Tunica fibrosa et Tunica vasculosa bulbi THE APPLE OF EYE Tunica fibrosa et Tunica vasculosa bulbi Dr. Andrea D. Székely

THE ORGAN OF VISION Composition of the EYE 􀂄 The eye is responsible for us to detect electromagnetis radiation as LIGHT as well as, to distinguish between BRIGHT/DARK contrasts and to see in COLOURS 􀂄 The visual organ comprises - the apple of the eye with a 3 layered structure and the accessories -eyelids and lacrimal apparatus Composition of the EYE -external fibrous coat (Tunica fibrosa bulbi) Sclera Cornea (+ Conjunctiva) -middle vascular coat (Tunica vasculosa bulbi, Uvea) Choroidea Ciliary body Iris -inner nervous coat (Tunica nervosa bulbi) Retina

EMBRYOLOGY 5. WEEK: 2 optic vesicles form in the lateral aspects of the DIENCEPHALON, then emerge to the surface to induce the LENS primordium the LENS will be engulfed by the optic vesicles which turns into a double walled „wine glass” the external wall keeps its single layer - transforms to pigment epithelium the internal wall develops into the pars optica of RETINA (4/5 of the retina) composed by 9 layers, whereas in the blind part, pars caeca (1/5 of the retina), 2 layers will be formed the 2 layered epithelium on the iris and the ciliary body is also part of the retina

TUNICA FIBROSA BULBI Sclera The sclera, or white of the eye, is the opaque (usually white), fibrous, protective, outer layer of the eye containing collagen (and elastic) fibers. It is derived from the neural crest. In the elderly, slightly yellow, fatty deposits may appear. The sclera forms the posterior 80% of the connective tissue coat of the eye. It is continuous with the dura mater and the cornea, and maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscles. The sclera is perforated by nerves and vessels passing through the posterior scleral foramen. At the optic disk, two-thirds of the sclera continues with the dural sheath, the other third with some choroidal tissue forms a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi). The thickness of the sclera varies from 1 mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. The main parts of the eye also include the conjunctiva. This is a thin layer covering the sclera.

TUNICA FIBROSA BULBI Cornea L C S I The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance. The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue. The cornea has no blood supply; it gets oxygen directly through the air. It borders with the sclera by the corneal limbus. L C S I

TUNICA FIBROSA BULBI CORNEAL LAYERS Corneal epithelium: stratified squamous epithelium. Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity. It is continuous with the conjunctival epithelium. Bowman's layer (anterior limiting membrane = a condensed layer of collagen) protects the corneal stroma, consisting of irregularly-arranged collagen fibers. 8- 14 microns thick. Corneal stroma (substantia propria) a thick, transparent layer, consisting of regularly-arranged collagen fibers (approx. 200 layers of type I collagen fibrils). 90% of the corneal thickness. Descemet's membrane (posterior limiting membrane) a thin acellular layer that serves as the modified basement membrane of the corneal endothelium. Corneal endothelium: a simple squamous or low cuboidal monolayer of mitochondria-rich cells responsible for regulating fluid transport between the aqueous and corneal stromal compartments.

TUNICA VASCULOSA BULBI The uvea (Lat. uva, grape), also called the uveal layer, uveal coat, uveal tract, or vascular tunic, is the pigmented middle of the three concentric layers that make up an eye. The name is possibly a reference to its reddish-blue or almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye. The uvea lyes between the corneoscleral unit and the retina. It is traditionally divided into 3 regions, the iris, ciliary body and choroid. These distinctions are based on their different structures as seen under light microscopy, and continued use of these terms is appropriate in anatomical studies.

TUNICA VASCULOSA BULBI Iris (Regenbogenhaut) 􀂇 The anterior part of the UVEA (acts as a diaphragm). 􀂄 It forms a mobile separation between the anterior and posterior chambers of the eye. 􀂇 Due to its muscular components, it may change its thickness and extension, while the PUPIL may shrink to 1,5 mm (Miosis) or widen to 12 mm (Mydriasis) in response to changes in illumination. 􀂄 The M. sphincter pupillae lyes near to the edge of the Pupil (Margo pupillaris iridis), it is composed of a circular layer of oriented smooth muscle cells. They will have cholinergic innervation. 􀂄 The M. dilatator pupillae setzt is also composed of smooth muscle cells, following a radial orientation, it reaches to the ciliary margin of IRIS. It is innervated by noradrenergic axons deriving from the Centrum ciliospinale (via Ganglion cervicale superius).

