The canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during normal eye development

Local control of cell signaling activity and integration of inputs from multiple signaling pathways are central for normal development but the underlying mechanisms remain poorly understood. Here we show that Dkk2, encoding an antagonist of canonical Wnt signaling, is an essential downstream target of the PITX2 homeodomain transcription factor in neural crest during eye development. Canonical Wnt signaling is ectopically activated in central ocular surface ectoderm and underlying mesenchyme in Pitx2- and Dkk2-deficient mice. General ocular surface ectoderm identity is maintained during development in Dkk2-deficient mice but peripheral fates, including conjunctival goblet cells and eyelash follicles, are ectopically permitted within more central structures and eyelids are hypomorphic. Loss of DKK2 results in ectopic blood vessels within the periocular mesenchyme and PITX2 expression remains persistently high, providing evidence for a negative feedback loop. Collectively, these data suggest that activation of Dkk2 by PITX2 provides a mechanism to locally suppress canonical Wnt signaling activity during eye development, a paradigm that may be a model for achieving local or transient inhibition of pathway activity elsewhere during embryogenesis. We further propose a model placing PITX2 as an essential integration node between retinoic acid and canonical Wnt signaling during eye development.

Eyelid fusion and epithelial differentiation at the ocular surface during mouse embryonic development

PURPOSE

To investigate epithelial differentiation at the ocular surface of the developing mouse eye by examining temporal and spatial changes in the expression of specific keratins.

METHODS

Ocular tissues, including the entire eyeball, conjunctiva, and eyelid, of mouse embryos at embryonic day (E) 12.5 to E18.5 as well as of adult mice were examined by hematoxylin-eosin staining and by immunohistochemistry with antibodies to keratins K4, K10, K12, and K14.

RESULTS

Hematoxylin-eosin staining revealed that eyelid fusion occurred at E17.5. Keratin immunohistochemistry demonstrated that: (1) K4 was expressed before K12, which in turn was expressed before K10; (2) expression of K4, K12, and K14 was spatially heterogeneous in the epithelia of the eyelid invaginations before eyelid fusion, but thereafter was continuous and homogeneous in the entire conjunctival epithelium, corneal epithelium, and basal cell layer of the surface epithelia, respectively; and (3) K10 immunoreactivity was not detected before eyelid fusion but was apparent in the epidermis of the eyelid thereafter.

CONCLUSIONS

Eyelid fusion is a critical period for differentiation of the ocular surface ectoderm into the epithelia of the conjunctiva, cornea, and eyelid skin. The conjunctival epithelium differentiates before the corneal epithelium, which in turn differentiates before the eyelid epidermis.

Ocular surface epithelial and stem cell development

Phenotypic features and developmental events involved in the genesis of the limbo-corneal and conjunctival epithelia are described. Together, these two epithelia define the ocular surface. They derive from a small cohort of optic vesicle-induced PAX6+ head ectodermal cells that remain on the surface following lens vesicle formation by the main PAX6+ cell cohort. Both epithelia are stratified, and display wet, non-keratinizing phenotypes. The most significant spatial feature of the limbo-corneal epithelium is the segregation of its supporting stem and early precursor cells to the limbus, the outer vascularized rim separating the cornea from the conjunctiva. These stem cells express ABCG2, a xenobiotic transporter present in stem cells from other organs. ABCG2 transport activity excludes the DNA dye Hoechst 33342, allowing the isolation of the ocular stem cells by flow cytometry, as a unique cohort known as a side 'side population'. Limbal stem cells do not form gap junctions and exist as metabolically isolated entities. Tracking of expression changes in Cx43, the main gap junction protein expressed in both the pre-epithelial ectoderm and in the mature central corneal epithelium, indicates that a limbal stem cell phenotype starts developing very soon after lens vesicle invagination, in advance of the appearance of any recognizable anatomical sub-epithelial limbal feature. Differences in Cx43 expression also reveal the very early nature of the divergence in limbo-corneal and conjunctival lineages. The putative involvement of several early genes, including gradients of PAX6 and differences in expression patterns for members of the Id or msh gene expression regulators are reviewed.

