Preparation of an epithelium-free mammary fat pad and subsequent mammogenesis in ewes

The objective of this study was to develop a surgical procedure for the pre paration of an epithelium-free mammary fat pad (cleared mammary fat pad; CF P) in ewes. At 7 to 10 d of age, ewe lambs (n = 43, mean BW 9.2 +/- .2 kg a t 14 d) were sedated and one mammary gland was locally anesthetized. An inc ision circumscribing the base of the teat was made and blunt dissection was performed through the extraneous mammary fat pad tissue to enable the pare nchyma and teat to be wholly removed. Failure to completely remove the epit helium enabled it to regenerate and grow into the mammary fat pad. Mean dia meter of the parenchymal rudiment at 7 to 10 d of age was 8.9 +/- .5 mm (ra nge of 5 to 16 mm). The excision site was closed with wound clips and recov ered lambs returned to their dams. The contralateral mammary gland remained intact, allowing it to undergo normal development. Live weight gain was un affected by this procedure. Ewes were subsequently slaughtered in groups at various stages of prepuberty, puberty, gestation, and lactation. Of 39 ope rated glands recovered, only one demonstrated epithelial outgrowth within t he CFP. Parenchyma within the contralateral, intact gland underwent phases of rapid growth in prepuberty, puberty, and late gestation and was capable of milk synthesis after steroid induction or parturition. Change in weight of the CFP paralleled that of the intact mammary gland to 100 d of pregnanc y. Sham CFP surgery was performed on four additional ewes wherein the paren chyma was completely excised and immediately replaced. Sham-operated epithe lium populated the mammary fat pad and synthesized milk that could be expre ssed from the teat. A CFP in sheep will be a useful model for future invest igations into the local growth regulatory mechanisms associated with rumina nt mammogenesis.