The bar indicates the thickness of the stratified squamous keratinized epithelium sske. Notice that the nuclei of the cells in the bottom layers tend to have a round shape, but that the nuclei seem to become flatter as you move towards the surface.
You have probably noticed that the bottom surface of the epithelium is not flat. The bumpy appearance is caused by connective tissue projections called dermal papillae. On all of the images on this page, the epithelium is stained more darkly than the connective tissue. The cells on the surface of stratified squamous keratinized epithelium are very flat. Not only are they flat, but they are no longer alive. They have no nucleus or organelles. This single layer is thin enough to form a membrane that compounds can move through via passive diffusion.
This epithelial type is found in the walls of capillaries, linings of the pericardium, and the linings of the alveoli of the lungs. Simple cuboidal epithelium consists of a single layer cells that are as tall as they are wide. The important functions of the simple cuboidal epithelium are secretion and absorption. This epithelial type is found in the small collecting ducts of the kidneys, pancreas, and salivary glands. Simple columnar epithelium is a single row of tall, closely packed cells, aligned in a row.
These cells are found in areas with high secretory function such as the wall of the stomach , or absorptive areas as in small intestine. They possess cellular extensions e. These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading hence pseudo impression that the epithelium is stratified when the cells are viewed in cross section.
Pseudostratified epithelium can also possess fine hair-like extensions of their apical luminal membrane called cilia. In this case, the epithelium is described as ciliated pseudostratified epithelium. Ciliated epithelium is found in the airways nose, bronchi , but is also found in the uterus and fallopian tubes of females, where the cilia propel the ovum to the uterus.
Stratified epithelium differs from simple epithelium by being multilayered. It is therefore found where body linings have to withstand mechanical or chemical insults. Stratified epithelia are more durable and protection is one their major functions. Since stratified epithelium consists of two or more layers, the basal cells divide and push towards the apex, and in the process flatten the apical cells.
Stratified epithelia can be columnar, cuboidal, or squamous type. However, it can also have the following specializations:. In keratinized epithelia, the most apical layers exterior of cells are dead and lose their nucleus and cytoplasm. They contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, and it is abundant in mammalian skin.
The lining of the esophagus is an example of a non-keratinized or moist stratified epithelium. Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra.
Privacy Policy. Skip to main content. Organization at the Tissue Level. Search for:. Epithelial Tissue. Characteristics of Epithelial Tissue The human body consists of four types of tissue: epithelial, connective, muscular, and nervous.
Learning Objectives Describe the primary functions and characteristics of epithelial tissue. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium.
The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body. Another kind of stratified epithelium is transitional epithelium , so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine.
It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended.
Figure summarizes the different categories of epithelial cell tissue cells. Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium? A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells.
The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.
Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.
Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine. The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity.
These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities.
Other multicellular exocrine glands release their contents through a tubular duct. The duct is single in a simple gland but in compound glands is divided into one or more branches Figure.
In tubular glands, the ducts can be straight or coiled, whereas tubes that form pockets are alveolar acinar , such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar tubuloacinar compound glands. In a branched gland, a duct is connected to more than one secretory group of cells. Methods and Types of Secretion Exocrine glands can be classified by their mode of secretion and the nature of the substances released, as well as by the structure of the glands and shape of ducts Figure.
Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion.
The eccrine glands that produce and secrete sweat are another example. Apocrine secretion accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break down; this causes body odor. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and golgi regions remain intact after secretion.
In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell.
The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. Glands are also named after the products they produce.
The serous gland produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth.
Mixed exocrine glands contain both serous and mucous glands and release both types of secretions. In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration.
Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia.
Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues. Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts.
Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. Glands are classified both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized. Apocrine glands release secretions by pinching off the apical portion of the cell, whereas holocrine gland cells store their secretions until they rupture and release their contents.
In this case, the cell becomes part of the secretion. In observing epithelial cells under a microscope, the cells are arranged in a single layer and look tall and narrow, and the nucleus is located close to the basal side of the cell.
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