Control of Mammary Gland Function in the Bitch and Queen: A Review

Introduction

Lactation can be divided into the consecutive stages of mammogenesis (development of mammary tissue), lactogenesis (production of colostrum and first milk) and galactopoiesis (maintenance of milk production under suckling and/or milking stimulus). Mammary growth and development, milk production, milk ejection and mammary involution are under the control of different hormone systems: progesterone and growth hormone act synergistically to cause mammary gland development, prolactin acts towards the end of pregnancy to start lactation and oxytocin causes milk letdown. Once suckling stimuli cease, insulin-like growth factor-I (IGF-I) is secreted in large amounts causing mammary gland involution, following which the mammary gland remains in a quiescent state for a few months. The control of different mammary functions relies on understanding how these different hormone systems and their stimulating/inhibiting factors exert their action. This paper will review the endocrine control of mammary functions in bitches and queens, as well as pharmacological treatments which can be used by small animal clinicians to treat bitches or queens with hyper- or hypo-function of the mammary gland. As the literature on physiology of the mammary gland of small animals and its normal or abnormal functions is very scant (canine) or almost non existant (feline) - with the only exceptions of mammary tumors or mastitis - the reader should be aware that a) majority of the information presented in this paper is drawn from work done (mostly) in humans or other animal species, and b) the majority of clinically relevant conclusions are extrapolated from data coming from humans or other animal species. However, it is noteworthy that mammary gland function is fairly similar across species, as it can be inferred by similarities in the action of endocrine drugs on lactation in different animal species as well as humans. Conditions such as mastitis or mammary neoplasia are beyond the scope of this paper and will not be discussed here.[...]