To understand the physiology of tear flow through the nasolacrimal passage, and also its pathophysiolo-gy (Chap. 2), basic knowledge of the whole lacrimal system is necessary. The ocular surface and its adnexa comprise the cornea, the conjunctiva with bulbar, for-nical and palpebral parts, the main lacrimal gland, and the glands of the eye lids, i.e., Meibomian, Moll, Zeis, and accessory lacrimal glands as well as the nasolacrimal system; the latter consists of the upper and lower puncta, the paired lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct (Fig. 1.1). The nasolacrimal ducts collect the tear fluid from the ocular surface and convey it into the nasal cavity, whereas all other structures contribute to formation of the preocular and cornea tear film. The tear film serves to protect and lubricate the ocular surface, allowing for protection of the cornea and consistent clarity of vision.
The preocular tear film contains water, protective antimicrobials, cytokines, lipids, and mucins, and is divided into three components: a lipid, an aqueous, and a mucus component. The lipid component is secreted by the Meibomian glands in the eyelid and forms the superficial layer of the tear film (Fig. 1.2). The aqueous component contains electrolytes, water, and a large variety of proteins, peptides, and glyco-peptides, and is secreted primarily by the lacrimal gland as well as the accessory lacrimal glands (glands of Krause, glands of Wolfring) of the lids (Fig. 1.2). The mucus component is the product of conjunctival goblet and epithelial cells, corneal epithelial cells , and acinar as well as excretory duct cells of the lacri-
Fig. 1.1. Ocular surface and nasolacrimal ducts. The ocular bulbus with cornea and bulbar conjunctiva, as well as tarsal conjunctiva, are visible. At the medial rim of the upper and lower lid open the lacrimal puncta leading into the lacrimal sac via the upper and lower canaliculi. The lacrimal sac is situated in the orbital lacrimal fossa and proceeds into the nasolacrimal duct. The nasolacrimal duct is surrounded by a bony canal created by the maxillary and lacrimal bones and opens into the inferior meatus of the nose. Both lacrimal sac and nasolacrimal duct are surrounded by a vascular plexus comparable to a cavernous body that is connected to the cavernous system of the nose. (From )
Fig. 1.2. Structures of the eyelid. a Overview. Sagittal section through an eye lid. tm tarsalis muscle, t tarsus, ppo palpebral part of orbicularis muscle, Mg Meibomian gland. White arrow: excretory duct of Meibomian gland; Mo Moll's gland. Black arrows: sections through eyelashes. b-e Representative magnifications of the areas marked by a large black arrow in a. b Conjunctival epithelium in the area of the tarsal plate near the fornix. The epithelium consists of columnar epithelial cells with integrated goblet cells (arrows). Tight connective tissue of the tarsus underlies the epithelium. c Conjunctival epithelium in the
Fig. 1.2. Structures of the eyelid. a Overview. Sagittal section through an eye lid. tm tarsalis muscle, t tarsus, ppo palpebral part of orbicularis muscle, Mg Meibomian gland. White arrow: excretory duct of Meibomian gland; Mo Moll's gland. Black arrows: sections through eyelashes. b-e Representative magnifications of the areas marked by a large black arrow in a. b Conjunctival epithelium in the area of the tarsal plate near the fornix. The epithelium consists of columnar epithelial cells with integrated goblet cells (arrows). Tight connective tissue of the tarsus underlies the epithelium. c Conjunctival epithelium in the area of the tasal plate near the rim of the eye lid. A non-cornified squamous epithelium covers the underlying tarsal plate. Parts of the Meibomian gland are visible. d The magnification shows an accessory lacrimal gland (Krause's gland; Kg). A small excretory duct opens into an infolding of the fornical conjunctiva (arrows). Above the gland parts of the tarsalis muscle (tm) are visible. e Magnification of a part of a Meibomian gland (Mg) reveals its sebaceous character. The gland is embedded in the tarsal plate. (Figure2a is from , Figures 2b-e are from )
mal gland, which have recently been shown to produce mucins (Fig. 1.2) [10, 16]. The mucinous layer helps to spread the lipid and aqueous layer across the cornea.
The surfaces of the ocular epithelia, both corneal and conjunctival, provide a specialized and important in terface between the tear fluid and the epithelium that stabilizes the fluid layer. That interface includes the undulating membrane ridges on the apical cell's apical membrane, termed microplicae, and emanating from their apices, a layer termed the glycocalyx. Membrane bound mucins (MUCs 1, 4, and 16) of corneal and conjunctival epithelial cells are present in the glycocalyx layer. Soluble mucins (MUC5AC) from conjunctival goblet cells, as well as MUC5B and MUC7 from lacrimal glands, are in solution in the
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.