Explain about the Transcellular Route - Calcium?

Transcellular Route: It operates primarily in the duodenum and proximal jejunum. It is stimulated when Ca ingestion is lower relative to requirement. It is a metabolically active process, which means energy in the form of ATP is required to absorb and transport the calcium across the intestinal mucosal cells, Transcellular movement involves 3 sequential steps, The first from the intestinal lumen to the intestinal mucosal cells across the brush border, the second within the cell from the lumen to the serosal side and finally the third, extrusion from the cell into the blood circulation. All three steps are regulated by calcitriol(1,25 di- hydroxy cholecalciferol), the biologically active hormone form of vitamin D₃.