Observations on Pseudopregnancy in the Mouse

Abstract

From 20 to 50 % of sexually mature female mice, grouped together in large cages but isolated from males since weaning, were found to be pseudopregnant as indicated by abeyance of oestrus, vaginal mucification, weight changes and mammary development. Such 'extra-coital' pseudopregnancies occurred independently of experimental handling in grouped females but were also observed, although much less frequently, in isolated mice subjected to daily examination.The duration of pseudopregnancy was found to be longer and much more variable than has previously been suggested.During the course of work involving the regular weighing of female mice it was observed that on several occasions untreated animals, completely isolated from male mice, showed marked weight fluctuations closely resembling the weight changes which occur during pseudopregnancy [Dewar, 1957]. Accordingly, the relation between these apparently spontaneous weight changes and the sexual state was investigated. The resulting observations and others associated with the existence of pseudopregnancy are recorded below. METHODSThe mice belonged to a closed colony of albinos maintained by random interbreeding for approx. 11 years. They were housed in cages containing up to twenty-five mice or, for experimental work, in smaller cages containing one to three animals. Except in breeding cages, the sexes were kept separate after weaning at 3-4 weeks of age.The mouse room was maintained at a temperature of 68-72°F. Mouse cake and water were supplied ad lib.The mice under observation were weighed twice daily and daily vaginal smears were taken. Comparisons were made with the behaviour of mice made pseudo¬ pregnant by mating with vasectomized males. Mammary gland development was investigated by preparing whole mounts of tissue stripped from the skin after fixa¬ tion in formol-saline, stained in iron haemalum, dehydrated and cleared in xylol. RESULTS Occurrence of' extra-coital ' pseudopregnancy Sixty virgin female mice, which had remained isolated from males from the time of weaning 4-5 months previously, were housed in groups of three and weighed daily for periods of 1-3 months. The majority, at some time, showed spontaneous weight

A Block to Pregnancy in the Mouse Caused by Proximity of Strange Males

“…This suggests that the latter situation supplies a stronger stimulus. When the test situation concerns females only (see Tables 2 and 3) there is no mutual interference with pregnancy, but mutual interference between females is well known to occur as regards the oestrous cycle (van der Lee & Boot, 1955, 1956Whitten, 1957b;Dewar, 1959) where the proximity of other females appears conducive to the formation of corpora lutea with the induction of pseudopregnancy. It seems probable that the absence of failed luteal function among the test groups containing only females is an expression of this reaction.…”

Effects of Hysterectomy on Corpus Luteum Activity in the Cyclic, Pseudopregnant and Pregnant Mouse

“…This is indicated also in the reduction from the 8th day onwards of the variability of this parameter (as shown by the increasing reduc¬ tion in the standard deviation-see Table 3 and Text- fig. 2) ; in this strain of mice, the duration of activity of the CL of pseudopregnancy is very variable (Dewar, 1959 and Tables 1 and 2), the range extending from 9 to 36 days, and a considerable proportion of the animals having pseudopregnancies as long as, or longer than, the gestation period (27% of 193 pseudopregnancies examined lasted 19 days or over). The abolition of this variability in CL activity in pregnancy suggests that the luteotrophic influence of the pituitary which initiates such activity not only becomes redundant in later pregnancy but is suppressed (after about the 13th day).…”

“…Newton (1935) noted that the interval between uterine evacuation and oestrus in a few animals was somewhat longer when the operation was performed on the 12th as compared with the 18th day of preg¬ nancy, but no reinvestigation of this observation has since been made. Although the earlier work has clearly established the dependence of the CL on placental endocrine function, the degree of this dependence may be questioned in the light of more recent observations on the duration of pseudopregnancy which, in some mice, can be as long as the normal gestation period (Dewar, 1959). The rôle of the uterus itself in the control of the CL of pregnancy requires further investigation although the indications are (Dewar, 1957a;McLaren, 1970) that it plays no significant part.…”

The Suppression of Ovarian Cyclical Activity in Groups of Mice, and Its Dependence on Ovarian Hormones

“…The oestrous cycle of the mouse is influenced by environmental conditions (Merton, 1937; Van der Lee & Boot, 1955, 1956Dewar, 1959). Contact with intact, but not castrated, male mice shortens the oestrous cycle (Whitten, 1958;Bruce, 1965).…”

“…The suppressive effect was abolished by ovariectomy, except that a small effect on the length of the cycle remained (Table 3). (Terenius, 1971 (Champlin, 1971), although Dewar (1959) has suggested that mounting behaviour is important, and Kimura (1971) …”