cells irradiated with 1000 r of acute X-rays in the former stage or with 2000r in the latter stages degenerate in the spermatid stage or turned into non-functional sperm. Thus, two types of cell death could be distinguished by their time of occurrence, namely the immediate death of irradiated spermatogonia and the later death of irradiated spermatocytes. Primordial germ cells, primary spermatogonia, spermatids and sperm were shown to be resistant to irradiation. 5. The regeneration of spermatogonia after irradiation was investigated. It was found that the fate of the regenerated spermatogonia differed with the stage of the animal at the time of irradiation. When irradiation was given in the sufficiently early stages of larval life, i.e., before the early third instar, the regenerated spermatogonia could develop into eupyrene and apyrene spermatozoa as in unirradiated insects. When X-rays were given during the fourth or fifth instar, the regenerated spermatogonia failed to differentiate into eupyrene spermatozoa. The stage limit for the production of functional spermatozoa from regenerated spermatogonia occurs near the early third instar. 6. Since the spermatocytes begin to differentiate around the middle of the second instar, irradiation should be given before this stage in order to obtain sperm samples treated as spermatogonia. When the irradiation is given in the late second instar, sperm utilized for fertilization will be contaminated by spermatozoa that developed from irradiated primary sperm3.tocytes, although their contribution to the total sperm sample probably is very small. 7. In the silkworm, sterility due to the destruction of late spermatogonia is not detectable, even though the cells are easily killed by radiation. Pronounced sterility after irradiation of the early fifth instar is the result of functional oligospermy because the prevalent cells at this stage, Le., spermatocytes in late meiotic prophase degenerate in the spermatid stage or turn into non-functional sperm. 8. These findings have been discussed in relation to comparable ones on mice and Drosophila, and it has been shown that a consistent picture of radiation induced sterility in animals can be obtained.続きを見る

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登録日 2012.07.20
更新日 2021.02.05

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