Steroidogenic cells of the adrenal cortex are thought to originate from a peripherally-located self-renewing population of undifferentiated stem or progenitor cells. These cells divide infrequently to give daughters, which proliferate transiently (termed transiently amplifying (TA) cells), migrate and differentiate to replenish the functionally-differentiated zones of the adrenal cortex, or remain in situ as undifferentiated stem cells. Theoretically such stem cells can be identified as taking up and retaining a bromodeoxyuridine (BrdU) label during infrequent cell divisions, but which, unlike terminally differentiating steroidogenic cells, do not migrate. After one weeks BrdU infusion into female mice, immunostaining of adrenocortical tissue sections showed that the majority of BrdU-labelled cells were located in the zona glomerulosa (ZG) and outer zona fasciculata (ZF). Six weeks following infusion, BrdU-labelled cells were distributed throughout the cortex, though a proportion remained in the ZG/outer ZF. Co-staining for aldosterone synthase (ZG-specific) and 21-hydroxylase (expressed by all steroidogenic cells) showed that very few of the BrdU-labelled cells in the ZG/outer ZF displayed a steroidogenic phenotype, indicating that most were relatively undifferentiated. However, most BrdU-labelled cells in the inner ZF co-stained for 21-hydroxylase suggesting that they were at least partially differentiated. The propensity of adrenocortical BrdU-labelled cells to proliferate further was tested by injecting mice with ACTH. Four hours after treatment, BrdU-labelled cell numbers were increased significantly in the ZG and in the outer and inner ZF; however additional staining for Ki67 (a marker of actively proliferating cells) indicated that ZF cells had responded more actively to ACTH.
(1) BrdU-labelled/non-steroidogenic/ACTH-responsive cells in the ZG/outer ZF may represent stem/progenitor cells.
(2) BrdU-labelled/steroidogenic/ACTH-responsive cells in the inner ZF may represent partially-differentiated late TA cells.
(3) Ki67-labelled/ACTH-responsive cells in the outer ZF may represent early proliferative TA cells, but require further characterisation to determine their steroidogenic phenotype.