General Histology Lec 3 PDF

General Histology Lec3 ‫سهيل لبيب حسون‬.‫د‬ Epithelium Histology Epithelium generally consists of closely grouped polyhedral cells surrounded by very little or no intercellular substance or tissue fluid, respectively. A basement membrane is located between these two tissue ty...

General Histology Lec3 ‫سهيل لبيب حسون‬.‫د‬ Epithelium Histology Epithelium generally consists of closely grouped polyhedral cells surrounded by very little or no intercellular substance or tissue fluid, respectively. A basement membrane is located between these two tissue types. Epithelium is avascular, having no blood supply of its own. Cellular nutrition consisting of oxygen and metabolites is obtained by diffusion from the adjoining connective tissue, which is usually highly vascularized, sharing its source of nutrition. This tissue is capable of rapid cellular turnover. In fact, epithelium is highly regenerative because its deeper germinal cells are capable of reproduction by mitosis. Epithelial cells usually undergo cellular differentiation as they move from the deeper germinal layers to the surface of the tissue to be shed or lost. An exception to the process of cellular maturation is the junctional epithelium of the gingival sulcular region that is attached to the tooth surface. Epithelial cells are usually tightly joined to each another by intercellular junctions provided for by the desmosomes except in the more superficial layers. The epithelial cells are also tightly joined in some cases to adjacent noncellular surfaces by hemidesmosomes, as is the case with its relationship to the basement membrane as well as the junctional epithelium of the gingival sulcular region that is attached to the tooth surface. A basement membrane is located between most epithelium and deeper connective tissue, such as with both the skin and oral mucosa. Components of basement membrane are produced by both the overlying epithelium as well as the adjoining connective tissue. Epithelium Classification Epithelium can be classified into two main categories based on the arrangement into layers of cells: simple and stratified. Simple epithelium Consists of a single layer of epithelial cells. The further classification of the tissue involves different types of epithelial cells according to cellular structure; they can be classified as either simple squamous, simple cuboidal or simple columnar. Simple squamous epithelium consists of flattened platelike epithelial cells, or squames, lining blood and lymphatic vessels, heart, and serous cavities, as well as interfaces in the lungs and kidneys. The term endothelium is used to refer to the simple squamous epithelium lining of these vessels and serous cavities. Simple cuboidal epithelium consists of cube-shaped cells that line the ducts of various glands, such as certain ducts of the salivary glands. Simple columnar epithelium consists of rectangular cells, such as in the lining of other salivary gland ducts, as well as the inner enamel epithelium of a maturing tooth germ, whose cells become enamel-forming ameloblasts. Epithelium can also be considered pseudostratified columnar epithelium, which is named as such since it falsely appears as multiple cell layers when viewed with lower power magnification due to the cells’ nuclei appearing at different levels. Pseudostratified columnar epithelium lines the upper respiratory tract, including the nasal cavity and paranasal sinuses. This type of epithelium may have cilia or be nonciliated at the tissue surface. Stratified epithelium Consists of two or more layer of cells, with only the deepest layer lining up to contact the basement membrane. It is important to note that only the cellular shape of the surface layer is used to determine the classification of stratified epithelium. Thus, stratified epithelium can consist of cuboidal, columnar, or squamous epithelial cells, or a combination of cell types, as seen in a transitional epithelium. Most epithelium in the body consists of stratified squamous epithelium, which includes the superficial layer of both the skin and oral mucosa and. Only the most superficial layers of this tissue are flat cells, or squames; the deeper cells vary from the deeper cuboidal to the more superficial polyhedral. Interdigitation of the outer epithelium with the deeper connective tissue occurs with the epithelial tissue forming rete ridges (or rete pegs); however, there is always a basement membrane located between these two tissue types. Stratified squamous epithelium can be nonkeratinized or keratinized. Nonkeratinized tissue can be found in certain regions of the oral mucosa as well as keratinized tissue. The keratin found within the keratinized tissue is a tough, fibrous, opaque, waterproof protein that is impervious to pathogenic invasion and resistant to friction. Keratin is produced during the maturation of the keratinocyte epithelial cells as they migrate from near the basement membrane to the surface of the keratinized tissue. Another example of keratinized stratified squamous epithelium is epidermis, which is the superficial layer of the skin. The epidermis overlies a basement membrane and the adjoining deeper layers of connective tissue (dermis and hypodermis, respectively). The skin has varying degrees of keratinization depending on the region of the body. Areas such as the palms of the hands and bottom of the feet have thicker layers of keratin, which form calluses. However, the keratin is less densely packed in both the skin and oral cavity, as compared with the densely packed hard keratin of the nails and hair. Basement Membrane Properties The basement membrane is a thin, acellular structure always located between any form of epithelium and the underlying connective tissue, as noted in both the skin and oral mucosa. This type of structure is even present between the components of the tooth germ during tooth development. The details of the basement membrane are not seen when it is viewed by scanning or lower-power magnification; only its location can be indicated. A higher-power magnification, such as that afforded by an electron microscope, is needed to see the intricacies of the basement membrane. The basement membrane consists of two layers: basal lamina and reticular lamina. The superficial layer of the basement membrane is the basal lamina, which is produced by the epithelium, and it is about 40 to 50 nm thick. Microscopically, the basal lamina consists of two sublayers: The lamina lucida is a clear layer that is closer to the epithelium, and the lamina densa is a dense layer that is closer to the connective tissue. The deeper layer of the basement membrane is usually the reticular lamina (the exception is lung alveoli and kidney, with fusion of basal laminae). The reticular lamina consists of collagen fibers and reticular fibers produced and secreted by the underlying connective tissue. Attachment mechanisms are also part of the basement membrane. These involve hemidesmosomes with the attachment plaque as well as 12 tonofilaments from the epithelium and the anchoring collagen fibers from the connective tissue. The tonofilaments from the epithelium loop through the attachment plaque, whereas the collagen fibers of the reticular lamina loop into the lamina densa of the basal lamina, forming a flexible attachment between the two tissue types. It is important to note that the interface between the epithelium and connective tissue of both the skin and oral mucosa where the basement membrane is located is not two-dimensional, as seen in microscopic cross sections of the tissue with its epithelial rete ridges and connective tissue papillae. Instead, in reality, the interface consists of three-dimensional interdigitation of the two tissue types. This complex arrangement increases the amount of surface area for the interface, thus increasing the mechanical strength of the interface, as well as the nutrition potential for the avascular epithelium from the vascularized connective tissue.