SYMPOSIUM ON OPHTHALMIC CYTOLOGY
Year : 2007 | Volume
: 24 | Issue : 1 | Page : 16--19
Anatomy, physiology, histology and normal cytology of eye
Yashoda Hospitals, Hyderabad, Andhra Pradesh, India
16-11-16/M/38, East Prashant Nagar, Moosarambagh, Malakpet, Hyderabad- 500036
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Sudha A. Anatomy, physiology, histology and normal cytology of eye.J Cytol 2007;24:16-19
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Sudha A. Anatomy, physiology, histology and normal cytology of eye. J Cytol [serial online] 2007 [cited 2020 Mar 30 ];24:16-19
Available from: http://www.jcytol.org/text.asp?2007/24/1/16/42084
Accurate cytological interpretation of ocular specimens requires a fundamental knowledge of normal ocular histology, anatomy and cytology. This is a brief general overview with emphasis on those areas where cytology has relevance.
The average adult eye measures about 25 mm horizontally, 23mm vertically, and 21-26mm anteroposteriorly. The eye has an external approximate volume of 7.6 ml; the aqueous has a volume of about 1.5 ml and the vitreous a volume of 4.0ml. The lacrimal gland is located supero-laterally in the orbit and is divided by the orbital septum. 
The conjunctiva covers the posterior surface of the eyelids (palpebral conjunctiva), curves anteriorly at the fornix to reflect onto the anterior surface of the eye as the bulbar conjunctiva. The conjunctiva covering the lid margin and bulbar conjunctiva is a modified non-keratinised stratified squamous epithelium. The tarsal and fornix conjunctiva is covered by stratified cuboidal to columnar epithelium of varying thickness. Goblet cells are abundant over the tarsus, fornix and specialized areas such as the plica semilunaris. Goblet cells are scarce near the lid margin and adjacent to the cornea at the limbus.  Epithelial layer covers a substantia propria that is thickest in the fornices and thinnest covering the tarsus. Constituents of this stromal layer include loosely arranged collagen fibres, vessels, nerves, resident lymphocytes, plasma cells and mast cells. Within the stroma of the caruncle are sebaceous glands, hair follicles, and accessory lacrymal glands. 
Cytology: Most swabs of the conjunctiva are taken from the inferior fornix and show clusters and single epithelial cells with abundant cytoplasm, eccentric nuclei and occasional single nucleoli. Goblet cells have clear vacuoles filled with mucin. The presence of keratinised epithelium in the conjunctival smear is distinctly abnormal unless the sample is taken from the caruncle or accidentally from the eyelid. 
The normal cornea is composed of five layers: epithelium, Bowman's layer, stroma, Descement's membrane and endothelium. The cornea is embryologically derived from the surface ectoderm and neural crest. The normal external surface of the cornea is composed of a stratified squamous, nonkeratinising epithelium ranging between five and seven layers in thickness.  The basal cells are smaller and have a higher nuclear-cytoplasmic ratio than the other epithelial cells in the cornea. Bowman's layer is a specialized layer of collagen. The stroma is composed of lamellar sheets of collagen arranged perpendicularly.
Cytology: Surface smears from the normal cornea will demonstrate cohesive sheets of non-keratinising squamous epithelium. Individual cells exhibit intermediate size, round nuclei with bland and uniform chromatin. The presence of keratinised cells in smears from the cornea is abnormal. 
Anterior Chamber and Trabecular Meshwork
The anterior chamber is bounded anteriorly by the corneal endothelium, posteriorly by the anterior surface of the iris-ciliary body and papillary portion of the lens, and peripherally by the trabecular meshwork. The depth of the anterior chamber is about 3.4-3.7 mm. The trabecular meshwork is derived from the neural crest.
The outermost corneo-scleral meshwork is composed of multiple layers of collagenous sheets that are lined by very thin endothelium. The uveal meshwork composed of beams or cords are covered by a thicker layer of endothelium. Schlemm's canal encircles the trabecular meshwork and is joined to the corneo-scleral meshwork by the internal collector channel. 
