Seasonal Changes in the Structure of the Neurohypophysis and Thyroid Gland of the Great Spotted Woodpecker, Dendrocopos major L.

Abstract

A maximal amount of neurosecretory material (NSM) is found in the median eminence (ME) and posterior pituitary (PP) of adult woodpeckers in winter. Most of the NSM in the PP is located along the capillaries and around the recessus hypophyseus. Intensively stained Herring bodies are present. A smaller amount of NSM is seen in the PP in the spring during migration, the mating season, and the beginning of gonadal development; NSM is not found in the external layer of the ME. In the latter, NSM disappears first from the zone surrounding the recessus hypophyseus, suggesting the release of neurohormones, mostly into the CSF. A minimal amount of NSM is found in the neurohypophysis (Nh) in the summer, during nesting, and the first stages of molting. NSM in the highly hyperemic PP disappears from the zone around the recessus hypophyseus and the capillaries. No stained Herring bodies are seen. This suggests an intensive release of neurohormones, into both the CSF and the general circulation. In the period from summer to winter, accumulation of NSM is observed in the Nh; Herring bodies appear, and the degree of hyperemia decreases. There is a difference between the annual cycle of the thyroid gland (TG) and that of the Nh. Moderate TG activity (epithelium colloid — E/C =0.803) is seen in later winter. A significant increase of TG activity is found during the mating season and spring migration. A lower activity of the TG (up to a moderate level) is observed during nesting and the beginning of molting (E/C =0.499); it is minimal by the end of molting, during autumn migration, and in early winter (E/C =0.343). The annual cycle of TG activity differs greatly from that of other passerine birds. Neither conspicuous correlation with seasonal, photoperiodic, and temperature changes nor considerable activation of the TG during molting is observed.