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. Key Takeaways Key Points Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another.
Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released. Key Terms epithelium : A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs.
Types of Epithelial Tissue Epithelial tissue is classified by cell shape and the number of cell layers. Learning Objectives Classify epithelial tissue by cell shape and layers. Key Takeaways Key Points There are three principal cell shapes associated with epithelial cells: squamous epithelium, cuboidal epithelium, and columnar epithelium. There are three ways of describing the layering of epithelium: simple, stratified, and pseudostratified.
Pseudostratified epithelium possesses fine hair-like extensions called cilia and unicellular glands called goblet cells that secrete mucus. This epithelium is described as ciliated pseudostratified epithelium.
Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult. In keratinized epithelia, the most apical layers exterior of cells are dead and and contain a tough, resistant protein called keratin.
An example of this is found in mammalian skin that makes the epithelium waterproof. Transitional epithelia are found in tissues such as the urinary bladder where there is a change in the shape of the cell due to stretching.
Key Terms simple columnar : A columnar epithelium that is uni-layered. Keratinized : To produce or become like keratin. Licenses and Attributions. 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. The specimen is what type of epithelial tissue? Which of the following is the epithelial tissue that lines the interior of blood vessels? Which type of epithelial tissue specializes in moving particles across its surface and is found in airways and lining of the oviduct?
The structure of a tissue usually is optimized for its function. Describe how the structure of the mucosa and its cells match its function of nutrient absorption. The mucosa of the intestine is highly folded, increasing the surface area for nutrient absorption. A greater surface area for absorption allows more nutrients to be absorbed per unit time.
In addition, the nutrient-absorbing cells of the mucosa have finger-like projections called microvilli that further increase the surface area for nutrient absorption. Skip to content The Tissue Level of Organization.
Learning Objectives By the end of this section, you will be able to: Explain the structure and function of epithelial tissue Distinguish between tight junctions, anchoring junctions, and gap junctions Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia Describe the structure and function of endocrine and exocrine glands and their respective secretions. The Epithelial Cell Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces.
Cell to Cell Junctions Cells of epithelia are closely connected and are not separated by intracellular material. There are three main layers: the epidermis , dermis and hypodermis. There are also sweat glands, and hairs, which have sebaceous glands, and a smooth muscle called the arrector pili muscle, associated with them.
Hairs are only found in thin skin, and not in the thick skin present on the fingertips, palms and soles of your feet. The epidermis : a thin outer portion, that is the keratinised stratified squamous epithelium of skin. The epidermis is important for the protective function of skin. The basal layers of this epithelium are folded to form dermal papillae. Thin skin contains four types of cellular layers, and thick skin contains five.
Click here to find out more about the epidermis and its layers. The dermis: a thicker inner portion. This is the connective tissue layer of skin. It is important for sensation, protection and thermoregulation. It contains nerves, the blood supply, fibroblasts, etc, as well as sweat glands, which open out onto the surface of the skin, and in some regions, hair.
The apical layers of the dermis are folded, to form dermal papillae, which are particularly prominent in thick skin. The hypodermis. This layer is underneath the dermis, and merges with it.
It mainly contains adipose tissue and sweat glands. The adipose tissue has metabolic functions: it is resonsible for production of vitamin D, and triglycerides. The outer layers of skin are towards the top. See if you can identify the epidermis, dermis, dermal papillae and sweat glands. Notice that there are no hairs in this region.
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