At approximately day 1

Three layers of mesenchyme are present. The deepest layer undergoes epithelial down growth.

Duration: 3 weeks

Reference [PubMed Link]

Gusterson BA, Stein T. 2012. Human breast development. Seminars in Cell and Developmental Biology 23(5):567-573.

At approximately day 1

The promordial breast cells are clearly delineated as a discrete population.

Reference [PubMed Link]

Howard BA, Gusterson BA. 2000. Human breast development. Journal of Mammary Gland Biology and Neoplasia 5(2):119-137.

At approximately day 1

BCL-2, a molecule that prevents apoptosis, is expressed in the stromal cells adjacent to the breast bud.

Reference [PubMed Link]

Howard BA, Gusterson BA. 2000. Human breast development. Journal of Mammary Gland Biology and Neoplasia 5(2):119-137.