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.
Blue Histology - Epithelia and Glands
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. 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.
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.
Overview and types of epithelial tissue
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. The structure of a tissue usually is optimized for its function. Describe how the structure of individual cells and tissue arrangement of the intestine lining matches its main function, to absorb nutrients.
Columnar epithelia, which form the lining of the digestive tract, can be either simple or stratified. The cells are long and narrow. The nucleus is elongated and located on the basal side of the cell. Ciliated columnar epithelium is composed of simple columnar epithelial cells that display cilia on their apical surfaces.
Skip to content Increase Font Size. Learning Objectives Describe the structural characteristics of the various epithelial tissues and how these characteristics enable their functions. By the end of this section, you will be able to: Explain the general 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.
External Website Summary of Epithelial Tissue Cells Watch this video to find out more about the anatomy of epithelial tissues. External Website. 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. Critical Thinking Questions The structure of a tissue usually is optimized for its function. Previous: 4. Next: 4. Depending on where they are on the body and whether they are protecting organs, they can be a single sheet of cells or multiple layers. As well as fighting invasion from viruses and the outside world, epithelial cells also help you experience it.
Epithelial cells are also responsible for producing sweat, which keeps you cool during exercise, or on hot summer days. Some epithelial cells contain receptors that allow you to appreciate every sensory experience. When you pick an apple from a tree and bite into it, it is these receptors that send signals to your brain so you can enjoy every moment of touch and taste, from the rough bark right to the sweet taste of the apple itself.
More epithelial cells come into action when that piece of apple is swallowed. Not only do they line the oesophagus and intestines, they also secrete the enzymes used in digesting everything you eat. Nutrients is absorbed through the epithelium of the intestines and transported around the body, where energy is needed for growth and repair.
The enzymes necessary for converting food energy into energy your body can use are also produced by these wonderful cells.
Epithelium Study Guide
As well as this, epithelial cells also secrete mucus in your nose and other cavities for cleaning and additional protection against dirt and microbes and hormones into your blood vessels to regulate bodily functions like the reproductive cycle, blood sugar levels and thirst, sleep and sex drives.
Different Shapes For Different Places In The Body Depending on where they are found and what they are used for, epithelial cells come in a range of shapes and sizes. All are tightly packed together, with minute space in between called Gap Junctions which allows for information and nutrients to travel between cells. Epithelium forms continuous sheets of cells that line internal surfaces and cover the external surface of the body. It is a selective barrier that protects tissues and is often involved in absorption or secretion.
A basement membrane separates an epithelium from the underlying connective tissue. Epithelial cells are highly polarized: Apical surface - faces the lumen or the external environment Microvilli, cilia, stereocilia Lateral surface - faces the sides of adjacent cells Tight junctions zona occludens , adherens junction zona adherens , desmosomes macula adherens , gap junctions Basal surface - attaches to the basement membrane Basement membrane, hemidesmosomes An epithelium does not contain blood vessels and receives nourishment via diffission from the underlying connective tissue.
Glands are formed by the down growth of an epithelium into the underlying connective tissue discussed in Chapter 12 - Exocrine Glands. It is not necessary to learn the names of specific tissues for this chapter, but rather learn to recognize variations in epithelia. Simple squamous epithelium consists of a single layer of flattened cells. The thinness of these cells facilitates the transfer of materials e. MH Simple Epithelia.
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