Abstract: Is lacrimal fluid composition more... fluid than we thought?

Really interesting stuff here. The idea is that what we commonly call the "aqueous" tear layer, produced by the lacrimal glands, is more complex than traditionally thought... that in fact some of the mucin in the tear film comes from the lacrimals, and that the lacrimals can vary the composition of what it puts out in response to what's going on in the eyes.

Not all lacrimal epithelial cells are created equal-heterogeneity of the rabbit lacrimal gland and differential secretion.

AIMS:
To test the hypotheses that some epithelial cells in the rabbit lacrimal gland (LG) are mucin-secreting cells that are also particularly rich in aquaporin 5 (AQP5) and sodium potassium ATPase β(1) subunit (NKAβ(1)), LG-secreted mucins contribute to the total mucin pool in tear film, and that the rabbit LG is a heterogenic gland where proteins secreted in response to different agonists are varied.

MATERIALS AND METHODS:
LGs were obtained from adult female rabbits and processed for paraffin sections for hematoxylin and eosin (HE) staining, periodic acid-Schiff (PAS), mucicarmine, and Alcian blue (pH 0.4, 1.0, and 2.5) for the detection of mucins. Serial sections were used for immunohistochemistry (IHC) and PAS. LG lysates and fluids were assayed by dot blot for detection of mucins, and by SDS-PAGE to detect differences in protein profiles of LG fluids stimulated by different agonists.

RESULTS:
HE staining demonstrated that the LG is a heterogeneous gland where most epithelial cells are serous, while all duct cells are mucous cells. Some acini and individual acinar cells within serous acini are also mucous or seromucous cells and these cells are particularly rich in AQP5 and NKAβ(1). Dot blot assay showed the presence of mucins in the LG fluids. The protein profiles of LG fluids from pilocarpine, phenylephrine, and isoproterenol varied significantly, particularly in the mid range.

CONCLUSIONS:
Our data indicated that the rabbit LG is a heterogeneous gland that is composed of both serous and mucin-secreting cells, and mucins produced by the LG contribute to the mucin pool in the tear film. The heterogeneity of the rabbit LG supports the notion of differential secretion, i.e. the volume and composition of the LG fluids vary depending on various circumstances in the ocular surface and the body's needs.

Curr Eye Res. 2011 Nov;36(11):971-8. doi: 10.3109/02713683.2011.602814.
Ding C, Huang J, Macveigh-Aloni M, Lu M.
Source
Department of Cell and Neurobiology, University of Southern California, Los Angeles, USA. cding@usc.edu