Because of great difficulties in understanding and even describing the elusive zonule, many terms have been used for it since the first complete description by the anatomist Zinn in 1753,¹ including the zonule of Zinn, the annular ligament of Zinn, the suspensory ligament of the lens, the ciliary zonule, and the tertiary vitreous.

Electron microscopy allowed the first significant insight into the nature of the zonule. In 1971 Raviola noted that the 10-nm size and tubular ultrastructure of the zonular fibers and their susceptibility to α-chymotrypsin digestion were similar to characteristics of the newly described microfibrils of elastic tissue.^14,15 These two types of microfibrils were found to be strongly cross-reactive immunologically¹⁶ and to have similar biochemical profiles, including an unusually high cysteine content.^11,12

With the advent of the molecular era, the primary component in all elastic system microfibrils was identified as a 350-kDa glycoprotein and named fibrillin,⁷ coded for by a gene on chromosome 15 q21.1.^17,18 Immunoreactivity of zonular fibers for fibrillin is strong^7,19 (Fig. 3). Classic Marfan syndrome with ectopia lentis was quickly linked to mutations on the fibrillin gene,^17,20 usually of the missense type. More than 70 such mutations have now been reported, including some mutations in cases of simple dominant ectopia lentis.²¹ Another fibrillin protein, fibrillin-2, was found at the same time on chromosome 5, causing congenital contractural arachnodactyly when mutated, but no ocular involvement.¹⁷ More than 10 proteins besides fibrillin-1 have since been found associated with the elastic system microfibrils, many of them also with zonular microfibrils, from which one of the new proteins was isolated.^22,23 It is unclear how many of these proteins are intimately involved in the structure of the microfibril, or what functions they may have. Most are known by acronyms referring to their microfibril glycoprotein nature, such as MAGP-1 and -2, MFAP-1 to -4, MAP, GP115/emilin, and LTBP-2,but also included are more familiar ubiquitous proteins including fibronectin and vitronectin.²⁴

The fibrillin-1 molecule has a large number of repeating domains resembling epidermal growth factor (EGF-like), all but two of the calcium-binding type, where most of the Marfan mutations have occurred.²⁴ This is consistent with evidence that the presence of calcium is important for the stability of this unique beaded microfibril.^25,26 The stability of the zonular bundles and their resistance to solubilization imply the presence also of strong cross-linkage sites, besides the great capacity for disulfide linkages between the many cysteines. Transglutaminase-induced glutamyl-lysine cross-linkages have also been identified in elastic microfibrils from several tissues.²⁷ Sites in the molecule for RGD-integrin binding to cell membranes andmatrix are also recognizable.²⁸ The adhesion mechanisms used by the zonular fibers are still incompletely known.

FUNCTION OF THE ZONULAR MICROFIBRILLAR SYSTEM

The elastic system is composed of three units,²⁹ the first composed entirely of microfibrils, as far as can be determined identical to the zonular microfibrils. These are the first units to be synthesized during elastogenesis,¹⁵ serving as a template on which the completely different elastomeric protein, elastin, may be deposited. Many bundles of microfibrils in the body like the zonular type never become elasticized, and these are sometimes called oxytalan (acid-resistant) fibers.³⁰ Like zonular bundles, they attach particularly to basement membranes of surfaces under stress, connecting them to each other just as zonular fibers connect the ciliary nonpigment epithelial basement membrane to the lens epithelial basement membrane. In deeper tissues these microfibrils can become partially elasticized bundles called elaunin fibers.²⁹ These then connect to the larger fully elasticized elastic fibers, completing the third unit of the system. The long-recognized elasticity or stretchability of the zonular bundles clearly has a different basis than that of the elastomer elastin and appears to reside in reversible relationships of the fibrillin filaments to each other and to their associated beads, as well as to adjacent microfi-brils, when stretched. As part of the widespreadelastic system, the zonule and fibrillin-1 appear toshare in relatively few other systemic diseases be-sides the Marfan syndrome, including the Weil-Marchesani syndrome³¹ and the pseudoexfoliation syndrome.^19,32,33

