Collagen and Microvascularization in Placentas From Young and Older Mares.
Authors: Neto da Silva Ana Catarina, Costa Ana Luísa, Teixeira Ana, Alpoim-Moreira Joana, Fernandes Carina, Fradinho Maria João, Rebordão Maria Rosa, Silva Elisabete, Ferreira da Silva José, Bliebernicht Miguel, Alexandre-Pires Graça, Ferreira-Dias Graça
Journal: Frontiers in veterinary science
Summary
# Collagen and Microvascularization in Placentas From Young and Older Mares Reproductive ageing in mares presents a paradox: whilst endometrial and oviductal fibrosis is associated with subfertility, the placental tissue itself may adapt differently to support successful pregnancies in older animals. This investigation examined collagen gene expression (COL1A1, COL1A2, COL3A1, COL5A1), total collagen protein content, collagen architecture, and microvessel density across placental samples from seven young and nine older multiparous mares of various breeds, with tissues analysed from the gravid horn, non-gravid horn, and placental body. Older mares demonstrated significantly elevated transcription of COL1A1, COL3A1 and COL5A1, higher total collagen protein, thicker chorionic plate connective tissue, and increased microvascularization in the gravid horn compared to their younger counterparts—changes typically associated with placental insufficiency and poor fetal outcomes in other species. However, older mares who remained fertile delivered heavier foals from heavier, more vascularized placentas, suggesting these structural and vascular changes represent productive physiological adaptations rather than pathological compromise. For practitioners, this work indicates that placental fibrosis observed in older mares may reflect compensatory mechanisms supporting adequate fetal nutrition across extended gestations, rather than inherent placental dysfunction, with implications for how we interpret imaging and tissue findings when assessing geriatric mare reproduction.
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Practical Takeaways
- •Increased collagen in older mares' placentas is not necessarily pathological—successful older mares appear to develop enhanced placental function with increased vascularity that supports heavier foals
- •When evaluating subfertility in older mares, placental fibrosis alone should not be assumed to indicate insufficiency without assessing overall vascularization and fetal outcomes
- •Older multiparous mares that have successfully carried pregnancies may have developed adaptive placental changes that actually improve fetal nutrition delivery
Key Findings
- •Older mares (n=9) showed significantly increased collagen gene transcription (COL1A1, COL3A1, COL5A1) and total collagen protein in gravid horn placenta compared to younger mares (n=7; P<0.05)
- •Microvascular density and chorionic plate connective tissue thickness increased in older mares' placentas despite increased fibrosis
- •Older fertile mares delivered heavier foals and showed no evidence of placental insufficiency despite increased collagen deposition
- •Placental adaptations in older mares appear compensatory rather than pathological, resulting in heavier, more vascularized functional placentas over longer gestations