On the Control of Calcium Secretion in the Avian Shell Gland (Uterus)

Shell glands (SGs) of white Leghorn hens were perfused in situ to determine the time course and magnitude of secretion of egg shell Ca and factors that control secretion, during laying cycles. With a surrogate wax egg present in the SG during all test periods (substituted for the natural egg when necessary), net Ca secretion averaged 20 μm/h/g dry wt before a natural egg arrived in the SG (1–4 hr after ovulation), increased sharply as an egg arrived (h 5–6) progressing to 150 μm/h/g (h 14–18) and returned to basal levels (by h 24) when a shell was completed, but 2 h before the egg normally left the SG. Secretion rates in SGs were always greater when distended in egg-like fashion than when not, but rates in active (shell forming) SGs whether distended or not were always substantially higher than in inactive controls. Ca secretion in the mature magnum and in SGs of mature prelaying hens and pullets was minimal (<30% of mature, inactive SGs) or undetectable. These observations indicate: 1) in the oviduct, capacity for functional Ca secretion is specific to the mature laying SG; 2) timing of Ca secretion is not cued by the previous oviposition, but is associated with occurrence of ovulation; 3) an egg in the SG (distention) enhances Ca secretion, but does not account for the normal magnitude of secretion. Severing the SG from the upper oviduct did not affect secretory cycles and decreasing the concentration of SG luminal Ca did not change secretion rates. Also, severing known autonomic nerves to the SG did not permanently affect egg laying or egg shell quality. The collective results dictate an hypothesis: Ca secretion by the SG is directed by hormonal factors thit control and coordinate both ovulation and Ca secretion. Accordingly, the time course of Ca secretion is discussed in relation to current knowledge of circulating levels of hypophyseal and gonadal steroid hormones during the laying cycle.