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J. M. BEDFORD
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Summary.

The fertilizing ability of rabbit spermatozoa is acquired in passage through the epididymis, during which certain changes take place in the spermatozoon. It has been found here that a significant diminution (P<0·01) in acrosome width and length occurs during passage of spermatozoa from caput to cauda epididymis. Within-group analysis of the caput sperm group reveals a significant correlation (P<0·01) of acrosome width with the position of the cytoplasmic droplet on the midpiece. A large difference in the cytoplasmic droplet distribution is apparent between samples of spermatozoa from the cauda and caput epididymis, respectively. The occurrence and disposition of the droplet in either of these regions was not obviously altered following a series of successive ejaculations. The change in acrosome size during epididymal passage is discussed in relation to the development of fertilizing ability of rabbit spermatozoa.

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J. M. BEDFORD
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Fluids instilled into the rabbit vagina are prevented from passing further along the tract by the valve-like disposition of the uterine cervix. In the present study, vaginal spermatozoa were killed at 2 or 5 min after coitus by instillation of 12 to 16 ml of 1% lauryl sulphate (sodium dodecyl sulphate) into the anterior vagina. When the vaginal spermatozoa were killed 2 min after a single mating, only 25% of eggs were subsequently fertilized and there were essentially no accessory spermatozoa about the eggs. The fertilization rate was increased to 93%, with a mean of 6·6 perivitelline spermatozoa/egg when killing of the vaginal spermatozoa was delayed until 5 min after a single mating. Thus, in the rabbit, sufficient numbers of vaginal spermatozoa for normal fertility enter the cervix within 5 min after coitus.

Early passage of spermatozoa into the cervix was much enhanced by a second successive coital stimulus, since destruction of all vaginal spermatozoa only 2 min after double mating allowed a 90% fertilization rate and a mean of 5·5 perivitelline spermatozoa/egg. Comparable results were achieved even if the second intromission was accomplished by a vasectomized male, pointing to the importance of the second coital stimulus rather than of the greater sperm numbers provided by double mating.

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J. M. BEDFORD
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The phagocytosis of rabbit spermatozoa has been studied with the electron microscope, and the process of ingestion of both intact and damaged spermatozoa by leucocytes in the oestrous uterus is described. The cell membrane and acrosome cap remained intact in many sperm heads wholly ingested by uterine leucocytes up to 12 hr after mating. However, only spermatozoa with damaged acrosomes and/ or damaged head membranes were ingested to any extent in the pseudopregnant uterus, in the pleural cavity, or following in-vitro incubation with leucocytes obtained from the oestrous uterus, intact sperm heads merely adhering to the leucocyte surface. It is suggested that some change occurs in the surface membrane of spermatozoa in the oestrous uterus which renders the intact sperm head acceptable to uterine leucocytes. Such surface changes may be associated with the process of sperm capacitation.

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J. M. BEDFORD
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The microstructure of the rabbit sperm head in ejaculate and in uterine spermatozoa has been examined with the electron microscope. The normal structure of the sperm head is described, and it is suggested that the properties of the plasma membrane may vary over different regions. No morphological change in the acrosome cap, which might account for the functional difference in capacitated spermatozoa, has been observed. There are indications, however, that the plasma membrane is more easily separable from the anterior and lateral edges of the underlying acrosome cap in spermatozoa recovered from the uterus.

Abnormal forms of acrosome cap development are described, and are discussed particularly in relation to similar abnormalities described previously in bull and boar spermatozoa.

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J. M. BEDFORD
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A potent agglutinin in the semen of a fertile male rabbit caused massive head-to-head agglutination of its own ejaculated spermatozoa and those of other rabbits, without reducing their motility, or destroying their fertility. By contrast, rabbit spermatozoa collected from the uterus 12 hr after insemination, and bull, rat, hamster, guinea-pig and human spermatozoa, were immobilized immediately when exposed to the active seminal plasma, the agglutinating and spermicidal properties of which were destroyed by heating to 55° C for 20 min. This indicates a specific change in the susceptibilities as well as in the functional competence of rabbit spermatozoa during the period of capacitation in the uterus. The agglutinin-active plasma also brought about marked clumping of leucocytes, but not of erythrocytes or granulosa cells. Brief comment is made on the nature of the sperm surface and on the significance of seminal antagglutins.

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J. M. BEDFORD
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A study of rabbit ejaculate and epididymal spermatozoa, and of spermatozoa recovered from various regions of the female tract, has revealed some consistent morphological differences in the state of the acrosome between these groups. Few acrosomes were missing or changed in epididymal or ejaculate spermatozoa. On the other hand, the acrosome was absent in spermatozoa released from the perivitelline space of fertilized ova. Uterine and tubal spermatozoa recovered 9 to 11 hr after insemination of ejaculates into oestrous or pseudopregnant females showed change or loss of the acrosome in a variable percentage (8 to 58%). The percentage acrosome loss from uterine samples taken after insemination of aliquots from the same ejaculate into pairs of females was closely similar within each pair, though variation occurred between pairs. Consistently higher acrosome losses (mean, 33%) were found following insemination of ejaculates collected after 1 month's sexual rest than were found after insemination of the third ejaculate from each of the same males (mean, 17·2%). Considerably fewer acrosomes were missing from uterine spermatozoa recovered after insemination of epididymal spermatozoa (mean, 5%) compared to the loss after insemination of ejaculate spermatozoa (mean, 27·4%) taken previously from the same males. Fertility trials with ejaculate and epididymal spermatozoa showed no difference in fertilization rate, or in numbers of supplementary spermatozoa, in favour of the ejaculate samples, i.e. those which showed the greater percentage acrosome changes.

