Ultrastructure of the hyaloid vasculature in primates. The eyes of 16 monkeys (Macaco, speciosa, M. irus, and M. cynomolgus), ranging in age from ... spindle-shaped nu-clei, and cytoplasmic

Ultrastructure of the hyaloid vasculaturein primates

Nancy A. Hamming, David J. Apple,* David K. Gieser, andCharles M. Vygantas

The histology and ultrastructure of the hyaloid vascular system was studied in 16 monkeys,ages 85 days in utero to 3 years old. The well-developed hyaloid artery is identifiable ultra-structurally as an arteriole consisting of a nonfenestrated intima, a multilayered smooth mus-cular media, a connective tissue adventitia, and a perivascular sheath. By the fourth weekafter birth, the vessel walk appear hyalinized and acellular; the lumen is occluded by athrombus. The time of the complete disappearance of the hyaloid artery varied considerablyin individual monkeys; remnants of the hyaloid artery were often present on the disc (Berg-meister's papilla) in adult life. The vasa hyaloidea propria and tunica vasculosa lentis aresmall branches of the hyaloid artery which fill the primary vitreous. Ultrastructurally, theyare Type A-l-a capillaries having a nonfenestrated endothelium, incomplete pericyte layer,and basement membranes surrounding each. Fluorescein angiography of several newbornmonkeys revealed nonleakage of dye from the hyaloid artery and its branches, thus correlatingwith the presence of endothelial tight junctions.

Key words: Hyaloid artery, embryonic intraocular vasculature, tunica vasculosa lentis,primates, ultrastructure of primary vitreous vessels.

.he embryonic hyaloid vascular systemis important for the growth and matura-

From the Theobald Laboratory of Ocular Pathol-ogy, Department of Ophthalmology and Pathol-ogy, University of Illinois Eye and Ear In-firmary, Chicago, 111.

Supported in part by a research grant from theAlexander von Humboldt-Stiftung, Schiller-strasse 12, D-5300 Bonn-Bad Godesberg, WestGermany, and United States Public Health Ser-vice Grant No. 24-14.

Reprint requests: Nancy A. Hamming, M.D., De-partment of Ophthalmology, University of IllinoisEye and Ear Infirmary, 1855 W. Taylor St.,Chicago, 111. 60612.

"Work completed in part during sabbatical leaveat the Histo-Pathologisch-Anatomisches Laborder Univ.-Augenklinik, Abteilung II (Prof. Dr.G. O. H. Naumann), D-7400 Tubingen, WestGermany.

tion of ocular structures during most ofgestation.13 Regression of these vessels isnormally competed by birth or shortlythereafter in human beings. Abnormal re-gression or proliferation of the vasculaturecan lead to numerous ocular anomalies,the most serious of which is persistent hy-perplastic primary vitreous.4"7

Although the histology of the embryonicvascular hyaloid system has been studiedin rodents,810 morphologic features of thevessels in primates have not yet been re-ported. This report is the first light andelectron microscopic study of the hyaloidvasculature in primates. The hyaloid sys-tem of the monkey is an advantageousmodel for histologic and ultrastructuralanalysis in that it most closely resemblesdevelopmentally and structurally that seenin man. Indeed, an important component

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Primate hyaloid vasculature ultrastructure 409

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Fig. 1. Photomicrograph showing the hyaloid artery (arrows) emerging from the vascularbulb (B) in the optic nerve. The perivascular sheath (arrows) is closely adherent to the vessel.(Mallory blue; x70.)

of the human primary vitreous, the elon-gated perivascular sheath of the hyaloidartery, has not been described in an ani-mal model. We have found such a well-developed sheath in the monkey, whichrepresents an anterior extension of theepipapillary sheath of Bergmeister. Thissheath, which is described in detail in alater study,17 is analogous to that seen inman and is not well developed in rodents.

Methods

The eyes of 16 monkeys (Macaco, speciosa, M.irus, and M. cynomolgus), ranging in age from85 days in utero to 3 years postnatally, wereenucleated immediately after sacrifice of theanimal. The eyes were fixed in 4 percent glutar-aldehyde-O.lM phosphate buffer solution.

