The Experimental Modification of the Oestrous Cycle in the Ferret by Different Intensities of Light Irradiation and other Methods

The general arrangement of the cages and the source of light were the same as in the original experiments (Marshall&Bowden, 1934). As before, the source of illumination was a large Sollux filament lamp of 1000 W., and the backs of the cages were formed by sloping glass mirrors. The illuminated cages were placed at distances of from 1 to 22 ft. away from the lamp. The light was automatically turned on about sunset and turned off about 8 hr. afterwards, the precise times varying according to the season of the year, that is to say, the light was turned on at times varying from about 4 p.m. to 9 p.m. and turned off at times varying from about 12.30 a.m. to 5 a.m. The condition of oestrus was in all cases clearly demonstrated by the increase in the size of the vulva which at its maximum is enormous.

In the first two years (1935-6 and 1936-7) the experiments were almost completely a failure, owing to mortality among the ferrets from distemper and other causes which resulted among other things in delay in starting. The results, such as they were, seemed to indicate that there was little difference in the times of onset of oestrus in ferrets placed at short distances (1-6 ft.) from the light. The results were, however, partly vitiated by the fact that artificial irradiation having begun late (not until December) the time when the ferrets came on heat (in each case in January) were nearer the normal sexual season. The ferrets placed at longer distances from the light unfortunately died in the course of the experiment and without coming on heat.

The results of the experiments for the two following years are given in Tables I and II. Experimental irradiation was begun on 9 October in 1937 and on 10 October in 1938. It was discontinued on 14 March in both 1938 and 1939. The tables show that the acceleration in the times of oestrus was very roughly in relation to the intensity of the irradiation, as shown by the distance of the ferrets from the lamp, but that there was considerable individual variation which may have been due to differences in the ages of the animals or other unknown individual factors.

In 1935 two ferrets were submitted to unfiltered ultra-violet irradiation at close quarters (about 1 ft. distance from the bulb). The bulb employed was an “Hanovia “Utility Model Analytic Lamp fitted with a “tilt type” mercury vapour quartz burner, supplied for operation on 200 V. alternating current mains. The radiation began on 2 December 1935 and ended on 16 March 1936. In each case the ferrets started coming on heat about 26 February and remained fully oestrous until 12 August 1936, when they began to go off heat. The period of full oestrus was 22 weeks in each case.

The ultra-violet irradiation experiments for 1936-7 concerned four female and two male ferrets.’ The irradiation started on 5 December and ended on 14 March. The bulb used was as before, the ferrets being placed horizontally to it, except where otherwise stated, and 1 ft. away.

Of the four female ferrets two came on heat on 13 and 20 January, but they died from unknown causes in March and early April.

Of the other two, one (no. 36) was put with a male, which received ultra-violet treatment, on 21 July but was not seen to copulate though it may have done so. It was still fully on heat on 28 July. It was then put with a male that had been submitted to light irradiation treatment and was believed to have copulated but the completion of the act was not seen. It went off heat on 14 August and was thought to have probably become pregnant; this, however, was a mistake as no young were born. It did not come on heat again. This ferret was 18 in. from the lamp which was placed above it and in this instance the rays were filtered by a copper sulphate glass plate.

The other female (no. 44) was put with another male which had received full ultra-violet irradiation on 21 July. It was taken out on 28 July after having probably copulated though copulation was not observed. It was going off heat on 28 July and was completely off on 11 August. Nevertheless, it did not become pregnant as no young were born. It remained off heat until 1938 when it came on at the normal time on 9 March.

The two male ferrets were used to test the effect of ultra-violet irradiation on the descent of the testes which usually occurs in the spring shortly before breeding. In each case the testes were observed to have descended as early as 16 December. The testes remained descended until 29 September when they once more ascended in the inguinal canals.

In 1937-8 six females were submitted to ultra-violet irradiation as before except that the bulb was renovated. This may have resulted in the treatment being too forceful as all the animals became very thin and four died in February, after first coming on heat. The irradiation began on 9 October and ceased for the remaining ferrets on 2 March. One of these came on heat on 23 February in spite of being in very poor condition. It was exceptional in going off heat on 30 March. It came on heat again, however, on 29 June and remained so until 14 September. The remaining ferret came on heat on 25 January and remained in that condition continuously until 5 October, a period of 34 weeks. This ferret, in view of its very poor condition, was taken away from the lamp for 1 week (17-24 November). It was given three drops of “radiostoleum “(a preparation containing vitamins A and D) every other day from 8 December to 9 February, and this treatment improved it. The two ferrets which survived to the autumn were placed horizontally at 12 in. from the lamp.

