The late fetal and neonatal rat thymus was studied by histological and histochemical techniques following adrenal cortical steriod administration. Albino rats from seventeenth day of gestation through thirtieth postnatal day were sacrificed six, 24, 48 and 96 hours following single or series of cortisone or hydrocortisone injections in pregnant or neonatal rats. Some of gestation periods were prolonged by progesterone injections. Stains included Harris hematoxylin and eosin, May-Grünwald Giemsa, Gomori's ('52) or Burstone's ('58) method for alkaline phosphatase, and the PAS stain. Autofluorescence was examined in some of the sections.
A reduction in distinctness of the cortico-medullary border of the fetal thymus followed maternal cortical steriod treatment. Similar treatment in neonatals one and two days of age led to pycnosis and phagocytosis of small lymphocytes. No histochemically stainable alkaline phosphatase was observed before sixteenth postnatal day, or following steriod administration on day one or two. However, a precocious increase in alkaline phosphatase followed a single injection on day 12. In the fetal and neonatal thymus PAS-positive material was present, was more prominent on the sixteenth day, and increased following steriod treatment, particularly on the twelfth day. Autofluorescent cells, present on the sixteenth day, increased following steriod administration. Histochemical and autofluorescent modifications were particularly prominent at the cortico-medullary border and are thought to represent postnatal maturation about the sixteenth day which changes can be induced precociously by cortical steroids.