Abstract
In older mares, increasing collagen fibers (fibrosis) in the endometrium and oviduct predisposes to sub-fertility and infertility. In this study, (i) gene transcription of collagen (qPCR: COL1A1, COL1A2, COL3A1, COL5A1); (ii) total collagen protein (hydroxyproline); (iii) collagen distribution (Picrosirius red staining; polarized light microscopy); and (iv) microvascular density (Periodic acid-Schiff staining), were evaluated in mares' placenta, and related to mares age, and placenta and neonate weights. Samples were collected from the gravid horn, non-gravid horn, and body of the placenta from younger (n = 7), and older mares (n = 9) of different breeds. Transcripts of COL1A1, COL3A1 and COL5A1, total collagen protein, chorionic plate connective tissue thickness, and microvascularization increased in the gravid horn of older mares' placentas, compared to the youngest (P < 0.05). Although in other species placenta fibrosis may indicate placental insufficiency and reduced neonate weight, this was not observed here. It appears that older fertile mares, with more parities, may develop a heavier, more vascularized functional placenta with more collagen, throughout a longer gestation, which enables the delivery of heavier foals. Thus, these features might represent morphological and physiological adaptations of older fertile mares' placentas to provide the appropriate nutrition to the equine fetus.
Highlights
The placenta is an overly complex organ of vital importance in equine pregnancy
Heavier foals were born from older mares (p < 0.0001), that had more parities (p < 0.0001), and longer gestations (p < 0.01)
There was a positive correlation between the weight of the placenta and the weight of the foal (r = 0.728; p = 0.001; Figure 2A); between the age of the mare and the weight of the placenta (r = 0.760; p = 0.0006; Figure 2B); and the age of the mare and the weight of the foal (r = 0.659; p = 0.005; Figure 2C)
Summary
The placenta is an overly complex organ of vital importance in equine pregnancy. Mare’s placenta as epitheliocorial type, enables the direct contact of the endometrial epithelium with the chorionic surface, through six layers of tissue separating the maternal circulation from the fetal circulation, throughout the entire pregnancy [1,2,3]. This organ allows the fetus to be nourished through metabolic exchange of nutrients, oxygen supply and elimination of debris, providing protection against internal and external aggressions [4]. Regardless of the animal species, the macroscopic examination of the placenta does not reveal microscopic lesions that are only detected by histological examination or electron microscopy observation, and that might be related to fetal death, neonate low body weight or sickness [11,12,13,14]
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