In 1938-9 the experiments were again partly vitiated by the ferrets getting into an unusually poor condition. The irradiation was started on 10 October and ceased on 14 March. It is possible that the irradiation was again too intense. The results are shown in Table III.

It will be seen that the tendency for ferrets treated with ultra-violet rays to remain on heat for an exceptionally long time was maintained, but that certain individuals in the last two years being in bad condition did not behave like the others submitted to this treatment.

In view of the action of ultra-violet irradiation in causing a long-continued condition of oestrus, the suggestion presented itself that the effects might be due to the production of vitamin D. Such a conclusion was, however, unlikely in view of the general evidence that the incidence of oestrus in animals is regulated by exteroceptive stimuli acting through the nervous system and pituitary. Nevertheless it was thought that the possible action of vitamin D as a factor ought not to be ruled out in the absence of experimentation. Accordingly, four ferrets were treated with the commercial preparation “radiostoleum “(British Drug Houses, Ltd.), which contains a standardized quantity of crystalline vitamin D (3000 1.U. per g.), as well as a standardized solution of vitamin A (15,000 1.U. per g.). Four drops were added daily to the usual bread and milk diet which was daily eaten. The experiments were begun on 27 October 1937, and continued until 15 February 1938. None of the ferrets came on heat during this period. They actually came into oestrus about 16, 23 and 30 March, and 13 April, which dates are approximately at the time when the normal sexual season commences. The feeding of radiostoleum, however, caused all the ferrets to become abnormally fat, far more so in fact than with any other ferrets that we have seen. This result was not anticipated, so we did not take records of the weights, but nevertheless there can be no doubt that it was significant. After ceasing to feed radiostoleum the ferrets gradually became thinner and returned to their normal appearance.

Since the experiments were carried out primarily to determine the influence of irradiation upon the incidence of oestrus and its duration the ferrets were usually kept alive until they had passed back into a condition of sexual rest. Consequently material was not usually available for the study of the reproductive organs of the treated animals. In some cases, however, the ferrets were killed during the oestrous periods and their organs were preserved and subsequently investigated. It had been noted that with those ferrets which were put with males (nos. 36 and 44) and were thought to have copulated, the effect of being with the male was to terminate the oestrous condition in a short time. None of these animals, however, became pregnant or palpably pseudo-pregnant, so the possibility presented itself that the Graafian follicles had become cystic as a result of the prolonged oestrus and consequently had not discharged their ova. The condition of the ovaries described below lends no support to this view.

Ferret no. 67 mentioned above in the “intensity series” (1938-9) was at a distance of 1 ft. from the lamp and was continuously on heat for 18 weeks before being killed on 16 March. The ovaries were entirely normal. Large protruding follicles could be seen on the surface with the naked eye. Sections showed that some of the follicles were mature and normal and there were other smaller ones at all stages. No recent corpora lutea were observed and no atrophic follicles. There was no evidence that follicles had become ripe in batches and subsequently degenerated. There was a great abundance of typical interstitial cells but no cells that could certainly be said to be old luteal cells.

The uterus as seen in section was in a general way similar to that of the normal ferret which had been for a long time oh heat described in an earlier paper (Hammond&Marshall, 1930). There was some congestion and the glands showed evidence of prolonged activity. The epithelium lining the cavity was normal and the cells did not present the appearance of a plasmodial condition such as is shown by ferrets which become pseudo-pregnant.

Ferret no. 78 in the same series was similar but it had been at a distance of 22 ft. from the lamp (the long distance extreme). It was on heat for 11 weeks before being killed on 16 March. There were numerous large follicles and many smaller ones. No corpora lutea were observed but there was one apparently collapsed follicle which was degenerate; it contained a persistent ovum. Interstitial tissue was abundant as in no. 67.

The uterus as seen in section was very similar to that of no. 67 ; there was some congestion and evidence of glandular activity was also seen.

Ferret no. 57 was in the ultra-violet series (1938-9). It had been on heat continuously for 38 weeks before being killed on 16 August. The ovaries contained very large ripe follicles but no cystic ones and no corpora lutea. There were, however, three old atrophic follicles. These had their cavities filled up in the usual way with a rather loose ingrowth of tissue from the walls but no remains of ova were seen. It is possible that these represented follicles that had ripened and then degenerated, being replaced by other follicles which in time enlarged in the manner described for the rabbit (Smelser et al. 1934). Interstitial cells of the usual type were fairly numerous in the ovarian stroma.

The uterus showed large active glands. The surface ones were not pitted and there was nothing to suggest a condition of pseudo-pregnancy. The epithelium was continuous and not plasmodial.