The iris, ciliary body and choroid constitute the uveal tract. It is embryologically derived from the mesoderm and neural crest. 
Iris: The iris is the pigmented diaphragm separating the anterior and posterior chambers.  It is composed of five layers: anterior border layer, stroma, muscular layer, anterior pigment epithelium and posterior pigment epithelium. 
Cytology: The iris is rarely sampled by cytologic techniques. However, normal iris may appear in intraocular washings from incidental ocutome cutting of the iris in an attempt to remove vitreous or lens fragments in the anterior chamber. Normal iris may also appear in fine needle aspiration specimens of iris neoplasms. In general, normal iris epithelium is so densely pigmented that cellular details are obscured. Iris stroma is characterized by the fine reticular meshwork of very cohesive and vascularised stroma. 
Ciliary Body: The ciliary body is composed of the ciliary processes, ciliary muscle, and ciliary epithelium. About 70 radially arranged ciliary processes form the pars plicata anteriorly and are joined posteriorly with the smooth portion of the ciliary body, the pars plana. The pars plana joins the retina and choroids at the ora serrata. Two layers of epithelium that include an inner non-pigmented layer and an outer-pigmented layer cover the ciliary body.
Cytology: Under normal circumstances, ciliary body structures will not appear in vitrectomy specimens. However, ciliary epithelium may be sampled by fine needle aspiration of adjacent tumours. It is important to recognize the two-layered structure of the epithelium with abundant cytoplasm and large pigmented granules. 
Choroid: The choroid is the pigmented vascular tissue that forms the middle coat of the posterior part of the eye. It extends from the ora serrata anteriorly to the optic nerve posteriorly and consists of 3 principle layers: lamina fusca, stroma and choriocapillaries. 
The crystalline lens is a soft, elastic, avascular biconvex structure derived from surface ectoderm.  It is encapsulated and is suspended by thin zonules that are attached to the ciliary body. The lens epithelium is located on the internal surface of the capsule. The interior of the lens is composed of cortical and nuclear cells.
Cytology: The lens is generally sampled during vitrectomy or lensectomy. The lens capsule can be identified on cytology preparations as a translucent (glass) membrane. Cortical fragments taken from the lens periphery or the bow sometimes demonstrate nucleated cells. The central part of the lens produces clusters of cells with transparent, lacy cytoplasm and almost pyknotic, central, small, round nuclei. Lens fragments appear as eosinophilic hexagonal structures by hematoxylin and eosin, and light green with Papanicolaou stain  and are Periodic-acid-Schiff reagent positive.
The vitreous cavity is simply an expanded extracellular space that normally contains 4.0 ml of clear gelatinous substance that is composed largely of water, hyaluronic acid, and collagen. The vitreous normally contains antero-posterior oriented collagen fibrils and occasional macrophages or hyalocytes. The presence of even small numbers of acute or chronic inflammatory cells within the vitreous is distinctly abnormal. The vitreous has distinct attachments to ocular structures. It is attached anteriorly in a circumferential band extending from the posterior pars plana to a few millimeters behind the ora serrata in what has been termed the vitreous base. Traction exerted by the vitreous body at the base results in hyper-pigmentation of the underlying pigment epithelium and is evident grossly. The vitreous is also attached to the retina over retinal blood vessels and at the optic disc. These attachments are important to understanding vitreous traction, retinal tears, and retinal detachment, for which vitrectomies are sometimes performed. 
The sensory neuro-epithelium of the eye is the retina, which is composed of many layers. These include the layer of outer and inner segments of the photoreceptor cells, the outer nuclear layer (cell bodies of photoreceptor cells), the outer plexiform layer, the inner nuclear layer, the inner plexiform layer, the ganglion cell layer, the nerve fiber layer, and the inner limiting lamina. The retina is loosely attached to the pigment epithelium, which is separated from the choroid by Bruch's membrane. 
