OV1113_OcularAnatomyAndPhysiology_pj_v1_2_c.pptx

OV1113: Ocular anatomy & physiology Pete Jones 〈[email protected]〉 Tear film 2 The tear film  When light rays impinge on the surface of the eye, the first thing they encounter is the tear film  A thin (~7 µl) layer of fluid with multiple...

OV1113: Ocular anatomy & physiology Pete Jones 〈[email protected]〉 Tear film 2 The tear film  When light rays impinge on the surface of the eye, the first thing they encounter is the tear film  A thin (~7 µl) layer of fluid with multiple functions: Lubricates the eye (lubrication) Barrier to foreign bodies and pathogens (protection) Maintains health of underlying epithelial cells (nutrition) Ensuring a smooth refractive surface for vision Tear film 3 The tear film  When light rays impinge on the surface of the eye, the first thing they encounter is the tear film  A thin (~7 µl) layer of fluid with multiple functions: Lubricates the eye (lubrication) Barrier to foreign bodies and pathogens (protection) Maintains health of underlying epithelial cells (nutrition) Ensuring a smooth refractive surface for vision  It is composed of a sophisticated mix of mucous, serous, and sebaceous substances, arranged in layers Tear film 4 Reminder  External secretions (incl. the tear film) emerge from exocrine glands, located within epithelial tissue  Given the complex of the tear film, a number of exocrine glands are involved… Tear film 5 Outer lipid layer (sebaceous)  Various roles, including: Minimises evaporation Limits contamination from particles & microorganisms Increases surface tension (stops tears overflowing onto the skin)  Secreted primarily by meibomian (“tarsal”) glands, located within tarsal plates of upper and lower eyelids The tarsal plate are regions of dense connective tissue located deep in the upper and lower eyelid Tear film 6 Outer lipid layer (sebaceous)  Various roles, including: Minimises evaporation Limits contamination from particles & microorganisms Increases surface tension (stops tears overflowing onto the skin)  Secreted primarily by meibomian (“tarsal”) glands, located within tarsal plates of upper and lower eyelidsThe The tarsal plate are regions of dense connective tissue located deep in the upper and lower eyelid  A simple, multicellular exocrine gland (i.e., they only have one duct, but have multiple secretory units) + alveolar  They open onto the ocular surface via pores at the eyelid margin Tear film 7 Central aqueous layer (serous)  Various roles, including: Washing away foreign bodies and contaminations Nourish the avascular cornea (oxygen, proteins, salts)  Secreted primarily by the lacrimal gland, located above the upper eyelid  A compound multicellular exocrine gland (i.e., multiple ducts) + alveolar Tear film 8 Inner mucin layer (mucous)  Various roles, including: Form a glycocalyx over the ocular epithelium that prevents pathogen adhesion Reduce friction during blinking  Secreted by goblet cells, located in the conjunctiva Conjunctiva being a mucous membrane covering posterior (inner) surface of the eyelid and the exposed surface of the eyeball up to the limbus  A unicellular exocrine gland (note its distinctive triangular nucleus) Tear film 9 Tear film summary  The tear film covers the anterior (front) surface of the eye  It has many important roles, and is composed of 3 components: 1. Sebaceous secretions (tarsal glands) 2. Serous secretions (lacrimal gland) 3. Mucous secretions (conjunctival goblet cells)  Once secreted, blinking is required to distribute the tear film over the ocular surface… Tear film 10 Blinking  Two skeletal muscles are the main muscles involved in opening and closing the eye 1. The orbicularis oculi runs in concentric bands around the eye  Part of the muscle lies within the face and part within the eyelid  and its contraction closes the palpebral aperture (the opening between the eyelids  Whenever the orbicularis oculi muscle contracts, the tears are drawn from the tear film Tear film 11 Blinking  Two skeletal muscles are the main muscles involved in opening and closing the eye 2. The levator palpebrae superioris runs along the top of the eyeball and sends tendons down into the eyelid  Its contraction raises the upper eyelid Tear film 12 Blinking  Two skeletal muscles are the main muscles involved in opening and SKELETAL closing the eye  Other muscles also involved.  