翼手類

The purpose of this paper is to report acrosomal reaction and effects of death of spermatozoa in the Japanese long-fingered bat (Miniopterus schreibersi fuliginosus). In this species, ovulation takes place a few days following fall copulation. It appears, therefore, that the spermatozoa must spend some time in the female genital tract before sperm is capable of penetrating the egg. In order to establish base lines for delineation of possible morphological changes in the spermatozoa stored in the female genital tract, we have examined epididymal spermatozoa (Fig. 1 a-d). Before ovulation, a part of spermatozoa brought in the uterus reaches the uterotubal junction (Fig. 2). In this region, neither autolysis of spermatozoa nor infiltration of leucocytes occurs. The accumulation of spermatozoa within the uterotubal junction would limit the amount of contact occurring between the spermatozoan reserve and leucocytes infiltrated into the uterine lumen through the endometrium. It is suggested that this spatial separation of the two kinds of cells, i.e., spermatozoa and leucocytes, plays an important part in the survival of an adequate population of spermatozoa in the uterotubal junction. Thus, a possible physiological significance of the uterotubal junction in sperm capacitation would be pointed out. In the spermatozoa stored in the uterotubal junction, acrosomal reaction is induced (Fig. 3). Meantime, vesiculation occurs between the plasma and outer acrosomal membranes. The process leads to the formation, around the acrosome, of a double-walled, fenestrated layer made up of the vesiculated remnants of plasma and outer acrosomal membranes. Acrosomal contents (enzymes etc.) would be liberated extracellularly through the pores of this layer. An unfertilized egg, surrounded by two or three layers of cumulus cells which tend clump together, is located in the upper ampulla tubae (Fig. 4). Fertilization may occur in the anterior portion of the ampulla tubae. In the egg with male and female pronuclei, there are still a number of cumulus cells loosely adhering the zona pellucida (Fig. 5). By the time the spermatozoa reach the zona pellucida, they lose their acrosomes, and the anterior regions of their heads are surrounded by the naked inner acrosomal membrane (Fig. 6). Since the zona pellucida allows penetration of several spermatozoa under normal physiological conditions, redundant spermatozoa are found in the perivitelline space of the fertilized egg as shown in Figs. 7 and 8. Thus, the mechanism which prevents polyspermy does not occur in the zona, but in the vitelline surface in this species. Most of spermatozoa reaching the perivitelline space through the zona pellucida lose obviously the equatorial segment of acrosomes (cf. Figs. 7 and 1). In the meantime, all spermatozoa deposited into the uterine lumen autolyse before ovulation (Fig. 9). That is, each of the spermatozoa in the uterus have a swelled saclike plasma membrane overlying the acrosomal cap region, and a bending acrosomal cap. In this species, a large number of leucocytes usually appear in the uterine lumen following copulation (Figs. 10 and 11). Many of the autolysed spermatozoa are engulfed by leucocytes infiltrated into the lumen through the endometrium, and the leucocytes themselves also seem to autolyse after phagocytosis (Fig. 12).