Observations on the adrenocortical structure of the female brush-tailed possum (Trichosurus vulpecula) and locations of cell division.

Twenty mature female possums (Trichosurus vulpecula) were captured at the Harry Waring Marsupial Reserve, some 30 km south of Perth in Western Australia, and injected simultaneously with tritiated thymidine in order to label dividing cells within the adrenal gland. The conservation status of the possum precluded killing all animals and adrenal glands were removed and examined histologically and autoradiographically in four experimental groups, timed for 1 hr, 1 week, and 2, 3, and 7 weeks following the injection. In two of the groups [untreated and FSH-1 (injected with an urogonadotrophin)] the left gland was obtained by unilateral adrenalectomy, whereas in the other two groups (ACTH and FSH-2, of pituitary origin) the animals were killed at the specified intervals. The precise location of labeled adrenocytes with time was measured in autoradiogrammes of adrenal sections. The labeled cells were counted and the distance from the outer connective-tissue capsule was measured with a MOP-Videoplan, calibrated for distance. The widths of the various cortical zones and the medulla were also estimated. The study focused primarily on the "Special Zone" (SZ) which is unique to the adrenals of the mature female possum. The only effect of hormonal treatments discerned was a slight hypertrophic effect of porcine FSH on the size of the SZ. Labeled cells indicative of cell division were found randomly distributed throughout the SZ, as well as in the cortex and occasionally, the medulla. Time series analysis gave no evidence of the progressive centripetal movement of cells within the SZ and our data are thus not consistent with the "Cell Migration Theory" concerning the differentiation of adrenocytes and accord instead with the "Zonal Theory" proposed to account for the maintenance of the classical zones in the eutherian adrenal cortex.