ZONULAR BUNDLE, FIBER, AND FIBRIL ARCHITECTURE

When seen during life through an iridectomy or under the dissecting microscope, the zonular fibers appear as glassy, straight rods of 5 to 30 μm in diameter (Fig. 4), occurring in diverging groups that appear as individual glassy rods when cut (Fig. 5). These groups of fibers are sticky when handled with instruments and show moderate elastic recoil after stretching by forceps. In one study of human eyes age 48 and 56 years, zonules attached to the lens capsule could be stretched for four times their normal length from capsule to ciliary body without apparent breakage or disinsertion.³⁴ Because ofthis extensibility, zonular fibers and bundles canvary considerably in diameter, depending on processing methods.

Understanding of the three-dimensional architecture of this almost invisible zonular system has been slow to build up by use of many types of microscopy and special stains,^35–38 but the combination of transmission and scanning electron microscopy has been the best tool to prove that the zonule is a system of fibers with a course that can be fairly accurately defined.

By light microscopy, zonular fiber bundles are eosinophilic, PAS-positive structures, often appearing to be ribbons connected by a paler interbundle “membrane” (Fig. 6). By scanning electron microscopy, the ribbons appear as groups of striated fiber bundles held together by a loose fibrogranular meshwork. Each fiber is composed of fibrils 10 nm in diameter, highly oriented and closely aggregated. Ultrastructurally, these fibrils have a tubular profile in cross section and a microperiodicity of 12 to14 nm seen along the fibril (Fig. 7). Dyes that precipitate and bind to polysaccharides show irregular granules and rod-like associated material on and between the fibrils.

The remarkably fine structural details of these zonular fibrils, or microfibrils as the identical fibrils are called in the rest of the elastic system, are shown best by rotary shadowing and electron microscopy of freshly homogenized zonular fibers.^39,40 The individual zonular fibril is a beaded structure (“beads on a string”) consisting of 29- to 32-nm electron-dense beads held together by filaments that may be four in number and 2 to 3 nm in diameter (Fig. 8). The filaments can be seen more clearly when the fibrils are in a looser conformation. When they are in tighter conformation, many show cross-banding across the filaments between two beads. The beads' usual periodicity is 50 to 55 nm, but it can vary from 35 to 57 nm or more with different methods of preparation and 80 to 100 nm when stretched, without visible breakage of the filaments.⁴¹ Several studies suggest that changes in periodicity of the beads cannot fully explain the high degree of elasticity exhibited by fibrillin-containing microfibrils in tissue such as the zonular fibers, or in arteries of lower forms, perhaps requiring some reorientation of the fibrils.^42,43 The nature of the bead is still not known, but the 148-nm filaments, the predicted length of a molecule of fibrillin, are thought to be arranged in a parallel staggered pattern with about one third overlap of adjacent molecules,²⁷ although other patterns have also been suggested.^44,45 The structure of elastic-system microfibrils throughout the body shows identical beaded fibrils.

The course of the zonule from the beginning of the pars plicata has been more difficult to elucidate and depends to some extent on the modality used to visualize it, because all have their artifacts and limitations. Scanning electron microscopy was a great step forward in understanding the three-dimensional nature of the zonule,^46–55 although defects in methodology and low resolution in the early years led to erroneous impressions. A longtime controversy of importance in accomodative theory is whether some zonular fibers originate on the pars plicata and have a different function from those that are continuous from the pars plana to the lens.⁴⁷ Recent investigators of both human and primate zonules have tended to conclude that most zonularfibers are continuous from pars plana to the lens,^53,55 but they differ in estimating the number that may originate or terminate on the pars plicata and the degree of zonular adherence to this tissue. The following description is a synthesis of views about zonular anatomy in the pars plicata, obtained by several different methods and diagramed in Figure 13.

