Evidence to suggest morphological and physiological alterations of lacrimal gland acini with ageing.

This study investigates changes in the morphology and physiology of lacrimal gland acinar cells with age. Changes in microstructural appearance of the acinar cells, the type and distribution of the different acini in glands and the secretory granules within the acini were examined in glands from animals of 3-5, 9, 12, 20, 24 and 28 month old rats. Differences in the secretory capacity of the acinar cells were also examined in animals of each age-group, with the exception of 28 months. The typical acini of young glands (3-5 months) were of the serous type. This was also true of 9 month glands, although there was a significant reduction in their overall distribution compared to young glands. The acini in the 12 month glands were predominantly of the seromucous type and appeared to be at the expense of the serous acini which were further significantly reduced compared to 3-5 and 9 month glands. This remained the prevalent acini type in 20 month glands, however by 24 months there was a significant increase in the occurrence of mucous acini and this time appeared to be at the expense of the seromucous acini which were significantly reduced in glands of this age-group. The predominant acinar cell in 28 month glands, like 24 month glands, was of the mucous variety. Qualitative EM studies revealed a progressive change in the secretory products of the lacrimal gland acini, strongly correlating to changes in acinar cell type. Typical acini of both 3-5 and 9 month glands contained numerous protein secretory granules. The seromucous acini also of these age groups contained both protein and mucous secretory granules, with the protein secretory granules in higher abundance. By 12 months the typical seromucous acini was packed with both protein and mucous secretory granules of equal proportions. However, by 20 months the predominant seromucous acini contained fewer protein secretory granules and elevated occurrence of mucous secretory granules. By 24 and 28 months the acini contained even fewer protein secretory granules and the typical acinar cell was of the mucous type containing exclusively mucous secretory granules. The secretory capacity of the acini was also altered with age. Maximum protein output in response to cholinergic stimulation resulted in an initial significant increase with ageing from 3-5 months to 9 and 12 months followed by a later significant age-dependent reduction in output. However, maximal peroxidase release from acinar cells of 3-5 and 9 month glands was the same. This was followed by a significant age-dependent reduction in peroxidase release. Furthermore, the concentrations required to evoke these responses differed with age. These results present evidence to suggest that acinar cells of the lacrimal gland undergo progressive alterations with age. The type of acini changing initially from serous to seromucous acini (intermediate phase) followed by a gradual transformation of the seromucous acini to mucous acini. This in turn changes the properties of the acini from protein producing and secreting acini to mucous producing and secreting acini. The results also suggest a reduction in the ability of the acini to synthesise proteins with age and altered responsiveness to cholinergic stimulation to secrete proteins. These findings may help in explaining the occurrence of altered protein/tear secretion with ageing.