Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Sense Organs, organa sensuum
Organ of Vision - Development of the Organ of Vision

Class="center">Development of the human Eyeball (Fig. 219, A).

The Development of the human eyeball is a complex process that begins during the 3rd week of intrauterine development and continues until the end of Prenatal ontogenesis.

The retina is formed from the neural tube, the corneal epithelium and lens from the ectoderm, while the corneal proper, sclera, choroid, and vitreous body originate from the mesenchyme.

During the 3rd week of embryonic development (at a length of only 1.5–4.5 mm), on both sides of the midline of the ectodermal groove, where the Brain vesicles arise, two pits with downward-facing floors appear at their apical ends. As the ectodermal groove closes and transforms into the neural tube, the pits shift laterally and turn into optic vesicles. The optic vesicles maintain their connection with the embryonic brain as hollow optic stalks. The cavity of the primary Forebrain vesicle communicates freely with the cavity of the primary optic vesicle. The apices of the optic vesicles come into very close proximity to the ectoderm, separated only by a narrow layer of mesoderm.

At the 4th week of development (embryo length is 4.5–7.5 mm), the lens begins to form from the ectodermal plate.

At the same time, the primary optic vesicle begins to transform into the secondary one—the optic cup. The anterior part of the primary optic vesicle invaginates into its cavity, As a result of which it acquires the shape of an optic cup, the wall of which consists of two layers. The ectoderm located opposite the opening of the optic cup thickens, grows into the optic cup, and subsequently separates, giving rise to the lens. The inner wall of the optic cup transforms into the retina, and the outer wall becomes the pigmented layer of the retina. The primitive optic disc is formed. Mesoderm begins to penetrate between the optic cup and the lens primordium.

At the 5th week of development (embryo length 7.5–12 mm), the embryonic optic fissure closes, the lens capsule, fibers, and capsule epithelium are formed, the vascular tunic of the lens arises, and the primitive neuroepithelium appears. The vascular network of the choroid proper begins to develop.

At the 6th week of intrauterine development (embryo length 12–17 mm), lens fibers are formed from the posterior Cells of the lens capsule. The pupillary membrane arises. The primary vitreous body and hyaloid vessels are formed. The main layers of the cornea, as well as the retinal layers, are established.

The anterior (Bowman's) limiting membrane of the cornea is formed from the anterior layers, while the posterior (Descemet's) membrane is derived from the endothelium lining the cornea from the side of the anterior chamber of the eyeball.

At the 7th week of development (17–24 mm), the stroma of the iris arises, the anterior and posterior ciliary Arteries are formed, and nerve fibers from the retina enter the optic canal.

* The development of the visual organ and developmental anomalies are presented according to Professor L. R. Mateshuk-Vatseba (Danylo Halytsky Lviv National Medical University).

Fig. 219 A. Development of the optic cup and lens.

A, B — embryos about 4 mm in length (after Heuser):

1 - surface ectoderm; 2 - wall of the forebrain; 3 - optic sulcus. C, D — embryo 5 mm in length (after Mall):

1 - wall of the forebrain; 2 - optic vesicle; 3 - lens placode. E, F — embryos about 7–10 mm in length (after Mann, Prantiss):

1 - pigmented layer; 2 - sensory layer; 3 - lens

At the 8th week (24–31 mm), the sclera develops, and pigmentation of the outer wall of the optic cup is observed.

At the 9th week (31–40 mm), the sclera thickens, the pupil is closed by the pupillary membrane, the hyaloid vessels disappear, and the secondary vitreous body and the internal limiting membrane of the retina develop.

At the 10th week (40–49 mm), rods and cones appear.

At the 11th week (49–59 mm), the ciliary body and the ectodermal part of the iris are formed.

At the 12th week of the intrauterine developmental period (fetus length is 59–70 mm), the embryonic Nucleus is formed in the lens. Schlemm's canal is formed, retinal arteries appear, and the vascular tunic of the lens is still preserved.

At the 4th month of intrauterine development (fetus length is 70–110 mm), the vascular plexus around the optic disc is formed, and the iris, ciliary body, and choroid proper develop.

At the 5th month (fetus length 110–160 mm), the Muscles of the iris develop from the marginal Regions of the optic cup, the Muscle fibers of the ciliary body are formed from the mesoderm, and all layers of the choroid proper differentiate, except for the choriocapillary layer.

At the 6th month (160–200 mm), glial sheaths are formed around the hyaloid vessels. The hyaloid artery departs from the central retinal artery at the site of its entry onto the optic disc and extends into the primary vitreous body, approaches the posterior surface of the lens, and forms the posterior part of the vascular tunic of the lens.

Following the resorption of the primary vitreous body and its hyaloid vessels, only Cloquet's canal remains (running from the posterior surface of the lens to the Optic nerve HEAD).

The fovea centralis of the retina is formed, and the development of the posterior pigment epithelium of the iris is completed.

By the 7th month (200-240 mm), the pupillary membrane undergoes resorption.

