Module 4: The Tissue Level of Organization

Lesson 2: Epithelial Tissue

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Mỗi bài học (lesson) bao gồm 4 phần chính: Thuật ngữ, Luyện Đọc, Luyện Nghe, và Bàn Luận.
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Dưới đây là danh sách những thuật ngữ Y khoa của module The Tissue Level of Organization.
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Medical Terminology: The Tissue Level of Organization

adipocytes
lipid storage cells
adipose tissue
specialized areolar tissue rich in stored fat
anchoring junction
mechanically attaches adjacent cells to each other or to the basement membrane
apical
that part of a cell or tissue which, in general, faces an open space
apocrine secretion
release of a substance along with the apical portion of the cell
apoptosis
programmed cell death
areolar tissue
(also, loose connective tissue) a type of connective tissue proper that shows little specialization with cells dispersed in the matrix
astrocyte
star-shaped cell in the central nervous system that regulates ions and uptake and/or breakdown of some neurotransmitters and contributes to the formation of the blood-brain barrier
atrophy
loss of mass and function
basal lamina
thin extracellular layer that lies underneath epithelial cells and separates them from other tissues
basement membrane
in epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue; made up of the basal lamina and reticular lamina
cardiac muscle
heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell contains a single nucleus, contracts autonomously
cell junction
point of cell-to-cell contact that connects one cell to another in a tissue
chondrocytes
cells of the cartilage
clotting
also called coagulation; complex process by which blood components form a plug to stop bleeding
collagen fiber
flexible fibrous proteins that give connective tissue tensile strength
connective tissue
type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems
connective tissue membrane
connective tissue that encapsulates organs and lines movable joints
connective tissue proper
connective tissue containing a viscous matrix, fibers, and cells.
cutaneous membrane
skin; epithelial tissue made up of a stratified squamous epithelial cells that cover the outside of the body
dense connective tissue
connective tissue proper that contains many fibers that provide both elasticity and protection
ectoderm
outermost embryonic germ layer from which the epidermis and the nervous tissue derive
elastic cartilage
type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity
elastic fiber
fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows the fibers to stretch and return to original size
endocrine gland
groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by blood
endoderm
innermost embryonic germ layer from which most of the digestive system and lower respiratory system derive
endothelium
tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous epithelium
epithelial membrane
epithelium attached to a layer of connective tissue
epithelial tissue
type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion
exocrine gland
group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body
fibroblast
most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space
fibrocartilage
tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate ground substance
fibrocyte
less active form of fibroblast
fluid connective tissue
specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins
gap junction
allows cytoplasmic communications to occur between cells
goblet cell
unicellular gland found in columnar epithelium that secretes mucous
ground substance
fluid or semi-fluid portion of the matrix
histamine
chemical compound released by mast cells in response to injury that causes vasodilation and endothelium permeability
histology
microscopic study of tissue architecture, organization, and function
holocrine secretion
release of a substance caused by the rupture of a gland cell, which becomes part of the secretion
hyaline cartilage
most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance
inflammation
response of tissue to injury
lacunae
(singular = lacuna) small spaces in bone or cartilage tissue that cells occupy
lamina propria
areolar connective tissue underlying a mucous membrane
loose connective tissue
(also, areolar tissue) type of connective tissue proper that shows little specialization with cells dispersed in the matrix
matrix
extracellular material which is produced by the cells embedded in it, containing ground substance and fibers
merocrine secretion
release of a substance from a gland via exocytosis
mesenchymal cell
adult stem cell from which most connective tissue cells are derived
mesenchyme
embryonic tissue from which connective tissue cells derive
mesoderm
middle embryonic germ layer from which connective tissue, muscle tissue, and some epithelial tissue derive
mesothelium
simple squamous epithelial tissue which covers the major body cavities and is the epithelial portion of serous membranes
mucous connective tissue
specialized loose connective tissue present in the umbilical cord
mucous gland
group of cells that secrete mucous, a thick, slippery substance that keeps tissues moist and acts as a lubricant
mucous membrane
tissue membrane that is covered by protective mucous and lines tissue exposed to the outside environment
muscle tissue
type of tissue that is capable of contracting and generating tension in response to stimulation; produces movement.
myelin
layer of lipid inside some neuroglial cells that wraps around the axons of some neurons
myocyte
muscle cells
necrosis
accidental death of cells and tissues
nervous tissue
type of tissue that is capable of sending and receiving impulses through electrochemical signals.
neuroglia
supportive neural cells
neuron
excitable neural cell that transfer nerve impulses
oligodendrocyte
neuroglial cell that produces myelin in the brain
parenchyma
functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or organ
primary union
condition of a wound where the wound edges are close enough to be brought together and fastened if necessary, allowing quicker and more thorough healing
pseudostratified columnar epithelium
tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract
reticular fiber
fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue
reticular lamina
matrix containing collagen and elastin secreted by connective tissue; a component of the basement membrane
reticular tissue
type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic tissue, spleen, and the liver
Schwann cell
neuroglial cell that produces myelin in the peripheral nervous system
secondary union
wound healing facilitated by wound contraction
serous gland
group of cells within the serous membrane that secrete a lubricating substance onto the surface
serous membrane
type of tissue membrane that lines body cavities and lubricates them with serous fluid
simple columnar epithelium
tissue that consists of a single layer of column-like cells; promotes secretion and absorption in tissues and organs
simple cuboidal epithelium
tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules
simple squamous epithelium
tissue that consists of a single layer of flat scale-like cells; promotes diffusion and filtration across surface
skeletal muscle
usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated
smooth muscle
under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fiber
stratified columnar epithelium
tissue that consists of two or more layers of column-like cells, contains glands and is found in some ducts
stratified cuboidal epithelium
tissue that consists of two or more layers of cube-shaped cells, found in some ducts
stratified squamous epithelium
tissue that consists of multiple layers of cells with the most apical being flat scale-like cells; protects surfaces from abrasion
striation
alignment of parallel actin and myosin filaments which form a banded pattern
supportive connective tissue
type of connective tissue that provides strength to the body and protects soft tissue
synovial membrane
connective tissue membrane that lines the cavities of freely movable joints, producing synovial fluid for lubrication
tight junction
forms an impermeable barrier between cells
tissue
group of cells that are similar in form and perform related functions
tissue membrane
thin layer or sheet of cells that covers the outside of the body, organs, and internal cavities
totipotent
embryonic cells that have the ability to differentiate into any type of cell and organ in the body
transitional epithelium
form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine
vasodilation
widening of blood vessels
wound contraction
process whereby the borders of a wound are physically drawn together
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Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type of epithelium.

