Actions of prolactin on the pigeon crop sac

As has been mentioned, in pigeons and doves the young are fed by a milk-like fluid regurgitated from the crop. This milk is formed by a sloughing of cells from the wall of the crop sac, a process which is stimulated by prolactin [96,97]. The hormone stimulates both an increase in proliferation of the cells which contribute to the milk and a change in the proteins and lipids contained within the cells [97,98], and these two types of response appear to be controlled independently [99]. The effect is a direct one, and prolactin can produce local effects when injected in the immediate vicinity of one side of the crop. 

There has been some work on the nature of the second messengers that mediate the mitogenic actions of prolactin on the crop sac. A role for cyclic GMP has been suggested [100]. Injection of this nucleotide with a small quantity of prolactin into the skin above the crop potentiated the actions of the hormone, although when injected alone the cyclic nucleotide had no effect. 

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