Mechanics of Ovulation

Abstract

The process of ovulation has been analyzed using a simplified idealized mathematical and physical model of the mammalian follicle. The follicle was assumed to be a thinwalled elastic sphere composed of a semipermeable membrane obeying Hooke's law. The liquor folliculi was assumed to behave as an ideal colloidal solution. Under equilibrium conditions, net osmotic pressure is equal to hydrostatic pressure. The stress-strain and pressure-volume characteristics of the follicle were derived using these assumptions. As follicular volume increases, the pressure will rise rapidly at first. Further increments of volume produce smaller increases in pressure until a maximum pressure is reached at 3.4 times the initial volume. The maximum pressure that can be achieved falls rapidly as growth of the follicle results in an increase in the unstressed volume. This model permits the definition of the conditions under which ovulation may occur. A volume-loaded system will rupture when the radius exceeds a critical value. A pressure-loaded system will rupture when the applied pressure exceeds the maximum pressure; this provides a trigger mechanism which is independent of the maximal tensile strength. A colloid-loaded system provides a delicate mechanism whereby only those follicles in a narrow “critical region” or pressure and radius will be eligible for ovulation. Stigma formation has been analyzed in terms of the distribution of tension in a sphere with uneven wall thickness and the site of rupture has been predicted using the physical characteristics of the model. Several predictions based on this model have been verified experimentally. The role of gonadotropins and other regulatory mechanisms is discussed in terms of this theoretical development.