Are yolk steroid concentrations at equilibrium with maternal plasma steroid concentrations?
Both yolk and plasma are complex mixtures of many substances, some lipophillic and some lipophobic. For our purposes of starting parsimoniously, we will make the simplifying assumptions that yolk is a pool of pure lipid whereas plasma is essentially water. Concentrations of yolk steroids are generally higher than the concentrations in maternal plasma (Johnston and Moore 2005). Does this mean they are actively deposited? No, this is the expectation of simple equilibrium chemistry. Steroids are generally thought to be able to pass freely through the phospholipid bilayer of cell membranes. If this is the case and if steroids are more soluble in lipids than in water, at equilibrium there will be much more steroid dissolved in the lipid (yolk) than in the water (plasma). Therefore, we do not have to invoke anything other than basic chemistry to explain the transfer of steroids from plasma to yolk or their high concentration in yolk. However, is this a true equilibrium? For this to be a true
