Lanthanum permeates the intercellular spaces, including cell-to-cell junctions such as desmosomes and tight junctions. It penetrates into small invaginations of the cell membrane, but does not go through the cell membrane. Lanthanum, therefore, stains intracellular organelles only when they are communicating with the extracellular spaces. Also, lanthanum combines with the cell-surface coating substance. When the epidermal Langerhans' cells were treated with lanthanum, it was found that some Langerhans' cell granules completely enclosed within the cytoplasm were stained, indicating that one end of these granules were open to the extracellular spaces at some level different from that of the section. The contents of such closed granules and of those open to the extracellular spaces were heavily stained with lanthanum, suggesting that they contained the cell-surface coating substance. This finding supports an endocytotic origin of Langerhans' cell granules.
Hashimoto K. Lanthanum Staining of Langerhans' CellCommunication of Langerhans' Cell Granules With Extracellular Space. Arch Dermatol. 1970;102(3):280-290. doi:10.1001/archderm.1970.04000090042007