Histology Of The Eye and Ear PDF

The human eye has three coats or layers: 1- Fibro-elastic outer coat (corneo-scleral coat) which consists of sclera and cornea. Sclera is the white posterior part and consists of dense collagen fibres and few elastic fibres. The cornea is the anterior transparent part of the outer coat. 2- Middle Vas...

The human eye has three coats or layers: 1- Fibro-elastic outer coat (corneo-scleral coat) which consists of sclera and cornea. Sclera is the white posterior part and consists of dense collagen fibres and few elastic fibres. The cornea is the anterior transparent part of the outer coat. 2- Middle Vascular coat which is called Uveal tract and consists of three parts : Iris, Ciliary body & Choroid. The choroid lies between the sclera and retina. The pigmented iris regulates the amount of light reaching the retina. The aperture of the iris is called the pupil. 3- Inner photosensitive nervous coat which is the Retina. LO-1 Cornea consists of the following five layers: 1. Corneal epithelium made of stratified squamous non-keratinized epithelium (5 or 6 layers). 2. Bowman’s membrane or anterior limiting membrane which is made of compactly packed collagen fibrils. 3. Stroma which is composed of around 200 layers of collagen fibrils with fibroblasts that are arranged in a highly organized manner. 4. Descemet’s membrane made of compactly packed collagen fibrils 5. Endothelium of anterior chamber made of simple squamous epithelium. LO-2 Histology Of The Retina The retina is principally composed of two parts: A- The pigmented cuboidal epithelium part which contains melanin pigments that absorbs the light after the photoreceptors have been stimulated in order to prevent back reflection of the light. LO-3 B- The multilayered nervous part. In the nervous part there are three main neurons: a- Photoreceptors(rods and cones) b- Bipolar neurons c- Ganglionic multipolar neurons In addition to the three main neurons the nervous part contains association neurons and glial cells such as: Horizontal cells- Amacrine cells - Muller’s cell Histologically, the retina is subdivided into 10 recognizable layers. LO-3 Starting from the outside to the inside of the eye, the 10 layers of the retina are: 1. Pigmented layer which is formed by a layer of pigmented simple cuboidal epithelium. 2. Photoreceptor Layer of rods and cones—made of outer and inner segments of rods and cones. 3. External limiting membrane 4. Outer nuclear 5. Outer plexiform layer LO-3 6. Inner nuclear layer —formed mainly by the cell bodies of bipolar neurons. 7. Inner plexiform layer. 8. Ganglionic cell layer formed by ganglionic cells which will form the optic nerve 9. Nerve fibre layer which is composed of axons that will form the optic nerve 10. Internal limiting membrane LO-3  Histology Of The Eyelid The eyelid consists of a dense fibro-elastic plate (the tarsal plate) which is covered externally by thin skin and lined internally by the conjunctiva. Orbicularis oculi muscle can be seen between the skin and tarsal plate. Within the tarsal plate lie 12-30 tarsal (Meibomian) glands Associated with the eyelashes are some ciliary sweat glands. The conjunctiva is the epithelium which covers the exposed part of the sclera and inner surface of the eyelids. It consists of non keratinized, stratified columnar or cuboidal epithelium, with goblet cells. LO-4  The Inner Ear  It consists of the bony labyrinth and membranous labyrinth filled with perilymph and endolymph respectively Parts of bony labryinth Vestibule Semicircular canals cochlea LO-5  The Inner Ear Parts of membranous Sensory receptors in Functions labryinth membranous labryinth Utricle and saccule maculae Balance Semicircular ducts Crista ampullaris Balance Cochlear ducts Organ of corti Hearing LO-5 What are the sensory functions of the inner ear? The inner ear contains sense organs serving both balance and hearing. All these sense organs are located inside the membranous labyrinth. A- Hearing is sensed by the organ of Corti within the scala media of the cochlea. B- Head vertical position is sensed by the otolith organs of the saccule and utricle C- Head rotation is sensed by the cristae ampularis of the semicircular canals LO-5  Histology Of The Cochlea The lumen of the cochlea is divided into three chambers: the scala vestibuli, the scala media (or cochlear duct) and the scala tympani. Both scala vestibuli and scala tympani are filled with perilymph while the scala media is filled with endolymph. The organ of Corti is located in the scala media where it is surrounded by the endolymph. LO-5  The organ of Corti rests on the basilar membrane and contains two types of hair cells: inner hair cells and outer hair cells. It also contain supporting cells. Basal part of the hair cells is directly related to the dendritic process of the afferent sensory neuron. The apical part of the hair cell show multiple cilia that are attached to the tectorial membrane. The fibrous tectorial membrane rests on top of the stereo cilia of the hair cells. LO-5  Histology Of The Vestibule The vestibule is the central part of the bony labyrinth in the inner ear. The name comes from the Latin vestibulum, which means the entrance hall. The vestibule has two membranous sacs which are the utricle and the saccule. LO-6 The vestibule contains the utricle and the saccule. Together both of the utricle and the saccule are known as the otolith organs. Each of the utricle and the saccule has on its inner surface a single patch of sensory cells called a macula. Each macula consists of hair cells and supporting cells resting on a basement membrane. LO-6 Within each macula, the stereocilia of the hair cells are embedded in a gelatinous mass known as the otolithic membrane. Upon movement of the head, the otolithic membrane bend the cilia in the direction of gravity and the position is recognized by the central nervous system. LO-6 Histology Of Semicircular Canals The semicircular canals are arranged in different planes. Each semicircular canal contains a tube of the membranous labyrinth which is filled with endolymph. Each canal has an expanded end called the ampulla which opens into the vestibule. Each membranous ampulla contains a group of tissues called the crista ampullaris. The cristae ampullaris of the semicircular canals contains hair cells and supporting cells. LO-7  On the apical surface of the hair cells there is a gelatinous structure called the cupula. The apical surface of the hair cells contains tufts of cilia which project into the cupula. When the head moves, the cupula moves too. The tufts of cilia will move as a result of the movement of the cupula Depending on the direction of the bending of the cilia the nerve fiber will be stimulated or inhibited. LO-7