Retinal pigment epithelium: The retinal pigment epithelium is a monolayer that lies between photoreceptor outer segments and Bruch's membrane. The epithelium has many functions including matrix production for photoreceptors, phagocytosis of outer segments, barrier protection and active transport. These cells are large; are polygonal in shape; and contain abundant cytoplasm, round nuclei and single nucleoli. The cytoplasm contains large distinctive ovoid and elliptical pigment granules. The retinal pigment epithelium has a remarkable potential to proliferate and undergo metaplastic transformation. 
Cytology: Normal and abnormal retina and pigment epithelium may be sampled in both vitrectomy and fine needle aspiration. In cytologic preparations, the retina usually appears as a plexiform pattern of cells with round nuclei and characteristic organoid architecture and distinctive nuclear haloes. Occasionally, ganglion cells may be sampled. It is important for the cytologist to report retinal fragments discovered in intraocular washings because full thickness breaks in the retina may lead to retinal detachment. Fragments of partial thickness retina that have been stripped in the process of peeling membranes from the retinal surface are not uncommon in intraocular washings and are not regarded presently as clinically significant.  In cases of retinal detachment, numerous pigmented retinal cells may be found in the aspirate of the fluid accumulated behind the detachment.
The eyelids can be subdivided into orbital and tarsal components. At the level of the tarsus, the eyelid consists of four main histologic layers, from anterior to posterior, i.e., skin, orbicularis oculi muscle, tarsus and palpebral conjunctiva. The skin consists of an epidermis of keratinising stratified squamous epithelium, which also contains melanocytes and antigen-presenting Langerhans' cells; and a dermis of loose collagenous connective tissue that contains cilia and associated sebaceous glands (of Zeiss), Apocrine sweat glands (of Moll), eccrine sweat glands and pilo-sebaceous units. The orbicularis oculi is composed of striated muscle. The tarsal plate, a thick plaque of dense fibrous connective tissue, contains the sebaceous Meibomian glands. Also present near the upper border of the superior tarsal plate are the accessory lacrimal glands of Wolfring; the accessory lacrimal glands of Krause are present in the conjunctival fornices. [Table 1] gives the normal function and common pathology of the glands of the eyelid. 
Seven bones (frontal, zygomatic, palatine, lacrimal, sphenoid, ethmoid and maxilla) form the boundaries of the orbit, all of them thick except those forming the medial and inferior walls, which are easily eroded and fractured. The orbital cavity is pear shaped with a volume of 30 cc. Other elements occupying the cavity are the following: globe, lacrimal gland, muscles, tendons, fat, fascia, vessels, nerves, sympathetic ganglia and cartilaginous trochlea. Inflammatory and neoplastic processes that increase the volume of the orbital contents lead to proptosis of the globe and/ or displacement from the horizontal or vertical position. 
Lacrimal gland: It is situated anteriorly in the supero-temporal quadrant of the orbit. The gland is divided into orbital and palpebral lobes by the aponeurosis of the levator palpebrae superioris muscle. The acini of the glands are composed of low cuboidal epithelium. The ducts, which lie within the fibro-vascular stroma, are lined by low cuboidal epithelium with a second outer layer of low flat myoepithelial cells. 
The optic nerve, embryologically derived from the optic stalk, is a continuation of the optic tract. Thus the pathology of the optic nerve reflects that of the central nervous system.
|1||Glasgow BJ, Foos RY. Normal anatomic and cytologic features. In: Ocular cytopathology. Oxford: Butterworth-Heinemann; 1993. p. 9-18.|
|2||Leisegang TJ, Deutsch TA, Grand MG. Conjunctiva, cornea, anterior chamber and trabecular meshwork, sclera, lens, vitreous, retina, uveal tract, eyelids, orbit, optic nerve. In: Ophthalmic pathology and intraocular tumours, Basic and clinical science course, Section 4. Sanfransisco: The foundation of the American academy of ophthalmology; 2001-2002. p. 62, 79, 89, 96-97, 109-110, 120-121, 153-154, 171-172, 191, 208.|