The Zonular Insertion: a Scanning Electron Microscopic Study

364 Streeten est. Ophthalmol. Visual Set. April 1977 21. Jones, R. E., and Moscona, A. A.: Effects 24. Hogan, M. J., Alvarado, J. A., and Weddell, of cytosine arabinoside on differential gene J. E.: Histology of the Human Eye. An Atlas expression in embryonic neural retina. I. Ac- and Textbook, Philadelphia, 1971, W. B. cumulation of glutamine synthetase with Saunders Company. suppression of macromolecular synthesis, J. 25. Hughes, W. F., and Coogan, P. S.: Pathology CellBiol. 61: 688, 1974. of the pigment epithelium and retina in rab- 22. Fine, B. S., and Yanoff, M.: Ocular Histol- bits poisoned with lead, Am. J. Pathol. 77: ogy. A Text and Atlas, Hagerstown, Md., 237, 1974. 1972, Harper & Row, Publishers. 26. Kuwabara, T., and Weidman, T. A.: De- 23. Rasmussen, K.-E.: A morphometric study of velopment of the prenatal rat retina, INVEST. the Miiller cell cytoplasm in the rat retina, OPHTHALMOL. 13: 725, 1974. J. Ultrastruct. Res. 39: 413, 1972. The zonular insertion: a scanning electron microscopic study Barbara W. Streeten The zonular insertion on the human lens capsule has been studied with scanning electron microscopy. The whole insertional area is covered by a variety of 35 to 55 nm. fibrils, including those of the major inserting zonules, the meridional zonules, capsuhr surface fibrils, and a nonoriented fibrillar layer over the zonules. All appear to be zonular in type. The meridional zonules span the equator and are a form of insertion shared by the three levels of major zonules, probably helping to distribute their lateral putt. Surface rippling over the zonular insertion appears to result from greater contraction of the oriented fibers than of the nonoriented fibrils and superficial capsule. Surface meshwork fibrils attach to the capsule by becoming embedded in a granular capsular matrix 0.5 to 1 n thick. Key words: zonule of the lens, scanning electron microscopy, zonular insertion, accommodation, zonular lamella. -Lhe mechanism of zonular insertion on sporadic attention in the modern era, al- the human lens capsule has received only though it is essential to an understanding of both normal accommodative function and pathology involving these structures. From the Departments of Ophthalmology and An enduring concept of zonular attach- Pathology, State University of New York ment has been that of a "zonular lamella" Upstate Medical Center, Syracuse, N. Y. or special layer on the lens surface into This study was supported by Research Grant EY which the zonules insert, as described by 01602 from the National Eye Institute. Berger1 in 1882. By transmission electron Reprint requests: Dr. Streeten, Eye Pathology microscopy (TEM) the zonular lamella of Laboratory, State University of New York 2 Upstate Medical Center, Syracuse, N. Y. 13210. light microscopy resolves into two layers: Downloaded from iovs.arvojournals.org on 09/29/2021 Volume 16 Zonular insertion: SEM study 365 Number 4 Fig. 1. Equator of lens. Zonular insertional area appears light against dark lens capsule background. Three levels of zonular insertion are seen: anterior (A), equatorial (E), and posterior (P). Tearing away of anterior hyaloid membrane has left posterior zonular insertion ragged. (Scanning electron micrograph, xG4.) (1) the loose superficial capsule surround- lems of zonular fiber distribution, since the ing the lens and (2) an outer layer of in- zonule is essentially a surface tissue. How- serting zonules in the equatorial region.1' * ever, reports of the zonular insertion by It is not clear whether the zonular portion SEM have been fragmentary in both is a continuous layer or intermittent, ap- man51- and animals.13"15 Technical prob- pearing only where the zonules directly lems with preservation of the fragile zonule attach. Since the number of zonules is said and diying of the dense lens material, to be considerably reduced in aging,2 with coupled with low resolution, decrease the those at the equator being especially sparse value of some of the early descriptions. or absent, an intermittent covering might In only a few studies has resolution been be expected in the adult. sufficiently high to visualize the individual 12 14> ls Scanning electron microscopy (SEM) fibrils of the normal zonule. - The should be ideal for studying these prob- presence of a zonular lamella into which Downloaded from iovs.arvojournals.org on 09/29/2021 366 Streeten Invest. Ophthalmol. Visual Sci. April 1977 Fig. 2. A, Anterior zonules terminating on lens capsule. Zonular surfaces show contraction furrows. (Scanning electron micrograph, x470.) B, Fibrillar structure of zonules near their terminus. Some fibrils blending into meshwork on capsular surface. (Scanning electron micrograph, x6,750.) the inserting zonules "disappear" has gen- fixed immediately in 2.5 percent glutaraldehyde. erally been accepted but not defined.5'Si 15 Twenty-four lenses were obtained from