Types of Tissues

8/8/2019 Types of Tissues

http://slidepdf.com/reader/full/types-of-tissues 1/7

Tissues

Types of Tissues

A tissue is a group of similar cells (together with

associated cell products) specialized for the performanceof a common function.

The study of tissues is called histology (Gr. histos, tissue,

_ logos, discourse).

All cells in metazoan animals take part in the formation of

tissues.

Sometimes cells of a tissue may be of several kinds, andsome tissues have a great many intercellular materials.

During embryonic development, the germ layers become

differentiated into four kinds of tissues.

These are epithelial, connective, muscular, and nervoustissues

This is a short list of only four basic tissue types that are

able to meet the diverse requirements of animal life.

Epithelial Tissue

An epithelium (pl., epithelia) is a sheet of cells that coversan external or internal surface.

Outside the body, the epithelium forms a protective

covering.

Inside, the epithelium lines all organs of the body cavity,as well as ducts and passageways through which various

materials and secretions move.

On many surfaces epithelial cells are modified into glandsthat produce lubricating mucus or specialized products

such as hormones or enzymes.

Epithelia are classified on the basis of cell form andnumber of cell layers.

Epithelium can be simple, consisting of only one layer of

cells, or stratified, consisting of two or more stackedlayers.

Simple epithelia are found in all metazoan animals, while

stratified epithelia are mostly restricted to vertebrates.



All types of epithelia are supported by an underlyingbasement membrane, which is a condensation of the

ground substance of connective tissue.

Blood vessels never penetrate into epithelial tissues, whichdepend on diffusion of oxygen and nutrients from

underlying tissues.

Individual epithelial cells can be:

flat (squamous epithelium)

cube shaped (cuboidal epithelium)

columnlike (columnar epithelium)

The cells of pseudostratified ciliated columnar epithelium

possess cilia and appear stratified or layered, but they are

not; hence, the prefix pseudo. They look layered becausetheir nuclei are at two or more levels within cells of the

tissues).

Simple columnar epithelium resembles cuboidal

epithelium, but the cells are taller and usually have elongate

nuclei. This type of epithelium is found in highly absorptivesurfaces such as the intestinal tract of most animals. The cells

often bear minute, fingerlike projections called microvilli that

greatly increase the absorptive surface. In some organs, suchas the female reproductive tract, the cells are ciliated.

Stratified squamous epithelium consists of two to many layers of cells adapted to withstand mild mechanical abrasion. The basal

layer of cells undergoes continuous mitotic divisions, producingcells that are pushed toward the surface where they are

sloughed off and replaced by new cells from beneath. This type



of epithelium lines the oral cavity, esophagus, and anal canal of many vertebrates, and the vagina of mammals.

Stratified squamous epithelium consists of two to

many layers of cells adapted to withstand mild mechanicalabrasion. The basal layer of cells undergoes continuous mitotic

divisions, producing cells that are pushed toward the surfacewhere they are sloughed off and replaced by new cells from

beneath. This type of epithelium lines the oral cavity,esophagus, and anal canal of many vertebrates, and the vaginaof mammals.

Connective Tissue Connective tissues support and bind.



They are so widespread in the body that removal of othertissues would still leave the complete form of the body

clearly apparent.

Connective tissue is composed of relatively few cells, agreat many extracellular fibers, and a ground substance

(also called matrix), in which the fibers are embedded. Connective tissues are of two general types, depending on

whether the fibers are loosely or densely packed.

In loose connective tissue strong, flexible fibers of

the protein collagen are interwoven with fine, elastic, and

reticular fibers, giving loose connective tissue its elasticconsistency and making it an excellent binding tissue (e.g.,

binding the skin to underlying muscle tissue).

Loose connective tissue, also called areolar connectivetissue, is the “packing material” of the body that anchors

blood vessels, nerves, and body organs. It contains fibroblasts that synthesize the fibers and

ground substance of connective tissue and wandering

macrophages that phagocytize pathogens or damagedcells.

The different fiber types include strong collagen fibers

(thick) and thin elastic fibers (branching) formed of theprotein elastin.

Adipose tissue is a type of loose connective tissue that

consists of large cells that store lipid. Most often, the cellsaccumulate in large numbers to form what is commonly

called fat.



In Dense connective tissue (fibrous connective

tissue), much of this type is composed of collagen the

collagen fibers are densely packed and may lie parallel toone another, creating very strong cords, such as tendons

(which connect muscles to bones or to other muscles) andligaments (which connect bones to bones).

Cartilage is a hard yet flexible tissue that supports such

structures as the outer ear and forms the entire skeleton of

such animals as sharks and rays. - Cells calledchondrocytes lie within spaces called lacunae that are

surrounded by a rubbery matrix that chondroblastssecrete.

- This matrix, along with the collagen fibers, givescartilage its strength and elasticity.



- Bone cells (osteocytes) also lie within lacunae, but the matrix

around them is heavily impregnated with calcium phosphate,making this kind of tissue hard and ideally suited for its

functions of support and protection.

- Blood is a connective tissue in which a fluid called plasma

suspends specialized red and white blood cells plus platelets.

- Blood transports various substances throughout the bodies of animals.

Muscular Tissue Muscle is the most abundant tissue in the body of most

animals. It originates (with few exceptions) frommesoderm, and its unit is the cell or muscle fiber,

specialized for contraction.

When viewed with a light microscope, striated muscleappears transversely striped (striated), with alternating

dark and light bands.

In vertebrates we recognize two types of striated muscle:skeletal and cardiac muscle.

A third kind of muscle is smooth (or visceral) muscle,

which lacks the characteristic alternating bands of thestriated type.

The unspecialized cytoplasm of muscles is called

sarcoplasm, and contractile elements within the fiber aremyofibrils.

Skeletal muscle is a type of striated muscle found in

both invertebrates and vertebrates. It is composed of extremely long, cylindrical fibers, which are multinucleate

cells that may reach from one end of the muscle to the

other. Viewed through the light microscope, the cellsappear to have a series of stripes, called striations, running

across them. Skeletal muscle is called voluntary muscle (in



vertebrates) because it contracts when stimulated bynerves under conscious cerebral control.

Nervous Tissue Nervous tissue is specialized for reception of stimuli and

conduction of impulses from one region to another. Two basic types of cells in nervous tissue are neurons (Gr.

nerve), the basic functional unit of the nervous system,and neuroglia (nu-rogle-a; Gr. nerve, glia, glue), a variety

of nonnervous cells that insulate neuron membranes andserve various supportive functions.

Documents

Types of Tissues