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Ovarian Stimulation in Immature Farm Animals¹

University of Wisconsin

Abstract

Various workers have pointed out that in the different species of laboratory animals, the immature female ovary does not react appreciably to gonad-stimulating hormones before the appearance of vesicular follicles. Investigation of farm animals has shown that vesicular follicles may be present at birth in the lamb and the foal and as a seven-month foetus in the calf. The pig, however, showed no vesicular follicles until an age of 10–14 weeks. Ovaries without vesicular follicles were found in only three of 190 veal calves (1–12 weeks of age) of slaughter house origin. Attempts to produce follicular stimulation by injection of gonad-stimulating hormones failed in a five-week old pig, and also in a veal calf in the ovaries of which no vesicular follicles were present at the exploratory laparotomy performed just before the hormone treatment.

Some attempt has been made to study the type of ovarian stimulation produced by various gonad-stimulating extracts in different species of farm animals at an early age when vesicular follicles were present. The extracts have been prepared by the pyridine method of Fevold and their physiological characterization is based on the reaction produced in the immature rat ovary.

Unfractionated sheep pituitary extract, active in the rat for both follicular development and luteinization, produced marked follicular development in the calf ovary when injected subcutaneously over a six day period. The dosage used was comparatively large, judged by the quantitative response obtained. When this treatment was continued over an eleven day period ovulation and corpus luteum formation occurred. Intravenous injection of this same extract over a six day period produced follicular stimulation but the response was much less than from subcutaneous administration of a similar dose.

Whole pyridine extract of desiccated pregnant mare blood serum has, in general, produced less luteinizing effect in test rats than unfractionated pituitary extract. When injected subcutaneously for six days into the pig and calf, however, ovulation and corpus luteum formation have been brought about, in addition to the follicular maturation. This, it may be recalled, is unlike the response from the similarly administered unfractionated pituitary extract in the calf. The success of workers at the California Experiment Station in producing ovulation in the anoestrus ewe with pregnant mare serum should be noted in this connection.

A pituitary extract, prepared as a partially purified follicular-stimulator was used in an attempt to simulate the results from pregnant mare serum. This preparation was one from which much of the luteinizing substance had been removed but which still produced some luteinization in the rat ovary. When small doses were injected subcutaneously into 3–5 month old lambs, follicular stimulation, ovulation, and formation of corpora lutea resulted. When administered in large doses, however, only marked follicular maturation was produced. Injection of the preparation one or two days subcutaneously, followed by one to three days intravenously, produced ovulation in the anoestrus ewe, whereas similar administration of the same extract produced only follicular maturation in the calf.

The most uniform success in producing ovulation in the calf and pig has resulted from the subcutaneous injection of a follicular-stimulating preparation for two to six days to mature the follicles and then following with intravenous injection of a luteinizing extract for one to two days to produce ovulation and luteinization. As with previously mentioned treatments, in many of the cases where ovulation has been obtained, only a few of the follicles which had been stimulated were ruptured (one to two in lamb and ewe; one to five in calf; three to nine in pig). In general, ovulation has occurred only in follicles which were no larger than mature follicles in the adult females of the species, rather than in the larger and more cystic follicles.

It is obvious at the present time that any attempt to analyze the characteristic species differences in their response to gonad-stimulating hormones must take into account not only the nature of the extract being used but also the degree of follicular development in the ovary at the time of treatment. The duration and intensity of the treatment (dosage) and the constancy of availability of the hormone throughout are also factors to be considered. The constancy of hormone availability may be determined by the purity of the extract and the frequency and mode of injection. The rate and type of change being induced in the ovary and genital tract by experimental treatment should at all times be compared with that in the different variations of the oestrus cycle for the particular species so that eventually abnormalities of the oestrus cycle may be explained and some measure of control over the abnormalities may be indicated.

¹ Paper from the Department of Genetics, Agricultural Experiment Station, University of Wisconsin. No. 176. Published with the approval of the Director of the Station.