E.g., The superior tarsal muscle is a smooth muscle adjoining the levator palpebrae superioris muscle that helps to raise the upper eyelid SMOOTH OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 13 Cornea & sclera 14 The cornea & sclera  The outer coat of the eye is mostly opaque sclera (the white of the eye) Composed of dense irregular connective tissue containing primarily collagen fibres  The central 1/6th is modified into the transparent cornea (see next slides)  The corneoscleral junction is known as the limbus Cornea & sclera 15 The cornea is a 5-layer structure 1. epithelium 2. Bowman’s layer  The cornea protects the eye, and helps to focus incoming light on the retina (it is the single main source of refractive power) 3. stroma  It is composed of 5 layers… 4. Descemet’s membrane 5. endothelium Cornea & sclera 16 1. epithelium Corneal epithelium 2. Bowman’s layer  The outer corneal layer is known as the corneal epithelium The corneal  It is a stratified (multilayered) 3. stroma epithelium: squamous epithelium A stratified squamous  It protects from dirt, eyelids, clumsy epithelium optometrists… 4. Descemet’s membrane 5. endothelium 1 2 Cornea & sclera 17 Corneal stroma  The stroma forms the bulk of the cornea  It is an example of a dense regular connective tissue, as it is made up of tightly packed, and regularly arranged collagen. This is what makes it transparent  In contrast the collagen in the sclera is irregular, and so this structure is opaque i.e., the fibres themselves are largely the same in sclera and cornea Cornea & sclera 18 Corneal stroma  The stroma forms the bulk of the cornea  It is an example of a dense regular connective tissue, as it is made up of tightly packed, and regularly arranged collagen. This is what makes it 1. epithelium 2. Bowman’s layer transparent  In contrast the collagen in the sclera is irregular, and so this structure is 3. stroma opaque  Fibroblasts can also be observed (appearing as dark patches sandwiched between lighter areas of tightly packed collagen fibres) 4. Descemet’s membrane 5. endothelium Cornea & sclera 19 1. epithelium Corneal endothelium 2. Bowman’s layer  Single layered (simple) epithelia would not be much good for protection. They are, however, ideal, for areas where 3. stroma the exchange of substances is The corneal required… endothelium:  A good example of a simple squamous A simple squamous epithelium is the inner surface of the epithelium cornea, known as the corneal 4. Descemet’s membrane endothelium 5. endothelium  It allows the cornea to exchange substances with the aqueous below, 3 since the cornea itself is avascular Nutrients are transported up, waste is 4 5 transported down OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 20 Anterior cavity 21 Anterior chamber  A space between the cornea and iris, ~3 mm in depth  Filled with aqueous humour: A transparent fluid, similar to plasma but with few proteins. 98% water Provides nutrition (amino acids & glucose) for the avascular ocular tissues (cornea, lens, etc.), immunity, and support (intraocular pressure) Also provides a small amount of refraction It constantly circulates, being secreted by the ciliary body and draining out through the trabecular meshwork (canal of Schlemm) and uveoscleral pathway (see later slides) Anterior cavity 22 Posterior chamber  The aqueous humour also extends into the posterior chamber – the space between iris and lens  The anterior & posterior chambers together form the anterior cavity (a.k.a., anterior segment) OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 23 Uvea 24 Uvea overview  The uvea is made up of: 1. Choroid 2. Ciliary body 3. Iris (that forms the pupil)  Will discuss each in turn Uvea = latin for “grape”, since if removed is dark red, wrinkled, and gape-like in size and shape Uvea 25 Choroid  A vascular layer sandwiched between the Sclera sclera and the retina. Its main function is to supply nutrients to the outer retina  Clinically, Bruch’s Choroid membrane, which separates the choroid and the retina, is particularly important. It is, for Retina example, where Drusen (parcels of undrained metabolic waste) are formed. Bruch’s Light membrane Uvea 26 Choroid  Rich in melanocytes Light (cells containing melanin pigments) that prevent intraocular light scatter The “Black Box Effect” Uvea 27 Ciliary body  The ciliary body extends between the choroid & iris  As we shall see, it has two main functions: to control the shape of the lens & to produce the aqueous humour Uvea 28 Ciliary body surface: overview  Structurally, the surface of the ciliary body is divided into 2 regions: 1. Anterior pars plicata 2. Posterior pars plana (smooth & thinner) Uvea 29 Ciliary body surface: overview  Structurally, the surface of the ciliary body is divided into 2 regions: 1. Anterior pars plicata 2. Posterior pars plana (smooth & thinner) Uvea Pars plana  The pars plana is a flat extension from the posterior aspect of the ciliary body to the ora serrata: Transition region (~3.5mm in length in which the complex, multi-layered, photosensitive region of the retina merges with the simple, non- photosensitive area of the ciliary body Scalloped (“serrated”) in appearance  The lens zonules that are used to control accommodation run along the pars plana surface, and are anchored to the ora serrata Uvea Pars plicata  The pars plicata is a ring of 70– 100 ciliary processes (“ridges”) with intervening “valleys” Uvea Pars plicata  The pars plicata is a ring of 70– 100 ciliary processes (“ridges”) with intervening “valleys”  Each ciliary process (both the ridges & valleys) is covered by a 2 layered epithelium called the ciliary epithelium bilayer Note not stratified, these are two separate layers with distinct developmental origins  Unpigmented epithelium: produces aqueous humour  Pigmented epithelium: part of the eye’s “black box effect” Uvea 33 Pars plicata: Aqueous humour Route is a lot less direct in practice!  