Expression of maf-B mRNA in the epithelium around the eyelid closure of the mouse eye at embryonic day 18

Maf encodes a transcription factor protein containing a typical basic leucine zipper domain structure, a motif for protein dimerization and DNA binding. We examined the expression of maf-B mRNA in the epithelium around the eyelid closure. Expression of maf-B mRNA was examined in C57Bl6 mice at the embryonic stages in 12.5 days of gestation (E12.5) and E18 using in situ hybridization with 35S-labeled antisense riboprobes. In embryos studied 12.5 days postconception, a message specific for maf-B was not detected around the developing eyelid. In contrast, maf-B was strongly expressed in the epithelium of the eyelid closure at E18. Expression of maf-B was strongly noted in the suprabasal differentiating cells derived from the basal layer of the conjunctiva and epidermis. In contrast, basal cells in the eyelid closure and in the epidermis, as well as keratinizing cells, did not express maf-B. These data indicate that maf-B mRNA is expressed during development of the eyelid closure.

The structure of the human semilunar plica at different stages of its development a morphological and morphometric study

In this study development of the semilunar plica was examined histologycally by making sections through the eyes of eleven foetuses at different stages of gestation, two newborns and an old man. We found that in the early stages of its development the semilunar fold covered a bigger part of the orbit and later did not keep up with the growth of the eyeball and the lids. In its development three different kinds of germinal glands could be seen in the semilunar plica. Beside poorly differentiated buds of the surface epithelium which can be classified as rudiments of the nictitating or Harderian gland, serous glands were detected which could be beginnings of Krause's glands. Additionally, a new kind of plica gland was identified in which ductular structures, with an onion skin appearance, could be discriminated from mucous acini. The surface of the plica developed slowly by an increase in the layers of its epithelium as well as by the maturation of the epithelial cells from a two-layered cuboidal to a multi-layered cylindrical epithelium. In general the palpebral side of the plica consisted of a higher number of epithelial layers and more goblet cells than its bulbar side. Moreover, the surface of the palpebral side appeared more irregular and enlarged by numerous pleatings of its epithelium. The initially loose mesenchymal connective tissue was soon condensed by firstly an increase of the number of cells and later by an increase in the fibre density. In the tight collagenous connective tissue no elastic or reticular fibres or cartilaginous structures could be found. Very dense vascularization of the plica semilunaris was seen early in development. The blood vessels rising from the root of the plica divided themselves into a central and a subepithelial vascular net. Later on, some of them showed an enlarged lumen and were covered by a thin layer of muscle cells. A few unmyelinated neurofibres were found next to blood vessels and glands. Muscle cells could not be detected. A very dense concentration of leukocytes in the plica even in early intrauterine development was very striking and could be observed in this kind for the first time. These were mostly lymphoplasmocytic elements but granulocytes and macrophages could also be seen around the blood vessels and in the subepithelial area where they were arranged as follicles. Some of these cells even passed through the epithelium and could be found in the conjunctival fissure. The discovery of dense infiltration with both specific and non-specific immune cells, abundant vascularization and secretory structures (goblet cells and surface enlargement) in the semilunar plica suggest that it plays an important role as a specialized organ in human eye protection. Its anatomical position at the medial border of the eye supports this theory. Its origin from the third eyelid of the mammals could be recognized to a certain extent.

Microvascularization of the mucocutaneous junction of the eyelid in fetuses and neonates

The aim of this study was to specify the microvascularization of the junctional region between the integuments of the superficial surface of the free margin of the eyelid and the palpebral conjunctival mucosa. The study was carried out using histological or transparified slices of upper and lower eyelids taken from fetuses and neonates, in which the vascular system was injected with agarized China ink. The mucocutaneous junction of the eyelid is located at the posterior border of the free margin and extends to its deep surface. It has vascular similarities to the oral cavity and the nasal pyramid. Under a thickened avascular epithelium, there is a papillary network composed of characteristic loops that are less raised than in the lips but more developed than in the nose. The superficial and deep vascular reticular networks are comparable in fundamental arrangement to those of other junctional zones. Thus, the palpebral mucocutaneous junction shows cutaneous-type microvascularization, just like the other junctional zones of the head.

