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The purpose of this morphometric study was to obtain detailed quantitative information on all cell types in the testis interstitium of Korean ring-necked pheasants combined with data on changes in the steroidogenic function of the testis during the breeding and nonbreeding seasons. Animals collected during the breeding season, testis weights, sperm production, serum testosterone levels, leuteinizing hormone-stimulated testosterone secretion, and the length of the seminiferous tubules were significantly (p ＜ 0.05) increased as compared to the nonbreeding season. Seminiferous tubules occupied 93.25% of testis volume in the breeding season. Leydig cells constituted 0.82% of the testicular volume. The mean volume of an Leydig cell was 1039 ㎛^3, and each testis contained about 24.53 million Leydig cells. Testes of the pheasants during the nonbreeding season displayed a 98% reduction in testis volume that was associated with a decrease in the absolute volume of seminiferous tubules (98% reduction), tubular lumen(100%), interstitium(90%), blood vessels(84%), lymphatic spaces(97%), Leydig cells(79%), mesenchymal cells(51%), and myoid cells(61%). The number of Leydig cells, mesenchymal cells, myoid cells per testis in the breeding season was higher (p ＜ 0.05) than in the nonbreeding season. Although the average volume of a Leydig cell was 74% lower in the nonbreeding season, the average volume of a myoid and mesenchymal cell remained unchanged. These results demonstrate tht there are a striking differences in the testicular structure of the Korean ring-necked pheasant in the breeding and nonbreeding seasons. Every structural parameter of the Leydig cell was positively correlated with both serum and LH-stimulated secretion concentrations of testosterone. Correlation of changes in hormonal status with morphometric alterations of all Leydig cell suggests that the Korean-ring necked pheasant may be used as a model to study structure-function relations in the avian testis.
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