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, diffuse into the bloodstream, and are delivered to cells that have receptors to bind the hormones. The endocrine system a major communication system coordinating the regulation and integration of body responses.
These glands will be discussed in much greater detail in a later chapter. Exocrine glands release their contents through a duct or duct system that ultimately leads to the external environment. Mucous, sweat, saliva, and breast milk are all examples of secretions released by exocrine glands. Exocrine glands are classified as either unicellular or multicellular. Unicellular glands are individual cells which are scattered throughout an epithelial lining.
Goblet cells are an example of a unicellular gland type found extensively in the mucous membranes of the small and large intestine. Multicellular exocrine glands are composed of two or more cells which either secrete their contents directly into an inner body cavity e.
If there is a single duct carrying the contents to the external environment then the gland is referred to as a simple gland. Multicellular glands that have ducts divided into one or more branches is called a compound gland Figure 4. In addition to the number of ducts present, multicellular glands are also classified based on the shape of the secretory portion of the gland.
Tubular glands have enlongated secretory regions similar to a test tube in shape while alveolar acinar glands have a secretory region that is spherical in shape. Combinations of the two secretory regions are known as tubuloalveolar tubuloacinar glands. Exocrine glands are classified by the arrangement of ducts emptying the gland and the shape of the secretory region. Methods and Types of Secretion In addition to the glandular structure, exocrine glands can be classified by their mode of secretion and the nature of the substances released Figure 4.
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, saliva containing the glycoprotein mucin is a merocrine secretion. The glands that produce and secrete sweat are another example of merocrine secretion. Apocrine secretion occurs when secretions accumulate near the apical portion of a secretory cell. That portion of the cell and its secretory contents pinch off from the cell and are released. The sweat glands of the armpit are classified as apocrine glands. Like merocrine glands, 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 the secretory event.
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 the cell bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are an example of a holocrine glands Figure 4.
Glands are also named based on the products they produce. A serous gland produces watery, blood-plasma-like secretions rich in enzymes, whereas a mucous gland releases a more viscous product rich in the glycoprotein mucin. Both serous and mucous secretions are common in the salivary glands of the digestive system. Such glands releasing both serous and mucous secretions are often referred to as seromucous glands. 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.
Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption , excretion, filtration, diffusion , and sensory reception. 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. Types of Cell Junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions. Classification of Epithelial Tissues Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed Figure.
Cells of Epithelial Tissue. Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. Goblet Cell. Stratified Epithelium A stratified epithelium consists of several stacked layers of cells. Summary of Epithelial Tissue Cells. Glandular Epithelium A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Endocrine Glands The secretions of endocrine glands are called hormones.
Exocrine Glands Exocrine glands release their contents through a duct that leads to the epithelial surface. Glandular Structure Exocrine glands are classified as either unicellular or multicellular. Types of Exocrine Glands. Modes of Glandular Secretion. These glands secrete oils that lubricate and protect the skin. They are holocrine glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost.
Chapter Review 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. Interactive Link Questions Watch this video to find out more about the anatomy of epithelial tissues. The inside of the mouth, esophagus, vaginal canal, and anus. Review Questions 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.
Critical Thinking Questions The structure of a tissue usually is optimized for its function. A single layer of epithelial cells is defined as a simple epithelium, while two or more layers of epithelial cells is defined as stratified epithelium.
Epithelial cells are supported by an underlying basement membrane and contact each other through protein complexes known as tight junctions. Epithelial cells are found throughout the body, including the gastrointestinal tract, the skin, and the airway. Epithelial cells in the gastrointestinal tract perform digestive and absorptive functions, while protecting the organ from ingested pathogens.
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