Characterization of Meibomian gland innervation in the cynomolgus monkey (Macaca fascicularis)

To characterize the innervation of the cynomolgus monkey (Macaca fascicularis) Meibomian (tarsal) glands, upper lids of six cynomolgus monkeys were investigated with electronmicroscopical and double-labeling immunocytochemical methods. Antibodies against calcitonin gene-related peptide (CGRP), dopamine-beta-hydroxylase (DBH), neuropeptide Y (NPY), nitric oxide synthase (NOS), protein gene product 9.5 (PGP 9.5), substance P (SP), tyrosine hydroxylase (TH), and vasoactive intestinal peptide (VIP) were used. In addition, sections were processed for NADPH-diaphorase (NADPH-d) histochemistry. Staining for PGP 9.5 and electron microscopy showed that Meibomian gland acini were surrounded by a network of unmyelinated nerves and terminal varicose axons. The terminals contained small agranular (30-60 nm) and large granular vesicles (65-110 nm), and were observed in close contact with the basal lamina of the acini, but never internally to the basal lamina. Meibomian axons showed like-immunoreactivity (LI) for the neuropeptides SP, CGRP, NPY, and VIP. In addition, the axons stained for TH, DBH, NOS, and NADPH-d. VIP-LI, NOS- and NADPH-d-positive axons appeared to be more numerous, TH- and DBH-positive axons more rare than others. Most SP-LI axons were double-labelled for CGRP-LI, some for VIP-LI or NPY-LI. In addition, some VIP-LI axons were double-labeled for NPY-LI. NPY/VIP-LI and NPY/SP-LI axons were only observed close to the Meibomian acini. Conversely, NPY-LI colocalized with TH-IR or DBH-IR predominated in perivascular nerves of Meibomian gland vasculature. The close association of varicose axons with the acini of Meibomian glands indicates that nervous signals modulate meibomian secretion. Meibomian gland nerve fibers in the cynomolgus monkey appear to utilize various neuropeptides, catecholamines and nitric oxide as transmitter substances, and seem to derive from the pterygopalatine, superior cervical and trigeminal ganglion respectively.