Definition of pellucid3/17/2023 ![]() The treatment of PMD is mostly conservative with spectacles and contact lens with certain patients requiring surgical intervention like collagen cross-linking or keratoplasty. PMD is a slowly progressive condition that more commonly occurs between the second and fifth decades of life. While the classic presentation of the disease is inferior thinning, there are reports with involvement of the superior, temporal, and nasal parts of the cornea. The corneal protrusion is more marked just superior to the area of thinning where the thickness of the cornea is usually normal. This “beer belly” configuration causes the cornea superior to the ectasia to protrude and produces a flat vertical meridian above the thinning and a high against-the-rule astigmatism. The degree of thinning of the cornea can be severe resulting in up to 80% stromal tissue loss. The corneal area between the thinned band-like region and the limbus is characterized by the absence of lipid deposition, scarring, or vascularization and is typically epithelialized. The term pellucid meaning “clear” to denote the corneal clarity was used for the first time by Schlaeppi. This review summarizes literature data on the possible mechanisms of zone hatching in the development of human embryos, obtained in experiments in vitro, as well as in animal models.Pellucid marginal corneal degeneration (PMD) is a progressive, noninflammatory ectatic corneal disorder that more commonly involves the inferior cornea separated from the limbus by a relatively uninvolved area of 1 to 2 mm in width. Only the involvement of the cytoskeleton of trophectoderm cells in the mechanism of blastocyst compression was shown, and the participation of desmosomes in the coordinated change in the form of trophectoderm cells during compression is suggested. The mechanisms of the active movement during blastocyst hatching are investigated to a lesser degree. Proteases, secreted by human trophectoderm, are not described. Depending on the species of the mammal, different classes of proteases participate in the zone hatching process: serine proteases, cysteine proteases, metalloproteinases. Lytic enzymes, secreted by cells of the trophectoderm for dissolving the zona pellucida, are different. Taking into account the singleness of the paper and the absence of further development of this subject by the authors in the following decade, the existence of specialized cells for zone hatching should be assumed with great care. There is only one paper (Sathananthan et al., 2003), which describes the specialized cells in the trophectoderm that locally dissolve the zona pellucida, promoting the emergence of the hole for blastocyst release. The break of the zona pellucida occurs under the influence of two forces: mechanical pressure of the growing blastocyst on the zone and chemical dissolution of the zone material with secreted lytic enzymes. While microscopic description of zone hatching is well known, biochemical and cytological basis of zone hatching remains poorly understood. During zona hatching the blastocyst breaks the zona pellucida and performs active movements to escape through a gap formed in the zona. The release of the embryo from the zona pellucida is carried out at the stage of the blastocyst and called zona hatching. The embryo needs to get out of the zona pellucida before implantation to establish cell contacts between the trophectoderm and endometrial epithelium. The zona pellucida is preserved after fertilization and surrounds the developing human embryo for a few days. The human oocyte is surrounded by the zona pellucida-an elastic, transparent extracellular matrix consisting of specific glycoproteins.
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