Photomicrographs illustrating motility and histoarchitecture of end buds. (a) Natural and experimentally induced motility 'behavior' of end buds in the mammary ductal system of a 5-week-old nulliparous mouse. The 'open' ductal architecture of the mammary tree leaves 80% or more of the gland epithelium-free. Large terminal end buds identify the most actively growing region of the gland (top arrows), and progressively smaller lateral end buds extend to each side of the center, indicating slowed forward growth as the end bud encounters a thinning fat pad. End buds may also reverse direction to grow back into accommodating stroma (side arrow). Bifurcating end buds (top arrows) are arrayed along the growth front. Original magnification approx. ×12. (b) Cross-section through end bud with accompanying diagram. End buds are bilayered structures; an outer layer of myoepithelial progenitor cells (cap cells) overlays a multilayered mass of luminal cells fated to form the walls of the ductal lumen (L). Stained with hematoxylin and eosin. Original magnification approx. ×300.