The globes were examined with a dissectingmicroscope, looking specifically for remnants ofthe hyaloid artery. When the artery was foundextending from the optic disc, a small segment


410 Hamming et al. Incest. Ophthalmol. Visual Sci.May 1977

Fig. 2. Electron micrograph of the wall of the hyaloid artery, showing the lumen (L), intimalendothelial cell (E), media (M), adventitia (A) containing a large flbroblast (F). S, Compressedperivascular sheath. The arrows indicate junction of the vessel and its sheath with thesecondary vitreous. Note basement membrane at this junction. (Uranyl acetate and leadcitrate; original magnification, x 17,000.)

of artery together with the optic disc was dis-sected away from the rest of the eye (Fig. 1).This specimen was processed for electron micros-copy by washing in phosphate buffer, postfixingin osmium tetroxide, dehydrating, clearing, andembedding in epoxy resin. Sections 1 p thick,stained with Mallory blue, were studied by lightmicroscopy; thin sections stained with uranylacetate and lead citrate were examined with theelectron microscope.

Results

In all eyes included in this series, ages85 days in utero to 3 years, the hyaloidartery was identified—either as a well-developed, patent vessel or as an occludedremnant. Seen at the height of develop-ment, the hyaloid artery emerges from avascular bulb in the optic nerve and ex-tends through the vitreous toward the lens(Fig. 1). Surrounding the artery is a glialsheath (Fig. 2). The wall of the hyaloid

artery is composed of three layers: theintimal, medial, and adventitial layers(Fig. 2). The intima is composed of thelayer of endothelium and its basementmembrane. The endothelial cells are non-fenestrated and maintain tight junctionsat the point of contact with each other(Fig. 3). Fluorescein angiography con-firmed this tight bonding in that no dyeleakage occurred. The nuclei of the endo-thelial cells are elongated; mitochondriaand ribosomes are prominent throughoutthe cytoplasm (Fig. 3).

The media of the hyaloid artery con-tains several layers of typical smooth mus-cle cells with their basement membranes(Figs. 2 and 4). Characteristic dense bod-ies which are helpful in the recognitionof the smooth muscle are seen adjacentto the cell membranes in Fig. 4. Thesmooth muscle cells are thin and elongated


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Primate hyaloid vascukiture ultrastructure 411

Fig. 3. Electron micrograph of the tight junction(arrows) of two endothelial cells surrounding thelumen (L) of the hyaloid artery. (Uranyl acetateand lead citrate; original magnification, x30,000.)

with fibrillar cytoplasm, spindle-shaped nu-clei, and cytoplasmic organelles distributedpredominantly at the longitudinal polesof the nucleus (Figs. 2 and 4). Inter-spersed between muscle cells are theirenveloping basement membranes (Fig. 4)and occasional collagenous bundles. At theorigin of the hyaloid artery at the opticdisc, the muscular media is less prominent.

The subendothelial space and the ad-ventitia are largely composed of collagen(Figs. 2 and 3). The collagen fibrils atthe outermost aspect of the arterial ad-ventitia merge with primary vitreous fibrilswhich occupy the space between the ar-tery and its perivascular sheath (Fig. 2).

By the first month after birth, as thehyaloid artery is undergoing active involu-tion, the lumen of the hyaloid artery nar-rows and becomes occluded by debrisderived from erythrocytes, leukocytes,platelets, and fibrin (Figs. 5 and 6). Thewall of the artery appears hyalinized and

Fig. 4. Electron micrograph of smooth musclecells showing intracytoplasmic fibrils (F) in themedia of the hyaloid artery. Arrows indicate densebodies along cell membrane, {Uranyl acetate andlead citrate; original magnification, x38,000.)

acellular (Figs. 5 and 6). Just prior to itscomplete disappearance, the hyaloid ar-tery remnant is visualized as a small pro-trusion of fibrotic tissue on the surfaceof the disc (Fig. 7).

The vasa hyaloidea propria are small,capillary branches extending from the maintrunk of the hyaloid artery throughout thevitreous (Figs. 8 and 9). Ultrastructurally,the walls of these capillaries (as well asthose of the tunica vasculosa lentis) con-sist of (1) a complete layer of nonfen-estrated endothelium with interveningtight junctions encircled by a basementmembrane and (2) an incomplete layer ofpericytes surrounded by another basementmembrane (Fig. 9).

