EYELID DEVELOPMENT, FUSION AND SUBSEQUENT REOPENING IN THE MOUSE

The process of eyelid development was studied in the mouse. The critic al events occur between about 15.5 d postcoitum (p.c.) and 12 d after birth, and were studied by conventional histology and by scanning elec tron microscopy. At about 15.5 d p.c. the cornea of the eye is clearly visible with the primitive eyelids being represented by protruding ri dges of epithelium at its periphery. Over the next 24 h, eyelid develo pment proceeds to the stage when the cornea is completely covered by t he fused eyelids. Periderm cells stream in to fill the gap between the developing eyelids. Their proliferative activity is such that they pr oduce a cellular excrescence on the outer surface of the line of fusio n of the eyelids. This excrescence had almost disappeared by about 17. 5 d p.c. Keratinisation is first evident at this stage on the surface of the eyelids and passes continuously from one eyelid to the other. E vidence of epidermal differentiation is more clearly seen in the newbo rn, where a distinctive stratum granulosum now occupies about one thir d of its entire thickness. Within the subjacent dermis, hair follicles are differentiating. By about 5 d after birth, a thick layer of kerat in extends without interruption across the junctional region. While a noticeable surface indentation overlies the latter, a similar depressi on is only seen on the conjunctival surface by about 10 d after birth. Keratinisation is also observed to extend in from the epidermal surfa ce to involve the entire region between the 2 eyelids at about this ti me. Numerous mature hair follicles are also present within the dermis at this stage, as well as differentiated muscle fibres of orbicularis oculi. By about 12 d after birth, squames of keratin are located betwe en the 2 eyelids, and eyelid opening occurs rapidly thereafter. While the sequence of events in the mouse is similar to that described in th e human, the histological events associated with the closure and subse quent reopening of the eyelids have not previously been described in d etail in any species.