AHVS 1314: Ocular Anatomy and Physiology 1 - Limbus PDF

AHVS 1314: OCULAR ANATOMY AND PHYSIOLOGY 1 Limbus Fatin Amalina Che Arif MHSc. (IIUM)  Outline: 1.) Limbus anato...

AHVS 1314: OCULAR ANATOMY AND PHYSIOLOGY 1 Limbus Fatin Amalina Che Arif MHSc. (IIUM)  Outline: 1.) Limbus anatomy: Histological features Palisades of Vogt Limbal blood vessels and lymphatics Limbal cell replacement 2.) Limbus physiology: Physiological features https://www.researchgate.net/publication/221925887_Limbal_ Stem_Cell_Transplantation_and_Corneal_Neovascularization/fi gures?lo=1 ANATOMY  Anatomy “Limbus" is a Latin word meaning "border" between two different types of tissues o transitional zone between cornea and conjunctiva, and cornea and sclera Externally- formed by the junction of corneal and conjunctiva epithelia ✓ change in the curvature, due to the different radii of curvature between the cornea and sclera ✓ this abrupt change of surface contour creates a shallow furrow or sulcus externally Internally - limbal zone is located at the junction of the extreme terminus of the Descemet's membrane and the anterior edge of Trabecular meshwork, that can be identified as the Schwalbe's line under gonioscopy  Several important transitions take place at limbus: The regularly arranged corneal lamellae give way to the more random array of lamellae in the sclera. The stratified squamous non-keratinized corneal epithelium with its parallel internal and external surfaces gives way to conjunctival epithelium, characterized by a folded basal surface and interdigitating subepithelial connective tissue (sometimes forming distinct papillae). Loops or arcades of conjunctival capillaries (derived from the anterior ciliary arteries) and lymphatic capillaries terminate at the limbus. Desçemet’s membrane and Bowman’s layer terminate in this region. The loose conjunctival subepithelial vascularized connective tissue (substantia propria), containing immunocompetent cell types such as mast cells, plasma cells and lymphocytes, tapers off at the limbus and is absent in the cornea.  Histological features Histologically, the limbus is a thin strip of approximately 1 to 1.5 mm in width which encircle the periphery of cornea At the limbus, the epithelium increase in thickness from a layer 5 cells thickness to 10 15 cells thick Melanocytes may present at the basal layer and pigmentation may be evident in the limbal and conjunctiva region Bowman’s layer tapers and terminates.  Limbus contains transitions from the very regular corneal lamellae to the irregular and random organization of collagen bundles in the sclera – gives rise to the opaque feature of sclera Within the limbal stroma at the corneal periphery, a group of collagen fibrils has been found lies circumferentially, forming an annulus Limbal stroma contains mesenchymal cells and is highly innervated and vascularized Descemet’s membrane tapers at the anterior limbal boundary, and the posterior non banded portion become interlaced with the connective tissue of the anterior sheets of the trabecular meshwork The corneal endothelium continues into the anterior chamber as the endothelial covering of the sheets of the trabecular meshwork https://link.springer.com/chapter/10.1007/978-3-319-98080-5_5  Palisades of Vogt A radial projection of limbal epithelium and stroma that extend into the cornea in spoke-like fashion. The epithelium in this area is the suspected site of stem cell origin (the source of corneal epithelial basal cell replication) This projection may appear as thin, grey pegs approximately 0.5 mm wide and 2-4 mm long. ✓ Research1 found that PoV are more discrete in younger and in more heavily pigmented individuals, and that they appear more regular and prominent at the lower limbus than at the upper limbus M. F. Goldberg and A. J. Bron, “Limbal Palisades of Vogt,” Transac-tions of the American Ophthalmological Society 80 (1982): 155–171 https://www.researchgate.net/publication/8551938_Characterization_of_Putative_Stem_Cell_Phenoty pe_in_Human_Limbal_Epithelia/figures?lo=1 https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-024-05603-y https://www.researchgate.net/publication/257731893_Tseng_2010_Critical_Appraisal_of_Ex_Vivo_Expansio n_of_human_LESC/figures?lo=1  Limbal blood vessels and lymphatics Limbal vessels originate from the ophthalmic artery with their anterior ciliary branches running forward and the arterioles ending in a complex capillary network to give a rich blood supply to the limbus The conjunctival vessels form a superior plexus that branches and extends toward the limbus to form the marginal corneal arcades (MCA) and vessels in the palisades of Vogt Immune complexes are present in the extravascular space Capillary loops from conjunctival and episcleral vessels form networks in the limbus, which surround the cornea and provides nourishment to the avascular cornea Limbal veins collect blood from the anterior conjunctival veins and drain into the radial episcleral veins, which then empty into the anterior ciliary veins  Lymphatic limbal vascular complexes (LLVC)- originate from the bulbar conjunctival lymphatics Two interconnected plexes (superficial and deep) - drain towards the palpebral commissures, where the fluid enters into the lymphatics of the eyelids and ultimately reaches the lymph nodes Lymphatic channels located in the limbal area do not enter the cornea https://www.sciencedirect.com/topics/neuroscience/lymphatic-vessel  Limbal cell replacement Cell in limbal basal epithelial layer - smaller and darker staining than the usual epithelial cells The histochemical features suggest presence of stem cells, which capable for cell proliferation limbal stem cells Centripetal movement from the limbal area is responsible for cell migration and replacement of corneal epithelial basal cells in both normal cell replacements and wound healing https://www.researchgate.net/publication/253649259_Review_Corneal_epithelial_stem_cells_t heir_niche_and_wound_healing/figures?lo=1 PHYSIOLOGY  Functions In general, it acts as: ❖ a barrier to prevent overgrowth and migration of conjunctival cells to the corneal region ❖ a stabilizer to absorb small pressure changes to maintain corneal curvature and refractivity ❖ a site richly infiltrated with vasculatures and lymphatics to nourish the peripheral corneal tissue ❖ a location of aqueous humor drainage through the trabecular meshwork to maintain a healthy level of intraocular pressure ❖ an important niche of stem cells for corneal tissue regeneration and wound repair Thank You