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W. G. BREED

Summary.

Lactation lasts for about 14 days in the field vole. Early in lactation the corpora lutea were large although some signs of degeneration were present; by Day 13 none could be recognized. Graafian follicles were present throughout the period but were slightly reduced in size during mid-lactation.

Vaginal smear patterns varied considerably between individuals although nucleated epithelial cells usually predominated.

Mating tests were carried out between Days 2 and 13 of lactation. 81% proved successful; coitus occurred on all days. 59% of the matings resulted in pregnancies that were carried to term. No reduction of litter size was apparent as a result of these matings. Gestation length was slightly prolonged when more young were being suckled. Most matings took place when there was a predominantly nucleated epithelial smear.

It is concluded that, in this stock, the only corpora lutea present during lactation are those of the previous pregnancy. The presence of Graafian follicles suggests that suckling does not entirely inhibit fsh secretion although the decrease in follicular size in mid-lactation may indicate slight fsh reduction. There were no 3- to 4-day oestrous cycles. The occurrence of mating and the predominant type of vaginal smear suggests continuous secretion of oestrogen. This may partly explain the lack of a marked implantation delay during lactation. There appears to be no inhibition of release of ovulatory-inducing hormone due to suckling nor of ovulation and resulting gestation even though the Graafian follicles are slightly reduced.

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W. G. Breed

The structural organization of the chromatin of cauda epididymal spermatozoa of the red veld rat Aethomys chrysophilus type B was investigated by fluorescence microscopy after staining with DNA specific dyes and by transmission electron microscopy after incubation with Triton X100, dithiothreitol, and SDS. Staining with DNA dyes showed variation in intensity of fluorescence of the sperm chromatin, with an anterior spherical region staining far more intensely than the surrounding chromatin. Transmission electron microscopy of these spermatozoa indicated that this region was composed of cords and fibres. This chromatin region dispersed more readily than the surrounding chromatin when spermatozoa were incubated with the detergents, and it is suggested that, unlike the rest of the sperm chromatin, it may be a histone-rich region, with protamine(s) being either scarce or absent.

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W. G. Breed

Department of Anatomy, University of Adelaide, Adelaide 5001, South Australia

It has been shown that laboratory-bred female hopping mice usually cease to have regular 7-9 day oestrous cycles when deprived of water and given only dry seeds as food (Breed, 1975). Body, ovarian and uterine weights tended to be lower, and it was therefore suggested that the availability of water, either free or in plant food, could regulate breeding in this species. This possibility has now been investigated in adult animals caught in the wild and then kept in controlled laboratory conditions.

Hopping mice (Notomys alexis) were obtained near Curtin Springs, Northern Territory, between 14 and 17 May 1975. Seven females were killed in the field, and the rest were returned within 3 days to the laboratory in Adelaide where they were randomly allocated to one of the following groups. Group 1 was the control group; the females were kept

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W. G. Breed

Summary. Morphological studies of the vagina and cervix from females of several species of Pseudomys (pseudo-mice) and Notomys (hopping-mice) have shown marked interspecific differences. In females of all four species of Pseudomys and in N. cervinus there was a relatively thin walled, dorsoventrally flattened, vagina with a large lumen, longitudinal epithelial folds, and conspicuous fornices. By contrast, in females of N. alexis and N. mitchelli the vagina had a much smaller lumen, few or no folds, minute fornices, but a well developed muscle coat. The cervix of the Pseudomys females was a conspicuous fibrous structure, rich in collagen, with two canals throughout its length. In N. cervinus females it was smaller and the two canals, although separated by a median septum for much of their lengths, joined just before the external os. In N. alexis and N. mitchelli females the cervix was much less extensive, its boundaries could not be discerned externally, and, histologically, it was more cellular and less fibrous. This variation in morphology of the female reproductive tract appears to complement that of the excurrent ducts, accessory sex glands and external genitalia of males of these species of Australian murid rodents.