With increasing height and complexity of the ciliary processes, the zonular bundles from their first entry between the posterior ciliary processes and minor plicae show considerable variation in distribution, but some general principles are apparent. Most of the bundles destined to insert on the posterior lens capsule become somewhat elevated above the level of the main zonular stream in the middle to anterior pars plana region. The one to three large bundles that pass over the crests of each ciliary process are the most highly elevated, especially over the prominent ciliary processes often found nasally. These bundles usually remain attached to the anterior hyaloid membrane up to the level of the ciliary processes. They then have some attachments to the crests and sides of the processes by direct adhesion or by joining branch bundles from below before passing to the posterior lens capsule at about a 30-degree angle, some still maintaining an attachment to the anterior hyaloid membrane. By the same direct and indirect attachments, the remaining posterior zonules become strongly adherent in the valleys between the ciliary processes or on the sides of the minor plicae, at intervals of 0.2 to 0.4 mm in from the posterior end of the pars plicata. These posterior zonulesin the valleys exit from the ciliary body at an angle of 50 degrees or more (Fig. 14). There is a continual exchange of fibers between bundles throughout their attachments to the valleys and walls of the pars plicata. Fibers from the deeper group of posterior zonules become associated with the anterior hyaloid membrane again only as it lies over their attachments on the lens posterior capsule.

When the anterior hyaloid membrane is removed for viewing or microscopy of this region, as it usually is, many of the posterior zonules are torn away or broken,⁵⁴ which can give the erroneous impression that no zonules pass over the ciliary crests or that a number terminate on the crests.⁵⁰ Fibers destined for the anterior and equatorial lens capsule are also strongly adherent to the valleys, between the minor plicae, or along the lower sides of the ciliary processes. They part company from the posterior zonules by continuing in an almost straight course to their insertion. Rohen has referred to the strong adherence of anterior and posterior zonules in the valleys and on the process walls as zonular “plexi” and the zone of divergence for these two main streams as the zonular “fork.”⁵⁵ The anterior zonules are extensively embedded in infoldings of ciliary epithelium in the valleys, so it is difficult to peel them back from this region without breakage. A single zonular bundle from the pars plana may send fibers to all three lens regions. This fact, plus the constant interbundle exchange, guarantees the interrelatedness of the whole zonular complex. Both anterior and posterior zonular fibers exit from the pars plicata in ribbon-like swaths, lining up parallel to the ciliary processes (Fig. 15). The few zonules passing to the equator of the lens arise from the midsides of the processes or from the valleys and usually derive from anterior zonular bundles (see Fig. 13).

The close attachment of almost all zonular fibers to the ciliary processes or to the plicae during their course is an important one. It occurs by direct adhesion of zonular fibers to the ciliary epithelial basement membrane at the edges and base of large bundles, and by fine strands from the larger bundles appearing to splay out over the epithelium (Fig. 16A), of importance to accomodation theory (see later discussion of accomodative mechanisms).^47,55On scanning microscopy, such bundles fan out in acharacteristic pattern at their attachment sites, partially embedded, or perhaps sometimes ending, in the rich fibrogranular matrix of the ciliary basement membrane (Fig. 16B) before regrouping to continue. Larger zonular bundles are attached here than in any other site, and they even become enmeshed in the early thickening of basement membrane occurring here (Fig. 17). Thus, between the attachment of small zonular fibers to the main zonular stream over the pars plana and the firm attachments of the main zonular fibers to the pars plicata, most of the system is closely bound to the ciliary body. This prevents the zonular fibers from forming a cord between their origin and their lens insertion that would interfere with their ability to transmit accomodative forces to the lens during accomodation.

