These fibers or fingerlike extensions of the plasma membrane are microvilli.

Do not mistake them for cilia: cilia possess 10 pairs of microtubules (one central pair surrounded by the 9 others) that run throughout their length. Microvilli do not.
You do not see the 9+1 pattern of microtubules in the cross section of these fingerlike extensions. You just see cytoplasm (e).

1: Microvilli
e: Cytoplasm inside the microvilli
f: Plasma membrane

In the longitudinal section at low magnification, observe the microvilli (1) on the top of the cells of the ABO. On this photomicrograph, you only see the apical part of the epithelial cells of the ABO (d).

If you increase the magnification you can see (in c) the continuity between the microvilli (1) and the cytoplasm of the cell it belongs to (b).
Notice the dilations (1a) along the length of these microvilli. This is very unusual: as a general rule, microvilli do not possess such things.