Although the accumulation of uterine fluid during pro-oestrus in the rat has been well documented (Long & Evans, 1922; Warren, 1938), its exact role in the reproductive process is still not clearly understood. Joshi, Yaron & Lindner (1970) isolated and characterized an endopeptidase from the uterine luminal fluid of pro-oestrous rats and from immature rats treated with oestrogen. This enzyme was later found to be unique to the uterine fluid and was not found in blood (Joshi & Murray, 1974). A protein immunologically similar to the rat endopeptidase has also been demonstrated in the uterine washings of oestradiol-treated mice of 3 different strains (Joshi & Rosenfeld, 1976). The role of this enzyme is unknown, but it has been suggested that this oestrogen-dependent peptidase may play a role in the lysis of the zona pellucida and the penetration of the ovum by the spermatozoon (Joshi & Murray, 1974).
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