As shown right, aqueous travels from the ciliary body, past the lens and supplies the avascular lens and cornea with nutrients  It is drained through the trabecular meshwork into the canal of Schlemm at the filtration angle (the angle between iris & cornea) Uvea 34 Pars plicata: Aqueous humour Histological section of the filtration angle  As shown right, aqueous travels from the ciliary body, past the lens and supplies the avascular lens and cornea with nutrients  It is drained through the trabecular meshwork into the canal of Schlemm at the filtration angle (the angle between iris & cornea) Uvea 35 Pars plicata: Aqueous humour  As shown right, aqueous travels from the ciliary body, past the lens and supplies the avascular lens and cornea with nutrients  It is drained through the trabecular meshwork into the canal of Schlemm at the filtration angle (the angle between iris & cornea)  Also a second outflow pathway, directly through uveosceleral tissue Uvea 36 Pars plicata: Aqueous humour  Interference with the drainage (“outflow”) of aqueous humour results in a build-up of pressure within the eye (as aqueous is continuing to be produced)  This will put pressure on the optic nerve, damaging the axons within it and eventually leading to loss of sight  This is (one form of) glaucoma Uvea 37 Ciliary muscle  Below the the anterior surface layer (pars plicata) and posterior surface layer (pars plana) lies the stroma and ciliary muscle  The stroma provides the blood that gets filtered to form aqueous humour (re: the unpigmented epithelium discussed previously) stroma literally means “bed cover,” and refers to a supporting tissue. Not to be confused with soma (“body”) Uvea 38 Ciliary muscle  Below the the anterior surface layer (pars plicata) and posterior surface layer (pars plana) lies the stroma and ciliary muscle  The ciliary muscle is a form of smooth muscle As we shall see later, it contracts to change the shape of the lens (accommodation) Uvea 39 iris Iris & pupil pupil  The iris is an ‘offshoot’ of the ciliary body sclera anteriorly  It has a central aperture (the pupil) Uvea 40 iris Iris & pupil pupil  The iris is an ‘offshoot’ of the ciliary body sclera anteriorly  It has a central aperture (the pupil)  Its posterior surface is covered by a 2 layered epithelium, but unlike in the ciliary body both layers contain melanin. This ensures that the only light that enters the eye is though the pupil Uvea 41 iris Iris & pupil pupil  The iris is an ‘offshoot’ of the ciliary body sclera anteriorly  It has a central aperture (the pupil)  Its posterior surface is covered by a 2 layered epithelium, but unlike in the ciliary body both layers contain melanin. This ensures that the only light that enters the eye is though the pupil  Its stroma is an example of a loose areolar connective tissue, allowing it to change shape (to dilate or construct the pupil) Uvea 42 iris Iris pupil  The iris is an ‘offshoot’ of the ciliary body sclera anteriorly  It has a central aperture (the pupil)  Its posterior surface is covered by a 2 layered epithelium, but unlike in the ciliary body both layers contain melanin. This ensures that the only light that enters the eye is though the pupil  Its stroma is an example of a loose areolar connective tissue, allowing it to change shape (to dilate or construct the pupil)  The iris contains 2 smooth muscles: The iris sphincter constricts the pupil. It is parasympathetically innervated The iris dilator opens up the pupil. It is sympathetically innervated NB: These muscles form an opposing pair (re: coactivation lab) Uvea 43 iris Pupil pupil  The pupil is the dark circular opening sclera in the centre of the iris, where light enters the eye  It controls the amount of light entering in the eye  In the healthy eye, pupil size varies in diameter (from 2 to 4 mm in bright light, to 4 to 8 mm in dark)  Pupil abnormalities (e.g., differences in size between the eyes – anisocoria – or in how they respond to light) can be indicative of a wide range of ocular and neural pathologies OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 44 Lens 45 Lens