Induction of squamous metaplasia: requirement for cell multiplication, and competition with lobuloalveolar development in cultured mammary glands

Mouse mammary glands were previously shown to undergo either of two courses of development and differentiation in whole organ culture. The combination of insulin, prolactin, aldosterone, and hydrocortisone induces a structural development of lobuloalveoli, followed by casein production. In the second course, the mixture of dibutyryl cyclic AMP, prostaglandins E1, E2 and B1, and papaverine brings about an extensive squamous metaplasia and excessive keratinization. In the present study, the foci of the metaplastic squamous cells appeared to originate from single or very few cells. A preferential stimulation of squamous cell multiplication was involved in the induction process. Twice the relative number of nuclei incorporated 3H-thymidine in the squamous metaplastic cells than in the surrounding cuboidal epithelium, according to autoradiography. The necessity for cell multiplication was indicated by the reversible and complete inhibitions of both the metaplastic squamous development and 3H-thymidine incorporation by 1 mM hydroxyurea in the culture medium. Simultaneous inductions of both courses of development and differentiation revealed a competitive and reciprocal relationship between the two pathways. The concurrent expressions of both courses were considerably less than those achieved when either pathway was induced alone. Only the combination of the three types of inducers of squamous metaplasia was able to compete effectively with the hormonal induction of lobuloalveolar development and differentiation. The findings suggest that individual metaplastic squamous foci may originate as clones of cells by processes that require cell multiplication, rather than through a direct non- replicative conversion of pre-existent cells of the cuboidal epithelium.