The Endocrine System

Abstract The endocrine organs of the rat are largely functional at birth but are morphologically immature. They continue to develop as the hypothalamic-pituitary axes between the pituitary gland and target endocrine glands mature in the postnatal period. In the pituitary gland, histologic changes primarily occur in the pars distalis through PND 35. The thyroid and parathyroid glands reach histologic maturity by PND 21 and PND 28, respectively. At birth, in the adrenal cortex, only the zona glomerulosa and fasciculata, but not the zona reticularis, are identifiable. The zona reticularis becomes identifiable by PND 10 to 14. At birth, the adrenal medulla is functional and consists of islands of cells which coalesce to form a contiguous medulla by PND 7. The pancreatic islets are well developed and functionally productive at birth, with little postnatal development.