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Mortality and hence the proportion of young weaned was markedly affected by the type of nesting material but not by maternal age or parity. With paper tissues for nesting purposes 476/791 (60%) young were lost compared with 216/1182 (18%) with woodwool. Similar results were obtained in 2 additional series. The proportion of young weaned was also affected by litter size; in general, pup mortality was heavier and took place earlier in the large litters (more than 9 young).

The machine was successfully used to induce and maintain anaesthesia by inhalation of methoxyflurane in Mongolian gerbils, mice and golden hamsters.

The scope of experimental approaches applicable to the study of mammalian eggs and embryos has advanced in recent years to provide unprecedented opportunities for understanding mammalian embryology. Amongst these significant advances has been the ability to alter the genetic constitution of eggs by pronuclear and nuclear transplantation as well as by the introduction of specific cloned genes into eggs and embryos. These techniques can be used in conjunction with the experimental reconstruction of preimplantation embryos to investigate more precisely a number of aspects of mammalian embryology. Recently, a most intriguing aspect of development has been uncovered, one that is apparently unique to mammals; experiments have revealed that the parental genomes are not functionally equivalent during embryogenesis. Hence, the parental origin of chromosomes determines their influence during embryogenesis. The mechanistic aspects responsible for the germ line modifications of homologous chromosomes, their role during development, and the wide-ranging implications of these findings for mammalian development have yet to be fully defined. An understanding of this process will provide the basis for developing genetic and reproductive strategies that can be applied to domestic animals and to humans.

Mongolian gerbils injected i.p. with a 1·25% solution of tribromoethanol (TBE) quickly lost the righting reflex and showed good surgical anaesthesia, the duration of which was positively related to the dose administered. All levels (225-450 mg/kg bodyweight) of TBE injected s.c. resulted in loss of the righting reflex, though surgical anaesthesia was not attained. Following recovery from anaesthesia induced with 2.25% TBE i.p., or high doses of 1·25% TBE i.p., visceral adhesions and deaths occurred. To avoid these TBE-induced fatalities such extreme care must be taken during the preparation, storage and administration of the agent that it is not recommended as a suitable anaesthetic for general use in the gerbil.

Summary.Lactating pregnant gerbils in which one oviduct was ligated during early pregnancy were used on Day 6–14 p.c.as recipients for eggs (morulae and blastocysts) recovered on Day 6–12p.c.from lactating donors. Eggs were transferred to the ligated (experimental) horn and the litter was removed. About half of the recipients had implantations in the non-ligated (control) horn 15 days after litter removal. Of these 40 females, 37 had implantations in the experimental horn and 32 had some normal fetuses present. Of 179 eggs transferred into the ligated uteri, 61% implanted and 41% developed into normal fetuses. The time after copulation of recipient and donor did not appear to influence the number of implantations or post-implantation development of the transferred eggs.

The lifetime reproductive performance of Mongolian gerbils from which an ovary had been removed 7 days before pairing at 100 days of age was only slightly inferior to that of normal females. A general decline in fertility was evident only in the oldest females. The mean number of litters, young born and age at last parturition were 7·6 ± 3·8, 37 ± 20 and 462 ± 196 days (2 ovaries) compared with 6·9 ± 3·5, 30 ± 16 and 417 ± 142 days (1 ovary) respectively.

We reported recently that the gene that encodes tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of catecholamines, is regulated by hypoxia in the dopaminergic cells of the mammalian carotid body (Czyzyk-Krzeska, M. F., Bayliss, D. A., Lawson, E. E. & Millhorn, D. E. (1992) J. Neurochem. 58, 1538-1546) and in pheochromocytoma (PC12) cells (Czyzyk-Krzeska, M. F., Furnari, B. A., Lawson, E. E. & Millhorn, D. E. (1994) J. Biol. Chem. 269, 760-764). Regulation of this gene during low O2 conditions occurs at both the level of transcription and RNA stability. Increased transcription during hypoxia is regulated by a region of the proximal promoter that extends from -284 to + 27 bases, relative to transcription start site. The present study was undertaken to further characterize the sequences that confer O2 responsiveness of the TH gene and to identify hypoxia-induced protein interactions with these sequences. Results from chloramphenicol acetyltransferase assays identified a region between bases -284 and -150 that contains the essential sequences for O2 regulation. This region contains a number of regulatory elements including AP1, AP2, and HIF-1. Gel shift assays revealed enhanced protein interactions at the AP1 and HIF-1 elements of the native gene. Further investigations using supershift and shift-Western analysis showed that c-Fos and JunB bind to the AP1 element during hypoxia and that these protein levels are stimulated by hypoxia. Mutation of the AP1 sequence prevented stimulation of transcription of the TH-chloramphenicol acetyltransferase reporter gene by hypoxia.

