THE ORIGIN OF THE EQUINE ENDOMETRIAL CUPS

Abstract

The gonadotrophin (pmsg) present in the serum of mares between the 40th and 120th days of pregnancy originates in the uterine endometrial cups. These structures begin to develop on the 36th day of pregnancy opposite a transitory, though well-defined, circumferential thickening of the chorion called the allantochorionic girdle (Ewart, 1897; van Niekerk, 1965; Allen, 1970). The endometrial cups are composed of a discrete and densely packed mass of very large, epithelioid, decidual-like cells. They develop before the allantochorion becomes attached to the endometrium and it has been widely accepted in the past that they are exclusively maternal in origin (Clegg, Boda & Cole, 1954; Amoroso, 1955). However, recent genetic evidence, derived from the study of interspecific equine hybrids, has indicated that fetal genotype influences the level of pmsg in the maternal blood (Bielanski, Ewy & Pigoniowa, 1965; Clegg, Cole, Howard & Pigon, 1962; Allen, 1969a). This led Allen (1970) to suggest that the allantochorionic girdle may play an important r\l =o^\l e in the develop-ment of the endometrial cups. We now wish to report on two experiments which provide morphological and biochemical evidence of a new concept of endometrial cup development. Horse conceptuses were removed at hysterotomy from three mares on the 35th day of pregnancy. They were immersed in a bath of sterile phosphate-buffered physiological saline (pH 7.2) and the allantochorionic girdle and pieces of other tissues were dissected free. Pieces of allantochorionic girdle, normal allantochorion, fetal skin and maternal endometrium from two mares were then transferred to a culture medium consisting of 20% fetal calf serum in M199 (Flow Laboratories) containing 50 ^g insulin, 50 µg ascorbic acid, 50 units penicillin and 50 units streptomycin/ml. The tissues were cultivated in this medium in plastic dishes or bottles (Sterilin) at 37° C in 5% C02 in air. The medium was changed every 4 days and the concentration of pmsg meas¬ ured in each sample using the immunoassay described by Allen (1969b). At chosen intervals, cultures were fixed in Bouin's fluid and examined histologically. In cultures of all four types of tissue, attachment of tissue fragments to the Petri dish or bottle occurred rapidly, and within a few days, large outgrowths of cells were visible around each expiant. The most rapidly growing tissue was the fetal skin which reached confluence within 4 days and consisted of elongated fibroblastic cells. The allantochorion grew more slowly and gave rise to sheets 313