6- Eye Microanatomy- Pt 1.docx

- **Eye Development (embryology)** - In embryology, the **ectoderm** forms the lens, cornea outer epithelium, and palpebrae epithelium (AKA: eyelids). - In embryology, the **mesoderm** forms the corneal stroma, sclera, extraocular muscles, ciliary muscles, and tunica...

- **Eye Development (embryology)** - In embryology, the **ectoderm** forms the lens, cornea outer epithelium, and palpebrae epithelium (AKA: eyelids). - In embryology, the **mesoderm** forms the corneal stroma, sclera, extraocular muscles, ciliary muscles, and tunica vasculosa. - In embryology, the **neuroectoderm** of diencephalon forms the optic cup that remains connected to the optic stalk. - The optic cup has 2 layers: the retina and pigment epithelium. - The optic stalk forms the optic nerve. - **Eye: General Info** - The eyeball (globe) is located in the skull's boney socket, known as the "orbit". - The eye is composed of a lens, and 3 layers of the: outer fibrous tunic, middle vascular (uveal) tunic, and inner retinal (neuroepithelial) tunic. - The **adnexa (accessory ocular structures)** are the supporting structures. - The adnexa includes the: palpebrae (eyelids), third eyelid and conjunctiva, and the lacrimal apparatus. - It is important to know the eye layers to help diagnose things clinically. - For example, to know how severe a corneal ulcer is, you would have to recognize which layer of the eye is infected. - The deeper the infected layer, the more severe the corneal ulcer. - **Eye color** follows the following trend, in order of least pigmented stroma to most pigmented stroma: Blue, gray, green, brown iris. - The blue iris has minimal pigment in the stroma of the iris. - **Ciliary processes** are located at the base of the iris. - The epithelial surface of ciliary processes have 2 layers of low columnar epithelium, which can be pigmented or nonpigmented. - Nonpigmented: ion transporting forming aqueous humor - Pigmented: basement membrane extends to form zonular fibers that suspend the lens - **Aqueous humor** occupies the anterior and posterior chambers of the eye, where it nourishes the cornea and lens (which is glucose rich). - Aqueous humor maintains the intraocular pressure, and it is formed by non-pigmented cells of the ciliary processes. - The aqueous humor requires constant drainage at the iridocorneal angle. - The **drainage pathway of aqueous humor** involves it draining: from the posterior chamber, through the pupil, to the anterior chamber, to the iridocorneal angle, past the pectinate ligaments, to the scleral venous plexus. - General concepts of the **optical retina**: - Light passes though layers of the retina, stimulating photoreceptor cells (rods and cones). - Impulse is passed to bipolar neurons, then to ganglion cells. - Axons of ganglionic cells form the nerve fiber layer, where these layers converge at the optic disk (papilla) and leave the eye as the optic never (cranial nerve 2). - Additional cells with supporting roles include: Amacrine cells (interneurons), horizontal cells (regulate input from photoreceptor cells), and Muller cells (glial cells). - **Eye: Tunics** - The eye tunics include the: fibrous tunic, vascular (uveal) tunic, and neuroepithelial (retinal) tunic. - The **Fibrous tunic** is composed of the sclera, cornea, and limbus. - **Sclera** - The sclera is the posterior portion of the eye that is composed of a white opaque layer of dense irregular connective tissue. - Functions of the sclera include: eye protection, maintain eye shape, and provide insertion point for extraocular muscle tendons. - **Cornea** - The cornea is the anterior portion of the eye which is avascular and transparent. - The transparency of the cornea is maintained by relative dehydration. - This dehydration is due to adenosine triphosphatase and carbonic anhydrase pumps transporting water out of the cornea. - The pumps are located in the anterior and posterior corneal epithelium. - The transparency is due to being avascular, and collagen arrangement with proteoglycans between the collagens. - The cornea is richly supplied with sensory nerves (from the ophthalmic branch of cranial nerve 5). - The cornea is highly capable of regenerating. - The cornea is composed of these 5 layers: - Anterior corneal epithelium - Anterior limiting lamina/subepithelial basement membrane - This supports the lining epithelium - Substantia propria - This is a corneal stroma (mostly of Type 1 collager) - Posterior limiting membrane (Descemet's membrane) - This supports the endothelium - Posterior epithelium (corneal endothelium) - **Limbus (corneoscleral junction)** - The limbus makes up the corneoscleral junction, and is where the opaque sclera overlaps the transparent cornea. - The corneoscleral junction has small blood vessels. - Nutrition from the cornea comes from the microvasculature of the limbus and aqueous humor. - The epithelium of the limbus is continuous with the conjunctiva that lines the eyelids. - The **Vascular (uveal) tunic** is composed of the iridocorneal angle, iris, ciliary body, and choroid. - The **Iris** contains the **stroma**, which is a pigment of loose connective tissue. - Iridial melanin is present in the stromal cells, where they determine the eye's color. - The iris has dilators and sphincter pupillae muscles. - The **posterior epithelium** of the iris contains iridic granules (**corpora nigra**), which are present in equids and ruminants at the dorsal ventral pupillary margins. - The **iridocorneal angle** is also known as the filtration angle or Drainage angle. - The iridocorneal angle is located at the convergence of corneoscleral junction (limbus), ciliary body, and the iris. - The iridocorneal angle is composed of pectinate ligament, trabecular meshwork, and trabecular (aqueous) veins. - The iridocorneal angle functions as a drainage point for aqueous humor. - The **ciliary body** is the anterior expansion of the choroid at the level of the lens, and it contains mechanical and secretory functions. - The ciliary body houses the ciliary muscle, which is composed of smooth muscle, and is surrounded by loose connective tissue (elastic fibers, vessels, and melanocytes). - The ciliary muscle contracts during accommodation, reducing the tension of the zonular fibers of the lens. - The ciliary muscle receives innervation from cranial nerve 3 (oculomotor nerve). - Posterior pigmented epithelium of the iris are continuous with the nonpigmented epithelium of the ciliary body. - The **choroid** is highly vascularized, and functions as the nutritional source for the retina. - The choroid contains 4 parts: - Anterior: continuous with the stroma of the ciliary body - External: connected to the sclera - Internal: connected to the retinal pigmented epithelium - Internal to vascular layer: tapetum lucidum - **Neuroepithelial (retinal) tunic** - The **Retina** has 3 parts: Sensory/optic, Non-sensory/ciliary, Non-sensory/iridal - Sensory/optic part: contacts the choroid. - This part of the retina sends visual images go the brain, and is composed of 10 layers, which are held in place by the vitreous body. - The combined nerve fiber layers converge on the optic disk to form the optic nerve. - Non-sensory/ciliary part: inner nonpigmented and outer pigmented ciliary epithelium "pars ciliaris retinae" - Non-sensory/ iridal: posterior pigmented epithelium "pars iridica retinae" - The retina is nourished by vessels of the choroid and by retinal vessels entering via the optic disk. - **Albinism** - Albinism is the inherited congenital disorder, characterized by a partial or total lack of melanin pigment in the eyes, skin, and hair- with the eyes being the rarest. - Albinos have melanocytes, which produce melanin, however, they lack genes for tyrosinase. - Albinism results from inheritance of recessive alleles. - Red colored eyes are a result of the lack of melanin pigment within the eyes, including the iris. - The red coloration is due to hemoglobin in the red blood cells.