By the 8th month (240-280 mm), the posterior wall of the vascular capsule of the lens disappears, retinal vascularization is finalized, the anterior chamber of the eyeball is fully formed, the relief of the iris is clearly defined, the choriocapillary layer of the choroid is completely established, and the Hemomicrocirculatory bed of the eyeball is virtually fully developed.

Developmental anomalies of the eyeball.

A significant number of developmental Anomalies of the eyeball are associated with impaired or delayed closure of the embryonic optic fissure. Improper embryonic development often leaves traces at the site of the embryonic optic fissure in the form of tissue defects known as colobomas. When the margins of the optic fissure fail to fuse in a particular area, a defect remains in the iris, choroid, retina, or lens.

Developmental anomalies of the iris include the following:

1. Iris coloboma typically has a pear-shaped appearance; the pupil in this pathology is elongated downwards and inwards, and the sphincter pupillae muscle extends to the edge of the tissue defect. The lower part of the coloboma is rounded. The size of the coloboma varies; sometimes this anomaly presents merely as a minor notch at the pupillary margin on the inner side, while in other cases it is accompanied by thinning of the iris tissue, with its posterior pigment layer clearly visible through the defect.

2. Aniridia (absence of the iris) manifests as a pupil whose size equals that of the cornea; the iris may be completely absent or, more frequently, persists as a very narrow rim behind the opaque portion of the limbus. The underdeveloped iris tissue retains its structural Organization—its mesodermal layer with Blood Vessels and the double pigment layer are observable, but the sphincter pupillae muscle is indistinct. This anomaly is predominantly bilateral, and the visual function of such eyes is reduced. The ciliary processes in this condition are underdevelopment, low, and barely noticeable.

3. The lens is opaque, the vitreous body is likewise clouded, and retinal detachment or fold formations resembling crests may be observed. Clinically, this pathology manifests as secondary glaucoma, low visual acuity, and nystagmus.

4. Displacement of the pupil (corectopia) occurs predominantly toward the periphery, giving the pupil an oval shape. The iris and lens sometimes remain normal; however, more frequently, a portion of the iris tissue undergoes atrophy, and the lens shifts in the direction opposite to the displaced pupil.

5. Polycoria is characterized by the presence of 2 to 3 pupils. This anomaly is extremely rare.

6. Eversion of the pigment epithelium of the iris at the pupillary margin is characterized by the posterior pigment layer extending outward, causing the iris to double in this area. The pupil loses its rounded shape.

7. Persistent pupillary membrane (membrana pupillaris persistens) is the most frequent anomaly. The external manifestations of pupillary membranes vary widely. Sometimes barely noticeable spiderweb-like filaments stretch across the pupil, attaching to the trabecular tissue of the iris above the minor arterial circle. Occasionally, the pupillary membrane forms a dense mesh of thick threads over the pupillary aperture, or the pupil may be covered by dense plates of various shapes.

8. Congenital cysts of the iris are located within its stroma. The cyst wall is lined with a double-layered cuboidal epithelium, and its content consists of proteinaceous fluid. The surface of the cyst is covered by the Fibrous Connective Tissue of the stretched iris. Expanding peripherally, the cyst can obstruct part of the anterior chamber angle and cause an elevation in intraocular pressure.

9. Congenital anomalies of iris pigmentation are associated with either a decrease or an increase in the number of chromatophores. In albinism, all chromatophores lack pigment; in iris heterochromia, a person exhibits different eye colors or distinct colored patches within a single iris.

An anomaly of ciliary body development is medulloepithelioma (or diktyoma), which represents an overgrowth of the non-pigmented epithelium of the pars plana of the ciliary body.

Developmental anomalies of the lens:

1. Lens coloboma refers to tissue defects in the lens that are not directly related to the closure of the embryonic optic fissure; it typically accompanies an iris coloboma, underdevelopment of the ciliary processes, and the absence of zonular fibers.

2. Posterior and anterior lenticonus. In posterior lenticonus, the curvature of the posterior surface of the lens forms an irregular circular segment, featuring a cone-shaped protrusion in its central part extending into the vitreous body. Visual acuity is impaired because different areas of the lens possess varying refractive power. Anterior lenticonus is characterized by a cone-shaped projection on the anterior surface of the lens pointing toward the anterior chamber.

3. Internal lenticonus is observed within the substance of the lens.

4. Congenital cataract presents as a thickening composed of lens capsule epithelial cells in the form of a cone-shaped outgrowth on the lens surface, causing the lens capsule to elevate. This anomaly is associated with the Embryonic period when the anterior chamber of the eyeball has not yet formed, leaving the lens in direct contact with the posterior surface of the cornea, or when The formation of the anterior chamber is delayed.

