Thomas A. Weidman

Histogenesis of the ferret retina.

The development of the ferret retina was studied with light and transmission electron microscopy from prenatal day 21 (E21; midgestation), through postnatal day 33 (P33; eye opening). Tissue samples were taken from the posterior pole of the retina. At E21, the optic cup was completely formed, the pigment epithelium was a single cell layer and the sensory retina consisted of an inner marginal layer and an outer layer of neuralblastic cell perikarya. Mitotic figures were observed among the nuclei of the outermost layer of neuralblastic cells through P10. At E27, the layer of neurablastic cell perikarya was divided into inner (light-staining) and outer (dark-staining) layers. The inner layer, presumbbly ganglion cells, was separated from the outer dark-staining layer at E36; at, E39, a distinct ganglion cell layer and inner plexiform layer was present. At birth light-staining nuclei each with a central dark-staining nucleolus were observed among the nuclei of the outermost layer of neuralblastic cells; these nuclei were presumed to be developing cone cell nuclei. At P2 light-staining nuclei, presumably horizontal cells, appeared within the remaining layer of neuralblastic cell perikarya. Conventional synapses were seen in the inner plexiform layer on P4. Separation of the layer of neuralblastic cell bodies into inner and outer nuclear layers was evident, on P9; by P12, the layers were clearly separated by the outer plexiform layer. Synaptic ribbons appeared in the outer plexiform layer on P12 and in the inner plexiform layer by P15. At the beginning of the second postnatal week (P15) membranous disks were seen in some photoreceptor cell outer segments. Vasculature was not observed prenatally; however, by the fifth week postnatally capillaries were observed as deep as the inner nuclear layer. From this study it is evident that, the ferret retina provides an easily obtainable postnatal tissue at an early stage of development.

Histochemical analysis of secretory vesicles in nongoblet conjunctival epithelial cells

Abstract. An Alcian‐blue/Periodic acid Schiff reagent (AB/PAS) pH dependent staining method was used to identify the mucus content of secretory vesicles of nongoblet epithelial cells of the human conjunctiva. Twenty subjects were selected for biopsy of the midcentral upper tarsal conjunctiva: 10 subjects had no contact lens wearing experience, and 10 subjects wore contact lenses. Human respiratory epithelium was used as a control for the staining procedure. Four‐micron paraffin sections of conjunctival and control tissues were stained with Alcian blue at pH 2.5 and 1.0 and counterstained with PAS. Non‐goblet epithelial cell secretory vesicles contained neutral mucin, sialmucin, and sulphomucin. This study provides evidence for a ‘second’ mucus system of the conjunctiva, that this system is in the non‐goblet epithelial cells, and that the mucus of this system has sulphomucins, sialomucins and neutral mucins.

Histogenesis of the cat retina

Abstract The development of the retina of the cat was studied with light and transmission electron microscopy from prenatal day 36 (E36) through postnatal day 9 (P9; eye opening), and at the adult stage (1 year). Tissue samples were taken from the posterior pole of the retina. At E36, the optic cup was completely formed, the pigment epithelium was a single cell layer, the retina consisted of an inner marginal fiber layer and an outer layer of neuroblastic cells, and the innermost cells of the neuroblastic cell layer, presumptive ganglion cells (which might still migrate through the inner plexiform layer) were displacing away from the neuroblastic cell mass. At E40, a distinct ganglion cell layer was seen. At E46, primitive horizontal cells appeared within the neuroblastic cell layer. Separation of the neuroblastic cell mass into inner and outer nuclear layers was first evident on E48; by E60, the layers were clearly separate. Conventional synapses were seen in the inner plexiform layer in the week prior to birth. Blood vasculature was observed prenatally as deep as the inner plexiform layer. In the newborn a few discs were seen in some photoreceptor cell outer segments. Synaptic ribbons appeared first in the outer plexiform layer in the newborn and then in the inner plexiform layer by P4.

