The structure indicated by the arrow, and visualized as a dark line at the apex of each of the columnar cells, marks the location of a thick layer of cytoskeletal actin filaments and intermediate filaments located at the base of the brush border of microvilli.

Note that some of the filaments in this layer are associated with the apical junctional complex that holds the cells together and some extend into the layer from the microvilli, which contain a core of actin microfilaments. This structure is known as:

The terminal web The cilia A desmosome Charlotte's Web

 

Now let's consider the structure of the cell.