McMurrich1914 Chapter 16

The Development of the Outer Coat of the Eye, of the Cornea, and of the Anterior Chamber

Soon after the formation of the optic bulb a condensation of the mesoderm cells around it occurs, forming a capsule. Over the medial portions of the optic cup the further differentiation of this capsule is comparatively simple, resulting in the formation of two layers, an inner vascular and an outer denser and fibrous, the former becoming the chorioid coat of the adult eye and the latter the sclera.

More laterally, however, the processes are more complicated. After the lens has separated from the surface ectoderm a thin layer of mesoderm grows in between the two structures and later gives place to a layer of homogeneous substance in which a few cells,

The Anterior Chamber Of The Eye

more numerous laterally than at the center, are embedded. Still later cells from the adjacent mesenchyme grow into the layer, which increases considerably in thickness, and blood-vessels also grow into that portion of it which is in contact with the outer surface of the lens. At this stage the interval between the surface ectoderm and the lens is occupied by a solid mass of mesodermal tissue (Fig. 278, co and tv), but as development proceeds, small spaces (ac) filled with fluid begin to appear toward the inner portion of the mass, and these, increasing in number and size, eventually fuse together to form a single cavity which divides the mass into an inner and an outer portion. The cavity is the anterior chamber of the eye, and it has served to separate the cornea (co) from the tunica vasculosa lentis (tv) , and, extending laterally in all directions, it also separates from the cornea the mesenchyme which rests upon the marginal portion of the optic cup and constitutes the stroma of the iris. Cells arrange themselves on the corneal surface of the cavity to form a continuous endothelial layer, and the mesenchyme which forms the peripheral boundary of the cavity assumes a fibrous character and forms the ligamentum pectinatum iridis, among the fibers of which cavities, known as the spaces of Fontana (Fig. 277, sf), appear. Beyond the margins of the cavity the corneal tissue is directly continuous with the sclerotic, beneath the margin of which is a distinctly thickened portion of mesenchyme resting upon the ciliary processes and forming the stroma of the ciliary body, as well as giving rise to the muscle tissue which constitutes the ciliary muscle (Figs. 277 and 278, mc).

Fig. 278. Transverse Section through the Ciliary Region of a Pig Embryo or 23 mm. ac, Anterior chamber of the eye; co, cornea; ec, ectoderm; I, lens; mc, ciliary muscle; p, pigment layer of the optic cup; r, retinal layer; tv, tunica vasculosa lentis. - (Angelucci.)

The ectoderm which covers the outer surface of the eye does not proceed beyond the stage when it consists of several layers of cells, and never develops a stratum corneum. In the corneal region it rests directly upon the corneal tissue, which is thickened slightly upon its outer surface to form the anterior elastic lamina; more peripherally, however, a quantity of loose mesodermal tissue lies between the ectoderm and the outer surface of the sclerotic, and, together with the ectoderm, forms the conjunctiva (Fig. 277, cj).

The Development of the Accessory Apparatus of the Eye. - The eyelids make their appearance at an early stage as two folds of skin, one a short distance above and the other below the cornea. The center of the folds is at first occupied by indifferent mesodermal tissue, which later becomes modified to form the connective tissue of the lids and the tarsal cartilage, the muscle tissue probably secondarily growing into the lids as a result of the spreading of the platysma over the face, the orbicularis oculi apparently being a derivative of that sheet of muscle tissue.

At about the beginning of the third month the lids have become sufficiently large to meet one another, whereupon the thickened epithelium which has formed upon their edges unites and the lids fuse together, in which condition they remain until shortly before birth. During the stage of fusion the eyelashes (Fig. 279, h) develop at the edges of the lids, having the same developmental history as ordinary hairs, and from the fused epithelium of each lid there grow upward or downward, as the case may be, into the mesodermic tissue, solid rods of ectoderm, certain of which early give off numerous short lateral processes and become recognizable as the tarsal (Meibomian) glands (m), while others retain the simple cylindrical form and represent the glands of Moll. When the eyelids separate, these solid ingrowths become hollow by a breaking down of their central cells, just as in the sebaceous and sudoriparous glands of the skin, the tarsal glands being really modifications of the former glands, while the glands of Moll are probably to be regarded as specialized sudoriparous glands.

Fig. 279. Section through the Margins of the Fused Eyelids in an Embryo of Six Months. ih, Eyelash; //, lower lid; m, tarsal gland; mu, muscle bundle; ul, upper lid. Seidl.)

-(Schweigger

A third fold of skin, in addition to^the two which produce the eyelids, is also developed in connection with the eye, forming the plica semilunaris. This is a rudimentary third eyelid, representing the nictitating membrane which is fairly well developed in many of the lower mammals and especially well in birds.

The lachrymal gland is developed at about the third month as a number of branching outgrowths of the ectoderm into the adjacent mesoderm along the outer part of the line where the epithelium of the conjunctiva becomes continuous with that covering the inner surface of the upper eyelid. As in the other epidermal glands, the outgrowths and their branches are at first solid, later becoming hollow by the degeneration of their axial cells.

The naso-lachrymal duct is developed in connection with the groove which, at an early stage in the development (Fig. 62), extends from the inner corner of the eye to the olfactory pit and is bounded posteriorly by the maxillary process of the first visceral arch. The epithelium lying in the floor of this groove thickens toward the begining of the sixth week to form a solid cord, which sinks into the subjacent mesoderm. From its upper end two outgrowths arise which become connected with the ectoderm of the edges of the upper and lower lids, respectively, and represent the lachrymal ducts, and, finally, the solid cord and its outgrowths acquire a lumen and a connection with the mucous membrane of the inferior meatus of the nasal cavity.

Fig. 280. Diagram showing the Insertions of the Lachrymal Ducts in embryos of 40 mm and 170 mm. The caruncula lacrimalis being formed in the latter. The eyelids are really fused at these stages but have been represented as separated for the sake of clearness. - (Ask.)

The inferior duct connects with the border of the eyelid some distance lateral to the inner angle of the eye, and between its opening and the angle a number of tarsal glands develop. The superior duct, on the other hand, opens at first close to the inner angle and later moves laterally until its opening is opposite that of the inferior duct. During this change the portion of the lower lid between the opening of the inferior duct and the angle is drawn somewhat upward, and, with its glands, forms a small reddish nodule, resting upon the plica semilunaris and known as the caruncula lacrimalis (Fig. 280).

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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.

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