Eyelid growth and fusion in fetal mice. A scanning electron microscope study.

During the last phase of mammalian morphogenesis, between days 14 and 16 of gestation in the mouse, the fetal eyelids grow across the eye and become tightly fused with each other. This paper describes the surface pattern of fetal eyelids, revealed by the scanning electron microscope, during normal eyelid growth and fusion in the ICR/MI stock of mice. Fusion proceeds from both inner and outer canthi and progresses toward the middle of the gap. The first changes in cell shape and distribution occur at the inner canthus. On day 14, a large clump of rounded cells appears on the inner surface of the inner canthus. A day later, two clumps of rounded cells are positioned to either side of, i.e. above and below, the inner canthus. As fusion progresses, the diminishing gap fills with a profusion of rounded cells that are extruded, flattened, and sloughed off from the area of completed fusion. The profusion of rounded surface cells during eyelid growth and fusion appears to be a major characteristic in which the eyelid fusion process differs both from permanent fusions, such as the fusion of the neural tube, lip or palate, and from other temporary fusions, such as fusion of the digits to each other or of the pinnae to the scalp.