TUNICA VASCULOSA BULBI IRIDAL LAYERS 􀂄 the anterior surface contains an incomplete layer of Mesothelial cells (ENDOTHELIUM CAMERAE ANTERIORIS) 􀂄 The Stroma contains a wide collection of cells (Melanocytes, Mast cells, Macrophages and Fibrocytes), as well as blood vessels and nerves, plus two muscles (M. sphincter and dilatator pupillae). 􀂄 the M. sphincter pupillae lies close to the pupil 􀂄 the M. dilatator pupillae forms a lose layer 􀂄 The posterior surface will be covered in 2 layers of pigment epithelium (derivative of the RETINA). 􀂇 Highly vascularized and innervated tissue. 􀂄 Circulus arteriosus iridis major – an anastomotic ring formed by the Aa. ciliares posteriores longae and the Aa. ciliares anteriores at the Margo ciliaris. 􀂄 Circulus arteriosus iridis minor – radial anastomotic vessels connecting the Margo ciliaris to the Margo pupillaris iridis. 􀂄 The IRIS does not absorb aqueous humor due to the lack of fenestrated capillaries

TUNICA VASCULOSA BULBI Corpus ciliare (CILIARY BODY) The ciliary body is the circular tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section, and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle. This double membrane is often regarded to be continuous with the retina and a rudiment of the embryological correspondent to the retina. The inner layer is unpigmented until it reaches the iris, where it takes on pigment. The retina ends at the ora serrata. The function of the ciliary body is accommodation, aqueous humor production and the production and maintenance of the lens zonules. It extends from the ora serrata to the root of the iris. There are three sets of ciliary muscles in the eye, the longitudinal, radial, and circular muscles. They are near the front of the eye, above and below the lens. They are attached to the lens by connective tissue called the zonule of Zinn, and are responsible for shaping the lens to focus light on the retina. When the ciliary muscle relaxes, it flattens the lens, generally improving the focus for farther objects. When it contracts, the lens becomes more convex, generally improving the focus for closer objects.

TUNICA VASCULOSA BULBI Corpus ciliare Histology 􀂇 The epithelium consists of two layers: 1. pigmented, 2. unpigmented. 􀂇 It is covered in both surfaces by Basement membranes :Membrana limitans interna and externa. The Membrana limitans externa is continouos with the BRUCH Membrane of the Choroid. 􀂇 The Stroma corporis ciliaris is similar to the Stroma iridis. 􀂇 The M. ciliaris consists of 3 parts: 1. An outer meridional muscle bundles (BRÜCKE muscle), 2. A middle, radial bundle (Pars obliqua), 3. An inner, circular bundle (MÜLLER muscle). 􀂄 The muscle contraction leads to the dilation of lens fibres therefore the lens may follow the curvature of the bulbus (e.g. will be rounder - Accomodation). The innervation derives from the parasympathetic cranial nuclei (EW)

TUNICA VASCULOSA BULBI Choroid The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina The choroid is supplied in humans by the posterior ciliary arteries, originating form the ophthalmic artery. The arteries of the uveal circulation, supplying the uvea and outer and middle layers of the retina, are branches of the ophthalmic artery and enter the eyeball without passing alongside the optic nerve. The retinal circulation derives from the central retinal artery, also a branch of the ophthalmic artery, but passes together with the optic nerve. They are branching in a segmental distribution to the terminal arterioles and do not form anastomoses. This is clinically significant for diseases affecting the choroidal blood supply. The macula, responsible for central vision, and the anterior part of the optic nerve are dependent on choroidal blood supply.

TUNICA VASCULOSA BULBI CHOROIDAL LAYERS: 1. HALLER layer (Arteries) 2. Lamina vasculosa (Arterioles) 3. Lamina choriocapillaris (capillaries) 4. Lamina vitrea (Complexus basalis, BRUCH Membrane) 􀂇 the HALLER layer is composed of the branches of the Aa. Ciliares posteriores breves (they break through the sclera in the vicinity of the Discus n. optici) and of an anastomotic mesh given by the Aa. ciliares posteriores longae and the Aa. ciliares anteriores. 􀂇 the capillaries establish a glomerular composition and will open through short venous tributaries into the 4 Vv. Vorticosae. These vessel lie in the Sclera and will be drained by the V. ophthalmica superior et inferior. 􀂇 The BRUCH membrane (cca. 2 µm thick collagen-elastin layer) contains 3 layers, lyes above the pigment cells of the retina. FUNCTIONS:maintaining the internal pressure, to form the blood – retina barrier, desaccomodation Macular degeneration

TRABECULAR MESHWORK An area around the base of the cornea, near the ciliary body, responsible for draining the aqueous humor from the eye via the anterior chamber The tissue is spongy (Fontana spaces) and lined by trabeculocytes; it allows fluid to drain into the Schlemm's canal (venous sinus) flowing into the episcleral veins. The corneal limbus is the border of the cornea and the sclera. (TUMORS!!)