overview  The crystalline lens provides variable refractive power, allowing the eye to focus on objects at different distances  It consists of 3 concentric layers; 1. An elastic capsule covering the entire surface 2. A simple epithelium underneath this 3. The bulk of the lens is made of elongated cells (lens fibres) Lens 46 Lens overview  The crystalline lens provides variable refractive power, allowing the eye to focus on objects at different distances  It consists of 3 concentric layers; 1. An elastic capsule covering the entire surface 2. A simple epithelium underneath this 3. The bulk of the lens is made of elongated cells (lens fibres) Lens 47 Lens overview  Lens fibres are long hexagonal cells that run from the front to the back surface of the lens  Adjacent lens fibres are joined by gap junctions to allow the flow of nutrients…  …As well as desmosomes and various mechanical junctions that present the fibres from tearing apart Lens 48 Lens overview  The elastic properties of the capsule ensure that the lens’s natural state is thick & round Lens 49 Zonules  The lens is suspended within a ring of fibrous strands called zonules (‘little bands’)  These fibres (8-12 nm) are non-collagenous and composed mainly of a cysteine-rich fibrillin, giving them elasticity  The zonule fibres run along the ciliary body and are anchored to the “dentate processes of the ora serrata” (i.e., in-between the scalloped feet of the pars plana) Lens 50 Zonules  The lens is suspended within a ring of fibrous strands called zonules (‘little bands’)  These fibres (8-12 nm) are non-collagenous and composed mainly of a cysteine-rich fibrillin, giving them elasticity  The zonule fibres run along the ciliary body and attached to the pars plana  Normally, the zonule fibres are pulled tight like the guide ropes on a tent. When the ciliary muscle contracts, the diameter of the ciliary body decreases, allowing the zonule fibres to go lax  due to the lens capsule the lens becomes thicker and rounder Lens 51 Accommodation Unaccommodated Accommodated  By contracting the ciliary muscles, the (Distance vision) (Near vision) lens becomes thicker and more curved, increasing its refractive power  This allows the eye to focus on near objects (accommodation) Lens 52 Accommodation  By contracting the ciliary muscles, the lens becomes thicker and more curved, increasing its refractive power  This allows the eye to focus on near objects (accommodation)  At full accommodation the lens can provide ~15 diopters of refraction (or 30D in infants!), though this declines with age (presbyopia)  Though the majority of refractive power (~45D) is provided by the cornea – but unlike the lens this value is fixed OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 53 Vitreous chamber 54 Vitreous chamber  Largest of the three chambers  Located behind the lens and in front of the retina  Also sometimes called the posterior cavity (not to be confused with the posterior chamber of the anterior cavity!)  Filled with vitreous humour (aka “vitreous body”) Vitreous chamber 55 Vitreous humour Vitreous humour  A clear gel, much more viscous than the aqueous  It is 99% water, with just a few cells (mostly just a few phagocytes to clear away cellular debris), as well as some salts, sugars, etc…  …plus a network of collagen fibrils Aqueous extend throughout to make it gel-like. It humour is more fluid-like near the centre, and more gelatinous near the edges  Provides support for the eyeball, plus additional refractive power Vitreous chamber 56 Vitreous humour  With age the collagen fibres degrade and the vitreous starts to liquify  The fibres holding the surrounding membrane to the retina can break, potentially leading to small floaters or even complete posterior vitreous detachment OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 57 Retina & optic nerve 58 Retina  The retina is what the eye is all about; the other structures are just there to ensure a good image on a healthy retina (“accessory structures”)  The retina lines the back of the eye, terminating anteriorly at the ora serrata Retina & optic nerve 59 Retina  The retina is what the eye is all about; the other structures are just there to ensure a good image on a healthy retina (“accessory structures”)  The retina lines the back of the eye, terminating anteriorly at the ora serrata  It is a 10 layered structure, composed primarily of neurons, glia, plus additional supporting elements (epithelial cells, blood vessels, etc.) Retina & optic nerve 60 Retina  The layers of the retina are also visible in vivo using OCT Retina & optic nerve 61 Retina  Curiously, these layers are arranged “back to front”, with the photoreceptors most posterior sclera Retina & optic nerve 62 Retina  Curiously, these layers are arranged “back to front”, with the