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Comparative Anatomy and Physiology of the Mammalian Eye
Published in David W. Hobson, Dermal and Ocular Toxicology, 2020
When damaged, the cornea may respond with stromal edema, epithelial cell migration and mitosis, migration of polymorphonuclear and monocyte cells to the injured site, secretion of collagenase, activation of stromal keratocytes, vascularization, secretion of new collagen, and a shift in GAGs.2,6 The initial response to surface injury is the exfoliation of damaged epithelial cells, followed within 24 h by the infiltration of polymorphonuclear cells (PMN) from the tear film, and possibly by migration from the limbus. This is followed by the infiltration of the stroma by mononuclear cells and the activation of stromal keratocytes to become phagocytic. Collagenase is secreted by the invading cells, epithelial cells, as well as the keratocytes and serves to remove the damaged stroma. This is replaced by type III collagen that is produced from the keratocytes. The epithelial cells will form a monolayer of cells at the margin of the wound and migrate to cover the wound surface. Once covered, they will undergo mitosis to return to the normal stratified, squamous, nonkeratinized epithelium and will secrete a new basement membrane. In those species which possess a Bowman’s layer, this is not replaced when lost to injury. Many of these processes are controlled or modulated by fibronectin, extracellular growth factor, prostaglandins, and leukotrienes.18–20 As noted previously, intraocular injury to the endothelium differs in that the endothelium does not possess the mitotic capabilities of the epithelium, with the possible exception of the rabbit.
Special Senses
Published in Pritam S. Sahota, James A. Popp, Jerry F. Hardisty, Chirukandath Gopinath, Page R. Bouchard, Toxicologic Pathology, 2018
Kenneth A. Schafer, Oliver C. Turner, Richard A. Altschuler
Bowman’s layer is a superficial, dense, acellular anterior limiting membrane that is present in humans and nonhuman primates (Merindano et al. 2002). This structure is not a membrane but composed of collagen fibers.
Ocular media
Published in Fiona Rowe, Visual Fields via the Visual Pathway, 2016
The epithelium is a non-keratinised stratified squamous epithelium. Bowman’s layer is an acellular layer consisting of fine interwoven collagen fibrils. The stroma comprises the majority of the cornea and consists of flattened lamellae composed of collagen fibrils with flattened stellate cells between the lamellae. Descemet’s membrane is a modified basement membrane of the endothelium and is composed of basement membrane glycoproteins, laminin and collagen in a lattice arrangement. The endothelium is a single layer of flattened hexagonal-shaped cells. A potential sixth layer, Dua’s layer, has been proposed between the corneal stroma and Descemet layer forming layer four of six (Dua et al. 2013).
Vanadium inhalation effects on the corneal ciliary neurotrophic factor (CNTF): study in a murine model
Published in Cutaneous and Ocular Toxicology, 2023
Isis Mendoza-Aldaba, Nelly López-Valdez, María Eugenia Cervantes-Valencia, Teresa Imelda Fortoul
The cornea occupies the sixth anterior part of the ocular globe and is its principal refraction element. It is structured by five layers: (1) The epithelium, (2) Bowman’s layer, (3) stroma, (4) Descemet’s membrane and (5) the endothelium. A new layer has been described recently in the human cornea as pre-Descemet’s layer or Dua’s layer which is rich in Type VI collagen and located in the posterior area of the stroma below the final layer of keratinocytes [2,3]. The anterior corneal epithelium consists of layers of squamous cells interspersed with dendritic cells and sensitive axons [4]. It has a notorious regenerative capacity, which is sustained from the stem cells in the sclerocorneal limbus and has numerous trigeminal free nerve endings. Additionally, it produces and releases neurotrophic factors to support nerve tropism and healing [5]. Bowman’s layer consists of interwoven collagen fibres which function as a barrier against infection dissemination. This layer has no regenerative properties. The stroma comprises 90% of the whole corneal thickness and builds parallel collagen fibres. Descemet’s layer is made up of Type IV collagen which is produced continuously by the endothelial cells. This layer has regenerative properties. The endothelium is a single squamous cell layer attached by desmosomes and occludens junctions that works as a semipermeable membrane, rich in Na+, K+-ATPase pump sites in the lateral membranes [6], and allows metabolic exchange between the cornea and the aqueous humour [7].
Studies on the Effectiveness of Ozone Therapy on the Treatment of Experimentally Induced Keratitis with Candida albicans in Rabbits
Published in Seminars in Ophthalmology, 2022
Kemal Varol, Ayşe Nedret Koç, Latife Çakır Bayram, Hatice Arda, İhsan Keleş, Metin Ünlü, Vehbi Güneş, Gencay Ekinci, İlknur Karaca Bekdik, Mustafa Altay Atalay
On light microscopic examination, the cornea was observed to consist of five layers. Basal columnar cells, intermediate polygon cells, and superficial squamous cells were present in the multi-layer squamous nonkeratinized layer. Under the corneal epithelium (Bowman’s layer). Secondary propria consisted of collagen fibers and scattered spindle-shaped stromal cells arranged at regular intervals. The Descemet membrane was present just below the stroma and was covered by the endothelium of Descemet. Normal corneal morphology was observed in the negative control group. In this group, the lamellar structure of the stroma was regular. There was no change in the size and shape of the corneal epithelial cells. Keratocytes are neatly sorted. Keratin is not available (Figure 2. A.1).
Corneal Stromal Regeneration: Current Status and Future Therapeutic Potential
Published in Current Eye Research, 2020
The most anterior part of the stroma consists of Bowman’s layer, an approximately 10 µm thick layer of compacted, randomly oriented collagen fibrils13 that merges continuously with the underlying corneal stromal lamellar structure. Bowman’s layer is non-regenerating, but when present is believed to facilitate a more rapid and complete anterior corneal nerve and anterior stromal regeneration and healing after injury.14 Below Bowman’s layer is the corneal stroma proper, consisting of approximately 300 distinct lamellae, each in turn consisting of parallel-oriented bundles of collagen fibers,15 mainly consisting of type I collagen. The major stromal cell, the keratocyte, is present throughout the stroma with the cell body often localized between stromal lamellae. Keratocytes have multiple functions including collagen synthesis, collagen degradation, and participating in wound healing through transformation to a (myo)fibroblast phenotype.16 Natural stromal regeneration through the normal keratocyte-mediated turnover of collagen is a relatively slow process, occurring over at least several years.17 This long turnover time may be a prerequisite for the regenerated collagen to adopt a proper lamellar structure and orientation to maintain corneal strength, structure and transparency (stromal regeneration); by contrast, short collagen turnover times are often associated with aberrant collagen production by myofibroblasts leading to stromal haze and permanent scar tissue deposition (stromal repair).18