OVERVIEW on ...
The MEIBOMIAN GLANDS
The Meibomian Glands produce the lipids for the superficial layer of the tear film that is of utmost importance for the stability of the tear film as such and, on top, for providing its a smooth outer layer for perfect light refraction.
Since the pre-corneal TEAR FILM is the target of all functional aims of the ocular surface because it is indispensable to maintain the moisture of the mucosal surface and thereby the transparency of the cornea - everywhere & everytime - the importance of the Meibomian glands can thus not be overrated.
AMOUNT of MEIBOMIAN GLAND TISSUE
The Meibomian Glands fill almost the complete tarsal plates
The volume of the Meibomian Glands, composed of the ductal system together with the secretory system, fills almost the complete volume of the tarsal plate in the upper and lower eye lids.
Their length thus depends on the maximal size of the tarsal plates which is about double in the upper lid compared to the lower one. The structure of the glands is also different because they are more slender in the upper lid but also more stout and wider in the lower lid.
The volume of the Meibomian glands is double in the upper lid
An exact calculation of the gland volume by Jack GREINER and colleagues revealed a gland volume of 26µl in the upper vs. 13µl in the lower that has a lid size relation of the glandular tissue roughly equivalent to the size of both tarsal plates . This may give us a very rough idea how much Meibomian oil may be available in total. This is of course the complete gland volume and the relation of volume of ducts vs. acini may be about double volume of the acini compared to the ductal system as derived from histological investigations. Therefore the volume of secreted oil inside the ductal system may be in the range of about 10µl to 15µl on average per eye. This is quite a bit compared to the large capacity of oil to spread in very thin layers on aqueous fluid.
Schematic Structure of the MEIBOMIAN GLANDS
The Schematic Structure of the Meibomian Glands consist of a long central duct with lateral narrow connecting ductules that connect the spherical acini that are filled with the secretory cells which produce the lipids.
THE DUCTAL SYSTEM is composed of:
an orifice that opens on the posterior lid margin just anterior to the Line of MARX
is encircled by cornified epidermis that also grows in for about half a millimeter
a short EXCRETORY/ TERMINAL duct
this is an ingrowth of the cornified epidermis for about half a millimeter
a long CENTRAL duct
several narrow shorter lateral DUCTULES that lead to
spherical secretory ACINI
which they connect to the central duct for the transport of the secretum
HISTOLOGIC Structure of the MEIBOMIAN GLANDS
The terminal duct of the Meibomian Glands has a physiological risk for obstruction
The terminal part of the central duct where the Meibomian oil is delivered onto the lid margin is termed terminal duct or excretory duct and is an ingrowth of the cornified epidermis from the terminal posterior part of the free lid margin.
Since the cornified epidermis encircles the orifice on the surface of the free lid margin and also the inside of the terminal duct inside the lumen of the terminal duct typically anuclear cornified epithelial squames occur in different amount (as seen in the histological photomicrograph below).
This is a valid indication that the TERMINAL PART of the Meibomian gland duct
is physiologically CORNIFIED and
can potentially be OBSTRUCTED when
either the mass of epithelial squames increases
due to increased cornification, as occurs in several chronic skin diseases
or the viscosity of Meibomian oily secretum increases and gradually solidifies
or when TYPICALLY Increased CornificationAND Increased Viscosity occur together
Which is actually the equivalent of the material composed of lipids with keratinized cell detritus that typically blocks the Meibomian glands in obstructive MGD as shown in several studies
The HOLOCRINE Secretion Mode of the Sebaceous Meibomian Glands has a clear structural basis
The holocrine secretion mode of the Meibomian Glands glands is easily understandable when they are investigated in light microscopy. ´Holo´-´crine´ means that the whole cells (Greek ´holos´) are constitute/ disintegrate/ produce (Greek ´crinein´) the secretum. Holocrine therefore describes that the whole cells disintegrate and eventually form the oily lipid secretum of the gland.
If this is true, it should be detectable in light microscopy if this technique is of any value for understanding the structure and potential function of tissues and organs. ... AND ... indeed, it can be seen in respective light microscopical images of sufficient resolution, that:
the secretory cells (termed Meibocytes) in the spherical secretory acini become larger in size,
whereas the nucleus is shrinking
at the same time the cytoplasm
becomes increasingly pale and empty in ordinary histology where the lipids are diluted
because the meibocytes are increasingly filled with lipid droplets
... so even though the meibocytes increasingly loose their information-store in the nucleus they still have a productive metabolic machinery running that is biased to produce large amounts of lipids
The holocrine Production of Meibomian Oil leads to death of the secretory cells and the oil consiSts of all the cell remnants
When the nucleus is shrinking while the cell body becomes larger, this is always a negative sign for the vitality of the cell or, at least, it indicates a negative prognosis for the further survival of the respective cell. Still, since the cells are actively producing lipids this process is termed a ´maturation´
As can be expected from the consideration that the ultimate sign of maturation likely is death, the meibocytes also eventually loose their nucleus, the cell membrane ruptures and the whole cell thus ´disintegrates´ or dissolves. By dissolving, the whole cell transforms into the lipid secretory product termed ´Meibum´.
The maturation of Meibocytes takes about one and a half weeks
The time of a meibocyte, from the start of maturation at the basement membrane until it disintegrates in the center of the acinus, takes roughly around one and a half week, as observed by OLAMI and colleagues in the rat, but may be slightly different in different species.
Since each Meibocyte is lost during the secretory process but the acinus does not end its existence after one ´round´of lipid production and instead continuously produces lipids, we must further assume that there should be a replacement of the meibocytes that are lost in action. This is indeed true, because in the basal cell layer in the periphery of the acinus on the basement membrane, there are stem cells that continuously divide and constitute new cells that start the maturation process.