Differentiation of thymus and thymocytes. A study in foetal guinea-pigs using alkaline phosphatase as a label of thymocytes.

The differentiation of the guinea-pig thymus and thymocytes was studied using alkaline phosphatase (AP) as a marker of the thymocytes. The stem cell was assumed to be AP-negative. Cortical thymocytes contained a high activity of AP, which was lost or greatly diminished during the differentiation into medullary thymocytes and into T cells. Only approximately 1 per cent of the peripheral lymphocytes contained AP-activity. The role of these cells is obscure. In the morphological study three developmental stages were observed. At the beginning the AP-activity was concentrated in the periphery of the thymus anlage, and was first observed in a 23-day-old foetus. At about 28 days the foetal thymus contained homogeneously distributed AP-positive and AP-negative cells. The cortex and medulla began to differentiate at the age of 31–33 days and the definite differentiation formed the third developmental stage. Medulla achieved its maximal proportion at birth. The results are in agreement with earlier studies concerning the development of the guinea-pig thymus.