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TISSUE - The term "tissue" is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions.

FOUR CATEGORIES OF TISSUE

• Epithelial tissue

• Also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines internal cavities and passageways, and forms certain glands.

• Connective tissue

• As its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body.

• Muscle tissue

• Is excitable, responding to stimulation and contracting to provide movement, and occurs as Three Major Types:

• Skeletal (voluntary) muscle

• Smooth muscle

• Cardiac muscle in the heart.

• Nervous tissue

• Is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.

TISSUE MEMBRANE

• A thin layer or sheet of cells that covers the outside of Zthe body (skin), the organs, internal passageways that lead to the exterior of the body, and the lining of the moveable joint cavities.

There are two basic types of tissue membranes:

• Connective Tissue

• Formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints.

• A synovial membrane is a type of connective tissue membrane that lines the cavity of a freely movable joint.

• Epithelial Membranes

• The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, your skin.

SEROUS MEMBRANE

• A serous membrane is an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that is supported by connective tissue.

CUTANEOUS MEMBRANE

• The skin is an epithelial membrane also called the cutaneous membrane.

• It is a stratified squamous epithelial membrane resting on top of connective tissue.

Epithelial Tissue

• 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.

• 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.

Frederik Ruysch, working in the Netherlands, introduced the term epithelia in the third volume of his Thesaurus Anatomicus in 1703.

Ruysch created the term from the Greek “epi”, which means on “top of”, and “thele”, which means “nipple”, to describe the type of tissue he found when dissecting the lip of a cadaver

EPITHELIAL CELLS

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.

ENDOTHELIUM

The epithelium that lines vessels in the lymphatic and cardiovascular systems derives from the mesoderm.

Adjoining cells form a specialized intercellular connection between their cell membranes called a cell junction.

BASAL LAMINA

Is 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.

Generalized Functions of Epithelial Tissue

• Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces.

• Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles.

• A glandular epithelium contains many secretory cells.

Epithelial Tissues

EPITHELIAL CELLS

are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces.

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

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.

Classification of Epithelial Tissues

Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed.

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.

In simple words, Epithelial Tissues are classified into two types: simple, which are one layer, and stratified which are multi-layered.

Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer

Simple Epithelium

The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells.

Simple Squamous Epithelium

• 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.

• Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.

Simple Cuboidal Epithelium

• 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 columnar Epithelium

• 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. 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.

Pseudostratified Columnar Epithelium

• 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.

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.

• Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body.

• 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.

• Stratified Columnar Epithelial Tissue is the rarest type of tissue.

• You are taller in the morning than in the evening.

• By the age of eighteen your brain stops growing.

• There are more than 600 individual skeletal muscles and 206 bones in your body.

• Your stomach manufactures a new lining every three days to avoid digesting itself