Morphological and ultrastructural postnatal development of thymus in guinea pigs

2. ABSTRACT In this investigation, the morphological and ultrastructural variations occurred in thymuses of guinea pigs in postnatal period were examined. In the study, 10 groups, one day and 1,2,4,6,8,12,16,20 and 24 weeks of groups consisting of 6 guinea pigs in each group (60 guinea pigs in total) were used. After birth, thymus weight increased parallel to body weight and reached to the maximum level in the sixth week. After the sixth week, a decrease in thymus weight was determined. One day old guinea pigs had a completely developed thymus. Cortex and medulla were easily distinguished. In cortex, a number of small lymphocytes, lymphoblasts, epithelial reticulum cells were seen. Whereas, in medulla, epithelial reticulum cells and Hassall's corpuscles were observed. The lipid cells, one of the involution signs, were first seen in the sixth week. Together with the increasing age, it was determined the lipid tissue amount to enter into cortex by increasing and lymphocytes number to decrease due to the lipid infiltration. Hassall's corpuscles decreased both in view of number and in view of volume. In medulla, increased connective tissue was observed. In electron microscopic examination, it was determined that whole organels in cell cytoplasms fully developed. It was seen that a number of mitochondria in lymphocytes cytoplasms were present. The cell limits were evident. The epithelial cell extensions had entered into interval of lymphocytes. In cortex, an intense correlation between epithelial cells and lymphocytes was seen. This correlation was important because of forming thymic microenvironment.Together with involution, it was determined that cytoplasm membranes of lymphocytes lose prominence, whereas, a notable change in epithelial reticular cells were not determined. A number of electron dense lysosomes and apoptotic bodies were seen in macrophage cytoplasms. In conclusion, it was determined that thymus was completed it's development with birth. It was also involuted after puberty and continued it's functional activity during all life. Key Words: Guinea pig, thymus, involution, the light and transmission electron microscopy.