Effects of age on Meissner corpuscles: a study of silver-impregnated neurites in mouse digital pads

Investigation focused on finding qualitative and quantitative evidence of age-changes in the quantity of neural surface within Meissner corpuscles. These mechanoreceptors were studied in 53 mice (nine age-groups) ranging from 1.7 to 24 months old. Forepaw digital pads were formalin-fixed and frozen-sectioned parallel to each digit and perpendicular to the skin. Serial sections were then silver-impregnated to allow light microscopic examination of the neurites (axons) in corpuscles. From young (1.7-7 months) to middle age (9-15 months), neurites became more coarse, tortuous, ramified, varicose, and thus more complex. At old age (18-24 months), neurites seemed attenuated and showed more of an irregular winding, twisted, or tangled pattern with less parallel orientation to the skin surface than the regular spiraled, looped, or arched pattern typical at young and middle ages. Corpuscle size appeared greatest at middle age, smallest at young age. Dermal papillae not occupied by corpuscular neurites were most abundant at old age. The number of corpuscles per area and neurites per corpuscle decreased significantly with age, whereas the number of neurite bifurcations per corpuscle increased significantly. Morphometric analysis of neurites projected by a camera lucida onto a planimeter showed that the length of neurites meandering through a fixed interval of tissue increased significantly until age 12 months-evidence of increased tortuosity; the area of neurites measured within the same fixed interval, and the area of neurite terminals changed significantly as inverse parabolic functions of age-evidence of increased volume until middle age, which decreased thereafter. The general trend of these changes implied growth at young age and atrophy at old age.(ABSTRACT TRUNCATED AT 250 WORDS)