Proliferation and apoptosis in the epithelium of the developing human cornea and conjunctiva

To determine the distribution of proliferating and apoptotic cells in the human cornea during prenatal and early postnatal development, we examined sections of the bulbar conjunctiva, the limbus as well as the central and peripheral cornea between 11 weeks of gestation and 6 months after birth. The objective was to localize dividing cells by proliferating cell nuclear antigen-like immunoreactivity (PCNA-LI) and apoptotic cells by terminal transferase-mediated nick-end labeling (TUNEL). Before the 17th gestational week, PCNA-LI was absent in all 4 regions examined, indicating negligible cell proliferation during early development. After 20 weeks, strong PCNA-labeling was observed in all regions examined suggestive of high proliferative activity not only in the limbus and the bulbar conjunctiva, but also in the central and peripheral cornea. This rise in proliferative activity was followed by a steady decline: after 28 weeks, anti-PCNA staining gradually disappeared in the central and peripheral cornea, so that, at 6 months after birth, it was confined to the limbus and the bulbar conjunctiva, resembling the picture described for the adult cornea. TUNEL-positive cells were virtually absent in all 4 regions examined before the 38th gestational week. Apoptotic cells only started to appear at 38 weeks; at this stage, they were confined to the bulbar conjunctival epithelium. At 6 months after birth, TUNEL-positive cells were observed in the bulbar conjunctival epithelium and the entire cornea; the limbus, however remained devoid of apoptotic cells throughout the entire prenatal and early postnatal period. The present study for the first time localizes proliferating and apoptotic cells in the epithelium of the developing human cornea. Three stages of development can be distinguished: Minimal proliferation (until 17th week), vigorous proliferation over the entire cornea including the limbus and the bulbar conjunctiva (until 28th week) and gradual decrease in proliferative activity (after 28th week) accompanied by the appearance of apoptotic cells.

Systematic immunolocalization of retinoid receptors in developing and adult mouse eyes

PURPOSE

To determine the localization of retinoic acid receptors (RAR) alpha, beta, and gamma and retinoid X receptors (RXR) alpha, beta, and gamma in developing and adult mouse eyes at the level of single cells.

METHODS

Immunohistochemistry was performed on paraformaldehyde-lysine-periodate-fixed cryosections of mouse eyes, from embryonic day 10.5 to adulthood, with polyclonal antibodies directed against each receptor isoform. Histologic sections from null mutant mice for each receptor served as negative controls.

RESULTS

RARalpha was present ubiquitously in the prenatal eye and preferentially located in the posnatal retina and ciliary body. RARbeta was detected predominantly in the periocular mesenchyme and ciliary body. RARgamma was distributed in the periocular mesenchyme, choroid, sclera, cornea, conjunctiva, and lids. RXRalpha was found preferentially in the prenatal periocular mesenchyme and retina and in the postnatal ciliary body, cornea, and conjunctiva. RXRbeta was ubiquitous at all the stages. RXRgamma was detected mainly in subsets of prenatal retinal cells and in postnatal ganglion cells as well as a subset of photoreceptor cells that were characterized as cones in adults.

CONCLUSIONS

RARalpha, beta, and gamma and RXRalpha and gamma exhibit specific and dynamic patterns of distribution in ocular tissues throughout the course of development. The abundance of RARbeta, RARgamma, and RXRalpha in the periocular mesenchyme suggests that this tissue represents an important site of retinoid actions during eye development and in adulthood.

Expression of fucosyltransferases in skin, conjunctiva, and cornea during human development