The “crossing” of anterior and posterior zonular fibers seen histologically as they pursue their different paths has interested microscopists for years.Salzmann noted in 1912 that “this double anchorage brings about a stronger fixation of the entire zonular mass at the posterior border of the corona ciliaris, and this is the reason why with few exceptions the zonular fibers break away at this place.”⁵⁷

POSTERIOR CILIARY GIRDLE AND ABERRANT ZONULAR FIBERS

The posterior girdle of zonular bundles lies within the vitreous just above the main zonular stream 1 to 2 mm anterior to the ora serrata (Fig. 18). This is also the area in which the median vitreous tract inserts, to use Eisner's terminology.⁶⁰ Vitreous collagen fibrils in the immediate vicinity of the zonular girdle fibers have a similar circumferential orientation. This suggests that the orientation of both types of fibers may be a response to the same mechanical stresses, such as vitreous traction during ocular movement and accommodative excursions.

Short bundles that stain like zonular fibers may pass back over the ora serrata into the vitreous at all ages, either from the posterior girdle or separately without any apparent attachment to the ciliary body. Aberrant zonular fibers are commonly found posterior to the ora serrata in association with peripheral abnormalities such as zonular traction tufts⁶² and also adjacent to meridional folds, enclosed ciliary bays, and peripheral retinal rosettes or “granulations.” These aberrant fibers generally do not join the main zonular stream but end close to the ciliary surface a few millimeters anterior to the ora serrata.

ANTERIOR CILIARY AND LENTICULAR GIRDLE

Prominent circumferential zonular fibers lie over the crests of the ciliary processes (Fig. 19), arising from branches of the posterior zonular bundles as they pass over the crests or from others attaching in the ciliary valleys. This anterior girdle lies superficially on and within the anterior hyaloid membrane, helping to bind the membrane close to the ciliary processes and resisting the pull of the coronary vitreous tract, which inserts here.⁶⁰ These fibers usually remain on the hyaloid membrane when the latter is torn away, along with some posterior zonular fibers. In young eyes the girdle fibers are very fine, but in old eyes they are stout cords. Some vitreous collagen fibers here also have a circumferential orientation. As diagrammed in Figure 20, another group of circumferential zonular girdle fibers is lenticular. These can also be seen in Figure 19B in the anterior hyaloid membrane above the anterior ciliary girdle. Most of the lenticular girdle fibers lie on the flattened and fanning posterior zonular bundles near the end of their lens insertions at the approximate site of Wieger's ligament. Eisner'shyaloid vitreous tract also inserts here.⁶⁰ Thesethree fibrillin-positive girdles represent sites where changes in tractional forces affect both the vitreous and the zonular system. The girdles may help maintain the posterior chamber and support the vitreous gel.

ANTERIOR ZONULAR INSERTION AND ZONULAR LAMELLA

The zonular fibers inserting on the lens form a layer that extends 1.5 mm onto the anterior surface of the lens capsule and 1.25 mm onto the posterior capsule in the adult. With increasing size of the lens, the inserting bundles appear flatter, with fewer underlying layers of attaching fibers visible, but at least two main layers are usually apparent. These rapidly spread out on the capsule as 5- to 10-μm bundles, presenting a remarkably symmetric appearance, with only occasional bundles passing ahead of the main insertion (Fig. 23). They continue to decrease in size, tapering over a flat insertional area whose length depends on the size and age of the lens, ending as 0.3- to 0.4-μm small bundles whose fibrils blend into the surface fibrillar layer of the capsule in a basket-weave fashion. For 1 mm central to the tips of the zonular inserting fibers, the surface fibrillar meshwork becomes gradually thinner until only the pebbly surface of the bare lens capsule is visible. All along the anterior insertion, the zonular bundles join a surface layer of more loosely arranged fibrils of similar size that often have a horizontally rippled appearance. When the capsular surface is abraded, as in Figure 24, by pulling away of some inserting zonular bundles, it is seen that the rumpled surface layer has fibrils like those of the meshwork around the inserting anterior zonular fibers, some of which remain adherent to the fibrogranular surface layer still on the anterior capsule. A rolling up and retraction of the distal abraded layer indicates that it has some elasticity and contains a few small linear zonular fibers within it.

