Eye Anatomy-Histology Correlate By: Michael Lu, Class of вЂ�07 - The superficial features of the eye and eyelids are detailed here. The palpebral fissure (not labeled )is the opening of the eye itself. The lateral and medial commissures (canthus or angles) form the lateral and medial corners of the eyes. The medial commissure contains the plica semilunaris (semilunar fold) and the lacrimal caruncle. - The conjunctiva can be divided into palpebral and bulbar portions which cover the inner surface of the eyelid and the eyeball, respectively. The conjunctival sac reflects at the superior and inferior conjunctival fornices, which are potential spaces filled with tears. - The lacrimal gland secretes tears directly into the superior fornix. Blinking the eyelids spreads the tears across the eyeball to keep it lubricated. The tears are collected at the lacrimal lake and then drained by the superior and inferior lacrimal papillae. The lacrimal caruncle prevents the papillae from getting stuck to each other. - The superior and inferior tarsal plates are dense fibrous plates that give support and form to the eyelids. Tarsal glands produce oily secretions to prevent tears from leaking out, while the eyelids stay dry. - The obicularis oculi muscle contains both palpebral and orbital parts and acts to close the eyelids. - The levator palpebrae superioris muscle elevates the upper eyelid. It is a skeletal muscle under voluntary control. Another muscle, the superior tarsal muscle, is a smooth muscle that attaches to the superior tarsus. - Some parts of the lacrimal apparatus were described earlier. The lacrimal gland is located superior and lateral to the eyeball. It is divided into orbital and palpebral parts by the levator palpebrae superioris muscle. The gland secretes tears directly into the superior conjunctival fornix. The tears are distributed across the eye by blinking, collected in the lacrimal lake, and drained by the superior and inferior lacrimal papillae through puncta (pores). The tears are drained via the canaliculi into the lacrimal sac. There is a suction action when the lids are closed due to attachments of the orbicularis oculi muscle into the lateral wall of the sac. From there, they drain via the nasolacrimal duct into the nasopharynx. - The lacrimal gland (bottom left) secretes tears, which contain lysozyme and electrolytes similar to plasma. They look similar to salivary glands, with the typical appearance of serous cells. - The conjunctival epithelium (bottom right) is stratified columnar with many goblet cells. The mucous secretions contribute to the protective layer of the exposed eye and allow the eyelids to move freely over the eye. - Note the various parts of the anterior segment of the eyeball. - Most of the eyeball is occupied by clear, hyaluronanrich vitreous body. The posterior chamber is located between the vitreous body and the iris. It contains the lens and zonular fibers (suspensory ligaments) and is filled with aqueous humor. - Anterior to the iris and posterior to the cornea is the anterior chamber. - Note also the ciliary body and processes, which will be covered later. - The opening of the iris, or the pupil, is controlled by the sphincter (parasympathetic; constricts the pupil) and dilator (sympathetic; dilates the pupil) pupillae muscles. - The lens is an avascular structure that is composed of highly arranged crystallins to let light pass through. - The ocular surface of the cornea is lined with nonkeratinized stratified squamous epithelium (bottom panel). - This epithelium sits on BowmanвЂ™s membrane. Below that, the corneal stroma consists of highly ordered arrays of collagen with specific types of proteoglycan. - The inner aspect of the corneal endothelium sits upon the DescemetвЂ™s membrane. The endothelium plays a very important role in maintaining a clear cornea by regulating the state of hydration within the corneal stroma. - The ciliary body contains ciliary muscle that is composed of smooth muscle. Contraction and relaxation of the ciliary muscles change the tension of the zonular fibers, or suspensory ligaments, of the lens. This allows the lens to change shape, a process known as accommodation. - The ciliary processes are folds of connective tissue that are covered by two layers of epithelium. There is also a complex vasculature that cannot be seen easily. Fluid from these vessels is processed and transported by the epithelial cells to the posterior chamber as aqueous humor. The epithelial cells constitute the blood-aqueous barrier. - The aqueous humor enters the anterior chamber through the pupil as it flows between the lens and the iris. - Aqueous humor leaves the anterior chamber through the trabecular meshwork and into the canal of Schlemm. This is an endothelial lines, circumferentially arranged vessel that communicates with veins in the sclera and returns the aqueous humor back to the general circulation. - Obstruction of the trabecular meshwork and canals of Schlemm are thought to be the major cause of elevated intraocular pressure, which could then lead to glaucoma. - The iris is detailed here in higher magnification. Note the anterior and posterior chambers to help orient yourself. - The anterior surface of the iris contains loose, variably pigmented stroma. It is open to the circulating aqueous humor within the anterior chamber. - Two layers of heavily pigmented epithelium cover the posterior surface of the iris. - Note that the sphincter pupillae muscle can be easily seen near the pupil margin. It is smooth muscle controlled by parasympathetics. The dilator pupillae muscle is more difficult to identify, but it dilates the pupil upon sympathetic innervation. - Now