Epithelial tissue how many types

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. 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. One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane.

The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures. Epithelial tissues are identified by both the number of layers and the shape of the cells in the upper layers.

There are eight basic types of epithelium: six of them are identified based on both the number of cells and their shape; two of them are named by the type of cell squamous found in them. Epithelial tissue is classified based on the number of cells, the shape of those cells, and the types of those cells.

Air sacs of the lungs and the lining of the heart, blood vessels and lymphatic vessels Allows materials to pass through by diffusion and filtration, and secretes lubricating substances Simple cuboidal epithelium. In ducts and secretory portions of small glands and in kidney tubules Secretes and absorbs Simple columnar epithelium.

Ciliated tissues including the bronchi, uterine tubes, and uterus; smooth nonciliated tissues are in the digestive tract bladder Absorbs; it also secretes mucous and enzymes. Pseudostratified columnar epithelium. Ciliated tissue lines the trachea and much of the upper respiratory tract Secrete mucous; ciliated tissue moves mucous Stratified squamous epithelium.

Lines the esophagus, mouth, and vagina Protects against abrasion Stratified cuboidal epithelium. Sweat glands, salivary glands, and mammary glands Protective tissue Stratified columnar epithelium. The male urethra and the ducts of some glands. Secretes and protects Transitional epithelium. Lines the bladder, urethra and ureters Allows the urinary organs to expand and stretch Types of Epithelial Tissue Epithelial tissue is classified by cell shape and the number of cell layers.

Most epithelial tissue is described with two names. The first name describes the number of cell layers present and the second describes the shape of the cells. For example, simple squamous epithelial tissue describes a single layer of cells that are flat and scale-like in shape.

Epithelial Tissue : There are three principal classifications associated with epithelial cells. Squamous epithelium has cells that are wider than they are tall.

Cuboidal epithelium has cells whose height and width are approximately the same. Columnar epithelium has cells taller than they are wide. Simple epithelium consists of a single layer of cells. They are typically where absorption, secretion and filtration occur.

The thinness of the epithelial barrier facilitates these processes. Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: 1 simple squamous; 2 simple cuboidal; 3 simple columnar; and 4 pseudostratified. Simple squamous epithelium cells are flat in shape and arranged in a single layer. 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. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia. Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular gland interspersed between the columnar epithelial cells of a mucous membrane Figure 4.

A stratified epithelium consists of multiple stacked layers of cells. This epithelium protects against physical and chemical damage. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells appear squamous, whereas the basal layer contains either columnar or cuboidal cells.

The top layer may be covered with dead cells containing keratin. The skin is an example of a keratinized, stratified squamous epithelium. Alternatively, the lining of the oral 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 relatively rare in the human body. Another kind of stratified epithelium is transitional epithelium , so-called because of the gradual changes in the shapes and layering of the cells as the epithelium lining the expanding hollow organ is stretched. Transitional epithelium 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-shaped apical cells with convex, umbrella shaped, surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition in appearance 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.

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.