Placental Steroid Production by the Basal and Labyrinth Zones During the Latter Third of Gestation in the Rat1

Progesterone (P) and testosterone (T) production were investigated in minced rat placental preparations on Days 16, 18, 20, and 22 of pregnancy. Whole placentae were minced or dissected into basal and labyrinth zones and minced prior to a 2-h incubation. Production of P from whole placental minces (pg X mg tissue-1 X 2 h-1) was greatest on Day 16 (P less than 0.05), dropped slightly on Day 18, and fell by nearly 50% on Days 20 and 22. Basal zone P production was greater on Days 16 and 18 (P less than 0.01) than that of the labyrinth zone, but then decreased to become equivalent to that of the labyrinth zone thereafter. Production of T from whole placental minces was greatest on Day 18 (P less than 0.05) and fell to low levels thereafter. The basal zone was almost entirely responsible for T production on all days. Incubation with the 17 alpha-hydroxylase inhibitor Su 10603 [7-chloro-3,4-dihydro-2-(3-pyridyl)-1-(2H)-napthalenone] completely blocked T production concomitant with increased P values. Placental weight increased from Days 16 to 20 because of rapid labyrinth zone growth. Estimated daily P production per placenta was not different among days, although production shifted from the basal to the labyrinth zone as term approached. Production of T per whole placenta reached maximum levels on Day 18 (P less than 0.05) and decreased thereafter. The marked (P less than 0.01) basal zone T production per placenta on Days 16 and 18 fell precipitously on Days 20 and 22.