we will look at the other portions of the eyeball. The eye can essentially be divided into 3 layers: - 1) sclera вЂ“ tough connective tissue that continues anteriorly as the cornea. - 2) choroid or uveal layer вЂ“ vascular, loose connective tissue that includes the ciliary body and the iris anteriorly. - 3) retina вЂ“ contains photosensory cells in the posterior portion and a non-sensory two-layered epithelium in the anterior portion. The boundary is marked by the ora serrata. - Note how the photosensory retina is interrupted by nerve fibers and blood vessels leaving the eye at the optic nerve. - The fovea centralis in the macula is a focal region of thinner retinal epithelium important for visual acuity. - The ora serrata is shown at higher magnification in the bottom panel. Note the transition from the tall, multilayered photosensory epithelium into the two-layered pigmented epithelium. - The layer adjacent to the vitreous body and aqueous humor is unpigmented, but the bottom layer remains pigmented. - Note the other two layers of the eye вЂ“ the choroid and the sclera вЂ“ as labeled. - The 3 layers of the eye are shown here again. Note the retina, the choroid, and the sclera. The top panel also indicates an extraocular muscle and some orbital fat. - Note the 10 distinct layers of the retina. Light travels through all layers of the retina and strikes the photoreceptor cells вЂ“ rods (more sensitive in dim light) and cones (more sensitive to bright light and color). Impulses are then sent back up through the integrative neurons to the ganglion cells, which finally bring the impulses to the optic nerve. - The photoreceptor cells are embedded in the retinal pigment epithelium, which is the weak spot and susceptible to retinal detachment. - The top panel shows the retina as viewed through the ophthalmoscope (with much practice). The numbered areas are magnified in the histological slides. - In the вЂњgenericвЂќ retina (#1, bottom left), note the multiple layers. There is a small vein in the ganglion cell layer and some small capillaries from retinal vessels in the inner nuclear layer (arrows). Rods and cones receive nourishment from the underlying network of capillaries in the choroid. - The fovea centralis (#2, bottom middle) in the macula lutea is the region of greatest visual acuity. Note the characteristic thinning of the epithelium. In the top left panel, the macula lutea is indicated by the circle, and the fovea centralis within the macula lutea is indicated by the arrow. The fovea contains no rod cells; it is composed of exclusively cone cells. - The optic disk (#3, bottom right) is the region where the retinal nerve fibers exit the eye as the optic nerve. It is also the region where blood vessels enter. Note discontinuity in the retinal epithelium in this area, creating the вЂњblind spotвЂќ. NOTE: - The extraocular muscles include the following: - levator palpebrae superioris muscle вЂ“ elevates the upper eyelid. - superior rectus muscle вЂ“ elevates and adducts the eyeball. - medial rectus muscle вЂ“ adducts the eyeball. - inferior rectus muscle вЂ“ depresses and adducts the eyeball. - inferior oblique muscle вЂ“ elevates and abducts the eyeball. - superior oblique muscle вЂ“ depresses and abducts the eyeball. - lateral rectus muscle вЂ“ abducts the eyeball. Inf. oblique Sup. rectus Sup. rectus Inf. oblique Lat. rectus Lat. rectus Med. rectus Sup. oblique Inf. rectus RIGHT EYE Med. rectus Inf. rectus Sup. oblique LEFT EYE - The eyeball itself is directly connected with, or part of, the optic nerve (CN II). - The extraocular muscles receive motor innervations primarily from cranial nerves III, IV, and VI. - Oculomotor nerve (III) вЂ“ The superior division innervates the levator palpebrae superioris and superior rectus muscles. The inferior division innervates the medial rectus, inferior rectus, and inferior oblique muscles. It also supplies parasympathetic innervation to the ciliary muscle and sphincter pupillae muscle via the ciliary ganglion and short ciliary nerves. The short ciliary nerves also carry sympathetics to the dilator pupillae muscle. - Trochlear nerve (IV) вЂ“ innervates the superior oblique muscle. - Abducens nerve (VI) вЂ“ innervates the lateral rectus muscle. - The ophthalmic division of the trigeminal nerve (CN V1) carries mainly sensory innervation along with parasympathetics. Its branches include frontal nerve that splits into the supraorbital and supratrochlear nerves, which are the most superior structures in the orbit. The lacrimal nerve carries both sensory and parasympathetic impulses to the lacrimal gland. The nasociliary nerve gives off multiple branches, all sensory, which include the infratrochlear, long ciliary, and anterior and posterior ethmoidal nerves. - The arterial supply to the eyeball, orbit, and eyelid comes primarily from the ophthalmic artery, which branches off the internal carotid artery. - Note the central artery that first branches off the ophthalmic artery and pierces the optic nerve. It is the only blood supply to the retina. - The supraorbital and supratrochlear arteries also branch off the ophthalmic artery, supplying the muscles, skin, and fascia of the forehead. - The anterior and posterior ethmoidal arteries supply the ethmoid and frontal sinuses and the nasal mucosa. - The lacrimal artery carries blood to the lacrimal gland and eyelids laterally. - The superior ophthalmic vein drains the eyeball, the superior portion of the orbit, the ethmoid sinuses, and the forehead into the cavernous sinus. - Note the other veins that drain the orbit, such as the supratrochlear, supraorbital, angular, and even the maxillary and retromandibular veins.