Discussion

In man, the main hyaloid artery, abranch of the primitive dorsal ophthalmicartery, enters the optic cup via the openembryonic fissure at the fourth week of


412 Hamming et at. Incest. Ophthalmol. Visual Sci.May 1977

Fig. 5. Light micrograph showing the occludedlumen and the surrounding hyalinized wall (W)of the hyaloid artery; 3 weeks postnatally. Theglial sheath {demarcated by arrows) is lined oninner and outer surfaces by a coiled basementmembrane. (Mallory blue; x,300.)

gestation.1"11 This vessel grows anteriorlyto branch at the posterior lens surface,forming the tunica vasculosa lentis. Thesevessels, in turn, course around the equatorof the lens to anastomose with vessels ofthe pupillary membrane. Additionally,small capillary branches, the vasa hyal-oidea propria, bud from the hyaloid arteryand fill the vitreous, anastomosing with thetunica vasculosa lentis and with each other.Thus, an intricate vascular network iscreated which nourishes the eye during

Fig. 6. A 3-week-ald monkey. Electron micro-graph of the hyaloid artery wall (HA) witherythrocytes, leukocytes, platelets, fibrin, anddebris occluding the lumen (L). (Uranyl acetateand lead citrate; original magnification, xl5,000.)

gestation and disappears normally duringperinatal life in man. In monkeys, we havenoted that this vascular system largely re-gresses during the first month of life (whichis slightly later than in man), althoughremnants may be found in the adult. Thestimulus for this regression is unknown.

The structure of the hyaloid vascularsystem has been studied in rodents byscanning electron microscopy,11 transmis-sion electron microscopy,9' in'12'14 and Indiaink preparation." Jack1214 has examinedthe ultrastructure of the hyaloid vas-cular system of rabbits in both develop-ment and regression. In our examinationof the vascular system in monkeys, wefound a structure very similar to that de-scribed by Jack in rabbits.


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Fig. 7. A 3-year-old monkey. Light micrograph of the occluded and hyalinized remnant ofthe hyaloid artery (A), and perivascular sheath (arrows) forming Bergmeister's papilla onthe optic nerve head. (Mallory blue; *220.)

Fig. 8. Electron micrograph of vasa hyaloidea propria, showing the complete endotheliallayer (E), incomplete pericyte investment (P), and their associated basement membranes.(Uranyl acetate and lead citrate; original magnification, x5,000.)


414 Hamming et at. Invest. Ophthalmol. Visual Sea.May 1977

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Fig. 9. Electron micrograph of vasa hyaloidea propria from the same animal as in Fig. 8.Note the tight junction between the two endothelial cells. P, Pericyte. (Uranyl acetate andlead citrate; original magnification, x25,000.)

The hyaloid "artery" is more correctlyclassified as an arteriole, according to thecriteria delineated by Rhodin.18 The hy-aloid "artery" is structured as follows. (1)The intima consists of flattened endo-thelial cells connected by tight junctions;underlying these cells is a basement mem-brane. (2) The media reveals concentriclayers of smooth muscle with basementmembranes around each fiber. Densebodies, also noted by Jack,™ are seen onthe inner surface of the plasma membraneof the muscle cells. Rhodinls stated thatthe dense "bars" are assumed to containcontractile proteins. (3) The adventitiacontains scattered fibroblasts and collagen.Unlike Jack's111 description in rabbits, wefind a well-developed glial sheath sur-rounding the hyaloid artery, which is simi-lar to that found clinically in man. Theperivascular sheath and its relation to theforming vitreous and Cloquet's canal willbe described in a subsequent paper.17

At the time of birth in monkeys, a well-developed intraocular vascular system, in-cluding hyaloid artery, tunica vasculosalentis, vasa hyaloidea propria, and pupil-lary membrane, is present. The regressionof the hyaloid vacular system beginsshortly thereafter. The artery remainspatent until approximately the third weekafter birth. At that time, the lumen be-comes narrowed and then occluded by athrombus (Figs. 6 and 7). The vesselgradually becomes attenuated, beginningat the middle of the artery and then ex-tending toward the lens and disc. Remnantsof the distal termination of the artery onthe lens form Mittendorf's dot, a smallopacity located inferonasal to the posteriorpole. The occluded remnant of the hy-aloid artery persisting at the disc, the so-called persistent Bergmeister's papilla, isby definition composed of the persistingvessel remnant as well as the vessel's glialsheath. Bergmeister's papilla was present


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in all monkeys studied during the firstmonth of life. This occluded remnant wasobserved in the oldest monkey in this ser-ies, who was 3 years old (Fig. 7), dem-onstrating that this remnant may persistinto adult life.