Most gerbils mating did so within 24 h of parturition, with a peak of activity occurring between 1600-2000 h on Day 0. Vaginal smears showed a gradual reduction both in the total number of cells present and leucocyte dominance, with a concomitant increase in cornification. A significant change to leucocyte and epithelial cell dominance occurred by 0930 on Day 1. The presence of distinctive fragmentary 'spicules', the absence of vaginal spermatozoa in many females showing vigorous mating behaviour, and significant differences between the sperm content at the time of mating (Day 0) and the following morning (Day 1), were associated with the presence of a concealed vaginal plug. The plug, which is quickly formed at the onset of mating, had usually disappeared by the following morning.

The reproductive performance of Mongolian gerbils mated post partum and allowed to nurse 1-12 young was recorded. More than 70% of the post-parturient females mated, the majority on Day 1; the incidence of mating was unaffected by nursing. However, both fertility and fecundity declined as the number of young nursed increased. Smaller litters were associated with longer gestations.

Keeping a sexually mature male with a weanling female rat advanced neither the time of vaginal opening nor that of 1st oestrus. In 2 of 3 experiments females kept singly after weaning reached sexual maturity significantly earlier than did grouped females. The reproductive performance of females mated at 1st oestrus was not significantly different from that of older primiparae. 26 rats gave birth to an average of 9·3 young at 59·5 days of age, and 22 of them reared 96% of the young to weaning.

32 of 35 sexually mature CFHB rats paired with proven males and thereafter allowed to mate post partum and nurse young concurrent with pregnancy, produced a total of 227 litters. The proportion of females producing litters decreased both with increasing age and parity, and with the number of young nursed. Litter size declined both as age and parity increased, and when gestation was extremely prolonged. The incidence and extent of delayed implantation (gestations >25 days) were affected by the number of young nursed but not by increasing age and parity. There was no evidence for the existence of implantation periods coinciding with multiples of oestrous cycles.

Mean age at vaginal opening was 41 days, range 33–53 days. Bodyweight at weaning and age at vaginal opening were negatively correlated, but there was no relationship between bodyweight and age at vaginal opening. The presence of a male before vaginal opening did not accelerate either the time of its occurrence or first conception. The vaginas of females caged singly from weaning opened earlier than in those kept in groups of 2 or 3.

A detailed analysis of the developmental potential of parthenogenetic embryonic stem cells (PGES) was made in vivo and in vitro, and a comparison was made with the development of cells from parthenogenetic embryos (PG). In vivo, in chimeras with normal host cells (N), PGES cells showed a restricted tissue distribution consistent with that of PG cells, suggesting faithful imprinting in PGES cells with respect to genes involved in lineage allocation and differentiation. Restricted developmental potential was also observed in teratomas formed by ectopic transfer under the kidney capsule. In contrast, the classic phenotype of growth retardation normally observed in PG<==>N chimeras was not seen, suggesting aberrant regulation in PGES cells of genes involved in growth regulation. We also analysed the expression of known imprinted genes after ES cell differentiation. Igf2, H19 and Igf2r were all appropriately expressed in the PGES derived cells following induction of differentiation in vitro with all-trans retinoic acid or DMSO, when compared with control (D3) and androgenetic ES cells (AGES). Interestingly, H19 was found to be expressed at high levels following differentiation of the AGES cells. Due to the unexpected normal growth regulation of PGES<==>N chimeras we also examined Igf2 expression in PGES derived cells differentiated in vivo and found that this gene was still repressed. Our studies show that PGES cells provide a valuable in vitro model system to study the effects of imprinting on cell differentiation and they also provide invaluable material for extensive molecular studies on imprinted genes. In addition, the aberrant growth phenotype observed in chimeras has implications for mechanisms that regulate the somatic establishment and maintenance of some imprints. This is of particular interest as aberrant imprinting has recently been invoked in the etiology of some human diseases.