Embryogenesis of the rabbit retina

The embryogenesis of the retina of the rabbit was studied with light and transmission electron microscopy from prenatal day 16 (E16; midgestation) through postnatal day 1 (P1; day of birth). Tissue samples were taken from the posterior central pole of the retina. At E16, the optic cup was formed. The neuroepithelium (sensory retina) consisted of a basally located marginal fiber layer and an apically located neuralblastic cell mass consisting of neuroblastic and glial cells. Mitotic figures were present among the most apical cells of the neuralblastic cell mass and persisted throughout the prenatal period. Numerous growth cones were observed throughout the sensory retina during the period studied. At E19, the neuralblastic cell mass was divided into basal (light-staining) and apical (dark-staining) layers. The cells of the basal layer, presumably ganglion cells, appeared to be displaced basally toward the internal limiting membrane by E21. At E24, a distinct multilayered ganglion cell layer was separated from the neuralblastic cell mass by the inner plexiform layer. At E27, the ganglion cell layer was a distinct single cell layer, conventional synapses presumably involving amacrine cells formed in the inner plexiform layer, light-staining (horizontal) cells appeared within the neuralblastic cell mass, and some nuclei among the apical layers of the neuralblastic cell mass were rounded, lightly stained, and presumed to be early cone cell nuclei. At E30 a gap formed at the level of the horizontal cells, which divided the neuralblastic cell mass into inner and outer nuclear layers. On P1 the outer plexiform layer appeared, ribbon synapses developed in the outer plexiform layer, and all nuclear and plexiform layers were established.

Histopathology of the Ocular Surface After Eye Rubbing

Purpose This study demonstrates the effects of eye rubbing on ocular surface tissue. Methods Rabbits (3–4 kg; n = 24) were killed at 0, 4-h, 8-h, and 12-h intervals after a 5-min period of eye rubbing. Ocular surface tissues were studied by light and scanning electron microscopy. Contralateral eyes served as controls. Eye rubbing was accomplished by using digital pressure over the closed eyelid with a force sufficient to appreciate by palpation the orbital rim. Biomicroscopic examination revealed marked vascular injection of the conjunctiva. Ocular surface tissues studied included the lid margins, the upper and lower tarsal conjunctivae, the bulbar conjunctiva, and the cornea. Results Changes in the ocular surface included dramatic alteration in the upper tarsal conjunctiva when compared with controls. The cornea and bulbar and lower tarsal conjunctiva were not altered when compared with control tissues, except for some increase in exfoliating cells in the cornea. The surface epithelial cells of the upper tarsal conjunctiva had a spheroidal structure and were markedly elevated, the microprojections were altered, and there was evidence of increased cellular exfoliation. These changes were most pronounced at the 0 and 4-h time points, less noticeable at 8 h, and no appreciable changes were observed when compared with control tissues at 12 h. Conclusion This study demonstrates that eye rubbing causes surface alterations in the stratified cuboidal to columnar epithelial surface of the upper tarsal conjunctiva while sparing the stratified squamous epithelial surface of the distal lid margins and cornea.

An Ultrastructural Study of the Cardiac Ganglia in the Bulbar Plexus of the Developing Chick Heart

The development of the chick bulbar cardiac ganglia has been studied ultrastructurally from the 3rd day of incubation to hatching. For descriptive purposes, their development has been divided into three phases. The first phase includes migration and aggregation of undifferentiated neuroblasts, stages 21-26 (day 3.5-5). Cardiac branches of the vagus first grow toward the heart at 3.5 days of incubation. The nerves are accompanied by migrating neural crest neuroblasts. The neuroblasts begin to aggregate at 4.5 days around the aortic arch arteries and truncus arteriosus. Some of these cells sprout axonal processes by day 5. A sparse population of developing supporting cells can be found near some neuroblasts and nerves. During the second phase of development, stages 27-36 (days 5-10), the neuroblasts assume a globular shape and have eccentric, indented nuclei. Supporting cells become more numerous as this phase of development progresses. Axodendritic synapses appear first at stage 34 in the ganglia. The neurons and supporting cells in the ganglia undergo maturation during the final phase from stage 37 to hatching (11-21 days). The immature/mature ganglion cells are large ovoid to pyriform neurons which become rounder as the cytoplasm accumulates. Axosomatic synapses are seen first at stage 38. Small supporting cells become more numerous. Clusters of catecholamine-containing cells near the cardiac ganglia are present during the final phase of development.

Ciliogenesis in photoreceptor cells of the retina

Ciliogenesis in the retinal photoreceptor of fetal, neonatal and adult ferrets has been investigated by electron microscopy. Ciliogenesis is described from the time (prenatal day 21) when a diplosome (two centrioles) is observed migrating toward the region of the external limiting membrane prior to eventual alignment beneath the apical plasma membrane of photoreceptor cells (approximately prenatal day 21). One of the centrioles becomes aligned perpendicular to the apical plasma membrane and is designated the basal body. Each developing basal body (composed of nine sets of triplet microtubules) has a rootlet extending basally and a cilium developing apically. Accessory structures associated with the basal body include foot processes and arm-like fibers. One arm-like fiber appears to be associated with each set of triplet microtubules. The arm-like fibers appear to attach to the plasma membrane along the cell apex, and it is at this point of attachment that the cell membrane invaginates. Invagination proceeds as the doublet microtubules of the developing cilium extend from the basal body toward the pigment epithelium. The invagination of the plasma membrane appears in cross-section as a ‘moat’ surrounding the evagination of the plasma membrane occupied by the developing cilium. This evagination containing the cilium eventually extends distal to the plane of the original photoreceptor cell apex. The developing cilium in the photoreceptor is unique in that it must overcome the resistance encountered from the adjacent pigment epithelial cell layer and does not merely grow into a lumen as in many other tissues. At maturation the invagination of the cell membrane around the cilium disappears, although the arm-like fibers are sometimes retained.

