Which connective tissue layer directly surrounds individual axons in a nerve?

The endoneurium

Explanation:

Each axon is covered by a layer of connective tissue called the endoneurium that consists of an inner sleeve made of a dense matrix of collagen. The endoneurium function is to serve as protection, preventing certain molecules to enter, but it also provides nutrients and energy to the enclosed neurons, finally, it guarantees a constant environment by maintaining a lightly positive pressure.

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