Tissue cells

General Characteristics of Epithelia Cells

Epithelial cells are tightly packed cells that form a continuous layer, covering the body's internal and external surfaces. They have a polarized structure, with distinct apical and basal surfaces. Epithelial cells are avascular, but they are innervated and receive nutrients from underlying connective tissue. They have a high rate of cell division and turnover, and they are involved in functions such as absorption, secretion, and protection. Epithelial cells can be classified based on their shape (squamous, cuboidal, or columnar) and the number of cell layers (simple or stratified).

what is a apical surface?

An apical surface is the surface of a cell that faces a lumen, cavity, or the external environment. It is typically the surface that is exposed to the outside of an organ or tissue and is often specialized for a particular function, such as absorption or secretion.

what is a basal surface?

A basal surface is the bottom or lowest layer of a cell or tissue that is in contact with the underlying substrate or basement membrane.

What is a basement membrane?

A basement membrane is a thin, fibrous, extracellular matrix that separates the epithelium (the cells that line organs and tissues) from underlying connective tissue. It provides structural support and acts as a barrier to prevent the passage of large molecules and cells between the two layers.

What is a lumen?

The opening in the middle of the intestinal tract or any hollow organ.

junctional complexes

Junctional complexes are specialized structures that form the points of contact between adjacent cells. They are composed of various proteins that help to anchor cells together and facilitate communication between them. Junctional complexes are found in many different tissues throughout the body, including epithelial tissues, cardiac muscle, and smooth muscle. Some examples of junctional complexes include tight junctions, adherens junctions, and gap junctions. Tight junctions are important for creating a barrier between cells, while adherens junctions help to anchor cells together. Gap junctions allow for the exchange of small molecules and ions between cells.

Do epithelial cells lack blood?

All epithelial cells lack blood vessels or capillaries. They are **avascular** and rely on underlying connective tissue to provide oxygen and nutrients.

**innervated**

Having a nerve supply.

Do all epithelia cells lack nerves?

Although some epithelia lack nerves, such as those in the stomach, intestines, and cervix, most epithelial cells are **innervated** and provide valuable sensory input.

Three major types of cellular junction

found between epithelial cells are *tight junctions*, *desmosomes*, and *gap junctions*

tight junction

formed by the fusion of the outermost layers of the plasma membranes of adjoining cells. The matrix- filled space between cells is lost at the site of a tight junction.

what is a tight junction?

A tight junction is a type of cell-to-cell junction in animal cells that forms a barrier to prevent the leakage of molecules between adjacent cells. It is composed of proteins that bind the plasma membranes of adjacent cells together, creating a seal that restricts the movement of ions and molecules through the space between the cells.

Where can tight junctions be found?

Tight junctions are found in tissues in which there can be no leaks, such as in the urinary bladder, where urine is held, or in the digestive tract, where they play a critical role in prevent- ing the leakage of digestive enzymes into the bloodstream

What is a plasma membranes?

A plasma membrane, also known as a cell membrane, is a thin, semi-permeable layer that surrounds the cell and separates its internal environment from the external environment. It is composed of a phospholipid bilayer with embedded proteins and carbohydrates, and plays a crucial role in maintaining the cell's shape, regulating the movement of molecules in and out of the cell, and facilitating communication with other cells.

What is a desmosome?

A desmosome is a type of cell junction that connects intermediate filaments of adjacent cells, providing mechanical strength to tissues.

What is diffusion?

The tendency for molecules to move from an area of high concentration to an area of low concentration

What is active transport?

The process that moves ions or molecules across the cell membrane and against the concentration gradient; requires energy.

What links epithelia cells to basement membrane?

Junctions that look like half of a desmosome are called **hemidesmosomes,**

What is a hemidesmosomes?

A hemidesmosome is a type of cell junction that anchors epithelial cells to the basement membrane. It is composed of transmembrane proteins and cytoplasmic plaque proteins that connect to intermediate filaments within the cell.

What cells that are connected by gap junctions are?

Connexons

What is a connexons?

A connexon is a protein channel that connects adjacent cells, allowing for the exchange of small molecules and ions between the cells. which extend from the cytoplasm of one cell to the cytoplasm of the other.

What is a cytoplasm?

The cytoplasm is a gel-like substance that fills the cell and contains various organelles, such as the mitochondria and ribosomes. It is responsible for supporting and protecting the cell's organelles and allowing for the movement of materials within the cell.

Where are gap junctions found?

most commonly found in intestinal epithelial cells, the heart, and smooth muscle tissue

What is a base membrane?

