Anat Cell Biol.  2013 Mar;46(1):49-56. 10.5115/acb.2013.46.1.49.

Qualitative changes in fetal trabecular meshwork fibers at the human iridocorneal angle

• Hosaka F
• Rodriguez-Vazquez JF
• Abe H
• Murakami G
• Fujimiya M
• Ohguro H

• KMID: 2046757
• DOI: http://doi.org/10.5115/acb.2013.46.1.49

Abstract

We examined a series of changes that occur in the trabecular meshwork fibers of human eyes during fetal development at 12-30 weeks of gestation. At 12 and 15 weeks, the uveal meshwork was stained black with silver impregnation (indicating the predominance of collagen types III and IV) in the endomysium of the ciliary muscle. At 20 weeks, in combination with Schlemm's canal, a dense fibrous tissue mass corresponding to the trabecular meshwork anlage appeared and was colored black. The anlage was continuous with the corneal endothelium rather than with the ciliary muscle. Until 25 weeks, the trabecular meshwork was identifiable as fragmented fiber bundles that stained red-black, suggesting a mixture of collagen types I, III, and IV. At 30 weeks, half of the ciliary muscle fibers were inserted into the scleral spur and not into the meshwork. Therefore, any contribution of ciliary muscle contraction to the differentiation of the trabecular meshwork would appear to be limited. We hypothesize that an uneven distribution of mechanical stresses in the area of the cornea-sclera junction causes a tear thereby creating Schlemm's canal and is accompanied by a change in the collagen fiber types comprising the meshwork.

Keyword

Trabecular meshwork; Schlemm's canal; Collagen; Silver staining

MeSH Terms

Collagen
Endothelium, Corneal
Eye
Fetal Development
Humans
Muscle Contraction
Muscles
Pregnancy
Silver
Silver Staining
Stress, Mechanical
Trabecular Meshwork
Collagen
Silver

Figure

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  Fig. 1 Early stage of iridocorneal angle development. Horizontal sections. (A, C) Silver impregnation and (B, D) smooth muscle actin immunohistochemistry. Panels (A) and (B) display sections from a 12-week fetus, while (C) and (D) show sections from a 15-week fetus. At these stages, the iridocorneal angle (star) is distant from the ciliary muscle. Smooth muscle is immunopositive (B, D) and is accompanied by types III and IV collagen fibers (black fibers in A and C). In the cornea and sclera, type I collagen fibers appear red with silver staining. At 15 weeks, small spaces appear in the loose mesenchymal tissue at the angle. The corneal endothelium (arrowheads in C) is well preserved in this specimen. Scale bar in (A)=0.2 mm (A-D).

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  Fig. 2 Iridocorneal angle without a definite trabecular meshwork at 20 weeks. Horizontal sections. Silver impregnation (A, B) and smooth muscle actin immunohistochemistry (C). Panel (A) displays a specimen different from (B) and (C). Panel (C) shows a section taken from near (B). Schlemm's and collector canal (Schlemm) is evident in both specimens. On the corneal side of the ciliary muscle, a loose mesenchymal tissue, or the uveal meshwork, is evident (asterisk in A and B), but parts of the tissue form a relatively dense fibrous mass at the future trabecular meshwork (a perspective anlage; arrows in A and B). In (B), a thin vein passes through the margin of the sclera. The corneal endothelium is well preserved in these specimens (arrowheads in A and B). Scale bar in (A)=0.2 mm (A-C).

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  Fig. 3 Iridocorneal angle with a definite trabecular meshwork. Horizontal sections. Silver impregnation. Panel (A; panel B) displays a 25-week fetus (a 30-week fetus). In both specimens, the trabecular meshwork is evident (arrows) and is composed of red-black fiber bundles. Schlemm's canal (Schlemm) is adjacent to the meshwork. Insert at the lower right-hand angle is a higher magnification view of the meshwork. The brown fibers have collagen fiber types that differ from the red fibers in the cornea and sclera, as well as from the black fibers in the ciliary muscle. The sclera spur (spur) is evident in both specimens. The connection between the ciliary muscle and trabecular meshwork appears to be very weak because of loose interposing tissue (asterisk). A black-colored mass in the anterior chamber in (A) is blood, possibly due to injury during abortion. The corneal endothelium is well preserved in (B, arrowheads), and is continuous with the trabecular meshwork. Scale bars in (A)=0.2 mm (A, B).

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  Fig. 4 Diagrams showing hypothetical induction of trabecular meshwork differentiation. Panel (A) displays four candidate generators of mechanical induction for early differentiation of the trabecular meshwork (TM): 1) growth of the iris, 2) contraction of the ciliary muscle, 3) thickening of the cornea, and 4) expansion of the sclera due to increased intraocular pressure. Panel (B) exhibits the lamellar configuration of the cornea and sclera in the early stage, while panel (C) shows lamellar distension and interlamellar dispersion due to sharing stress at the cornea-sclera junction. In (C), stars indicate the site at which the anlage of the TM appears. First, the sharing stress forms Schlemm's canal as a split, subsequently inducing the TM anlage to cover the tissue loss. Second, composite fibers of the TM anlage show a change in collagen types (becoming a mixture of types I, III, and IV), becoming fasciculated to form thick bundles and woven into a lattice-like meshwork.

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