SYNTHESIS OF CASEIN-LIKE MATERIAL BY RAT MAMMARY GLAND ANLAGEN UNDER HORMONAL STIMULATION IN VITRO

In the human newborn, the appearance of 'witch's milk ' is a common event(Mayer & Klein, 1961). Similarly, the foetal mammary rudiment of the rat develops into a highly ramified structure with secretion present in the ducts at birth (Myers, 1917). Insulin is required for the synthesis of casein-like material by rat mammary rudiments. This casein-like material runs as a single band with the fastest component of adult rat casein in acrylamide-gel electrophoresis (Ceriani, 1968) and is bound by rabbit antibody to rat casein (R. L. Ceriani & M. Feldman, unpublished). Mammary anlagen from Fischer rat embryos 16 days after conception were ex-planted as organ cultures to eliminate possible stimulation by foetal, placental and maternal hormones. Incorporation of 32P into casein (a phosphoprotein) was used as an indicator of the secretory responses of the mammary gland. Sixteen 2nd and 3rd mammary gland anlagen per experimental group were removed and prepared as organ cultures, in Waymouth-agar medium (Lostroh, 1965) which contained one-third of its specified phosphate buffer, in the presence or absence of hormones. The cultures were kept at 37 ° under a constant flow of humidified 5 % C02:95 % 02, a mixture which maintained the pH at about 7-4. After 3 days they were transferred for anothe