A base membrane is a thin layer of material that is placed between a subgrade and a concrete slab to prevent the two from coming into contact and to provide a stable base for the slab. It can be made of various materials such as asphalt, polyethylene, or geotextile fabric.

Characteristics of basement membrane:

Its strength and elasticity help prevent the cell from being torn off by intraluminal pressures, such as stretching or erosion caused by the rubbing of luminal material.

What extends into the cytoplasm?

In addition to these linkages **intermediate filaments** called **tonofilaments** extend from the plaque into the cytoplasm of each cell like anchors, forming stabilizing bases for the membrane junction.

* The basement membrane can be found in the extracellular matrix that separates and supports the epithelial and endothelial cells of various tissues and organs in the body.
* The basement membrane in skin, for example, is thin, but in the trachea it is much thicker.
* \

Where can the base membrane be found?

The BSE membrane, also known as the brain-specific endothelial membrane, can be found in the blood-brain barrier.

Oxygen and nutrient molecules are supplied to the epithelial cells by diffusing through the basement membrane from capillaries in the underlying connective tissue.

What happens to nutrient substances in the basement membrane?

Are absorbed and waste that is excreted by the epithelium diffuse across the basement membrane into the blood supply of the connective tissue.



the basement membrane acts as a partial barrier between the epithelial cell and the underlying connective tissue.

The many surfaces of epithelial cells

* Depend on where they are in the body
* what role it plays as a function in the tissue

Example: The epithelia that line blood vessels, for example, have smooth surfaces to allow the easy passage of blood cells.

What do epithelial cells of irregular surfaces have?

may be covered with many fingerlike projections, called **microvilli**, or thousands of tiny hairs, called **cilia**

Surface Specialization

* smooth
* cilia
* microvilli(brush border)
* kertain

The surface of a cell covered with microvilli is called

a brush border

What does a brush border do?

* Increases the surface area of the cell, thereby increasing the absorptive ability of the cell.
*

Where can microvilli( brush border) be found?

* microvilli usually occur on cells that are involved in absorption or secretion, such as the epithelia in the intestinal and urinary tracts.
* a cell with microvilli has about *20 times* the surface area of a cell without them.

What is cilia ?

Hairlike processes of the luminal surfaces of cells that assist in the movement of mucus, fluid, and solid material across the cell surface.

What are luminal surfaces?

Luminal surfaces refer to the inner surfaces of a hollow organ or structure, such as the inside lining of a blood vessel or the interior of the digestive tract. These surfaces are in direct contact with the contents that pass through the organ or structure.

Where are cilia found in the body?

* Cilia are found in various parts of the body, including the lining of the respiratory tract, the fallopian tubes, and the brain. They are also present in the middle ear and the kidneys.
* Cilia are also found on the free surface of cells, usually in the respiratory and urogenital tracts.
* ex: In the trachea, for example, the cilia help to propel mucus and debris up and away from the lungs toward the mouth

Keratain surfaces (*keratinized epithelium)*

* Epithelial cells of the skin become filled with a protective, waterproof substance called **keratin**.
* The accumulation of keratin occurs as the cell matures and moves from the basal

layer to the superficial layer of the integument.

List seven functions performed by epithelial cells.

Epithelial cells (1) protect, cover, and line other tissues; (2) filter biochemical substances; (3) absorb nutrients; (4) provide sensory input; (5) manufacture secretions; (6) manufacture excretions; and (7) act as an interface layer that separates and defines the beginning and ending of different types of tissues.

What four attributes characterize epithelial tissue in

general?

. Epithelial cells are polar: that is, they have a sense of direction relative to surrounding structures. Each epithelial cell has an apical surface and a basal surface, which are quite different from each other. The apical surface is the side of the cell that faces the lumen or body cavity, and the basal surface is the side of the cell that faces the underlying connective tissue.

2\. Epithelial cells have lateral surfaces that are connected to neighboring cells by junctional complexes. These junctions bring the cells into close apposition to one another, leaving little room for extracellular matrix. The matrix that surrounds epithelia therefore exists in very small quantities, if at all.

3\. All epithelial cells lack blood vessels or capillaries. They are avascular and rely on underlying connective tissue to provide oxygen and nutrients.

4\. Although some epithelia lack nerves (for example, those in the stomach, intestines, and cervix), most epithelial cells are innervated and provide valuable sensory input.

List four types of cellular junction. Can you describe them?

