Wnt-1 and Wnt-4 regulate thymic cellularity.

Thymic primordium, formed by cells derived from the endoderm, the ectoderm and the neural crest-derived mesenchyme, receive fetal liver derived lymphoid precursors. Reciprocal cell-cell interactions between thymic stromal cells and lymphoid precursors are critical in the expansion and maturation of thymocytes. Transcription factor TCF-1 is critical for the expansion of thymocytes because deletion of TCF-1 results in a significant decrease in the number of thymocytes without affecting the developmental pattern. In this report we show that Wnt-1 and Wnt-4 are expressed in the thymus and the deletion of Wnt-1 or Wnt-4 result in a substantial decrease in the number of thymocytes without affecting the pattern of maturation. Wnt-1 and Wnt-4 both regulate developing thymocytes because a double deficiency results in a significantly greater decrease of immature and mature thymocytes compared to deficiency in either Wnt-1 or Wnt-4.