During human development, type-1-precursor, sialyl-Le a, and Le x antigens were present in the periderm of skin and eye at week 6. The Le x antigen disappeared from cornea at 10 weeks and then from skin at 20 weeks. H-type-1, Le a, Le b, sialyl-Le a, H-type-2, sialyl-Le x, and Le y were found in cornea, conjunctiva, and periderm between 10 and 20 weeks. They disappear from the skin (at week 20) and progressively reappear in skin derivatives, especially in the epithelium of sweat glands. The secretory part of the sweat gland is type-1-precursor and H-type-1 positive while its excretory part is Le a, Le b, sialyl-Le a, and Le y positive. On the eye surface the disappearance of Le x at 10 weeks and of the H-type-1, sialyl-Le x, and Le y at week 35 starts in the central cornea in front of the lens. The corneal epithelium and the conjunctiva have similar antigens to those of excretory and secretory parts of the sweat gland, respectively. Invaginations and folding of the epidermis might preserve the embryonic staining. We propose that fucosylation patterns are associated with the embryonic origin and differentiation stage of tissue. The early and transient presence of Le x is associated with FUT4 or FUT9 activities, while the late appearance of Lewis antigens is related to other alpha3-fucosyltransferases.

A caruncular dermoid with contiguous eyelid involvement: embryologic implications

An 11-year-old girl with a mass arising from the caruncle and the upper eyelid was examined for spontaneous epibulbar bleeding. Excision of the lesion involved reconstruction of the proximal nasolacrimal system, lid margin, and conjunctiva. The histopathologic diagnosis was consistent with a dermoid of the caruncle. This case is notable not only for a dermoid accompanied by epibulbar bleeding but for its caruncular origin and attachment to the upper eyelid. This case suggests that the caruncle and the upper eyelid arise from similar embryologic structures.

Morphological differentiation of the conjunctival goblet cells in the chick (Gallus domesticus)

BACKGROUND

These is no consensus in the literature regarding the differentiation of conjunctival goblet cells in vertebrates.

METHOD

The conjunctival epithelium of the chick was studied before and after hatching in order to demonstrate the morphological evolution of the goblet cells. The entire conjunctiva was processed for light microscopy either on semithin sections stained with toluidine blue-pironine or on traditional sections stained with Alcian blue pH 2.5-PAS.

RESULTS

It was possible to demonstrate that goblet cells underwent remarkable changes in their secretory activity. At 12 h after hatching, isolated Alcian blue-positive cells were present in the fornix. At 24 h after hatching, cells positive for both Alcian blue and PAS were scattered among epithelial cells. Two days after hatching, cells which reacted positively only to PAS were also present.

CONCLUSION

It is suggested that the differentiation of conjunctival goblet cells occurs first in the fornix, probably due to the particular vascular environment of this region, and then spreads all over the conjunctiva.

Localization of basement membrane-associated protein isoforms during development of the ocular surface of mouse eye

The developmental localization patterns of collagen type IV alpha1-5 chains, laminin-1, laminin-5, and laminin alpha2 chain were analyzed in the embryonic mouse eye using isoform specific antibodies and immunofluorescence microscopy. Laminin-1 isoform and alpha1-2(IV) were ubiquitously expressed along the ocular surface basement membranes at a very early stage of eye development. Alpha3-5(IV) were first detected at later stages of development, and exhibited a variable distribution pattern along the ocular surface basement membrane. In contrast, expression of the laminin alpha2 chain was restricted to the conjunctival basement membrane, and was first detected during the same developmental period in which keratin K4-positive, differentiated conjunctival epithelial cells were observed. Although laminin-5 was uniformly expressed along the adult ocular surface basement membrane, during embryogenesis it was first incorporated into the conjunctival basement membrane structure. These data suggest that some of the laminin isoforms, including laminin alpha2 and laminin-5, may play a role in the formation of a conjunctival-type basement membrane. The temporal relationship between the localization of these molecules to the conjunctival basement membrane and the appearance of differentiated conjunctival epithelial cells suggests a role for external influence on the differentiation pathways of ocular surface epithelium.

The Pax-6 homeobox gene is expressed throughout the corneal and conjunctival epithelia

PURPOSE

Heterozygous defects in the highly conserved PAX6 homeobox gene are associated with aniridia, an inherited human disorder affecting several ocular structures, including the adult cornea. This work establishes the pattern of Pax-6 gene expression in the surface epithelia of the late embryonic and adult eye.

METHODS

Chick embryo sections and wholemounts, as well as adult mouse and monkey tissues, were analyzed by in situ hybridization and immunohistochemistry with probes specific to Pax-6. Western immunoblots were used to detect Pax-6 protein, and mRNA expression was analyzed by quantitative reverse transcription-polymerase chain reaction.

