Comprehensive Histology and Embryology of the Human Eye
Histology of the Eye: Anterior Region and External Tunics
The anterior region of the eyeball is characterized by three distinct tunics. The outermost tunica is fibrous in nature and consists of the cornea and the sclera (esclerótica). The sclera is a thick, acidophilic coating composed of dense connective tissue. Moving inward, the middle tunica is the vascular tunica, which includes the iris, the ciliary body (corpo ciliar), and the choroid (coroide). These structures are highly vascularized and pigmented. In typical laboratory preparations, the choroid and retina may appear separated from the sclera due to tissue rupture. The innermost tunica is the retina, which is organized into several layers and delimits the vitreous chamber (câmara vítrea) internally.
In the living eye, the space between the lens (cristalino) and the cornea is divided into the anterior chamber (câmara anterior) and the posterior chamber (câmara posterior). These spaces are normally occupied by aqueous humor, although this fluid is typically not preserved in histological preparations. During the preparation process, the lens may be displaced anteriorly or exhibit structural ruptures.
Histology of the Cornea
The cornea is an avascular structure (it contains no blood vessels) and is composed of five specific layers. The first and most superficial is the corneal epithelium (epitélio corneano), which is a non-keratinized stratified squamous epithelium. At the corneal periphery, this epithelium transitions into the conjunctival epithelium. The second layer is Bowman's membrane (membrana de Bowman), an acellular, homogeneous-looking layer situated between the epithelium and the underlying stroma.
The third and thickest layer is the corneal stroma (estroma or substância própria), consisting of collagen fibers and slender fibroblasts interspersed among the fibers. The fourth layer is Descemet's membrane (membrana de Descemet), which functions similarly to a thick basement membrane between the stroma and the innermost layer. The final layer is the endothelium of the anterior chamber (endotélio da câmara anterior), a simple squamous epithelium that delimits the space between the cornea and the iris where the aqueous humor once circulated.
Histology of the Lens (Cristalino)
The lens is composed of acidophilic, elongated cells known as lens fibers. Many of these fibers are anucleated, although cells at the equator of the lens may still retain their nuclei. One of the faces of the lens is covered by a simple cuboidal epithelium. External to this epithelium is the lens capsule (cápsula do cristalino), which is glycoprotein in nature. Histological sections viewed at and magnifications reveal these structural details, though the lens is often partially destroyed during slide preparation.
Ciliary Body and Iris
The ciliary body is identified by its proximity to the iris and its voluminous structure. It contains ciliary processes (processos ciliares), which are responsible for the secretion of aqueous humor, and the ciliary muscle (músculo ciliar). The epithelium of the ciliary body consists of two distinct layers: a pigmented layer adjacent to the connective tissue and a second layer characterized by pink-colored cells. The conjunctiva, located nearby, consists of stratified epithelium and loose connective tissue, resting upon the dense connective tissue of the sclera.
The iris features an anterior face covered by connective tissue cells. The stroma of the iris is composed of loose connective tissue containing numerous blood vessels and melanocytes (pigment-producing cells). The epithelium covering the posterior face of the iris is a continuation of the retina and is composed of two layers of highly pigmented epithelial cells. In some preparations, the iris may appear distorted or twisted, appearing behind the lens between the lens and the ciliary body.
Histology of the Medial Region and Retina
The medial and posterior contour of the eyeball is formed by the sclera, choroid, and retina. The sclera serves as the outermost dense connective tissue layer. Immediately internal to it is the choroid, which is thinner than the ciliary body and is composed of loose connective tissue rich in blood vessels and melanocytes.
The retina consists of various nuclear layers separated by lightly stained layers where synapses occur. Key layers from outer to inner include:
The pigmented epithelium (epitélio pigmentar), which adheres directly to the choroid.
The layer of cones and rods (cones e bastonetes), which are the extensions of the photoreceptor cells.
The external nuclear layer (camada nuclear externa), containing the nuclei of the photoreceptors.
The internal nuclear layer (camada nuclear interna), containing the nuclei of bipolar cells.
The ganglion cell layer (camada de células ganglionares).
The optic nerve fiber layer (camada de fibras do nervo óptico).
Embryology of the Eye: 4th to 6th Weeks
The development of the eye begins in the 4th week of development (equivalent to a whole chick embryo at the same stage). At this time, a dilation called the optic vesicle (vesícula óptica) appears on each side of the prosencephalon (forebrain). By the 5th or 6th week, the optic vesicle invaginates to form the optic cup (taça óptica), which remains connected to the diencephalon by the optic stalk (pedículo óptico). The optic cup will eventually differentiate into the retina.
The lens forms from the surface ectoderm. This tissue deepens toward the optic cup to form the lens vesicle (vesícula do cristalino). The optic cup itself has two layers: an outer layer (folheto externo), which is thin and pigmented, and an inner layer (folheto interno), which has multiple layers of cells. The outer layer gives rise to the retinal pigmented epithelium, while the inner layer develops into the neural layers of the retina. The surrounding mesenchyme (embryonic connective tissue) differentiates into the choroid and the sclera.
Embryology of the Eye: 8th to 11th Weeks
By the 8th week of development, the posterior epithelial cells of the lens vesicle extend forward to occupy the entire cavity; these cells are the lens fibers, many of which lose their nuclei. The anterior face of the lens retains its simple epithelium. During this stage, the eyelid primordium (esboço da pálpebra) appears as a protrusion covered by ectoderm. The surface ectoderm at this stage gives rise to the epidermis of the eyelid skin, the conjunctival epithelium, and the corneal epithelium.
In horizontal or transverse sections from the 8th week, several structures are identifiable:
The optic cup and its borders.
The primordium of the nasal fossae.
The optic nerve and the hyaloid artery (artéria hialoide). The proximal portion of this artery will later give rise to the central retinal artery.
Hyaloid vessels within the vitreous chamber, which are destined to degenerate.
By the 11th week, the eye is much more differentiated. The retina shows distinct layers (pigmented and neural), and the extrinsic eye muscles (músculos extrínsecos) are in the process of differentiation. The optic nerve is easily identified. Hyaline cartilage can be seen separating the nasal fossae from the orbit. The anterior region at this stage includes the developing cornea, though the iris and pupil may not always be visible depending on the exact plane of the histological section.