The layer of lenticular zonular fibers and the superficial lens capsule into which they insert was named the zonular lamella by Berger.⁶⁵ This superficial 0.6 to 0.9 μm of the capsule in the insertional area is more loosely arranged than the rest of the capsule and has a higher content of glycosaminoglycans with Alcian blue staining, also shown by the zonule (Fig. 25). A high content of proteoglycans is found in this junctional region, with a large chondroitin sulfate proteoglycan (chondroitinase ABC-sensitive) surrounding bundles of zonular fibers and their junction with the capsule, and extensive hep-aran sulfate (heparitinase-sensitive) in the adjacent superficial capsule. Evidence of chondroitin sulfate associated with the zonule was found earlier biochemically^9,11 and more recently by impressive immunohistochemistry and synthesis studies.^66,67 Other moieties related to adhesion of zonular and other elastic microfibrils to basement membranes, such as fibronectin,^68,69 vitronectin,⁷⁰ and integrin αvβ3 are being further investigated.

By transmission electron microscopy, the zonular lamella of the superficial lens into which the zonule inserts has a loose fibrogranular vacuolated appearance, the latter perhaps related to water held by its proteoglycan. Its fibrils when visible are 1 to 3 nm in diameter (Fig. 26) compared with the 10-nm zonular fibrils. The zonular fibers insert by sending individual fibrils and small 0.07- to 0.5-μm fibers into the superficial capsule to a depth of 0.6 to 1.6 μm. The horizontal surface rippling seen between the anterior inserting zonular bundles is associated with rippling of the underlying superficial capsule as well, where a small number of their zonular fibers insert, emphasizing the malleability of this zonular lamellar capsule.

Staining with fibrillin-1 gives a new view of the familiar “capsular inclusions” that are said to begin accumulating in the superficial and deep capsule from the beginning of the second decade.^71,72 Anteriorly they are seen dramatically in flat preparations as fibrillin-positive, comma-shaped forms, slightly deeper than the inserting zonular fibers themselves (Fig. 27), and especially clear where anterior zonules are torn away. In some capsules they can be very profuse even at age 16, extending halfway back over the insertional area, having no clear connection to zonular fibers. In tangentially cut sections of the insertion they are curved, thin fibrogranular forms about 1 to 6 μm long, mostly 2 to 3 μm, but a few up to 9 μm long when joined together. Besides immunoreacting for fibrillin-1, they are also posi-tive for vitronectin and another binding protein,HNK-1. Whether they have a normal adhesivefunction or are to be considered degenerative isuncertain. Studies of intact fibrillin microfibrilshave shown that they can be quickly degraded by nuetrophil elastase, chymotrypsin, and trypsin,⁷³ but what enzymes might be involved in any normal turnover processes is not known.

MERIDIONAL ZONULAR FIBERS

The zonular layer thickens from 1.0 to 1.7 μm behind the anterior zonular insertion, where it shows a marked radial ribbing produced by the deep layer of meridional zonular fibers (Fig. 28A). These small fibers vary from 3 to 10 μm in width and are covered by the loose surface meshwork fibrils seen elsewhere. They are intimately involved in the insertion of the regional zonular fibers, as seen in Figure 28B, where many branches of an equatorial zonular bundle appear to merge onto the surfaces of the meridionals or enter between them for their own insertions. Zonular bundles inserting directly on the equator are quite infrequent and vary in number but can be seen at all ages, even into the tenth decade. Although some are up to 60 μm in diameter, most are 10 to 15 μm and attach almost perpendicularly, quickly merging with the meridional fibers so they cannot be individually distinguished. By transmission electron microscopy the midequatorial meridional bundles insert superficially in the capsule as small bundles (Fig. 28C).

