Action of Progesterone in Determining Persistence of Placental Elements During Parturitional Delay in the Rat

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Abstract
When the uterus of a rat is ligated so as to retain one placenta beyond parturition, trophoblast giant cells in the retained placenta involute within 2 days post partum. If, however, the partially delivered female either is allowed to suckle her litter or subsequently becomes pregnant, giant elements of trophoblast persist for as long as 6 days post partum. That such persistence is an action of progesterone has been demonstrated by injecting exogenous progesterone during the last several days of pregnancy, a procedure which both delays parturition and prolongs giant-cell persistence beyond the equivalent of term. Evidence is presented to suggest that: 1) during the terminal stages of a typical pregnancy, the number of giant cells normally decreases with concomitant attenuation and interruption of the giant-cell layer, and 2) the action of progesterone on the junctional zones of such placentae is to prolong the persistence of giant cells beyond a period of time equivalent to term. The possible significance of fixed giant-cell persistence with regard to the mechanisms of parturition is discussed.