Steriological analysis of tissue compartments of the thymus of Giriraja birds (Gallus domesticus)

Morphometric changes in the thymus of various age groups such as posthatch, mature, involutary and recrudescent stages were studied in giriraja strain of chicken. The posthatch changes could be classified as maximum proportion of body weight of thymus, equal volume density of thymic cortex and medulla, high mitotic index and high corticomedullary ratio of thymic follicles. During the mature phase the volume density of medulla was higher while cortical thickness and corticomedullary ratio were low. Involutary phase was characterized by low proportions of thymic weight, maximum capsular thickness, high volume densities of medulla and stroma, lowest corticomedullary ratio and drastic drop in mitotic index. Recrudescence was characterized by maximum thymic weight and a drastic increase in volume density of thymic cortex from a near zero value at involutary phase. A noteworthy fact was that the corticomedullary ratio increased twice after involution which may be due to the effect of breeding cycle. Maximum diameter of follicles was noted at 20 weeks. During terminal involutary phase, decreased proportional weight of thymus, increased values of volume densities of stroma and medulla, decreased follicular diameters, irregular capsular thickness and mitotic index of cortical changes were noted in the immediate post hatch and recrudescence period while the medullary expansions were predominant at the start of involuntary phase. These changes should be known and differentiated from experimental effects while studying steriology of the thymus.