The meridionals appear to extend over the entire zonular insertion as an initial zonular layer into which the larger bundles merge (Fig. 29), best seen when there is good preservation of the larger zonular bundles. Most of this zonular complex is torn from the lens during an intracapsular cataract extraction or separation of lenses from donor eyes. What typically remains is only the inserting tips of the anterior zonular fibers and occasional remnants of meridional fibers (Fig. 30A). Pulling forward on some of the zonular complex, as seen in Figure 30B, shows how easy it is to remove the whole complex like a blanket, leaving bare capsule visible.

The most unusual feature of the meridionals is their strong attachment in the postequatorial region, approximately over the terminal meridional nuclear rows of lens epithelial cells (Fig. 31), as seen in flat preparations of whole lens capsules immunostained for fibrillin-1.⁷⁴ The very regular outline of these10-μm columns of meridional zonular insertion may remain when most of the zonular complex has been completely removed. The insertional columns are seen to have small zonular fibers inserting on them as larger bundles are pulled away, sometimes with paler spaces across them forming a partial “waffle” pattern. The meridional zonules that attach strongly to this area can be seen to continue both above and below this attachment region. This pattern is obscured when more of the meridional zonules remain attached and may even be hidden if large numbers of posterior zonules cover the area. When the zonular fibers have been mostly removed, the base of their attachments is visualized as profuse aggregates of small fibrillin-positive filaments. Coronal cross sections through this meridional insertion show three features. Two rows of fibrillin-positive zonular material lie on the lens capsule, the outer row segmented into 3.3-μm ellipses of zonular material and the innermost layer appearing somewhat more irregular but continuous (Fig. 32). The overlying zonular bundle seen in this figure is 9.9 μm in toto. This complex appears to represent a single meridional insertion made up of three small zonular fibers with an overlying more loosely attached bundle.

In young eyes, some evidence of this radial pattern can extend up almost to the anterior zonular insertions, but it becomes increasingly restricted to the equatorial region. This intriguing site may be related to the 10-μm vertical bands of fibrillin staining reported by Wheatley and colleagues^75,76 in postequatorial frozen sections of lens capsule cut coronally. Interpretation in these studies was hindered by incomplete preservation of the zonular insertional region, such that no connection was found between the anterior inserting zonular tips and the equatorial inserting zonules, which were thought to be the only region of zonular insertion on the lens. The great regularity and persistence of this area overlying the most metabolically active site for new lens fiber formation is of interest, also the fact that the peripheral lens fibers average 10.2 μm in diameter.⁷⁷ Perhaps this postequatorial region is established in an early period of zonular aggregation on the lens capsule to help direct a radial zonular pattern, rather than a haphazard coating of the insertional region by fibrillin matrix. Regulatory and adhesive factors from the underlying lens cells might well be required.

POSTERIOR ZONULAR INSERTION

The posterior zonular bundles insert in two or three layers over a 0.4- to 0.5-mm area. The superficial bundles have a slightly more fanned insertion than do the anterior zonular bundles, with fewer interbundle connections (Fig. 33). Their insertion begins at the posterior edge of the lens equator, just beyond the terminus of the subcapsular lens epithelial cells. The innermost layer of posterior zonules begins to insert earlier, with some adhesion to the meridional zonular fibers. All are covered by a denser layer of loosely arranged meshwork fibrils than over the anterior zonular fibers (Fig. 34). When exposed,however, the insertions show the usual pattern of fibrils blending into a fibrogranular capsular matrix. The superficial capsule or zonular lamella into which they insert is especially loose in its outer2 μm, and the zonular fibers penetrate in multilay-ers for almost the whole 2-μm thickness (Fig. 35).No penetration of any zonular fiber deeper than2.6 μm into the lens capsule has been found.