Eye Bank eyes, 4 to 12 hours after death. These lenses The ultimate attachment of the zonules were prepared in a variety of ways, including in to the lens is known to involve penetration situ fixation and extraction while unfixed, with of zonular fibers into the superficial lens the use of a capsule forceps. Most lenses were capsule,3'll! but may also include some type removed before fixation by cutting the zonules and of physicochemical binding commonly re- anterior vitreous close to the ciliary processes under the dissecting microscope, to preserve the ferred to as "glue." With one exception the maximum number of zonules on the lens capsule. SEM studies have not thrown light on this Eight lenses were fixed in 2.5 percent glutaralde- problem. Recently Farnsworth and co-work- hyde and the remainder in 10 per cent neutral ers1- have described a fibrillar ridging of buffered formalin. Most of the specimens were the anterior lens surface which they sug- postfixed in osmium, dehydrated in graded gest may serve as zonular attachment sites acetones, and dried in a critical-point drier. They were then coated with carbon and gold-palladium of the Velcro type. The surface fibrils on and examined in an ETEC or a Coates and Welter these ridges were considered by the authors scanning electron microscope. Eight lenses were to -be capsular in origin, although they postfixed in osmium-thiocarbohydrazide (TCH) were of the same 50 to 70 nm. diameter and coated only with gold-palladium. as the fibrils of the zonule. Results The present study will describe all areas of the zonular insertion as seen by SEM, When a lens is extracted with its at- with special consideration of the zonular tached zonules the entire width of the lamella and capsular attachments. zonular insertion can be viewed in the scanning electron microscope (Fig. 1). Methods and materials This insertional region appears as a well- Thirty human lenses were examined with SEM. demarcated light area against a darker Six were lenses removed at cataract surgery and lens capsule background. The attached an- Downloaded from iovs.arvojournals.org on 09/29/2021 Volume 16 Zonular insertion: SEM study 367 Number 4 Fig. 3. Final termination of smallest anterior zonular bundles (Z). Fibrils partially obscured by interfibrillar material as they join surface fibrillar meshwork. (Scanning electron micrograph, x21,000.) Fig. 4. Anterior lens capsule. A, 1 mm. in front of zonular insertion showing mixture of random fibrils and pebbled surface. (Scanning electron micrograph, xl6,800.) B, Center of lens with pebbled texture. (Scanning electron micrograph, xl6,800.) Downloaded from iovs.arvojournals.org on 09/29/2021 368 Streeten Invest. Ophthalmol. Visual Sci. April 1977 Fig. 5A. Abrasion of flbrillar surface layer in anterior insertion. Note variable rippling of the remaining layer. (Scanning electron micrograph, x645.) terior, equatorial, and posterior zonules 2, A). (Linear measurements are not accu- join this band at fairly regular intervals, rate due to a 20 percent shrinkage of the often in double rows. lens in processing.) An interfibrillar ma- The anterior and posterior zonules in- terial often obscures the fibrils of the larger sert as bundles of 25 to 60 p. diameter in zonules, but where visible they are 35 to their contracted state. The equatorial bun- 55 nm. in diameter. Loose peripheral fibrils dles can be as large (Fig. 1) but are often blend into a flbrillar meshwork on the small, from 10 to 15 \x. in diameter, seen capsular surface (Fig. 2, B). This mesh- at all ages into the ninth decade. As the work is thrown into consistent but irregular zonules spread out on the capsule the ridges in the inserting area. The anterior larger ones rapidly separate into 5 to 10 zonules end as 0.3 to 0.4 /± fanning bundles, /J. bundles. The anterior zonules continue blending into the surface layer in basket- to intermingle and decrease in size over a weave fashion (Fig. 3). 0.3 to 0.4 mm. flat insertional area (Fig. The fibrils on the capsular surface are Downloaded from iovs.arvojournals.org on 09/29/2021 Volume 16 Zonular insertion: SEM study 369 Number 4 Fig. 5B. Edge of abraded area shown in box in Fig. 5A. Fibrils of surface layer adherent to deep meshwork. Bare surface is granular with evidence of interfibrillar matrix. (Scanning electron micrograph, xlO,75O.) also 35 to 55 nm. in diameter. They con- Where the base of the fibrillar layer is tinue for 0.2 to 0.3 mm. beyond the ap- exposed the attachment of its fibrils to a parent ends of the zonules. Random fibrils deep meshwork of fibrils can be appre- may be seen for another 0.4 to 0.6 mm. ciated (Fig. 5B). An admixture of inter- centrally (Fig. 4, A). The typical pebbly fibrillar matrix and capsular granules covers surface shown by basement membranes is them. Some radially oriented longer fibers increasingly evident among these surface of 0.3 to 0.7 p.

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