Experimental Studies on the Histology of the Mammalian Thymus

Abstract

ABSTRACT A number of male mice 4-6 weeks old were subjected to irradiation and killed at intervals of 2 hours to 12 days after the treatment. A detailed study was made of the resulting histological changes in the thymus. Immediately after irradiation most of the small thymus cells comprising the cortex of the organ degenerate. Simultaneously the existing connectivetissue elements in the thymus proliferate amitotically, and fresh connective tissue and blood-vessels invade the periphery of the organ from its sheath. Numerous ciliated cysts develop in the thymus, derived from the blood-vessels and their adjacent connective tissue; the majority of these disappear 28 hours after irradiation when resorption of the cell debris is complete. Regeneration is initiated 3 days after irradiation by an enlargement of the nuclei of the thymus cells persisting at the edge of the organ; these, by repeated division, reconstitute the cortex. Fresh medullary areas are formed by the aggregation of the connective tissue and blood-vessels which are carried centripetally during regeneration; most of this tissue degenerates, giving rise to a fresh series of corpuscles of Hassall and cysts. These are transitory structures and have almost all disappeared from the medulla by the time histological regeneration of the thymus is complete, 12 days after irradiation. Small pieces of thymus from an advanced cat embryo were cultivated in normal maternal plasma; the implants were fixed after various periods of incubation, embedded in paraffin and sectioned. The histological changes after incubation are similar to those in the irradiated thymus, before the latter begins to regenerate; they include pycnosis of the small thymus cells and proliferation of fibrous connective tissue. This forms a border round the implant in which fresh corpuscles of Hassall develop, their outer cells being continuous with the fibrous connective tissue. The significance of these results with regard to the normal histology of the thymus is discussed, as are the conclusions of other workers who have studied similar changes in the thymus after experimental involution. Both these experimental studies of the histology of the thymus indicate the complete absence of an epithelial reticulum, such as is frequently postulated; the corpuscles of Hassall, and cysts generally associated with it, are derived from the blood vascular and accompanying connective tissue. A note is added on the reaction of typical lymph nodes to the irradiation; this is slight in comparison with that of the thymus.