Placentation between 80 and 220 days of gestation was studied by measurement of fetal dimensions and gross, light and transmission and scanning electron microscopic examinations of the allantochorion and endometrium from 4 fertile mares with no, or very mild, endometrial histopathology and 6 subfertile Thoroughbred mares suffering varying degrees of age-related chronic degenerative endometritis (endometrosis). Spontaneously occurring twin gestation was observed in 3 animals. Several of the subfertile mares had endometrial cysts which showed 2 distinct features during pregnancy: those located near the lumenal surface of the endometrium showed no sign of microcotyledon development in both the endometrium and the opposed, flat and smooth allantochorion, whereas endometrial cysts located deeper in the stroma were covered by apparently normal glands and microcotyledons. Fetal dimensions were lower in the subfertile mares, but still within the reference range. Microcotyledon development appeared to be delayed and the chorionic macro- and microvilli looked shorter and blunter between 80 and 120 days in the 2 subfertile mares compared to the 2 fertile mares at the same gestational ages. At 120 days, the subfertile mare had significantly less microvilli per surface area than her fertile counterpart and her fetus weighed only half that from her counterpart, despite a similar crown-rump length. However, between 160 and 220 days the difference in microcotyledon development between the young fertile and old subfertile mares was much less marked, despite the continued existence of considerable numbers of distended 'gland nests' in the endometrium of the latter group.

Summary Videoendoscopy of the reproductive tract was performed in 87 Thoroughbred mares with histories of reduced fertility. During hysteroscopy samples for cytological, microbiological and histological examinations were obtained under visual control. Common findings in these broodmares included: (a) endometrial degeneration, as assessed by an uneven distribution or atrophy of endometrial folds and/or a scarred appearance of the endometrium (49 mares, 56%); (b) endometrial cysts of various sizes and locations within the uterus with the most common location being at the base of the uterine horns (48 mares, 55%); (c) fluid accumulation in the uterine lumen (28 mares, 32%). A few mares had transluminal adhesions (7 mares, 8%) and in 2 mares the adhesions appeared to obstruct one uterine horn completely. A solitary discrete lump was detected in the wall of the uterine body in one mare and the suspicion of it being a leiomyoma was confirmed histologically with the aid of a visually directed biopsy sample. Free intraluminal structures were present in the uterine lumen in 3 mares, including one inspissated blood clot and two suspected remnants of resorbing pregnancies. Flexible biopsy forceps and scissors passed through the working channel of the endoscope were used to sever small thin adhesions, but this method proved inadequate for multiple adhesions or cysts. Solitary endometrial cysts were removed by means of conventional rigid biopsy forceps passed alongside the endoscope, although bleeding from the operation site usually limited this type of intervention.

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