Development and histogenesis of the thymus in dog. A light and electron microscopical study.

The development of the thymus was studied with histological, transmission electron microscopic (TEM) and histochemical methods in 100 dog fetuses (beagle), between day 19 of gestation and day 21 after birth. Thymus development could be divided in three stages: 1/Formation of epithelial palisades; 2/Initiation of lymphopoiesis; 3/Differentiation of the medulla and Hassall's bodies (HB). The epithelial anlagen were seen at day 23 of gestation showing the characteristic palisade structure of the endodermally derived epithelium. Ten days later the beginning of lymphoiesis and the reticularization of the epithelial cells could be seen. The first HB could be found at day 38 when cortical-medullary differentiation is recognized. The histochemical observation demonstrated a rich content of PAS positive coarse granules in the cytoplasm of reticulo-epithelial (RE) cells. On the other hand, the HB showed a diffuse PAS positive reaction. The ultrastructural investigations demonstrated the presence of desmosomes connecting RE cells to one another. Desmosomes were not found between RE and lymphocytes. The growth of the developing thymus into the mesenchymal matrix resulted in the lobulation of the organ by connective tissue cells and fibers. The first mast cells were seen at day 35 of gestation, most abundantly in the interlobular connective tissue (ICT) although a few were present in the cortex and somewhat more in the medulla, near the HB. At the end of development small groups of neutrophil cell precursors appeared in the ICT and the cortex. Cysts were not present up to day 21 after birth.