5. Absence of the lens resulting from the absence of the lens placode/primordium.

Developmental anomalies of the choroid:

1. Choroidal coloboma frequently occurs in combination with colobomas of the iris, retina, and optic nerve. The margins of the coloboma are sharply demarcated from the normal choroid and are almost always pigmented. When a choroidal coloboma is combined with an optic nerve coloboma, their boundaries merge. The Base of the coloboma consists of altered retinal tissue, through which the underlying sclera is visible beneath the transparent retina.

2. Multiple defects of the choroid proper are predominantly located in the lower half of the eyeball. The edges of the defects are bordered by an irregular pigmented strip.

Developmental anomalies of the macula — macular Dysplasia.

Developmental anomalies of the retina:

1. Disruption of the normal retinal Structure. White membranes or dense deposits obscuring retinal vessels, as well as septum-like or ridge-like elevations, are observed.

2. Retinal pigmentation characterized by clusters of small foci.

Developmental anomalies of the optic disc:

1. Optic disc aplasia occurs due to delayed ingrowth of nerve fibers into the optic canal.

2. Optic disc excavation resulting from a partial delay in nerve fiber ingrowth into the optic canal.

Developmental anomalies of the cornea:

1. Size anomalies include corneal enlargement (macromega) and reduction (micromega). An enlarged cornea exhibits irregular curvature, leading to astigmatism. A reduced cornea has decreased curvature and may even be flat.

2. Corneal shape anomaly — its oval shape.

3. Impairment of corneal transparency — opacification of the posterior corneal layers.

4. Embryotoxon — an embryonal arc of the widened limbus.

Developmental anomalies of the sclera:

1. Thin sclera — the choroid is visible through the sclera.

2. Scleral melanosis — pigmentation of the sclera (the sclera acquires a yellowish-brown color).

3. Epithelial cysts within the sclera.

4. Underdevelopment of the scleral venous sinus causes congenital glaucoma.

Developmental anomaly of the vitreous body — remnants of the Branches of the hyaloid artery (arteria hyaloidea) obstruct light transmission within the vitreous body.

Disruptions in the Proportions of the eyeball lead to congenital myopia (elongated visual axis) or congenital hyperopia (shortened visual axis).

Development of accessory eye structures.

Development of the extraocular muscles, Orbital Fasciae, and the eyeball.

The extraocular muscles develop from the myotomes of 3 pairs of head somites. By the 7th week of Human embryonic development, the primordium of the extraocular muscles is formed. By the 4th month of human fetal development, the levator palpebrae superioris muscle develops.

Development of the eyelids.

The eyelids begin to develop in the 2nd month of the Prenatal period of ontogenesis. At this time, the eyelids appear as two Skin folds. In the 3rd month, the skin folds fuse. By the 5th month, the palpebral fissure is formed. The eyelid glands develop from the ectoderm, while the so-called tarsal plate develops from the mesoderm.

Age and individual CHARACTERISTICS OF THE eyelids.

In newborns, the palpebral fissure is narrow, and the medial canthus is rounded. Subsequently, the palpebral fissure rapidly enlarges. By the age of 14-15, it becomes wide, making the eye appear larger than that of an adult.

Developmental anomalies of the eyelids.

Developmental anomalies of the eyelids manifest as defects in their embryonic primordia (coloboma of the eyelid). These defects are predominantly triangular in shape, with the base facing the eyelid margin. Their size can vary considerably.

Another developmental anomaly of the eyelids is cryptophthalmos, in which the eye is covered by what appear to be fused eyelids. In such cases, the eyeball can be palpated beneath the eyelids, sometimes even with preserved light perception. The Essence of this anomaly is not the fusion of normal eyelids, but rather the replacement of the cornea by their tissue.

Epicanthus is also classified as an eyelid anomaly—it is a skin fold that extends from the upper eyelid to the lower one, covering the inner corner of the eye.

A rare anomaly is the aberrant growth of eyelashes emerging from the openings of the meibomian glands. A delay in the regression of mucous membrane duplicatures during the Formation of the eyelid fornices results in an anomaly known as epitarsus (duplicatures of the mucous membrane in the area of the upper eyelid tarsus and transitional fold).

Development of the Lacrimal Apparatus.

By the 7th week of human embryonic development, the lacrimal canaliculi begin to form as epithelial cords. At the 8th week, the orbital portion of the lacrimal gland develops from outgrowths of the conjunctival epithelium in the lateral part of the upper eyelid, which is also forming during this period. By the 5th month of intrauterine development, the nasolacrimal duct is formed. It develops from two segments (an upper segment originating from the lacrimal sac and a lower one from the Nasal cavity). At the junction of these two segments, a thin membrane may persist for a long time, causing tear accumulation in the lacrimal sac and suppuration. (A single probing Procedure is usually sufficient to restore the patency of the nasolacrimal duct).

Developmental anomalies of the lacrimal apparatus.

1. Absence of lacrimal puncta with preserved lacrimal canaliculi.

2. Absence of both lacrimal puncta and lacrimal canaliculi.

3. Additional lacrimal puncta.

4. Accessory lacrimal canaliculi.

5. Lacrimal punctum in the form of a lacrimal sac fistula.