Relationship between Hassall's corpuscles and thymocytes fate in guinea-pig foetus.

The development and structure of the Hassall's corpuscles are studied with the light and electron microscope in 53 guinea-pig foetuses from 30 to 58 days gestation. The different stages of their life cycle are described. Prior to the appearance of Hassall's corpuscles, no medullary lymphocytes can be found in the early medulla. Hassall's corpuscles are seen initially between 35-42 days gestation. They are surrounded by large and medium-size lymphocytes, while medullary lymphocytes are in close contact with the "external cells" which exhibit cytoplasmic characteristics of a secretory function. The data reported in this paper suggest that Hassall's corpuscles are not the only sites of lymphoid cells destruction but are also "privileged" areas for maturation of medullary lymphocytes at least for the guinea-pig.