photoreceptors most posterior  The photoreceptors are the sensory r r r receptors that transduce (convert) the optical image into nervous potentials (phototransduction)  Cones (c) are sensitive at high light c c c (photopic) levels, while the shorter rods (r) subserve low light (scotopic) vision Retina & optic nerve 63 Retina  The retinal pigment epithelium (RPE) supports the photoreceptors in many important ways. It transports nutrients into and removal of waste products from photoreceptor cells, enables retinoid transport and regeneration, and performs phagocytosis of photoreceptor outer segments,  One RPE cell supports 30–50 photoreceptors, which shed daily ~5% of their outer segment mass Retina & optic nerve 64 Retina: Neurons & glia  The retina is made up of 5 main types of neuron; photoreceptors, horizontal cells, bipolar cells, amacrine cells and ganglion cells. Additionally, there are supporting glial cells (mainly Műller cells)  Light traverses all of the inner retinal neurons before reaching the photoreceptors(!)  The electrical signals leave via ganglion cell axons, which make up the optic nerve (cranial nerve II) Retina & optic nerve 65 Retina: Optic nerve  The optic nerve is a myelinated bundle of 1-2 million nerve fibres, all of which leave the orbit (eye socket) via the optic canal Technically part of the CNS, but often treated as part of the PNS Retina & optic nerve 66 Retina: Blood vessels  In a fundus photo one can also see 2 major blood vessels emanating from the optic disc  These are the central retinal artery (CRA) and central retinal vein (CRV), branches of which supply and drain blood to/from the inner 5 layers of the retina  The fundus also reveals an area where there are no blood vessels. This is the macula region, the centre of which is the fovea Retina & optic nerve 67 Retina: Fovea  In a histological section, the fovea is characterised by a ‘dip’ in the retina, where the inner retinal layers are pushed aside (rendering the photoreceptors relatively unobscured)  It is also where the photoreceptors are most densely packed  As a result, the fovea subserves the highest acuity vision  we therefore often move our eyes to image objects of interest on this region (“foveate”) Cones photoreceptors OV1113: Ocular anatomy & physiology ▪ Tear film ▪ Cornea & Sclera ▪ Anterior cavity & aqueous humour ▪ Uvea: ▪ Choroid ▪ Ciliary body ▪ Iris & Pupil ▪ Lens ▪ Vitreous chamber & vitreous humour ▪ Retina & optic nerve ▪ Adnexa (incl. extraocular muscles) 68 Adnexa 69 The end of the tour? Adnexa 70 Adnexa  Adnexa is latin for attachment – in biology refers to gross supporting structures  The Ocular Adnexa incorporates those parts of the orbital region that are outside the eyeball, including: Extraocular muscles (eye movement) Lacrimal gland (tear film production) Eyelids (spreading of tear film) Conjunctiva (additional tear film production + immune response) Eyebrows & eyelashes (barriers to contaminants + facial expression) Adnexa 71 Extraocular muscles  Six sets of skeletal muscles (4 rectus muscles, 2 oblique) Adnexa 72 Extraocular muscles Don’t need to remember all these terms  Six sets of skeletal muscles (4 for OV1113, but will need to know them in future years rectus muscles, 2 oblique)  Can produce a range of eye movements: Move the eye inwards, towards the nose (adduction) Move the eye outwards, away from the nose (abduction) Move the eye upwards (supraduction) Move the eye downwards (infraduction) Rotate the top of the eye towards the nose (intorsion) Rotate the top of the eye away from the nose (extorsion) Adnexa 73 Extraocular muscles  Six sets of skeletal muscles (4 rectus muscles, 2 oblique)  Can produce a range of eye movements: Move the eye inwards, towards the nose (adduction) Move the eye outwards, away from the nose (abduction) Move the eye upwards (supraduction) Move the eye downwards (infraduction) Rotate the top of the eye towards the nose (intorsion) Rotate the top of the eye away from the nose (extorsion) Adnexa 74 Extraocular muscles  Six sets of skeletal muscles (4 rectus muscles, 2 oblique)  Subserve various different types of eye movements, including:  Vestibulo-ocular reflex (to counteract head rotation)  Smooth pursuit (to track objects)  Saccades (rapid shifts of gaze)  Vergence eye movements (to maintain binocular vision)  Further Reading  [Overview] Chapter 40 (“The Eye”) of Standring, S. (2008). Gray’s Anatomy 40th Edition City, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 5060 E: [email protected] W: https://www.city.ac.uk/people/academics/peter-jones  77 Key groupings to consider All 4 tissue types: Muscle: skeletal & smooth Connective Nervous Epithelial Refractive components (cornea, lens, & humours) Sensory receptors: all in retina (everything else accessory structs) 2 segments 3 chambers