The effects of an advanced uterine environment on embryonic survival in the mare.

REASONS FOR PERFORMING THE STUDY: During embryo transfer (ET) the equine embryo can tolerate a wide degree of negative asynchrony but positive asynchrony of >2 days usually results in embryonic death. There is still confusion over whether this is due to the inability of the embryo to induce luteostasis or to an inappropriate uterine environment. OBJECTIVES: To assess embryo survival and development in an advanced uterine environment. HYPOTHESIS: Embryo-uterine asynchrony, not the embryo's inability to induce luteostasis, is responsible for embryonic death in recipient mares with a >2 days chronologically advanced uterus. METHODS: Experiment 1: Thirteen Day 7 embryos were transferred to the uteri of recipient mares with luteal prolongation, occasioned by manual crushing of their own conceptus, such that donor-recipient asynchrony was between +13 and +49 days. Experiment 2: Day 7 embryos were transferred to recipient mares carrying their own conceptus at Days 18 (n = 2), 15 (n = 2), 14 (n = 4), 12 (n = 4) or 11 (n = 4) of gestation. In addition, Day 8 embryos were transferred to 4 pregnant recipient mares on Day 11 of gestation. RESULTS: No pregnancies resulted following transfer of Day 7 embryos to recipients in prolonged dioestrus with asynchronies between +13 and +49 days. However, the use of early pregnant mares as recipients resulted in 5/20 (25%) twin pregnancies, 4 of which came from the transfer of a Day 8 embryo to a Day 11 recipient. All transferred embryos showed retarded growth, with death occurring in 4/5 (80%). CONCLUSIONS AND POTENTIAL RELEVANCE: The results emphasise the importance of an appropriate uterine environment for embryo growth and the inability of equine embryos to survive transfer to a uterus >2 days advanced even when luteostasis is achieved. It is possible that in normal, non-ET equine pregnancy, embryo-uterine asynchrony may account for some cases of embryonic death.

Post natal oestrogen administration stimulates precocious endometrial gland development in the horse.

REASONS FOR PERFORMING STUDY: Fillies completely devoid of endometrial glands (uterine gland knockout; UGKO) would make ideal experimental models in which to study the role of endometrial histotroph in embryogenesis and early fetal development in the mare. HYPOTHESIS: Administration of a synthetic progestagen plus oestrogen to newborn filly foals and, thereafter, at regular intervals to age 6 months, would permanently suppress endometrial gland development. METHODS: Nine half-sister Thoroughbred filly foals were treated, in 3 groups, with: A) the weakly active progestagen, norgestomet, administered from birth to age 6 months, in subcutaneous implant form plus oestradiol valerate and norgestomet i.m. at fortnightly intervals; B) the strongly active oral progestagen, altrenogest, administered daily from birth to age 6 months plus fortnightly injections of oestradiol valerate and norgestomet; C) nothing (untreated controls). Endometrial biopsies were recovered from all fillies at ages 6 months and 2 years to assess the degree of endometrial gland morphogenesis and to determine immunohistochemically the presence or absence of oestrogen and progesterone receptors in the endometrial tissues. RESULTS: Groups B and C showed no endometrial gland development, whereas Group A fillies showed a high degree of endometrial gland development, plus strong staining for both oestrogen and progesterone receptors at age 6 months. All 9 fillies showed full normal endometrial gland morphogenesis, development and function at age 2 years. CONCLUSIONS AND RELEVANCE: While the administration of a strongly active progestagen over-rode the actions of the concomitantly administered oestrogen and suppressed endometrial gland development during the period of administration, treatment with oestradiol valerate together with a weakly active progestagen, stimulated precocious endometrial gland development. Neither steroid was able to create the desired UGKO experimental model and all fillies showed normal endometrial gland development and fertility after puberty. Hence, ovarian oestrogen, not progesterone, appears to be the basic stimulus for endometrial gland morphogenesis in the horse.

Influence of breed and oestrous cycle on endometrial gland surface density in the mare.

REASON FOR PERFORMING STUDY: The diffuse noninvasive epitheliochorial equine placenta develops an intimate and complex interdigitation with the maternal endometrium throughout gestation to maximise surface contact and, consequently, optimise nutritional and gaseous maternofetal exchanges. A significant reduction occurs in the surface density of microcotyledons on the placentae of Welsh Pony vs. Thoroughbred mares that may relate to a difference in either the number or density of endometrial glands between these breeds. OBJECTIVES: To examine this hypothesis and to determine the influence of the oestrous cycle upon the development and surface density of endometrial glands. METHODS: Endometrial biopsies were taken under videoendoscopic visual control from the base of a uterine horn from young, fertile, Welsh Pony and Thoroughbred mares at defined stages of the oestrous cycle. Computer-assisted morphometric analysis then permitted the surface density of endometrial glands within the stratum spongiosum to be assessed. CONCLUSIONS: There was a statistically significant reduction in endometrial gland surface density in the Welsh Pony vs. Thoroughbred mares during both oestrus and dioestrus. POTENTIAL RELEVANCE: A substantial upregulation of epidermal growth factor (EGF) mRNA in the epithelial cells lining the apical portions of endometrial glands has been demonstrated in pregnant mares between Days 35 and 40 after ovulation, coincident with the onset of interdigitation between the allantochorion and endometrium to form the microcotyledonary placenta. The increased surface density of endometrial glands noted in the uteri of Thoroughbred mares might account for the greater surface density of placental microcotyledons in this breed.

Endometrial gland surface density and hyperaemia of the endometrium during early pregnancy in the mare.

REASON FOR PERFORMING STUDY: The morphology and surface density of endometrial glands responsible for the nutrition of the placenta requires further study employing computer assisted morphometric analysis. OBJECTIVES: To analyse changes in the morphology and surface density of endometrial glands situated adjacent to, and remote from, the conceptus during the first 30 days of gestation. METHODS: Endometrial biopsies were recovered, under videoendoscopic control, from both nonpregnant and pregnant mares 12 days after ovulation, as well as from pregnant mares on Days 16, 20, 25 and 30 of gestation. One biopsy was taken at the base of the nongravid uterine horn and the other from beneath the conceptus at the base of the gravid horn. The samples were evaluated by computer assisted morphometric analysis to determine the surface area per unit volume (Sv) of endometrial glands in the stratum spongiosum. RESULTS: No differences in morphology or density of the glands were discernable between pregnant and nonpregnant mares on Day 12 day after ovulation. On Day 16, however, the glands beneath the conceptus appeared to be at a significantly lower density than the clearly more tortuous glands at the base of the nongravid horn. Between Days 20 and 30 of gestation the endometrium in direct contact with the conceptus was visibly more reddened and hyperaemic than elsewhere in the uterus. However, the Sv of glands beneath the conceptus had now increased again so they appeared similar to those in the nongravid horn, due mainly to a reduction in the level of stromal oedema in the former. CONCLUSIONS AND POTENTIAL RELEVANCE: The embryo-maternal interactions that are vital during early pregnancy to suppress the cyclical luteolytic cascade and support nidation of the conceptus, were characterised here with local endometrial hyperaemia, histotroph production and a decrease in endometrial gland Sv at the commencement of the stationary phase. Both the components secreted by the endometrial glands and the factors liberated by the conceptus during early pregnancy need further investigation.