The fibre dimensions of uterine smooth muscle of the rabbit following treatment by female sex steroids

The effects of female sex hormones on the dimensions of myometrial smooth m uscle fibres were studied by using ovariectomized rabbits, After one month of treatment, the fiber dimensions of the outer myometrial layer were measu red, using cryostat sections, Calculated smooth muscle fiber volume was fou nd to be in the sequence: control < medroxyprogesterone < estradiol < estra diol + medroxyprogesterone < estradiol alone. The measurements show that me droxyprogesterone-treated uteri contain the narrowest and the longest smoot h muscle fibres, while estradiol treatment have the largest cells. This stu dy complements previous observations in showing that medroxyprogesterone al one, or in combination with other modulators, contributes to sustain pregna ncy by increasing internal resistance of estradiol-primed myometrial smooth muscle fiber fascicles. Our discussion, based on recent literature, shows that this resistance is u ltimately controlled by changes in the myometrium innervation, in the repre ssion of some controlling myofibrillar components, in the expression of spe cific membrane receptors and ionic channels, and in favoring the: switching of molecular connexins in gap junctions, making P paramount in maintaining pregnancy. Moreover, other recent observations have also shown that probab ly an hcG-like hormone actually control P receptors expression in myometria l smooth muscles. (C) 2000 Harcourt Publishers Ltd.