Development and age-related features of the organ of vision

The organ of vision in its development has traveled from separate ectodermal origin of photosensitive cells (in coelenterates) to complexly arranged pair eyes in mammals. Vertebrate eyes develop difficult. From the lateral outgrowths of the brain a photosensitive membrane - the retina is formed. The middle and outer shells of the eyeball, the vitreous humor are formed from the mesoderm (the middle germinal leaf), the lens - from the ectoderm.

The inner membrane (retina) in form resembles a two-walled glass. The pigment part (layer) of the retina develops from the thin outer wall of the glass. The visual (photoreceptor, photosensitive) cells are in a thicker inner layer of the glass. In fish, the differentiation of the visual cells into rod-shaped (sticks) and cone-shaped (cones) is poorly expressed, in reptiles there are some cones, in mammals in the retina - mainly sticks. In aquatic and nocturnal animals, cones in the retina are absent. In the composition of the middle (vascular) membrane, a ciliated body is already formed in fish, which becomes more complicated 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 lizard-like and single-pass). In mammals, the outer shell is constructed only from fibrous (fibrous) tissue. The anterior part of the fibrous membrane (cornea) is transparent. The lens of fish and amphibians is rounded. Accommodation is achieved due to the movement of the lens and the reduction of a special lens that moves the lens. In reptiles and birds, the lens can not only move, but also change its curvature. In mammals, the lens occupies a permanent place. Accommodation is due to changes in the curvature of the lens. The vitreous humor, which initially has a fibrous structure, gradually becomes transparent.

Simultaneously with the complication of the structure of the eyeball, the auxiliary organs of the eye develop. The first appear six oculomotor muscles, which are transformed from myotomes of three pairs of head somites. The eyelids begin to form in the fish in the form of a single annular skin fold. In terrestrial vertebrates, upper and lower eyelids are formed. In most animals, the medial angle of the eye also has a blinking membrane (third eyelid). The remains of this membrane remain in monkeys and humans in the form of a semilunar fold of the conjunctiva. In terrestrial vertebrate animals, the lacrimal gland develops, a lacrimal apparatus is formed.

The human eyeball also develops from several sources. The photosensitive membrane (retina) comes from the lateral wall of the cerebral bladder (the future intermediate brain); the main lens of the eye - the lens - directly from the ectoderm, the vascular and fibrous membranes - from the mesenchyme. At the early stage of development of the embryo (the end of the first - the beginning of the second month of intrauterine life) on the side walls of the primary cerebral bladder there is a small pair protrusion - eye blisters. Their terminal sections widen, grow toward the ectoderm, and the legs connecting with the brain narrows and subsequently turn into optic nerves. In the process of development, the wall of the eye bladder is invaginated inside of it and the bubble turns into a two-layered eyeglass. The outer wall of the glass is then thinned and transformed into the outer pigment part (layer), and the intricate light-receiving (nervous) part of the retina (photosensor layer) is formed from the inner wall. At the stage of formation of the eye glass and differentiation of its walls, on the second month of intrauterine development, the ectoderm adjacent to the optic cup first thickens, and then the lens fossa is formed, which turns into a lens vial. Separated from the ectoderm, the vial is immersed inside the eye glass, loses the cavity, and the lens is subsequently formed from it.

At the 2 nd month of intrauterine life, the mesenchymal cells penetrate into the eye cup through the gap formed on the lower side of the glass. These cells form a vascular network inside the glass in the vitreous body that forms here and around the growing lens. From the mesenchymal cells adjacent to the eye glass, a vascular membrane forms, and from the outer layers - a fibrous membrane. The anterior part of the fibrous membrane becomes transparent and turns into a cornea. In the fetus, 6-8 months, the blood vessels in the capsule of the lens and vitreous body disappear; the membrane closing the aperture of the pupil (pupillary membrane) is resorbed.

The upper and lower eyelids begin to form on the 3rd month of intrauterine life, at first in the form of ectoderm folds. The epithelium of the conjunctiva, including the cornea covering the front, comes from the ectoderm. The lacrimal gland develops from the outgrowths of the conjunctival epithelium, appearing on the third month of intrauterine life in the lateral part of the emerging upper eyelid.

The eyeball of the newborn is relatively large, its anteroposterior size is 17.5 mm, the mass is 2.3 g. The visual axis of the eyeball passes laterally than in the adult. The eyeball grows faster in the first year of the 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 - by 3 times compared with the newborn.

The cornea in the newborn is relatively thick, its curvature during life almost does not change; the lens is almost round, the radii of its anterior and posterior curvature are approximately equal. Especially rapidly grows the lens during the first year of life, and subsequently its growth rate is reduced. Iris is convex anteriorly, there is little pigment in it, the diameter of the pupil is 2.5 mm. As the child's age increases, the thickness of the iris increases, the amount of pigment increases, the pupil diameter becomes large. At the age of 40-50 the pupil slightly narrows.

The ciliary body of the newborn is weakly developed. The growth and differentiation of the ciliary muscle is quite fast. The optic nerve of a newborn is thin (0.8 mm), short. By 20 years of life, its diameter almost doubles.

Muscles of the eyeball in the 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 only from the 2nd month of life.

The lacrimal gland of the newborn has small dimensions, the excretory canals of the gland are thin. The function of lacrimation appears on the 2nd month of the child's life. The vagina of the eyeball in the newborn and infants is thin, the fatty body of the eye socket is weakly developed. In people of elderly and senile age, the fatty body of the orbit decreases in size, partially atrophies, the eyeball less protrudes from the orbit.

The eye gap in the newborn is narrow, the medial angle of the eye is rounded. In the future, the eye gap increases rapidly. In children up to 14-15 years of age, it is wide, so the eye seems larger than that of an adult.

[1], [2], [3]