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Growth properties of multicellular units (organoids) of mouse mammary parenchyma have been analyzed. These intact units grew differently in collagen-matrix cultures than did dispersed cells prepared from them. The latter actively migrated in the collagen matrix and reorganized themselves into multicellular structures before producing three-dimensional protuberances in gel. Terminal unit (end-bud/alveoli)-enriched fractions grew more extensively than did ducts, as predicted from growth patterns in vivo. To assess the growth potential and the relationships between replication history in vivo and replication potential in vitro in mammary parenchyma, intact terminal units from mammary glands of mice of different ages and with different reproductive histories were isolated and their growth characteristics compared. Terminal-unit organoids were cultured in collagen gel matrix and passaged weekly for up to 5 weeks. Morphology, growth rates, and growth fractions were compared among organoids from young virgin, old virgin, monoparous, and multiparous mice. Morphologies observed in various passages of organoids from the groups of mice were similar. Organoids from old virgin and multiparous mice declined in growth rate for four passages and then growth rate increased again during the fifth passage. (However, fifth-passage organoids failed to form tumors if implanted in syngeneic mice in vivo.) Growth of organoids from either old or young virgin mice was less at any given passage than tissue from multiparous mice of similar age. Growth fractions of organoids from old parous mice were the same as those from old virgin mice but reached the same maximum fraction faster. Later passage organoids from the different mouse groups responded morphologically to the hormone combination of estrogen, progesterone, prolactin, and cortisone but not respond to cholera toxin. These results suggest that an animal's hormonal history (altered profoundly by pregnancy and lactation) may be as important as chronological age in determining subsequent growth potentials of mammary epithelium.
After three or more passages, stromal cell outgrowths from explants of normal rabbit corneas and from corneas previously burned with alkali did not produce collagenase until they were stimulated by the addition of rabbit blood-derived mononuclear cells or media conditioned by them. Normal stromal cells required stimulation by lymphocytes or their products from alkali-burned animals, whereas those from alkali-burned corneas were stimulated by lymphocytes from either normal or alkali-burned rabbits.