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Abstract

This study sought to investigate the structure, cytology and possible function of the palisades of Vogt. These structures were found to be concentrated in the vertical meridian, where the conjunctival overlap of the comea is greatest. Of the three primate groups studied, palisades were found to be exclusive to the limbal conjunctiva of man.

Palisades varied greatly in their form and degree of associated pigment. They were more regular inferiorly, whereas superiorly they often consisted of a number of interrupted papillae. Normally, in a Caucasian eye palisades were unpigmented and difficult to see against the white sclera. In an Asian eye, they were often outlined by thin lines of pigment giving the biomicroscopic appearance of a pair of pigment lines. In a Negroid eye palisades often appeared as thin white lines surrounded by diffuse melanin pigment.

Palisades appeared as a series of characteristic stromal elevations projecting into the overlying epithelium in histological section. A flat conjunctival surface was retained by epithelial cells filling the troughs between adjacent palisades. The thickness of the epithelial layer varied between 4-5 cells thick at palisade crests and 12-15 cells at the troughs. Dichotomy of basal cells of the palisade epithelium (and subsequently the remainder of the limbal conjunctival epithelium) was described in the current study for the first time. A heterogeneous distribution of melanin within the two basal cell types was found in pigmented conjunctivae. The more numerous small cells accumulated more cytoplasm within their perikaryon than their larger counterparts. Pigment donation was found to be the driving force in pigmentation of basal cells in primate limbal conjunctival epithelia.

Immunohistochemical staining was used to investigate the cell cycling time of the two native basal cell types. From the results of histology it was hypothesised that less densely pigmented cells have a quicker turnover than more pigmented cells. Large cells were found to be faster cycling than small cells, confirming the initial hypothesis. These findings have been correlated to comeal epithelium stem cell properties to suggest that small cells conform to the properties of these important progenitor cells whilst large cells conform to their immediate offspring- transient amplifying cells.

Incidental to a quest to find palisades in the other primates, a peculiar native cell type was found in the limbal conjunctival epithelium of the cynomolgus monkey. “Star” cells were present in the basal and suprabasal layers. Their long, broad, insinuating processes suggested a capacity for movement. Coupled with the finding that these accumulated less pigment than their neighbours without processes, these cells would make ideal candidates for migration to the comeal epithelium.

The stability of the palisade pattern was investigated over a duration of one year under normal conditions and during hydrogel contact lens wear. Palisade morphology was found to be unaltered in both groups of subjects. The results may reflect the conditions and duration of the study. On the other hand, the ability to resist change in the short term may be an important property if the palisade zones are involved with generating cells for the corneal epithelium.

The apparent exclusiveness of the palisades of Vogt in man probably represents species variation. The palisade zones are highly organised areas that have been shown in the current study, to house cells conforming to the properties of comeal epithelial stem cells. Their presence at the corneo-conjunctival junction in man may serve to enhance the concentration of these important progenitor cells in this species.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RE Ophthalmology
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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