Summary.Ovarian follicles were counted and classified in control and unilaterally ovariectomized Mongolian gerbils, which had ceased to breed, at 306–1020 days and 340–902 days of age respectively. With increasing age there was a steady reduction in the non-antral follicle population, but a reduction in the number of antral follicles only occurred in extreme old age. Approximately twice as many non-antral follicles were present in control as in unilaterally ovariectomized females of similar ages, whereas the number of antral follicles was similar in both groups. There were no instances of total depletion of the follicle stock, the lowest numbers recorded being 288 and 74 in control and unilaterally ovariectomized females respectively.

The developmental potential of inner cell mass (ICM) and trophectoderm (TE) derived from parthenogenetic or biparental gynogenetic embryos was examined in reconstituted blastocysts with normal TE or ICM, respectively. The results demonstrate that when a normal ICM was introduced inside a trophectoderm vesicle derived from parthenogenetic or gynogenetic blastocysts, postimplantation development was characterized by the almost complete failure of trophoblast proliferation and without compensating cellular contribution from the normal ICM to the outer trophoblast lineage. Consequently, the normal ICMs also failed to develop adequately and only a few retarded embryos were detected on day 11–12 of pregnancy. In most respects, development of these reconstituted blastocysts resembled that obtained with unoperated gynogenetic and a parthenogenetic blastocyst. By contrast, an ICM from a parthenogenetic or gynogenetic embryo introduced inside a normal trophectoderm vesicle induced substantial proliferation of the trophoblast but again without a detectable cellular contribution from the ICM to the outer trophoblast lineage. However, with the improved development of the trophoblast, both the parthenogenetic and gynogenetic ICMs developed substantially better and without a detectable cellular contribution from the TE to the embryo. Almost all the embryos developed at least up to the 25-somite stage and many of them reached the 30- to 40-somite stage. Some of the most advanced day-11 and -12 gynogenones and parthenogenones yet seen have now been obtained in this way. Nevertheless, all the embryos were still smaller than the equivalent control embryos and showed signs of some tissue degeneration. The yolk sac was also suboptimal with poor blood supply and may need to be improved to obtain further improvement in the development of the embryos. The combined results demonstrate that the trophoblast proliferates very poorly even in the presence of a normal ICM, if the TE tissue lacks a paternal genome. However, ICM tissues which lack a paternal genome can develop to an advanced embryonic stage if they are introduced inside a normal trophectoderm vesicle. The results give further insight into the differential roles of maternal and paternal genomes during development of the embryo and extraembryonic tissues in the mouse.

Summary.Within the same female, concurrent lactation had no apparent effect on the development and hatching of embryos artificially retained in the oviduct while effectively inhibiting zona shedding and implantation in the contralateral uterine horn. This result is believed due to a specific inhibitor(s) present in the uterine fluid or to physical constriction by the uterus. These extrinsic, maternal factors act either singly or in combination to influence the mature blastocyst.

Summary.Observations were made on vaginal smear pattern, mating, rate of egg transport and preimplantation development in cuis. In 39 of 45 females (87%) vaginal opening occurred within 2 days of pairing, and in 44 (98%) within 5 days; 36 of the 44 females (82%) mated. By the morning of Day 2, eggs were no longer recovered from the ampulla, having all passed into the isthmus, where they remained till Day 4, p.m. Uterine entry was complete by Day 5, a.m. Uterine blastocysts were no longer recoverable on the morning of Day 6. There was no evidence of blastocyst expansion or zona shedding before implantation.

Summary.The efficacy of several exteroceptive factors including exposure to a 'strange' male, were tested to induce pregnancy failure in newly mated Mongolian gerbils. Only physical separation of monogamous pairs consistently induced pregnancy failure. This phenomenon was restricted to the immediate period after mating and is believed to be due to disruption of pair bonding, with associated luteal and implantation failure. Concurrent lactation annulled this effect.

