Photomicrograph of a longitudinal section through an end bud and its subtending duct. A lateral end bud stained to reveal sulfated glycosaminoglycans and mitotic cells with the cap cell layer is indicated by the dashed line. Constriction of the end bud to ductal dimensions coincides with induction of a collagenous extracellular matrix along the end bud flank. Note that this sheath is continuous with the subtending duct as well as the duct of origin (dotted lines). The basal lamina along the end bud flank (large arrows) as well as in the cleft of a bifurcation (triangle) stained deeply for sulfated glycosaminoglycans (Alcian blue stain). This contrasts with weakly stained basal lamina around the end bud tip indicative of non-sulfated hyaluronate (short arrows). Silver grains (dark spots) are from tritiated thymidine autoradiography and mark mitotic cells in the end bud and subjacent ducts. Original magnification approx. ×300.