Seasonal histological changes in the gonads, thyroid and adrenal of the Canada goose (Branta canadensis interior)

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Abstract
Cyclic histological changes in the testis, ovary, thyroid and adrenal of the North American eastern prairie population of Canada geese (Branta canadensis interior) breeding in Ft. Churchill (Manitoba, Canada) and wintering in the Swan Lake refuge in Missouri (USA) were studied. The investigation was carried out during the following six periods of the yearly life cycle. Phase 1: pre-spring migration at Swan Lake (early March); phase 2: post-spring migration at Ft. Churchill (early May); phase 3: breeding (early June); phase 4: moult (early August); phase 5: pre-fall migration (early September); phase 6: post-fall migration at Swan Lake (mid-October). The development of the testis and the initiation of spermatogenesis up to the formation of secondary spermatocytes was begun in phase 1. All stages of spermatogenesis, including the formation of spermatids and spermatozoa, were reached by the end of phase 2 and remained so through most part of phase 3. The weight of testis recorded was highest in phases 2 and 3. Testicular regression characterized by progressive loss in weight, reduction of spermatogonia in the tubules and appearance of sudanophilic material in the intersititium commenced in phase 4 and lasted through phases 5 and 6. Though the initiation of ovarian development probably started in phase 1, the ovary weighed the highest in phase 2. At this time, the ovary contained numerous maturing ova in different stages of development and lipoidal atresia. Lipoidal atresia was more prominent among young small ova. The larger maturing ova were characterized by a more distinct granulosa layer around the oocyte. Numerous sudanophilic interstitial cells were present scattered throughout the ovarian stroma. Atretic follicles and scars from discharged follicles were found in all the phases. The thyroid showed a significant increase in weight during moult (phase 4). No marked histological changes were discernible in all the phases. The increase in weight during moult is