Summary.A series of experiments based on 150 newly mated, uni- and multi-parous Mongolian gerbils provided no evidence for the existence of pregnancy blocking due to a strange male effect. The reduced level of fertility observed was attributed to stressful factors at critical times in the pregnancy. Treatment with 10 i.u. PMSG on the day following mating interrupted pregnancy and the majority of females mated again within 4 days.

Observations are presented on the incidence of mortality between birth and weaning at 30 days of age in the Mongolian gerbil, Meriones unguiculatus. The proportion of young weaned was approximately 80%; it was little affected by litter size or whether or not adjustment had taken place. A slightly higher proportion of males was lost than females. The heaviest mortality occurred within the first 5 days of birth, apparently due largely to neglect. No case of cannibalism was recorded.

In the Mongolian gerbil, vaginal opening occurs at 40 to 60 days of age (Nakai, Nimura, Tamura, Shimizu & Nishimura, 1960; Schwentker, 1963; Marston & Chang, 1965), whilst the age of sexual maturity has been given as 9 to 12 weeks (Schwentker, 1963; Marston & Chang, 1965). Recently, Norris & Adams (1972a) estimated that the mean age at first conception was 75 days under the most favourable conditions, namely 60-day-old females paired with sexually experienced males. Normal growth data from birth to 150 days of age have been presented by Norris & Adams (1972b). The present study was undertaken in order to obtain information on ovarian development in relation to puberty and sexual maturity in this species. Recently, Stark (1973) has reported on ovarian development in gerbils aged 0 to 21 days.Observations were made on

The incidence of fighting was significantly higher in male-male (70%) than in female-male (16%) or female-female (10%) pairings.The use of a deodorant spray or anaesthesia to overcome aggression was examined. Application of the spray before pairing did not reduce the level of aggression. Anaesthesia before pairing reduced the level of aggression in 83% of pairs, but not in groups of 4 animals. When successfully paired animals were separated for a week and then re-united they again behaved aggressively.Anaesthesia could have a practical application in the pairing of species that behave aggressively.

The incidence and severity of ovarian cysts were stongly age dependent, increasing from 7% in 89 female gerbils aged 135-399 days to 47% in 45 females aged 400-899 days. Cysts varied in size from less than 1 mm up to 50 mm in diameter, and accounted for from 1 to 16% of the female's live weight.Litter sizes were reduced in the presence of ovarian cysts, and reproduction ceased prematurely. Ovulation and formation of corpora lutea could still occur in cystic ovaries but the proportion of eggs developing normally was reduced from 90 to 29%, probably because of impaired gamete transport.

Since introducing the Mongolian gerbil to our laboratory in 1965, we have accumulated a variety of data concerning the reproduction of this species. The present report is based on the 4-year period, 1967-70.Details of the origins and management of our colony have been given elsewhere (Norris & Adams, 1972a). Breeding stock were kept as monogamous pairs. If no litter resulted within 3 months of pairing, a different male was introduced and, whenever necessary, this procedure was repeated after a further 3 months. If the female still proved barren, autopsy was performed. Any males or females which showed a strong tendency to behave aggressively were culled. Routine procedures, such as inspection of cages for litters and taking vaginal smears, were carried out twice daily at 09.00 to 09.30 hours and 17.00 to 17.30 hours.

The Mongolian gerbil is normally a very tame and tractable animal. In certain situations, however, it may behave aggressively as, for example, when sexually mature animals are paired. Schwentker (1963) regarded the Mongolian gerbil as monogamous, observing that when old females lost their mate they did not usually accept another. Marston & Chang (1965) reported that polygamous mating groups were less successful than monogamous pairs and that careful supervision of mating groups was essential. They also noted that it was not possible to carry out test matings owing to the female's aggressiveness towards a strange male. In accord with these observations, Barfield & Beeman (1968), who made a study of the oestrous cycle, found it necessary to introduce each female to a specific male to avoid aggression during the non-receptive periods. The