The branches of the hyaloid trunk(the vasa hyaloidea propria and tunicavasculosa lentis) in monkeys have TypeA-l-a structure of capillaries. This typeis characterized by a continuous basementmembrane underlying an endotheliumwithout fenestration, with incomplete peri-cyte investment. This same structural ap-pearance has been noted in hyaloid andretinal capillaries in rodents.10' ™ The factthat the endothelial cells maintain tightjunctions at the point of contact with eachother explains the resistance to fluoresceindye leakage. This finding has been sub-stantiated clinically in a human patientwith persistence of the tunica vasculosalentis in which dye leakage from theaborizing retrolental vessels was minimal.10

REFERENCES1. Barber, A. N.: Embryology of the Human Eye,

St. Louis, 1955, The C. V. Mosby Company.2. Duke-Elder, S.: System of Ophthalmology.

Vol. 3: Normal and abnormal development.Part 1: Embryology, St. Louis, 1964, TheC. V. Mosby Company.

3. Mann, I.: The Development of the HumanEye, London, 1964, Butler and Tanner, Ltd.

4. Reese, A. B.: Persistent hyperplastic primaryvitreous, Am. J. Ophthalmol. 40: 317, 1955.

5. Mann, I.: Developmental Abnormalities ofthe Eye, Philadelphia, 1957, J. B. LippincottCompany.

6. Duke-Elder, S.: System of Ophthalmology.Vol. 3: Normal and abnormal development.Part 2: Congenital deformities, St. Louis,1964, The C. V. Mosby Company.

7. Apple, D. J., and Rabb, M. F.: Clinico-pathologic Correlation of Ocular Disease, St.Louis, 1974, The C. V. Mosby Company.

8. Cairns, J. E.: Normal development of thehyaloid and retinal vessels in the rat, Br. J.Ophthalmol. 43: 385, 1959.

9. Cartner, J.: The fine structure of the zonularfibre of the rat. Development and agingchanges, Z. Anat. Entwicklungsgesch. 130:129, 1970.

10. Braekevelt, C. R., and Hollenberg, M. J.:Comparative electron microscopic study de-velopment of hyaloid and retinal capillariesin albino rats, Am. J. Ophthalmol. 69: 1032,1970.

11. Hollenberg, M. J., and Dickson, D. H.:Scanning electron microscopy of the tunicavasculosa lentis of the rat, Can. J. Ophthal-mol. 6: 301, 1971.

12. Jack, R. L.: Ultrastructural aspects of hyaloidvessel development, Arch. Ophthalmol. 87:427, 1972.

13. Jack, R. L.: Ultrastructure of the hyaloidvascular system, Arch. Ophthalmol. 87: 555,1972.

14. Jack, R. L.: Regression of the hyaloid vas-cular system, Am. J. Ophthalmol. 74: 261,1972.

15. Gloor, B.: Zur Entwicklung des Glaskorpersund der Zonula. I. Uberblick und chronolo-gischen Ablauf der Glaskorper und Zonula-entwicklung der Kaninchen und Maus, vonGraefes Arch. Klin. Exp. Ophthalmol. 186:299, 1973.

16. Gloor, B.: Zur Entwicklung des Glaskorpersund der Zonula. II. Glaskorperzellen wahrendEntwicklung und Riickbildung der Vasahyaloidea und der Tunica vasculosa lentis,von Graefes Arch. Klin. Exp. Ophthalmol.186: 299, 1973.

17. Hamming, N. A., Apple, D. J., Gieser, D. K.,et al.: Developmental anatomy of the sheathof the hyaloid artery in primates. Specialreference to the developing vitreous and itsrelation to Cloquet's canal. Submitted toArch. Ophthalmol.

18. Rhodin, J.: The ultrastructure of mammalianarterioles and precapillary sphincters, Ultra-struct. Res. 18: 181, 1967.

19. Gieser, D. K., Goldberg, M. F., Apple, D. J.,et al.: Persistent hyperplastic primary vitreous:a fluorescein angiogram demonstrating thelesion in an 18 year old man. Submitted toArch. Ophthalmol.


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Ultrastructure of the hyaloid vasculature in primates. The eyes of 16 monkeys (Macaco, speciosa, M. irus, and M. cynomolgus), ranging in age from ... spindle-shaped nu-clei, and cytoplasmic