Eye Microanatomy- Pt 1

Eye Development (Embryology)

• Ectoderm forms the lens, cornea outer epithelium, and palpebrae epithelium (eyelids)
• Mesoderm forms the corneal stroma, sclera, extraocular muscles, ciliary muscles, and tunica vasculosa
• Neuroectoderm of diencephalon forms the optic cup, which remains connected to the optic stalk
• Optic cup has two layers: retina and pigment epithelium
• 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 three layers: outer fibrous tunic, middle vascular (uveal) tunic, and inner retinal (neuroepithelial) tunic
• Adnexa (accessory ocular structures) are the supporting structures, including palpebrae (eyelids), third eyelid and conjunctiva, and the lacrimal apparatus
• Knowing the eye layers is important for clinical diagnosis, such as determining the severity of a corneal ulcer

Eye Color

• Eye color follows the trend: blue, gray, green, brown iris, with blue iris having minimal pigment in the stroma of the iris

Ciliary Processes

• Ciliary processes are located at the base of the iris
• Epithelial surface of ciliary processes has two layers of low columnar epithelium, which can be pigmented or nonpigmented
• Nonpigmented epithelium forms aqueous humor, while pigmented epithelium forms zonular fibers that suspend the lens

Aqueous Humor

• Aqueous humor occupies the anterior and posterior chambers of the eye, nourishing the cornea and lens
• Aqueous humor maintains the intraocular pressure and is formed by non-pigmented cells of the ciliary processes
• Aqueous humor requires constant drainage at the iridocorneal angle

Drainage Pathway of Aqueous Humor

• Aqueous humor drains from the posterior chamber, through the pupil, to the anterior chamber, to the iridocorneal angle, past the pectinate ligaments, to the scleral venous plexus

Optical Retina

• Light passes through 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 nerve (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

Fibrous Tunic

• The fibrous tunic is composed of the sclera, cornea, and limbus
• Sclera is the posterior portion of the eye, composed of a white opaque layer of dense irregular connective tissue
• Sclera functions include eye protection, maintaining eye shape, and providing insertion point for extraocular muscle tendons
• Cornea is the anterior portion of the eye, avascular and transparent
• Cornea is richly supplied with sensory nerves (from the ophthalmic branch of cranial nerve 5)
• Cornea is highly capable of regenerating
• Cornea is composed of five layers: anterior corneal epithelium, anterior limiting lamina/subepithelial basement membrane, substantia propria, posterior limiting membrane (Descemet's membrane), and posterior epithelium (corneal endothelium)

Limbus (Corneoscleral Junction)

• Limbus is where the opaque sclera overlaps the transparent cornea
• Limbus has small blood vessels
• Nutrition from the cornea comes from the microvasculature of the limbus and aqueous humor
• Epithelium of the limbus is continuous with the conjunctiva that lines the eyelids

Vascular (Uveal) Tunic

• The vascular tunic is composed of the iridocorneal angle, iris, ciliary body, and choroid
• Iris contains the stroma, which is a pigment of loose connective tissue
• Iridial melanin is present in the stromal cells, determining the eye's color
• Iris has dilators and sphincter pupillae muscles
• Posterior epithelium of the iris contains iridic granules (corpora nigra), present in equids and ruminants at the dorsal ventral pupillary margins
• Iridocorneal angle is also known as the filtration angle or drainage angle
• Iridocorneal angle is located at the convergence of corneoscleral junction (limbus), ciliary body, and the iris
• Iridocorneal angle is composed of pectinate ligament, trabecular meshwork, and trabecular (aqueous) veins
• Iridocorneal angle functions as a drainage point for aqueous humor

Ciliary Body

• Ciliary body is the anterior expansion of the choroid at the level of the lens, containing mechanical and secretory functions
• Ciliary body houses the ciliary muscle, which is composed of smooth muscle, surrounded by loose connective tissue (elastic fibers, vessels, and melanocytes)
• Ciliary muscle contracts during accommodation, reducing the tension of the zonular fibers of the lens
• Ciliary muscle receives innervation from cranial nerve 3 (oculomotor nerve)
• Posterior pigmented epithelium of the iris is continuous with the nonpigmented epithelium of the ciliary body

Choroid

• Choroid is highly vascularized, functioning as the nutritional source for the retina
• Choroid contains four parts: anterior, external, internal, and internal to vascular layer (tapetum lucidum)

Neuroepithelial (Retinal) Tunic

• Retina has three parts: sensory/optic, non-sensory/ciliary, and non-sensory/iridal
• Sensory/optic part contacts the choroid, sending visual images to the brain, and is composed of 10 layers held in place by the vitreous body
• Combined nerve fiber layers converge on the optic disk to form the optic nerve
• Non-sensory/ciliary part consists of inner nonpigmented and outer pigmented ciliary epithelium "pars ciliaris retinae"
• Non-sensory/iridal part consists of posterior pigmented epithelium "pars iridica retinae"

Albinism

• Albinism is an inherited congenital disorder, characterized by a partial or total lack of melanin pigment in the eyes, skin, and hair
• Albinos have melanocytes, which produce melanin, but lack genes for tyrosinase
• Albinism results from inheritance of recessive alleles
• Red-colored eyes are a result of the lack of melanin pigment in the eyes, including the iris
• The red coloration is due to hemoglobin in the red blood cells