Mystery of the Chicken Grid

What is it?
As odd as it looks - its natural!

Apterial muscles

Unusual grid like pattern under the skin of a Foster Farms chicken thigh.

This photo shows a piece of Foster Farms 100% Natural chicken
This piece went went directly from the package to the pan (smooth pan) and were half baked when the pattern was noticed on the now semi-transparent skin.
No pattern was noticed at time of purchase or unpackaging.
The chicken was cooked in a standard convection oven and not a microwave.
The skin was cut and this white string like fiber is physical and UNDER the skin and is soft.

Unusual grid like pattern UNDER the skin of Foster Farms chicken thighs

Dr. O'Conner, the Vet for Foster Farms discovered the reason for the mysterious grid pattern:

Anatomic diagram from AVIAN ANATOMY, Integument, Part II, Agriculture Handbook 362,
Lucas & Stettenheim, Chpt. 8, "Feather and Apterial Muscles", page 439.

The attachment shows the "apterium" muscles of the skin covering the thigh. Acting collectively these muscles are capable of tensing the featherless skin of the bird. As described in the reference, "apterial muscles are composed of bundles, joined end-to-end by elastic tendons, and arranged in a sheet across non-feathered spaces."

Mystery solved!