RESULTS

In days 5 and 6 chick embryos, Pax-6 protein is found in the nuclei of all cells within the corneal epithelium and in the future conjunctiva. Although not detected in the cornea by in situ hybridization, Pax-6 mRNA is, in fact, present at levels comparable to those observed in the retina. In the mature mouse, Pax-6 protein was expressed in all cells of the corneal epithelium, the limbus, and the entire conjunctiva. Similar results were obtained for the monkey cornea.

CONCLUSIONS

These data indicate that in addition to its role in the embryo, Pax-6 is expressed strongly in surface epithelia of the adult cornea and conjunctiva. In cells of these tissues, the gene may function by regulating structural or secretory specializations. Pax-6 might play a direct role in the maintenance and proliferation of corneal stem cells, a vital process that appears to be defective in aniridia.

Expression of keratins K12, K4 and K14 during development of ocular surface epithelium

The 55 kDa keratin K12 and the 59 kDa keratin K4 were used as biochemical markers of differentiated corneal and conjunctival epithelium, respectively, to follow the temporal and spatial appearance of these cell types during embryonic development of the mouse eye. K12 was first detected in corneal epithelial cells of day 15 mouse embryos in a small subpopulation of superficial cells. At later developmental stages only suprabasal corneal epithelium expressed K12, however, in post-natal and adult cornea all cell layers were K12-positive. K4 was first observed, in 14 and 15 day embryos, in a subpopulation of epithelial cells which had invaginated from surface ectoderm to form the lid buds. From embryonic day 16 on K4 was detected in all areas of developing conjunctival epithelium. Some superficial corneal epithelial cells also expressed K4 during embryonic development, but by immunofluorescence microscopic criteria, this keratin was localized exclusively to the conjunctiva in post-natal and adult eye. Expression of the 50 kDa 'basal-type' keratin K14 was also examined in this study. Similarly to K4, K14 was first noted in epithelium comprising the lid bud at embryonic day 14. Between 14 and 17 days of development some epithelial cells in the putative fornix of the conjunctiva did not express K14. Although corneal epithelial cells expressed K14 during development, in adult cornea only certain basal cells did so. These results suggest that the invagination of surface ectoderm to form the presumptive eyelid may be coupled to the initiation of differentiation of ocular surface epithelium.

Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Neural tissue has been considered to be immunologically privileged and major histocompatibility complex (MHC) class II antigens not expressed in normal human brain grey matter and retina. In the present study we compare phenotypic characteristics of human retinal microglia and dendritic Langerhans cells, including their morphologies and distribution, MHC class II and CD45 antigen expression and nucleotidase reactivity. Levels of class II expression were measured using optical densitometry in combination with standard immunohistochemical techniques applied to retinal flatmounts. The results indicate that ramified retinal microglia have features in common with dendritic antigen presenting cells of cornea and conjunctivum, including the constitutive expression of MHC class II antigens.

[Morphological criteria of early embryonal histogenesis exemplified by the development of the auxiliary ocular apparatus in humans]

In 50 human embryos at the age of 21 days--12 weeks of development, including the stages X-XXIII and beginning of the fetal period according to Carnegie's classification, spatial-temporal regularities on rearrangement of cellular material in the mesenchymal derivatives of the ocular auxiliary apparatus have been investigated. The main attention has been paid to ascertain the moments, when the first signs of differences appear in the previously homogenous cellular material of the differentiating mesenchyme around the ocular cup and in the eyelids, taking into account appearance of derivates in the ocular auxiliary apparatus: sclera, oculomotor muscles, stroma of the lacrimal gland, cartilagenous laminae in the eyelids and in the orbicular muscle. Regularities of cytochemical differentiation of the mesenchyme and its derivatives have been studied, taking into account the organ's changing topography. Morphometrical investigations of cells in the anlages mentioned are expanded and correlated with the time of their cytochemical differentiation. Correlation of the morphometrical and cytochemical data with the histological investigations has been performed.