Comparative histogenesis of Bruch's membrane (complexus basalis)

We documented the comparative histogenesis and timing of development of Bruchs membrane (complexus basalis) in five mammalian models with different lengths of gestation. Retinas with attached choroid from the central posterior pole of the eye were obtained from hamster, vole, rabbit, ferret and cat, from mid-gestation onward until the time at which all components of Bruchs membrane could be detected. The sequence of events in the development of Bruchs membrane was similar among species, however the time of appearance of the components varied. At mid-gestation, the basement membrane of the retinal pigment epithelium (RPE) was present to some degree in all species and microfibrils were developing in the connective tissue space external to the RPE basement membrane. The appearance of an initially scant endothelial basement membrane around the choriocapillaries occurred on post-natal (P) day 1 (P1) in hamster, pre-natal (E) day 20 (E20) in vole, E16 in rabbit, P2 in ferret, and E48 in cat. Among the microfibrils within the connective tissue space external to the RPE, electron-dense islands presumed to be elastin were detected on P7 in hamster, P9 in vole, E24 in rabbit, P19 in ferret, and P1 in cat. Relative to birth date and eye opening, the components of Bruchs membrane in the ferret were detected at a later date than that of the other species studied. However, relative to the interval from conception to the appearance of the components of Bruchs membrane, the ferret and cat are similar, paralleling their similar intervals from conception to eye opening.

THE INFLUENCE OF SERIAL REMOVAL OF FELINE FOETUSES ON RETINAL DEVELOPMENT

The cat, albeit an expensive animal to breed and maintain, affords a relatively large eye ideal for studies on the embryonic retina. This study reports on the efficacy of serial removal of feline foetuses for studies on the histogenesis of the retina. Single feline foetuses were serially removed from the uterus of pregnant cats to determine if there were histologic changes in the foetal retina of the remaining foetuses during subsequent development. The retinas of neonates from mothers that had undergone hysterotomies were compared by light and transmission electron microscopy to those from animals whose mothers had not had hysterotomies. No histologic differences in the retinas were detected between the 2 groups. The duration of gestation was unaffected. This study indicates that it is possible to obtain embryonic retinal tissue at more than one stage of development from the same pregnant female without inducing histologically observable anomalies.

Response of the Ocular Surface to Histamine

Eye rubbing results in ocular surface changes, such as irregular epithelial cell morphology, alterations in microprojections, and increased cellular exfoliation. It is unknown whether such changes are related entirely to the mechanical trauma of eye rubbing or manifest secondary to the effects of histamine released from degranulated mast cells associated with eye rubbing. To distinguish between these effects, the present study examines the effects of topical histamine application in the absence of applied mechanical trauma. The bulbar and tarsal conjunctiva of rabbits (n=10) exposed to 25 μl of histamine (25 mg/ml) for 10 minutes were examined using scanning electron microscopy. Contralateral untreated eyes served as controls. Regions studied included the bulbar and tarsal conjunctivae. The bulbar conjunctiva was divided into two regions, the interpalpebral bulbar conjunctiva and that portion unexposed to the environment. In contrast to control eyes, the interpalpebral bulbar conjunctiva of treated eyes had surface cells with irregular morphology and elevated apices. The goblet cell intercellular crypt openings in treated eyes had minimal mucus, while in control conjunctiva most intercellular openings contained mucus. The upper tarsal conjunctiva of treated eyes had an irregular surface and empty intercellular crypt orifices when compared to the smooth tarsal conjunctival surface and mucus-con- taining crypts of control eyes. The lower tarsal conjunctiva had minimal differences in surface cell morphology and did not appear notably altered, in contrast to the upper tarsal conjunctiva. In summary, topical histamine resulted in alteration of the exposed bulbar conjunctiva and upper tarsal conjunctiva, whereas the unexposed bulbar conjunctiva and lower tarsal conjunctiva surfaces exhibited only subtle changes. Perhaps the changes observed in the upper tarsal conjunctiva resulted from the blinking (wiping) action of the upper lid over the chemotic exposed bulbar conjunctiva. The paucity of mucus in the goblet cell orifices after histamine exposure was unexpected.