A 55-year-old man has had a foot ulcer for 2 months that has not healed. Physical examination shows a 2-cm shallow, nonhealing ulceration of the left medial malleolus. There is a symmetric decreased sensation in the distal regions of the lower extremities. He has a history of multiple urinary tract infections resulting from difficulty in completely emptying the bladder. He is impotent. Which of the following pathologic findings is most likely to be present in the peripheral nerves?