Physiological traits as affected by heat stress in sheep—A review

Invited review: Heat stress effects during late gestation on dry cows and their calves

Effect of late-gestation maternal heat stress on growth and immune function of dairy calves

The placenta is the primary interface between the fetus and mother and plays an important role in maintaining fetal development and growth by facilitating the transfer of substrates and participating in modulating the maternal immune response to prevent immunological rejection of the conceptus. The major substrates required for fetal growth include oxygen, glucose, amino acids and fatty acids, and their transport processes depend on morphological characteristics of the placenta, such as placental size, morphology, blood flow and vascularity. Other factors including insulin-like growth factors, apoptosis, autophagy and glucocorticoid exposure also affect placental growth and substrate transport capacity. Intrauterine growth restriction (IUGR) is often a consequence of insufficiency, and is associated with a high incidence of perinatal morbidity and mortality, as well as increased risk of cardiovascular and metabolic diseases in later life. Several different experimental methods have been used to induce placental insufficiency and IUGR in animal models and a range of factors that regulate placental growth and substrate transport capacity have been demonstrated. While no model system completely recapitulates human IUGR, these animal models allow us to carefully dissect cellular and molecular mechanisms to improve our understanding and facilitate development of therapeutic interventions.

/pdf/placental-adaptations-in-growth-restriction-4qdzabzpt5.pdf

Placental adaptations in growth restriction.

To establish physiological mechanisms for fetal growth restriction in pregnant adolescent ewes we studied uterine, fetal, and uteroplacental metabolism in ewes offered a high (n = 12) or moderate (...

https://www.physiology.org/doi/pdf/10.1152/ajpregu.00465.2001

Blood flows and nutrient uptakes in growth-restricted pregnancies induced by overnourishing adolescent sheep

Aspects of placental protein and energy metabolism were examined in pregnant ewes subjected to either thermoneutral (TN, 18 to 20 degrees C, 30% humidity, n = 7) or hot (H, 30 to 40 degrees C, 40% humidity, n = 5) temperatures through mid and late gestation. Fetal and placental weights and total content of protein, RNA, and DNA were reduced (P less than .001) in H ewes. Placental protein and RNA concentrations (mg/g) were not different, and DNA concentrations were slightly greater (P less than .1), in H vs TN ewes. Thus, heat seemed to greatly reduce total cell number and placentome size and only slightly decrease cell size. Ratios of RNA to DNA indicated a reduced capacity for protein synthesis in H placenta. However, in vitro fractional rates of protein synthesis in tissue slices from the fetal and maternal placenta and from the myoendometrium were not different between TN and H ewes. The H ewes had greater placental protein concentrations of hydroxyproline and glycine, perhaps suggesting a greater collagen content. In vitro oxygen consumption of fetal placenta, but not of maternal placenta or myoendometrium, was lower in H than in TN ewes. This lower oxygen consumption was partially due to a lower Na+,K+ ATPase-dependent oxygen consumption.

https://science.widener.edu/~vatnick/JAS_1991.pdf

R. J. Early

Papers

Chronic heat stress and prenatal development in sheep: II. Placental cellularity and metabolism