Anatomy, physiology and pathology of the amniotic and allantoic compartments in the sheep and cow

In both cows and sheep the allantoic fluid is confined to 2 sacs connected by a flattened isthmus, one in the non-pregnant horn and one in the upper part of the pregnant horn. The chorion encloses the amniotic and allantoic compartments, forming the amniochorion and chorioallantois, respectively. In the last third of gestation the compositions of both amniotic and allantoic fluids differ substantially from each other and from those of foetal urine, and maternal and foetal plasma. There is less variation in composition than in volume for a given gestational age. Abnormalities of volume are more common in cows than sheep, and hydrallantois is more common than hydramnois. Data obtained from both physiological experiments and pathological cases suggest that the foetal membranes play an important role in the regulation of composition and volume of foetal fluids. Evidence is presented that the permeability of the membranes to various solutes, as well as their capacity to produce and respond to a number of hormones, can affect the foetal fluid composition and/or volume. Progesterone, oestrogens and prolactin are some of the hormones known to affect foetal fluids. Foetal adrenal insufficiency has been associated with hydramnios implying a lack of hormones from this gland in this disease. The changes in allantoic fluid composition from normal to that closely resembling maternal/foetal extracellular fluid, in hydrallantois, suggests an alteration of membrane function as an aetiology and the continued production of fluid, after removal of the foetus in some cases, favours this hypothesis.