In conclusion it is suggested that the quantitative variation in acrosome reaction between samples of uterine spermatozoa reflects differences in the samples, rather than differences in the female environment. The acrosome changes in the uterine and tubal spermatozoa of the domestic rabbit are probably not a morphological concomitant of capacitation, but more probably reflect impending sperm senility.

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J. M. BEDFORD
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The first barrier presented to rabbit spermatozoa at the site of fertilization is the cell mass which comprises the cumulus oophorus and, immediately surrounding the ovum, the corona radiata. Fertilization in the mouse, rat and rabbit occurs before dispersion of these granulosa cells (Lewis & Wright, 1935; Austin, 1948a, b; Chang, 1951a). In studies of the interaction of spermatozoa with the cumulus of the rat, rabbit and hamster in vitro, Austin ( 1960) observed that epididymal or ejaculate spermatozoa were apparently unable to penetrate the cumulus oophorus, whereas under similar conditions spermatozoa were seen moving freely through the cumuli obtained from mated animals. This observation led Austin to suggest that capacitation, which occurs while the spermatozoa are in the female tract (Austin, 1951; Chang, 1951b), may facilitate sperm passage through the cumulus oophorus to the surface of the zona pellucida. This suggestion has been tested in the rabbit, in vivo, in the manner described below.

Spermatozoa flushed from rabbit uteri with Hanks' solution 12 to 14 hr after natural mating were concentrated by centrifugation at 1000 rev/min for 5 min.

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K. M. Sultan
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J. M. Bedford
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In several mammals studied, ovulation appears to stimulate coordinated translocation of a few potential fertilizing spermatozoa to the ampulla from the caudal isthmus of the Fallopian tube. The present experiments demonstrate that, in the rat, this movement is regulated to a considerable degree within each duct by the ipsilateral ovary. Unilateral ovariectomy had no effect on ipsilateral sperm transport into the uterus, but this brought a unilateral quenching of sperm transport to the ampulla of the oviduct, from which spermatozoa were often totally or almost absent. This suppression was always complete on the ipsilateral side in unilaterally ovariectomized females anaesthetized with sodium pentobarbital soon after ovulation, and was evident bilaterally also at a highly significant level among intact females that were anaesthetized in this way. Unilateral ovariectomy and sodium barbital anaesthesia could provide experimental situations through which to decipher the mechanisms of normal sperm transport in the Fallopian tube.

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Patricia M. Saling
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J. M. Bedford
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Summary. Spermatozoa and eggs of one species were transferred into the oviducts of other species to examine the specificity of mammalian sperm capacitation in vivo. Using mice, rats, hamsters and rabbits, 11 different combinations of gametes and host were tested, with cumulus-intact and cumulus-free eggs being placed in opposite oviducts of a recipient. In all combinations compatible with the maintenance of sperm motility over a period sufficient for capacitation, fusion occurred between homologous gametes without regard to the identity of the host environment. The period required for capacitation, as judged by the timing of egg penetration, was a function of the specific character of the spermatozoon more than of the 'oestrous' oviduct. Fertilization level was not enhanced significantly by the presence of the cumulus oophorus, with the single striking exception of mouse eggs in the hamster. Sperm viability was not always optimal in a foreign tract; mouse and hamster spermatozoa became immotile within 2 h in the rabbit. Despite this limitation on sperm viability, some mouse eggs were fertilized in the rabbit, indicating that capacitation is possible even when the environment is suboptimal for motility maintenance. Therefore, while the results indicate that there is no specificity for capacitation as such, it should not be inferred that there is a total absence of specificity in the relationship between the female reproductive tract and gametes.

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J. M. BEDFORD
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J. W. OVERSTREET
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Summary.

X-irradiation with 6000 to 10,000 rad can be used as a method of `marking' a population of rabbit spermatozoa without affecting their fertilizing ability. Following a mixed insemination, irradiated spermatozoa compete equally well with non-irradiated spermatozoa from the same population, as judged by differential egg development 50 hr after insemination, at which time eggs fertilized by irradiated spermatozoa display obviously retarded cleavage.

In this paper, the technique has been used to demonstrate: (a) a subtle decrease in the fertility of spermatozoa after incubation in vitro at 40° C, which was not detectable by insemination of the treated sample alone, (b) the ability of spermatozoa from the proximal cauda epididymidis to initiate contact with and fertilize ova as readily as ejaculated spermatozoa following mixed insemination, there being no delay in the time of egg penetration by the epididymal spermatozoa, (c) the existence of selective fertilization between individuals of the same breed, as well as those differing genetically.

The irradiation marking technique could have many applications in comparative assessment of the fertility of semen from different individuals, irrespective of their genetic make-up, and in testing the effect of various treatments or environments on the fertilizing capacity of spermatozoa.

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