1\. Tight junction: formed by the fusion of the outermost layers of the plasma membranes of adjoining cells. The matrix-filled space between cells is lost at the site of a tight junction. For centrally placed cells, the fusion occurs as a strip that wraps around the entire circumference of the cell like a belt. In this way, an impenetrable barrier is formed that prevents the passage of substances from the luminal end to the basal end of the cell and vice versa. Only by passing through the body of the cell can substances pass through the epithelial layer. Tight junctions are found in tissues in which there can be no leaks—for example, in the urinary bladder, where urine is held, or in the digestive tract, where tight junctions play a critical role in preventing the leakage of digestive enzymes into the bloodstream.

2\. Desmosome: strong, welded plaque that connects the plasma membranes of adjacent cells. The bond is a mechanical coupling formed by filaments that interlock with one another, just as plastic fibers do in Velcro. Tonofilaments, or intermediate filaments, may also extend from the desmosomic plaque into the cytoplasm of each cell like anchors, forming stabilizing bases for the membrane junction. In this way, desmosomes form tough bonds between cells and therefore are found most commonly in tissues that undergo repeated episodes of tension and stretching, such as the skin, heart, and uterus.

3\. Hemidesmosome: junctions that look like half-desmosomes and link epithelial cells to the basement membrane.

4\. Gap junction: made of tubular channel proteins called *connexons* and extends from the cytoplasm of one cell to the cytoplasm of another. These transmembrane proteins allow the exchange and passage of ions and nutrients (e.g., nucleotides, sugars, and amino acids) from one cell to another. Gap junctions are most commonly found in intestinal epithelial cells, the heart, and smooth muscle tissue. The function of gap junctions in epithelial cells is not yet fully understood, but their ability to quickly transport electrical signals from one cell to another explains their presence in cardiac and smooth muscle cells, where they help coordinate contraction.

How does the basement membrane act as a partial

barrier between the epithelial cell and the underlying

connective tissue?

Oxygen and nutrient molecules are supplied to the epithelial cells by diffusing through the basement membrane from capillaries in the underlying connective tissue. Similarly, nutrient substances that are absorbed and waste that is excreted by the epithelium diffuse across the basement membrane into the blood supply of the connective tissue.

Why do some epithelial cells have cilia and microvilli?

What role do they play? Where are the cells with these specialized surfaces found in the body?

Microvilli increase the surface area of cells and allow more absorption and secretion. They are found on epithelial cells in the intestines and urinary tract. Cilia are found on the free surfaces of cells, usually in the respiratory and urogenital tracts. Ciliary movement occurs in coordinated “beats” that enable the efficient transport of material. In the trachea, cilia help propel mucus and debris up and away from the lungs toward the mouth. In the uterine tube, the beating motion of cilia encourages newly released ova into the oviduct, or *infundibulum*.

**CLASSIFICATIONS OF EPITHELIA**

* *Number of layers of cells*
* *Shape of the cells*
* *Presence of surface specializations*.

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Number of layers of cells

If there is only a single layer of epithelial cells, the tissue is classified as *simple*. If there is more than one layer of cells, the tissue is called *stratified*.

**Simple epithelia**

Epithelium composed of a single layer of cells.

**Stratified epithelia**

Epithelium composed of layers of cells.

**stratified squamous epithelium**

Epithelial tissue composed of multiple layers of flat squamous cells; found in the epidermis of the skin, vagina, mouth, and anus; possesses the ability to regenerate rapidly.

**cuboidal cells**

Cube-shaped cells with centrally located nuclei.

**squamous cells**

Flat cells that make up squamous epithelia

**keratinized stratified squamous epithelium**

found in the skin

nonkeratinized stratified squamous epithelium

found in the lining of the mouth

Can increase surface area as well

Made of ground substance, cells, and fibers

Loose CT

Dense CT

Dense Regular CT

**What is Dense Regular CT?**

Dense Regular CT is a tightly bound, minimally vascularized fibrous connective tissue found in ligaments, tendons, and fascia. It binds joints in ligaments, binds muscle to bone in tendons, and helps support surrounding tissues in fascia.

Dense Irregular CT

A collagen-based fibrous connective tissue that is found in the dermis, spleen, and liver. It has thicker bundles of fiber than dense regular connective tissue and is designed to withstand tension from multiple directions.

Dense irregular connective tissue is a type of connective tissue characterized by densely packed collagen fibers arranged in a random pattern. It provides strength and support to areas subjected to tension from multiple directions, such as the dermis of the skin and the capsules of organs.

