In the medical nomenclature for hollow organs, the term luminal is often used to refer to the pole which faces the lumen. The pole oriented away from the lumen is called abluminal. In other parts, epithelial tissues form the inner lining and external lining of body parts. To summarize, the apical pole faces the surface, while the basal pole is attached to the connective tissue located below the epithelium.
There are 3 different types of epithelial tissue: squamous, cuboidal, and columnar. The differentiation and classification of the types of epithelial tissue are based on the shape of the cell e. Image: Simple classification of epithelium. By Lecturio. Image: Stratified classification of epithelium. Image: Pseudo-stratified and transitional classification of epithelium. General functions of the epithelial tissue include a barrier protection of tissues from radiation, desiccation, and invasion by pathogens and toxins , secretion substance release of hormones, sweat, mucus, and enzymes and absorption substance intake.
The epithelial tissue has a general characteristic that their cells are closely bound to each other through specialized structures such as tight junctions, desmosomes, interdigitations, gap junctions, and intercellular bridges. These gap junctions help in communication with 2 cells together by cementing each other.
They constitute the linings of organs and body parts. They have 1 surface that is not attached to any cell, either exposed to the external environment or the lumen of an organ. They do not possess blood vessels and their nutrients requirement is fulfilled by connective tissue forming the basement membrane. Image: The 3 characteristics of any epithelium.
Epithelial cellular organizations are located on the basal membrane. The basal membrane is a thin extracellular matrix. It consists of the basal lamina, which is itself divided into the laminae rara externa and interna, the lamina densa, and the lamina fibroreticularis except in renal corpuscles.
The function of the basal membrane is to stabilize the epithelial and endothelial cell layers. The attachment of cells is achieved with the help of syndecan or laminin and integrin. Microfilaments attach the lamina fibrorecticularis to the perlecan in the lamina densa. Epithelial cells regenerate on the basal membrane. Image: Electron microscopic image of the basal lamina. By Robert M. Hunt, License: Public domain.
The surface epithelium primarily has barrier and transport functions. It is a mechanical and chemical barrier that prevents the uncontrolled substance exchange. It takes substances in absorption or releases them into the tissue secretion. Image : Schematic representation of the endothelium with astrocytes in the blood-brain barrier. The simple epithelial tissue is a closed network of flat epithelial cells. These are located on the basal membrane.
It is composed of a single layer of cells that are specialized in diffusion, osmosis, filtration, secretion, and absorption. The simple epithelial tissue is found in the alveolar epithelium pulmonary alveolus , the endothelium lining of blood vessels and lymph vessels , and the mesothelium lining of the body cavities.
Medically, the simple epithelium is further divided into 2 types: simple cuboidal epithelium and simple columnar epithelium. The cells of the cuboidal epithelium are equally tall as they are wide and have a central round nucleus. The simple cuboidal epithelium is found, e.
They have large cytoplasmic content so, they perform important complex functions like absorption and secretion. They are found in exocrine glands, endocrine glands, and ducts of endocrine glands. Epithelial cells come in different shapes depending on where in the body they're found. These shapes are called squamous, cuboidal, columnar, and ciliated columnar.
Squamous epithelial cells are flat and are usually found lining surfaces that require a smooth flow of fluid, such as your blood vessels. They also line areas that require a very thin surface for molecules to pass through, such as the air sacs in your lungs.
In addition to these shapes, epithelial cells can be described as being either simple or stratified. These terms refer to how many layers are present. Simple tissue has only one layer of epithelial cells, while stratified tissue has many layers stacked on top of each other. Stratified cells are found in places that need to withstand a lot of wear and tear from their environment.
An example of this would be your skin, which is made up of many stratified layers of epithelial cells. As the top layer wears down, cells from the bottom layers constantly grow up to replace them. Epithelial Cells. By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteers page to get the process started. Digging Deeper. 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. 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. 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 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. 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.
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