Zippering on tomato

Zippering on tomato is the presence of a long, thin brown tissue resembling a zipper, which usually runs from the stem end to the blossom end. 

It's caused by an abnormality during fruit development, usually when an anther or other flower part remains attached to the newly forming fruit. Sometimes temperature during fruit development is a factor. There is no control for this abnormality.

The fruit is completely edible, provided that there is no other damage to the tomato causing a secondary bacterial infection. 

Source:

Ozores-Hampton, M., Kiran, F., MacAvoy, G. (n.d.). Blossom drop and reduced fruit set in tomatoes. Retrieved March 24, 2021, from https://swfrec.ifas.ufl.edu/docs/pdf/veg-hort/tomato-institute/presentations/ti2011/ozores.pdf