[Karyometric characteristics of the accessory apparatus of the eye in embryonal histogenesis]

In 50 human embryos, distributed according to the age scale from the 21st day up to 12 weeks of the intrauterine development, which includes stages X-XXIII and beginning of the fetal period (classification of Carnegie Institute), the diameter and volume of the cell nuclei have been followed in the process of formation of epithelial and mesenchymal derivatives of the eye accessory apparatus. In early histogenesis of the eye accessory apparatus the dimensions of the cell nuclei of all epithelial and mesenchymal anlages decrease linearly. In the cells of the epithelial derivatives decrease of the nuclear dimensions in expressed more significantly than the mesenchymal ones. The dynamics of the nuclear dimensions is compared with the form of the same nuclei in each anlage. The nuclear form changes. The differentiation in all the anlages occurs similarly: cells with nuclei having various size appear. Each tissue by the moment, when it appears from the tissue source, possesses cells with nuclei, significantly differing from the nuclei in the latter. In accordance with the modern cytological data, problems and matters making the base of the changes in the nuclei in the course of embryonal histogenesis are discussed.

Ultrastructural observations on the development of the human conjunctival epithelium

In human embryos with a gestation age of 8.6-22 weeks, the palpebral conjunctival epithelium was examined by transmission electron microscopy (TEM). During the gestation period studied, the structural integrity of the tissue is established by an increase in the quantity of tonofilaments, desmosomes, and hemidesmosomes as well as by the undulating appearance of the cell membranes, the widening of the intercellular space, and the development of cytoplasmic protrusions into it. The superficial cells display a chronological sequence in the elaboration of transport mechanisms. A precursor stage is described for hemidesmosome formation at the interface between the basal cell membrane and the conjunctival stroma.

Patterns of cytokeratin and vimentin expression in the human eye

We studied the expression of the various cytokeratin (CK) polypeptides and vimentin in tissues of the human eye by applying immunocytochemical procedures using a panel of monoclonal antibodies as well as by performing biochemical analyses of microdissected tissues. Adult corneal epithelium was found to contain significant amounts of the cornea-specific CKs nos. 3 and 12 as well as CK no. 5, and several additional minor CK components. Among these last CKs, no. 19 was found to exhibit an irregular mosaic-like staining pattern in the peripheral zone of the corneal epithelium, while having a predominantly basal distribution in the limbal epithelium. Both the fetal corneal epithelium and the conjunctival epithelium were uniformly positive for CK no. 19. In the ciliary epithelium, co-expression of CKs nos. 8 and 18 and vimentin was detected, whereas in the retinal pigment epithelium, CKs nos. 8 and 18 were dominant. The present data illustrate the remarkable diversity and complexity of CK-polypeptide expression in the human eye, whose significance with respect to histogenetic and functional aspects is, as yet, only partially clear. The unusual distribution of CK no. 19 in different zones of the corneal epithelium may be related to the specific topography of corneal stem cells. The occurrence of the expression of simple-epithelium CKs in the ciliary and pigment epithelium demonstrates that, despite their neuroectodermal derivation, these are true epithelia.

Cell proliferation in condensing scleral ectomesenchyme associated with the conjunctival papillae in the chick embryo

The role of cell proliferation in the formation of scleral ectomesenchymal condensations underlying the conjunctival papillae was examined with in vivo tritiated thymidine labelling in chick embryos ranging in age from 8 days 0 h to 10 days 12 h. Percentages of labelled nuclei were determined in both ectomesenchyme and the deeper fibrous sclera for short-term and continuous tritiated thymidine incubations. During formation of the ectomesenchymal condensations the percentages of labelled nuclei were consistently higher within the condensations than in corresponding non-condensing ectomesenchyme between papillae. The consistent differences of labelling percentages observed within the condensing versus non-condensing ectomesenchyme were not found in the fibrous sclera at any stage. All areas of both the ectomesenchyme and fibrous sclera showed decreases in the percentages of labelled nuclei from 8 days 0 h to 10 days 12 h, although the decline in the ectomesenchymal condensations beneath papillae occurred more slowly than in areas between papillae. The data suggest that the conjunctival papillae directly influence the proliferation in the subjacent condensing ectomesenchyme but have no effect on the ectomesenchyme between papillae or any region of the deeper fibrous sclera. The observations of this investigation are discussed in relation to other studies of the development of the pre-ossicular mesenchyme.