Epithelial cells derive from all three major embryonic layers. The epithelia lining the skin, parts of the mouth and nose, and the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm. The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an endothelium.

All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a cell junction. The epithelial cells exhibit polarity with differences in structure and function between the exposed or apical facing surface of the cell and the basal surface close to the underlying body structures. The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue, forming a basement membrane that helps hold it all together.

Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.
Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes.

Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.
Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.

Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach.
Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure 1).

At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to act as selective barriers. An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells. These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basal lamina. While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue.

In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.
Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed (Figure 2). Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina. Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.

A. Simple Epithelium

The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.

In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.

In simple columnar epithelium, the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.

Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. 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 mucous membranes (Figure 3).

B. Stratified Epithelium

A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. 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 are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth 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 uncommon in the human body.

Another kind of stratified epithelium is transitional epithelium, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It 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 apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition 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. Figure 4 summarizes the different categories of epithelial cell tissue cells.
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. A gland can be classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).

A. Endocrine Glands

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.

B. Exocrine Glands

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.

C. Glandular Structure

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 5). 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.

D. 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 6). 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. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells (Figure 7).

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.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.

Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells.

(a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells. (b) The arrows in this micrograph point to the mucous-secreting goblet cells. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Exocrine glands are classified by their structure.

(a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell is released, as well. (c) In holocrine secretion, the cell is destroyed as it releases its product and the cell itself becomes part of the 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. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

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  1. 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.
  2. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration.
  3. Cells are bound together by tight junctions that form an impermeable barrier.
  4. 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.
  5. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia.
  6. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia.
  7. Very few capillaries penetrate these tissues.
  8. Glands are secretory tissues and organs that are derived from epithelial tissues.
  9. Exocrine glands release their products through ducts.
  10. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream.
  11. Glands are classified both according to the type of secretion and by their structure.
  12. Merocrine glands secrete products as they are synthesized.
  13. 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.
  14. In this case, the cell becomes part of the secretion.
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