Glands of Moll: history, current knowledge and their role in ocular surface homeostasis and disease

Abstract

Over the last 20 years, research into the Meibomian glands of the eyelids has increased exponentially and is now widely recognized as a field of research. It is all the more astonishing that knowledge about another type of gland in the eyelids, the Moll glands or ciliary glands, has almost stagnated and there has been little to almost no progress, even though this type of gland as a whole takes up a relatively large volume in the upper and lower eyelids. There is not much information about the namesake Moll or the function of the glands although these are listed in nearly every textbook of anatomy, histology and ophthalmology. For this reason, we set out to compile the existing knowledge about the Moll glands of the eyelids in order to create a basis for follow-up studies and to stimulate research into this type of gland. In our literature research, we went back to the middle of the 19th century and made contact with a descendant of the Moll family and illustrate their relevance for the present. The structure of the secretory part of the Moll glands is very well described, a number of secretory products are known, but the current state of research allows only very rough speculations about their function. The overview provides numerous interesting insights, which, however, raise more questions than they provide answers.

Keywords: Apocrine gland; Ciliary glands; Glandulae ciliares conjunctivales; Jacob Anthonie Moll; Lid margin; Moll glands; Ocular surface; Sweat gland; Tear film.

Fig. 1.

Jacob Anthonie Moll aged around 65, private property of the Moll family (Jaap Moll, second cousin of coauthor Frans Moll). At the bottom reference to the portrait painter: “painted by Dora Aried Paschied ? in Kiel.

Fig. 2.

Family tree of Jacob Antonie Moll (shown in bold). [The youngest son of Jacob Anthonie Moll after whom the gands of Moll are named bore the same name as hist farther].

Fig. 3.

Title page of the dissertation by Jacob Anthonie Moll (1957): Translation: Contributions to the anatomy and physiology of the eyelids - Dissertation for the degree of Doctor of Medicine at the University of Utrecht - defended on March 18, 1857 by J. A. Moll - Utrecht - P.W. van de Weijer 1875.

Fig. 4.

Distribution of the Moll glands along the upper and lower eyelid (drawing by Jörg Pekarsky). The illustration shows that the totality of all Moll glands in the upper and lower eyelids results in an extensive glandular volume, which in its entirety accounts for more volume than the main lacrimal gland.

Fig. 5.

Depiction of a Moll gland (gland of Moll) on the hair funnel (epithelial root sheet). (drawing by Jörg Pekarsky) and histological representation of the secretory part of a Moll gland between the muscle fibers of the Riolan muscle. A shows a “Moll gland unit”. This consists of a proximal nodular secretory glandular part and a ductal part. The proximal part lies partly between muscle fibers of the Riolan muscle (arrowheads in B, distal part of the orbicularis oculi muscle, whose function has not been conclusively clarified). The distal part of the Moll gland runs straight towards the hair bulb of an eyelash (A) and joins the epithelial root sheet of the eyelash distally before the junction of a gland of Zeis (= sebaceous gland) but before the lid margin. B shows the secretory part (stars) of a Moll gland between the Riolan muscle (arrowheads). Image with Toluidine blues staining, scale bar 100 μm.

Fig. 6.

A illustration of an opened Moll gland with secretory part and excretory duct part. The various functional states of the Moll epithelial cells and the wide glandular lumen can be seen in the secretory part. The secretory part is surrounded by myoepithelial cells. (Drawing by Jörg Pekarsky). B The Moll gland with a single layer of inactive/flat glandular cells (arrows) and myoepithelial cells (arrowheads). C The Moll gland with a single layer of active secretory cells (red arrows), and myoepithelial cells (black arrowheads). Active apocrine gland cells with high apical cells (brown arrow) and glandular vesicles in the lumen. D The excretory ducts consist of two cell layers made up of typical luminal (arrows) and basal cells (asterisks). All images in B-D with toluidine blue staining, scale bars 20 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7.

Transmission electron micrographs of a Moll gland with inactive and active glandular parts. A Depiction of the epithelium of an excretory duct consisting of two cell layers made up of typical luminal (black arrow) and basal cells (yellow stars on cell nuclei). B In the area of the glandular part, myoepithelial cells lie beneath the epithelium, which appear here as dark cells (red arrowhead). The myoepithelial cell appears with typical dense bodies and contractile filaments. The image was taken from the inactive area of a Moll gland with a single cell layer (orange-coloured star on cell nucleus). C A single layer of active secretory cells (red star on nucleus) and underlying myoepithelial cells (red arrowheads). The cytoplasm of the gland cells contains granules of different sizes with different electron densities and also luminescent lipid material. D Active apocrine gland cells (red star) of a Moll gland with high apical cells and secretory vesicles stained green in the inset and marked by red arrows. Bars 2,5 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8.

Schematic representation of an eyelash with Zeis gland and Moll gland opening into the hair funnel. A If the Moll glands function normally, the secretion products of the Moll glands could hypothetically contribute to the fact that no or only very few Demodex mites appear on the eyelashes and support the homeostasis of the eyelids. This theory needs to be scientifically tested in the future (drawing by Jörg Pekarsky). B Reduced Moll gland function (red X on the Moll gland) or dysfunction of the Moll glands thus hypothetically could contribute to a reduced defence against Demodex mites, which multiply and contribute to inflammatory reactions at the eyelid margin (blepharitis) (drawings by Jörg Pekarsky). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9.

To date, it is completely unknown whether Moll glands have a thermoregulatory function at all and if so whether central proteins that are important for thermoregulation in sweat glands such as the transient receptor potential cation channel subfamily V member 4 (TRPV4) channel, the voltage-gated calcium-activated anion channel anoctamine 1, the water channel aquaporins 5 and others have a thermoregulatory function for Moll glands and which nerve fibres stimulate the secretion of Moll glands. The schematic illustration shows a Moll gland in high magnification with a section magnification in which this hypothesis is shown (drawing by Jörg Pekarsky). Further research must clarify whether Moll glands are involved in maintaining the health at the lid margin and maybe also the ocular surface.