Localisation of 15‐hydroxy prostaglandin dehydrogenase (PGDH) and steroidogenic enzymes in the equine placenta

15‐hydroxy prostaglandin dehydrogenase (PGDH) is the critical enzyme that determines metabolism of primary prostaglandins. Its expression is determined in part by steroid hormones, particularly progesterone, formed from Δ₅ steroids through 3β‐hydroxysteroid dehydrogenase (3β‐HSD) activity. To assess whether the regulation of PGDH might occur in a paracrine, autocrine or intracrine fashion, we used immunohistochemistry (IHC) to determine the localisation of key steroidogenic enzymes in the equine placenta and compared these patterns to the distribution of immunoreactive (IR‐) PGDH. Placental tissue was obtained from pony or Thoroughbred mares at about Days 150, 250–280 and >300 of pregnancy (term 320–360 days; n=5–8 each group). IR‐PGDH, 3β‐HSD, cholesterol side chain cleavage enzyme (P450_scc) and 17‐hydroxylase/lyase (P450_c17) were localised using specific antibodies and the avidin‐biotin peroxidase technique and visualised using diaminobenzidine as substrate. IR‐P450_scc was present in trophoblast cells, but not in maternal tissues of the microcotyledons or elsewhere in the endometrium. Specific staining for P450_C17 was not detected in either maternal or fetal tissues at any stage of gestation studied. IR‐3β‐HSD was present in trophoblast cells, but not in maternal tissues of the microcotyledons. In contrast, at Days 150 and 280, IR‐PGDH was present in maternal epithelial and interstitial cells in the microcotyledons, but was not detected in trophoblast epithelium, chorioallantois or endometrial glands. After Day 300, IR‐PGDH was present in the maternal epithelium and interstitial cells of the placenta and it was also present in trophoblast cells in some specimens. We conclude that the equine placenta has the enzymes necessary to produce C₂₁Δ₄ steroids from C₂₇Δ₅ substrates in trophoblast cells by Day 150 of gestation. The localisation of these enzymes is compatible with paracrine regulation of PGDH by products of 3β‐HSD activity for most of pregnancy, although autocrine or intracrine regulation of placental PGDH may also be possible nearer to term.