Ferret no. 58 was also in the ultra-violet series. As recorded above, it was exceptional in not remaining on heat the first time for more than 5 weeks. This behaviour suggested that it might possibly have ovulated or that some of the tissue had become lutealized. It came on heat again in January when it remained oestrous until killed on 16 August. The ovaries showed a lot of old luteal tissue. Some of this was definitely surrounded by walls so as to present the appearance of old corpora lutea. The luteal cells were similar to those found in ferrets’ corpora lutea at the end of pregnancy or just after parturition. (Compare the ferret a fort-night after pseudo-pregnancy in Hammond&Marshall’s series (1930).) The ferret had never been with a male. It is possible, therefore, that the luteal tissue had been formed as a postponed result from the former treatment with ultra-violet light acting on the anterior pituitary, the normal cyclic changes which depend upon that organ having been deranged.

The uterus of no. 58 as seen in section showed a somewhat abnormal appearance. There were numerous small glands as well as some very large glands. The appearances were such as to suggest a normal ferret coming on heat again a little while after pregnancy or pseudo-pregnancy. (Cf. no. 15 in Hammond&Marshall’s series (1930), a ferret 15 days after parturition with young removed, but in no. 58 in the present series some of the glands were still larger.) This ferret, however, had been on heat continuously from mid-January to mid-August. It was possible that the ultra-violet irradiation had caused the pituitary to secrete some amount of the lutealizing hormone along with the follicle-stimulating hormone, but that the oestrogenic effect was dominant the greater part of the time. The fact that this ferret went off heat for a time in December may perhaps be similarly explained. The uterine epithelial cells were continuous and not plasmodial.

The effect of artificial irradiation in accelerating the oestrous cycle in various species of animals, both vertebrate and invertebrate, has been well summarized in articles by Bissonette (1936a, b) and Rowan (1938) and need not be further described here. The evidence relating to the nervous paths of transmission of impulses to the brain and thence to the anterior pituitary is given in a paper by Le Gros Clark et al. (1938) which adds considerably to our knowledge. The experiments recorded above suggest that exteroceptive stimulation by ultra-violet irradiation may result not only in the release of the follicle-stimulating hormone but also in that of the lutealizing hormone from the pituitary, the former, however, always being dominant. As is well known, ferrets only ovulate after coition (Marshall, 1904) and the existence of well-defined luteal tissue in the ovary of a ferret which had never been with a male may probably have been due to the release of the lutealizing hormone consequent upon ultra-violet irradiation. It is to be noted that a similar result may follow in some animals after the injection of anterior pituitary extracts as described by Hill & Parkes (1930).

With female ferrets subjected to different degrees of intensity of light irradiation as measured by placing them at different distances from a 1000 W. lamp, the acceleration of the oestrous cycle, generally speaking, was correlated with the degree of intensity. There were, however, individual exceptions. The distances between the ferrets and the lamp varied from 1 to 22 ft.

Female ferrets subjected to ultra-violet irradiation usually remained on heat until much later in the year than those submitted to light irradiation. With male ferrets ultra-violet irradiation caused descent of the testes in December and the testes remained descended until the autumn.

Irradiated female ferrets (both with light and ultra-violet rays) went off heat shortly after being put with males and probably as a result of copulation (which, however, was not always observed), but pregnancy did not supervene. This may have been due to the animals not being in a state for ovulation.

Feeding vitamin D to anoestrous ferrets did not result in accelerating the cycle, the ferrets not coming on heat until the normal time. The treatment, however, resulted in their becoming abnormally fat. Vitamin D was given in a commercial preparation which also contained vitamin A.

Sections through the ovaries of irradiated oestrous ferrets showed large ripe follicles and other follicles in varying degrees of development. Some apparently unruptured degenerate follicles were also seen. There were no cystic follicles in any of the ferrets’ ovaries observed. Typical interstitial cells were present usually in great abundance.

In a ferret that had been submitted to ultra-violet irradiation the ovaries contained a quantity of old luteal cells, the presence of which suggested that some of the follicles had become lutealized without rupturing.

The uterus in three of the irradiated ferrets was somewhat congested and showed signs of glandular activity, being, generally speaking, similar to that of normal ferrets which have been on heat a long time. The uterus of a female ferret, in which the ovaries contained old luteal cells, presented the appearance of one belonging to an animal that was coming on heat again after pregnancy or pseudo-pregnancy.

I desire to express my indebtedness to Dr F. P. Bowden for his valuable assistance at the beginning of these experiments and my regret that his subsequent absence from Cambridge rendered his collaboration no longer possible. I wish also to thank Mr J. Pike for his help in superintending the experiments. Furthermore, I desire to express my obligations to the firm of Hanovia Ltd. for the information which they willingly supplied.