Structure and Function of Epithelial Tissues

Definition and types of tissues

Tissue is  a collection of similar cells specialized to perform a specific function

  • The human body consists of trillions of cells, but they do not work independently
  • The cell must work together to perform various tasks to keep the body in homeostasis
  • For cells to work together, they form tissues
  • To help study the various cells and the tissues they make, anatomists have subdivided the tissues into four major groups and those groups are further subdivided according to their cellular makeup

There are four types of tissues namely

 1. Connective tissue

 -Connective tissue supports other tissues and binds them together (bone, blood, and lymph tissues)

2. Epithelial tissue

 -Epithelial tissue provides a covering (skin, the linings of the various passages inside the body)

 3. Muscle tissue

 -Muscle tissue includes striated (also called voluntary) muscles that move the skeleton, and smooth muscle, such as the muscles that surround the stomach

 4. Nervous tissue

 -Nerve tissue is made up of nerve cells (neurons) and is used to carry "messages" to and from various parts of the body

Functions of Epithelial Tissue

• Protection

-Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria, and excessive loss of water

• Sensation

 -Sensory stimuli penetrate specialized epithelial cells

 -Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose, and the tongue

• Secretion

 -In glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones, and lubricating fluids

• Absorption

-Certain epithelial cells lining the small intestine absorb nutrients from the digestion of

food

• Excretion

-Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine

 -Sweat is also excreted from the body by epithelial cells in the sweat glands

• Diffusion

 -Simple epithelium promotes the diffusion of gases, liquids, and nutrients.

Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of

capillaries and lungs).

• Cleaning the Ciliated epithelium assists in removing dust particles and foreign bodies which

have entered the air passages.

• Reduces Friction

•  Signal transduction- refers to any process by which a cell converts one kind of signal or

stimulus into another. Most processes of signal transduction involve ordered sequences of

biochemical reactions inside the cell, which are carried out by enzymes and activated by

second messengers, resulting in a signal transduction pathway.

Characteristics of Epithelial Tissue 

  •  Form continuous sheets (fit like tiles)
  •  Apical Surface

        - All epithelial cells have a top surface that borders an open space – known as a lumen 

  • Basement Membrane

      -The underside of all epithelial cells anchors them to connective tissue

  • Avascularity (a = without)

       -Lacks blood vessels

       -Nourished by connective tissue

        -Regenerate & repair quickly

General Structure of Epithelial Tissues

• Epithelial tissues are widespread throughout the body.

• They form the covering of all body surfaces, line body cavities, and hollow organs, and

are the major tissue in glands.

• The cells in epithelial tissue are tightly packed together with no intercellular material

between the cells or very little intercellular materials.

• It also forms all glands including endocrine and exocrine glands.

• Cells of epithelial tissue appear in different shapes and structures.

• These differences reflect the functions of the epithelium.

• The cells are attached to the underlying connective tissue known as the basement

membrane and have three cell surfaces namely the basal surface, lateral surface,,, and the

apical or luminal surface.

• Each surface has its own specialization

• The apical surface is for absorption and secretion

• The basal surface is for attachment with basement membrane and diffusion of material to

the cell

• The lateral surface is an attachment with other cells and for cell protection, because it forms a tight junction with other cells, therefore preventing the entry of material into intercellular space

• The epithelial tissues are not penetrated by the blood vessels.

• The blood vessels end at the basement membrane and therefore nutrition is achieved

through diffusion from the connective tissue beneath.

Basement Membrane

-The basement membrane is a connective tissue structure where the epithelial cells are

attached.

• It is made up of collagen fibers, reticular fibers, and polysaccharide material.

• These fibers are secreted by the connective tissue cells and the cells associated with them.

Functions of the Basement Membrane

• Support to the epithelium

• Barrier to the epithelium

• It regulates the exchange of materials between the cells and blood

Classification of Epithelial Tissue

1. Cell Shape

  •  Squamous – flattened like fish scales or an irregular flattened shape
  •  Cuboidal –   have a shape like  a cube, meaning its width is the same size as its height
  •  Columnar – columns like are taller than they are wide

2. According to based layers

  • simple epithelia(one layer)
  • stratified epithelia (many layers)
  • pseudostratified epithelia.
  •  Specializations



Simple Epithelium

Structure

There is a single layer of cells

• Cells are organized in a single layer and therefore all cells are attached to the basement

membrane

• The cells that make simple epithelium can be squamous, cuboidal, or columnar cells

  •  Simple squamous epithelium
  •  Simple cuboidal epithelium
  •  Simple columnar epithelium

• They are found in different organs depending on their function

let start with Simple Squamous Epithelium

 Structure

-Single Layer of flattened cells

 Function

-Absorption, and filtration

-Not effective protection – a single layer of cells.

 Location

 -Walls of capillaries, air sacs in lungs

 -Form serous membranes in the body cavity can be found in the filtration tubules of the kidneys,

Simple Cuboidal Epithelium

 Structure

 -Single-layer of cube-shaped cells

 Function

-Secretion and transportation in glands, filtration in kidneys

 Location

-Glands and ducts (pancreas & salivary), kidney tubules cover ovaries

-Squamous cells are relatively inactive metabolically and are associated, with the diffusion of water, electrolytes, and other substances

Simple Columnar Epithelium

 Structure

 -An elongated layer of cells with nuclei at the same level

 Function

 -Absorption, Protection & Secretion

-When open to body cavities – called mucous membranes

 Special Features

 -Microvilli, a bumpy extension of apical surface, increase surface area and absorption rate.

-Goblet cells, single-cell glands, produce protective mucus.

 Location

 -Linings of the entire digestive tract

Pseudostratified Epithelium

 Structure

 -Irregularly shaped cells with nuclei at different levels – appear stratified, but aren’t.

 -All cells reach the basement membrane

 Function

 -Absorption and Secretion

 -Goblet cells produce mucus

 -Cilia (larger than microvilli) sweep mucus

 Location

 -Respiratory Linings & Reproductive tract

Stratified Squamous Epithelium

 Structure

 -Many layers (usually cuboidal/columnar at bottom and squamous at top)

 Function

-Protection

 -Keratin (protein) is accumulated in older cells near the surface – waterproofs and toughens

skin.

 Location

 -Skin (keratinized), mouth & throat

Transitional Epithelium

 Structure

-Many layers

 -Very specialized – cells at the base are cuboidal or columnar, at the surface will vary.

 -Change between stratified & simple as tissue is stretched out.

 Function

-Allows stretching (change size)

 Location

 -Urinary bladder, ureters & urethra

Glandular Epithelium

-One or more cells make and secrete a product. Secretion = protein in aqueous solution: hormones, acids, oils.

 Endocrine glands

  • No duct, release secretion into blood vessels
  •  Often hormones
  • Thyroid, adrenal, and pituitary glands

 Exocrine glands

  • Contain ducts, empty onto the epithelial surface
  • -Sweat, Oil glands, Salivary glands, Mammary glands.

 Shapes of Exocrine glands

1. Branching

  • Simple – single, unbranched duct
  • Compound – branched.

2. Shape: tubular or alveolar

  • Tubular – shaped like a tube
  • Alveolar – shaped like flasks or sacs
  • Tubuloalveolar – has both tubes and sacs in glands

Modes of Secretion

 1. How the gland’s product is released

 2. Merocrine

 -Just released by exocytosis without altering the gland at all.

 -Example: Sweat glands and salivary glands

3. Holocrine

 -The gland ruptures and releases secretion and dead cells as well.

 -Sebaceous (oil glands on the face)


Key points here