Submandibular gland morphogenesis: Stage‐specific expression of TGF‐α/EGF,IGF, TGF‐β, TNF,and IL‐6 signal transduction in normal embryonic mice and the phenotypic effects of TGF‐β2, TGF‐β3, and EGF‐r null mutations

Branching morphogenesis of the mouse submandibular gland (SMG) is dependent on cell‐cell conversations between and within epithelium and mesenchyme. Such conversations are typically mediated in other branching organs (lung, mammary glands, etc.) by hormones, growth factors, cytokines, and the like in such a way as to translate endocrine, autocrine, and paracrine signals into specific gene responses regulating cell division, apoptosis, and histodifferentiation. We report here the protein expression in embryonic SMGs of four signal transduction pathways: TGF‐α/EGF/EGF‐R; IGF‐II/IGF‐IR/IGF‐IIR; TGF‐βs and cognate receptors; TNF, IL‐6, and cognate receptors. Their in vivo spatiotemporal expression is correlated with specific stages of progressive SMG development and particular patterns of cell proliferation, apoptosis, and mucin expression. Functional necessity regarding several of these pathways was assessed in mice with relevant null mutations (TGF‐β2, TGF‐β₃, EGF‐R). Among many observations, the following seem of particular importance: (1) TGF‐α and EGF‐R, but not EGF, are found in the Initial and Pseudoglandular Stages of SMG development; (2) ductal and presumptive acini lumena formation was associated with apoptosis and TNF/TNF‐R1 signalling; (3) TGF‐β2 and TGF‐β3 null mice have normal SMG phenotypes, suggesting the presence of other pathways of mitostasis; (4) EGF‐R null mice displayed an abnormal SMG phenotype consisting of decreased branching. These and other findings provide insight into the design of future functional studies. Anat Rec 256:252–268, 1999. © 1999 Wiley‐Liss, Inc.