[Skin changes in advanced age--biochemical findings corresponding to morphology?]

The clinically relevant morphological changes of the skin during aging can be summarized by the term "senile atrophy". The changes are a diminished thickness of epidermis with a reduced mitosis rate of epidermal basal cells, shortened and attenuated rete ridges, reduction of epidermal appendages, and a decreased number of fibroblasts and capillaries in the dermis. Corresponding to these morphological findings regarding the cell number in the senile skin (cutis) we found a slight decrease in the DNA concentration of human and rat cutis. The specific DNA activity (3H-thymidine incorporation rate related to DNA concentration) decreased in presenile versus adult animals. The mesenchymal changes in the dermis have been morphologically described by the term "senile elastosis" or "elastoid collagen degeneration", but in fact they correspond to a progressive collagen denaturation with aging. The total collagen concentration, here determined as the hydroxyproline concentration in the human cutis, shows almost constant values from the 3rd until the 9th decade of life in both sexes. This is also true for the skin of two different rat strains. The insoluble collagen fraction shows a relative increase to the disadvantage of the soluble collagen fractions, which can be interpreted as an indicator of a decelerated collagen turnover. In spite of the decelerated turnover, i.e. a prolonged half-life of the collagen metabolism in the skin, the indicators of the collagen neosynthesis (14C-proline incorporation rate, specific hydroxyproline activity, prolyl-hydroxylase activity) are significantly elevated in the cutis of presenile versus adult rats. Any connection of these findings with a possible change in the distribution of collagen types in the senile skin (e.g. pericapillar fibrosis with increase of collagen type I as well as changes in the distribution of type I, III, IV and V) can only be discussed at present. The glycosaminoglycans in the cutis show a minimal increase of the total content of hexosamines and uronic acids with a significant shift in the ratio of the glycosaminoglycan components in favour of dermatan sulfate and keratan sulfate and to the disadvantage of hyaluronic acid and partly also of chondroitin-4-sulfate and -6-sulfate. The neosynthesis of sulfated glycosaminoglycans (indicator method: 35S-sulfate incorporation rate) is only slightly increased whereas the enzyme activities being specific for the glycosaminoglycan catabolism (beta-glucuronidase, beta-N-acetyl-glucosaminidase) are significantly decreased with aging of the skin.(ABSTRACT TRUNCATED AT 400 WORDS)