In the beginning: the establishment of the mammary lineage during embryogenesis

The mammary primordium is comprised of an aggregate of immature, undifferentiated mammary epithelial cells and its associated mammary mesenchyme, a specialised tissue which harbours mammary-inductive capacity. The mammary primordium forms during embryogenesis as a result of inductive interactions between its two component tissues, the mammary mesenchyme and epithelium. These two tissues constitute a signalling centre that directs the formation of the mammary gland through a series of reciprocal mesenchymal-epithelial interactions. A rudimentary mammary ductal tree and stroma is formed prior to birth as a result of these interactions. The subsequent mammary outgrowths that arise upon hormonal stimulation during puberty originate from this rudimentary tissue. The initial appearance of the embryonic mammary primordium during embryogenesis represents the earliest morphological evidence of commitment to the mammary lineage. Classic tissue recombination studies of mouse mammary primordial cells have demonstrated that the epithelial cells are already functionally determined as mammary at the embryonic mammary bud stage. Recent studies have determined the molecular identity of the embryonic mammary cells by transcriptomic profiling and these have provided new insights into signalling components that mediate early embryonic mammary inductive signalling and lineage commitment. This review highlights what is currently known about the morphogenesis, function, and behaviour of embryonic mammary cells and examine current knowledge of the genetics underlying mammary cell fate and establishment of the mammary lineage during embryogenesis.