Unusual features in the conjunctiva and cornea of the normal guinea-pig: clinical and histological studies

Clinical and histological studies of the outer eyes of newborn and adult female guinea-pigs were carried out together with histological studies of fetal eyes. Fine superficial corneal vessels, extending from the limbus, were seen in most newborn and adult eyes and were also present in histological preparations of fetal eyes. These vessels were therefore considered to be features of the normal guinea-pig eye. Masses of tissue composed of lymphoid cells, which were often organised into germinal centres, were found in the fornices of newborn and adult eyes. Smaller accumulations of lymphoid cells were found in fetal eyes, and this lymphoid tissue was therefore also considered to be a normal feature of the guinea-pig eye. In adult eyes mild but short-lived oedema or hyperaemia of the palpebral conjunctiva was quite common. There were no differences in the amounts of conjunctival cellular infiltration in these eyes c found in fetal eyes, and this lymphoid tissue was therefore also considered to be a normal feature of the guinea-pig eye. In adult eyes mild but short-lived oedema or hyperaemia of the palpebral conjunctiva was quite common. There were no differences in the amounts of conjunctival cellular infiltration in these eyes c found in fetal eyes, and this lymphoid tissue was therefore also considered to be a normal feature of the guinea-pig eye. In adult eyes mild but short-lived oedema or hyperaemia of the palpebral conjunctiva was quite common. There were no differences in the amounts of conjunctival cellular infiltration in these eyes compared with normal eyes, and these inflammatory changes were therefore not considered to be significant.

[Microcirculatory bed of the bulbar conjunctiva in 3- to 9-month-old fetuses]

In 32 corpses of 3-9-month-old fetuses the diameter, length amount of the microcirculatory bed vessels and their wall thickness have been studied in different areas of the eye bulb conjunctiva by means of impregnation after Kuprianov. General morphofunctional vascular peculiarities have been revealed. They are connected with age and structure of the developing tissues in the eye bulb conjunctiva. Two stages of development have been defined: 3-6 and 7-9 months.

Morphogenesis of conjunctival papillae from normal and scaleless chick embryos

Morphogenesis of avian conjunctival papillae follows a predictable temporal and spatial pattern and is in some manner directly related to the introduction of the underlying scleral ossicles. We have been able, using Scanning Electron Microscopy (SEM), to correlate all of Murray's ('43) histological stages (1--6) of papillae development, with changes in elevation and morphology of the surface of the conjunctiva. The first indication of morphogenesis is the formation of "papillae primordia." The centers of these primordia exhibit decreased intercellular contact, and become elevated as radially symmetrical humps whose surfaces are composed of rounded cells with numerous microvillar projections. As the papillae become asymmetrical and elongate, cells near the tip of the papillae enlarge and develop microridges. During regression of the papillae, single clusters of cells appear to become lost from the surfaces of the papillae into the surrounding fluid. In contrast to normal chick embryos, those homozygous for papillae and underlying scleral ossicles (Palmoski and Goetinck, '70). SEM of the mutant conjunctival surface indicates that these papillae do not exhibit all of Murray's ('43) histological stages and are morphologically abnormal. Data from the present SEM study of the normal and scaleless conjunctiva are discussed in relation to those data of other investigators, and we suggest that Stage 4 in papillae development is critical to scleral ossicle formation.

[Age-specific features of the vessels of the microcirculatory bed of the conjunctiva]

The microcirculatory bed of various parts of the human bulbar conjunctiva in ontogenesis has been studied in 63 preparations of the eyeball by means of complex morphological methods. Structural peculiarities and functions of all parts of the micro-vessels have been revealed in fetuses, their conjunctiva being not yet subjected to any environmental effect, and in successive age groups of the postnatal development. The period of stabilization and differentiation for all parts of the microcirculatory bed in the eyeball conjunctiva has been stated. Certain age changes of the microcirculatory bed have been revealed in all sectors and in various parts within the sectors; these changes should be taken into account at biomicroscopy and when analysing the data obtained.