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W. G. Breed

Summary. The relative size and histology of the testes of sexually active adult Pseudomys and Notomys differed markedly between species. Average weight of testes (as % of body wt) ranged from about 3% in P. australis and P. nanus to about 0·15% in most Notomys. In the testes of most species, interstitial tissue was relatively sparse, but in P. apodemoides, P. delicatulus and P. hermannsburgensis it was abundant and consisted of a mass of lipid-rich epithelioid Leydig cells. Seminiferous tubule diameters were similar in all species, but epididymides ranged from about 500 mg in weight (about 1% body wt) in P. australis to about 20 mg (about 0·05% body wt) in most Notomys. Likewise numbers of spermatozoa in the paired cauda epididymidis ranged from about 500 × 106 in P. australis and P. nanus to about 2 x 106 in 3 of 4 Notomys species.

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W. G. BREED

Summary.

Australian hopping mice (Notomys alexis) were subjected to short or long photoperiods, different social environments and/or water deprivation. The age at which vaginal opening and first oestrus occurred was delayed by short photoperiods and by the absence of male proximity. Water deprivation prevented normal maturation. The normal 7 to 8 day oestrous cycle was prolonged by short photoperiods or water deprivation. Animals deprived of water had lighter body, ovarian and uterine weights, and follicular growth seemed to be impaired. No differences in organ weights were found between animals in the two photoperiods. The significance of the findings is discussed.

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W. G. Breed

Summary. Hopping mice were examined to study two interrelated questions: (1) when groups of adults of both sexes are kept together in one cage in the laboratory is there evidence that the females copulate with only one male, and (2) is a copulatory plug formed in the female tract after ejaculation? The findings indicate that a female will sometimes lock with more than one male in the group during an oestrous period induced by administration of exogenous gonadotrophins, and that a small 'plug' of soft material forms post coitum in the more caudal parts of the female tract. Individuals of this species, therefore, do not appear to be strictly monogamous, at least in this artificial laboratory situation. Although a coagulum is formed, this is quite different from the typical hard copulatory plug that occurs in common laboratory murids; it may possibly reduce sperm backflow from the lower region of the female reproductive tract.

Keywords: hopping mouse; copulation; plug

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W. G. Breed

The morphology and structural organization of unusual spermatozoa from two species of bandicoot rats (Bandicota spp) from Southern Asia were studied with the light and transmission electron microscope. Unlike those described for nearly all other eutherian species the heads of Bandicota savilei and B. indica spermatozoa were bulbous or globular in shape and had no narrow uniformly defined apex nor a clearly delineated perforatorium. The nucleus, which had one or more prominent vacuoles, was capped by a huge, bulky, acrosome with no obvious morphologically recognizable equatorial segment. The postacrosomal dense lamina was extremely short and a posterior ring lay anterior to the caudal pole of the nucleus. In addition, the tail of the spermatozoon was only 28–45 μm in length. The organization of the spermatozoon of these two species of Bandicota is thus quite unlike that described for any other murid rodent or even mammalian species. Its highly divergent features suggest that the processes of zona penetration and incorporation of the fertilizing spermatozoon into the egg may differ markedly from those of all eutherians so far investigated.

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W. G. BREED and JESSAMINE HILLIARD

Summary.

Ovarian function was studied in thirty-eight does suckling at least four, and usually six to eight, young each. A balanced pellet diet was available at all times. Under these conditions, ripe follicles were present in the ovaries but the corpora lutea regressed rapidly following parturition and the size of the ovaries and uterus decreased as suckling continued. A basal progesterone output could not be quantified but the release of 20α-hydroxypregn-4-en-3-one, expressed as μg/g ovary/hr, tended to rise throughout lactation. In six of the seven suckled does which were allowed to mate post partum, pregnancy was maintained, indicating that sufficient oestrogen was produced by the follicles to maintain luteal function. The results support the concept that neither lactation nor the suckling stimulus suppresses fsh and lh release, since the presumed oestrogen production continues when adequate nourishment is available.

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W. G. BREED and H. M. CHARLTON

Summary.

Electrical stimulation of the anterior hypothalamus of the vole, Microtus agrestis, usually results in ovulation, whereas stimulation of areas in the posterior hypothalamus do not. Ovulation can also be elicited by injections of sheep median-eminence extracts, and by preparations of sheep luteinizing hormone. Observations on the number of corpora lutea formed, and ripe Graafian follicles remaining after treatment, suggest that mating is the most potent stimulus for ovulation, followed by electrical stimulation and hormone injection.