The presence of sporadic bundles of fibrils in the equatorial and midperipheral lens capsule close to the lens epithelium was once thought to represent deep zonular insertions, but no direct connection to surface zonules has been found, and these inclusions were not seen before the second decade.^72,73 Some are prominently banded at 45 to 50 nm (Fig. 36), which is the periodicity of fibrillin fibrils, and stain for fibrillin-1. Others found in midcapsule in increasing numbers with age are fibrogranular with tubular profiles. They are fibrillin- and vitronectin-positive, like the anterior ones, and show no zonular fiber connection. In the exfoliation syndrome, similar capsular inclusions are much larger and very profuse. All types appear to represent aberrant synthesis by the lens epithelium, but some relation to a normal lens secretory function is not ruled out.

POSTERIOR ZONULAR ATTACHMENT TO THE ANTERIOR HYALOID MEMBRANE

The posterior zonular insertion is closely related to the attachment of the anterior hyaloid membrane on the lens capsule. This anterior hyaloid attachment was defined by Wieger (cited by Vail)¹ as an 8- to 9-mm circular area at the edge of the posterior lens, which he called the hyaloideocapsular ligament (Fig. 37). This junctional area is also known as Egger's line when seen at the slit lamp.⁶⁰ By microscopy the attachment site is found at variable distances behind the equator, depending on the method of preparation and the age of the patient. In childhood, the anterior hyaloid membrane is normally closely attached to the whole posterior zonular insertion and lens (Fig. 38). In the first two or three decades, removal of the intact lens is accompanied by the vitreous due to the strength of this adhesion. In later adult life, the anterior hyaloid membrane more readily peels back to the end of the zonular insertion and separates completelywith slight traction. The reason for the strength of Wieger's “ligament” is not evident, because no special differentiation of capsule or zonule has been recognized here, except for some circumferential girdle zonular fibers. During intracapsular cataract extraction it was not uncommon for a superficial flap of either capsule or anterior hyaloid to be torn from Wieger's area, often with vitreous loss. Many of the avulsed posterior zonules and circumferential zonular fibers remain adherent to the anterior hyaloid membrane after an uneventful intracapsular extraction (Fig. 39).⁵⁴

It is likely that none of these zonular changes contributes to the development of presbyopia as a primary factor, but rather reflects the continuous lens growth with later aging pathology.

Rohen and colleagues^47,55 proposed that there are two different types of zonules: the main zonular fibers passing directly from the pars plana to the lens, and “tension zonules” connecting the main stream to the ciliary processes. They suggested that during accommodation the tension fibers are placed on stretch, allowing the main zonular fibers to slacken. This theory assumes that the pars plana has no anterior movement during accommodation, conflicting with the traditional view and warranting further investigation. Others have not found these two separate systems of zonules.^50,53 There is no doubt, however, that most zonules are extensively attached to the ciliary processes, especially on the ciliary valleys and walls where the large number of puncta adherens junctions between the epithelial cells is evidence of tractional stress, as described by Ober and Rohen.⁵⁶ During extreme accommodation the inserting zonular fibers were observed to fold over on themselves by videography,⁸⁸ suggesting that they are isolated at least partly functionally from the pars plana fibers by their strong ciliary attachments. Relaxation of the human zonule during near accomodation has also been noted by ultrasound biomicroscopy.⁹² A new theory of accomodation offered by Schachar and colleagues with a proposed physical model^93,94 is based on the equatorial zonules being the active components in accomodation, where they are envisioned as being under tension, reducing tension on the anterior and posterior zonules and allowing the lens to round up. Anatomically the minimal number of equatorial zonules observed by most seems inadequate to allow them to exert such independent action. This theory is still being actively investigated.

Factors controlling the synthesis and turnover of zonular components are unknown. McKanna and Casagrande⁹⁵ presented evidence that there is a feedback mechanism among the variables of zonular fiber development, accommodation, and lens growth during “emmetropization” in the infantile eye. In the elongated eyes of tree shrews with unilateral lid-suture myopia, they found the lens and zonule to be hypoplastic. They postulated that blurred vision induced excessive chronic accommodation, reducing tension on the zonular fibers and lens and leading to hypoplasia of both structures by feedback loops.

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