Elastic CT

* Connective tissue composed of large numbers of elastic fibers; found in tissues that expand and contract, such as the lungs and vocal cords.
* Elastic connective tissue is a type of connective tissue that contains elastic fibers, which allow it to stretch and recoil. It is found in various parts of the body, such as the lungs, arteries, and vocal cords.

What is ground substance?

* Ground substance is the amorphous, gel-like substance that fills the space between cells and fibers in connective tissue. It is composed of glycosaminoglycans, proteoglycans, and glycoproteins, and plays a crucial role in providing support, lubrication, and hydration to the extracellular matrix.
* The shapeless, viscous matrix present in connective tissue in which cells receive nutrients and void waste products. It also helps to protect the body from infectious agents by acting as a barrier.
* The **ground substance** in connective tissue is an **amorphous**, **homogeneous** material that ranges in texture from a liquid or gel to a calcified solid

What are example of extracellular matrix?

* The extracellular matrix (ECM) is a complex network of proteins and carbohydrates that provides structural and biochemical support to cells. Examples of ECM components include collagen, elastin, fibronectin, laminin, and glycosaminoglycans (GAGs) such as hyaluronic acid and chondroitin sulfate.
* The mixture of fiber and ground substance is called the **extracellular matrix**

Wide Range of Connective Tissue;

* Due to Variations in the ground substance, fibers, and cellular components

Loose or areolar connective tissue. Loose or areolar tissue is a model-type of connective tissue because it contains all three types of fiber (elastic, collagen, and reticular) and a wide variety of cells (lymphocytes, mast cells, neutrophils, fibroblasts, adipocytes, and plasma cells) suspended in ground substance. These three components—fibers, cells, and ground substance—are found in varying amounts in all connective tissue.

Specialized CT

Cartilage, bone, and blood

Epithelial tissue is characterized as simple, stratified, or pseudostratified. What does this mean?

If there is only a single layer of epithelial cells, the tissue is classified as *simple.* If there is more than one layer of cells, the tissue is called *stratified*. Pseudostratified columnar epithelium is an epithelial layer that is not truly stratified. The epithelial cells appear to be stratified because the nuclei are found at different levels across the length of the tissue layer. In addition, not all the cells reach the luminal surface; therefore, the cells appear to be at different levels, as though stratified. In reality, each cell forms a distinct attachment, however subtle, with the basement membrane. In this way, pseudostratified columnar epithelium forms a single layer and therefore is considered a simple epithelium.

What are the three basic shapes of epithelial cell?

Squamous, cuboidal, and columnar

Draw a picture of each of the following types of epithelia and give an example of where each of them can be found in the body.

• Simple squamous \n • Simple cuboidal \n • Simple columnar

• Stratified squamous \n • Pseudostratified columnar • Transitional

• Simple squamous epithelium can be found in the inner lining of the lung (where oxygen is absorbed and carbon dioxide released) and in the filtration membranes of kidneys (where water and other small molecules are excreted as urine).

• Simple cuboidal epithelium can be found on the surface of ovaries; in the secretory portions of glands, such as the thyroid; and in the lining of the ducts of the liver, pancreas, kidney, and salivary gland.

• Simple columnar epithelium is found lining the length of the gastrointestinal tract from the stomach to the rectum.

• Stratified squamous epithelium is found lining the mouth, esophagus, vagina, and rectum.

• Pseudostratified columnar epithelium is found in the respiratory tract and in portions of the male reproductive tract.

• Transitional epithelium is found in portions of the urinary tract where great changes in volume occur (urinary bladder, ureters, urethra, and calyxes of the kidney).

**Secretions**

are specialized protein molecules that are produced in the rough endoplasmic reticulum, packaged into granules by the Golgi apparatus, and discharged from the cell.

**FIBERS OF CONNECTIVE TISSUE**

*collagenous*, *reticular,* and *elastic*.

wandering cells

There are many types of wandering cell that move in and out of connective tissue as needed. In this section, three common types of wandering cells are discussed: leukocytes, mast cells, and macrophages.

In general, connective tissue is divided into two broad categories:

**connective tissue proper** and **specialized connective tissue**

**CONNECTIVE TISSUE PROPER**

*Connective tissue proper* is the largest classification and contains every subtype of connective tissue except bone, cartilage, and blood.

two subclasses of connective tissue proper

*loose connective tissue* and *dense connective tissue*

**Loose connective tissue**

includes *areolar*, *adipose,* and *reticu- lar* tissue

**dense connective tissue**

*dense regular*, *dense irregular,* and *elastic* tissue.