Development and age-specific characteristics of the organ of vision

The organ of vision in its formation has made a way from separate ectodermal origin of light-sensitive cells (in coelenterates) to complexly structured paired eyes in mammals. In vertebrates, the eyes develop in a complex way. The light-sensitive membrane - the retina - is formed from the lateral outgrowths of the brain. The middle and outer membranes of the eyeball, the vitreous body are formed from the mesoderm (the middle germinal layer), the lens - from the ectoderm.

The inner shell (retina) resembles a double-walled goblet in shape. The pigment part (layer) of the retina develops from the thin outer wall of the goblet. Visual (photoreceptor, light-sensitive) cells are located in the thicker inner layer of the goblet. In fish, the differentiation of visual cells into rod-shaped (rods) and cone-shaped (cones) is weakly expressed, reptiles have only cones, and mammals have predominantly rods in the retina. Aquatic and nocturnal animals do not have cones in the retina. As part of the middle (vascular) shell, the ciliary body is already formed in fish, which becomes more complex in its development in birds and mammals.

Muscles in the iris and ciliary body first appear in amphibians. The outer shell of the eyeball in lower vertebrates consists mainly of cartilaginous tissue (in fish, partly in amphibians, in most lizards and monotremes). In mammals, the outer shell is constructed only of fibrous tissue. The anterior part of the fibrous membrane (cornea) is transparent. The lens of fish and amphibians is round. Accommodation is achieved by moving the lens and contracting a special muscle that moves the lens. In reptiles and birds, the lens is capable of not only moving, but also changing its curvature. In mammals, the lens occupies a constant place. Accommodation is achieved by changing the curvature of the lens. The vitreous body, which initially has a fibrous structure, gradually becomes transparent.

Simultaneously with the increasing complexity of the eyeball structure, the auxiliary organs of the eye develop. The first to appear are the six oculomotor muscles, transformed from the myotomes of three pairs of head somites. The eyelids begin to form in fish in the form of a single ring-shaped skin fold. In land vertebrates, upper and lower eyelids are formed. Most animals also have a nictitating membrane (third eyelid) at the medial corner of the eye. The remains of this membrane are preserved in monkeys and humans in the form of a semilunar fold of the conjunctiva. In land vertebrates, the lacrimal gland develops, and the lacrimal apparatus is formed.

The human eyeball also develops from several sources. The light-sensitive membrane (retina) originates from the lateral wall of the cerebral vesicle (the future diencephalon); the main lens of the eye, the crystalline lens, comes directly from the ectoderm, and the vascular and fibrous membranes come from the mesenchyme. At an early stage of embryonic development (end of the 1st - beginning of the 2nd month of intrauterine life), a small paired protrusion appears on the lateral walls of the primary cerebral vesicle - the optic vesicles. Their terminal sections expand, grow toward the ectoderm, and the legs connecting to the brain narrow and later turn into the optic nerves. During development, the wall of the optic vesicle invaginates into it and the vesicle turns into a two-layer optic cup. The outer wall of the cup subsequently becomes thinner and transforms into the outer pigment part (layer), and the inner wall forms a complex light-perceiving (nerve) part of the retina (photosensory layer). At the stage of formation of the optic cup and differentiation of its walls, in the 2nd month of intrauterine development, the ectoderm adjacent to the optic cup in front initially thickens, and then a lens pit forms, turning into a lens vesicle. Having separated from the ectoderm, the vesicle plunges into the optic cup, loses its cavity, and the lens is subsequently formed from it.

In the 2nd month of intrauterine life, mesenchymal cells penetrate into the optic cup through a slit formed on its lower side. These cells form a blood vascular network inside the cup in the vitreous body forming here and around the growing lens. The vascular membrane is formed from the mesenchymal cells adjacent to the optic cup, and the fibrous membrane is formed from the outer layers. The anterior part of the fibrous membrane becomes transparent and turns into the cornea. In a fetus aged 6-8 months, the blood vessels located in the lens capsule and vitreous body disappear; the membrane covering the pupil opening (pupillary membrane) is resorbed.

The upper and lower eyelids begin to form in the 3rd month of intrauterine life, initially as folds of the ectoderm. The epithelium of the conjunctiva, including that covering the cornea in front, originates from the ectoderm. The lacrimal gland develops from outgrowths of the conjunctival epithelium that appear in the 3rd month of intrauterine life in the lateral part of the forming upper eyelid.

The eyeball of a newborn is relatively large, its anteroposterior size is 17.5 mm, weight - 2.3 g. The visual axis of the eyeball is more lateral than in an adult. The eyeball grows faster in the first year of a child's life than in subsequent years. By the age of 5, the mass of the eyeball increases by 70%, and by 20-25 years - 3 times compared to a newborn.

The cornea of a newborn is relatively thick, its curvature hardly changes during life; the lens is almost round, the radii of its anterior and posterior curvature are approximately equal. The lens grows especially quickly during the first year of life, and then its growth rate decreases. The iris is convex anteriorly, there is little pigment in it, the pupil diameter is 2.5 mm. As the child ages, the thickness of the iris increases, the amount of pigment in it increases, the pupil diameter becomes larger. At the age of 40-50 years, the pupil narrows slightly.

The ciliary body of a newborn is poorly developed. The growth and differentiation of the ciliary muscle occurs quite quickly. The optic nerve of a newborn is thin (0.8 mm), short. By the age of 20, its diameter increases almost twice.

The muscles of the eyeball in a newborn are developed quite well, except for their tendon part. Therefore, eye movements are possible immediately after birth, but the coordination of these movements is possible only from the 2nd month of life.

The lacrimal gland in a newborn is small, the excretory ducts of the gland are thin. The function of lacrimation appears in the 2nd month of the child's life. The vagina of the eyeball in a newborn and infants is thin, the fatty body of the orbit is poorly developed. In elderly and senile people, the fatty body of the orbit decreases in size, partially atrophies, the eyeball protrudes less from the orbit.

The palpebral fissure in a newborn is narrow, the medial angle of the eye is rounded. Later, the palpebral fissure quickly increases. In children up to 14-15 years old, it is wide, so the eye seems larger than in an adult.

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