Tissues

TISSUES

Groups of cells that are similar in structure and perform a common or related function are called tissues.

Four primary tissue types interweave to form the fabric of the body.

• These basic tissues are:
• 1.Epithelial Tissue
• 2.Connective Tissue
• Muscle Tissue
• 4.Nervous tissue

Each has numerous subclasses or varieties.

Describe its general role, the terms would most likely be:

• Covering (epithelial)
• Support (connective)
• Movement (muscle)
• Control (nervous).

However, these terms reveal only a fraction of the functions that each tissue performs.

Tissues are organized into organs such as the kidneys and the heart.

Most organs contain all four tissue types, and their arrangement determines the organ’s structure and capabilities.

The study of tissues is called Histology

EPITHILIAL TISSUE

Epithelial tissue, or an epithelium, is a sheet of cells that covers a body surface or lines a body cavity.

Covering and lining epithelium forms the outer layer of the skin, dips into and lines the open cavities of the cardiovascular, digestive, and respiratory systems, and covers the walls and organs of the closed ventral body cavity.

Epithelia form boundaries between different environments.

For example, the epidermis of the skin lies between the inside and the outside of the body, and epithelium lining the urinary bladder separates underlying cells of the bladder wall from urine.

Nearly all substances received or given off by the body must pass through an epithelium

In its role as an interface tissue, epithelium accomplishes many functions, including

• 1.Protection
• 2.Absorption
• Filtration
• Excretion
• 5.Secretion
• Sensory reception

CLASSIFICATION OF EPITHELIA

Each epithelium is given two names.

The first name indicates the number of cell layers present; the second describes the shape of its cells.

• Based on the number of cell layers, there are simple and stratified epithelia.

Simple epithelia are composed of a single cell layer.

They are typically found where absorption and filtration occur and a thin epithelial barrier is desirable.

Stratified epithelia, consisting of two or more cell layers stacked one on top of the other, are common in high-abrasion areas where protection is important, such as the skin surface and the lining of the mouth.

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In cross section, all epithelial cells have six sides, and an apical surface view of an epithelial sheet looks like a honeycomb.

This polyhedral shape allows the cells to be closely packed.

However, epithelial cells vary in height, and on that basis, there are three common shapes of epithelial cells.

1.Squamous cells are flattened and scalelike

• 2.Cuboidal cells are boxlike, approximately astall as they are wide.
• Columnar cells are tall and column shaped.

In each case, the shape of the nucleus conforms to that of the cell.

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The nucleus of a squamous cell is a flattened disc; that of a cuboidal cell is spherical; and a columnar cell nucleus is elongated from top to bottom and usually located close to the cell base.

SIMPLE EPITHELIA

The simple epithelia are most concerned with absorption, secretion, and filtration.

Because they consist of a single cell layer and are usually very thin, protection is not one of their specialties

The cells of a simple squamous epithelium are flattened laterally, and their cytoplasm is sparse.

In a surface view, the close-fitting cells resemble a tiled floor.

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Thin and often permeable, this epithelium is found where filtration or the exchange of substances by rapid diffusion is a priority.

In the kidneys, simple squamous epithelium forms part of the filtration membrane; in the lungs, it forms the walls of the air sacs across which gas exchange occurs.

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Endothelium provides a slick, friction-reducing lining in lymphatic vessels and in all hollow organs of the cardiovascular system—blood vessels and the heart.

Capillaries consist exclusively of endothelium, and its exceptional thinness encourages the efficient exchange of nutrients and wastes between the bloodstream and surrounding tissue cells.

Mesothelium is the epithelium found in serous membranes lining the ventral body cavity and covering its organs.

Simple cuboidal epithelium consists of a single layer of cells as tall as they are wide .

Important functions of simple cuboidal epithelium are secretion and absorption.

This epithelium forms the walls of the smallest ducts of glands and of many kidney tubules.

Simple columnar epithelium is seen as a single layer of tall, closely packed cells, aligned like soldiers in a row.

It lines the digestive tract from the stomach through the rectum.

Columnar cells are mostly associated with absorption and secretion, and the digestive tract lining has two distinct modifications that make it ideal for that dual function:

1) Dense microvilli on the apical surface of absorptive cells

• Goblet cells that secrete a protective lubricating mucus.

Some simple columnar epithelia display cilia on their free surfaces, which help to move substances or cells through an internal passageway.

The cells of pseudostratified columnar epithelium vary in height.

All of its cells rest on the basement membrane, but only the tallest reach the free surface of the epithelium.

Because the cell nuclei lie at different levels above the basement membrane, the tissue gives the false (pseudo) impression that several cell layers are present; hence “pseudostratified.”

The short cells are relatively unspecialized and give rise to the taller cells.

• This epithelium, like the simple columnar variety, secretes or absorbs substances.

A ciliated version containing goblet cells lines most of the respiratory tract.

Here the motile cilia propel sheets of dust-trapping mucus superiorly away from the lungs.

STRATIFIED EPITHELIA

Stratified epithelia contain two or more cell layers.

They regenerate from below; that is, the basal cells divide and push apically to replace the older surface cells.

Considerably more durable than the simple epithelia, the major role of stratified epithelia is protection.

Stratified squamous epithelium is the most widespread of the stratified epithelia.

Composed of several layers, it is thick and well suited for its protective role in the body.

Its free surface cells are squamous; cells of the deeper layers are cuboidal or columnar.

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This epithelium is found in areas subjected to wear and tear, and its surface cells are constantly being rubbed away and replaced by division of its basal cells.

Transitional epithelium forms the lining of hollow urinary organs, which stretch as they fill with urine.

• Cells of its basal layer are cuboidal or columnar.

The apical cells vary in appearance, depending on the degree of distension of the organ.

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When the organ is distended with urine, the transitional epithelium thins from about six cell layers to three, and its domelike apical cells flatten and become squamous like.

The ability of transitional cells to change their shape allows a greater volume of urine to flow through a tubelike organ.

In the bladder, it allows more urine to be stored.

CONNECTIVE TISSUE

Is found everywhere in the body.

It is the most abundant and widely distributed of the primary tissues, but its amount in particular organs varies.

For example, skin consists primarily of connective tissue, while the brain contains very little

There are four main classes of connective tissue and several subclasses.

• The main classes are
• Connective tissue proper
• Cartilage
• Bone tissue
• Blood.

Its major functions include

• Binding and support
• Protection
• Insulation
• Transportation of substances within the body.

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For example, bone and cartilage support and protect body organs by providing the hard underpinnings of the skeleton; fat cushions insulate and protect body organs and provide reserve energy fuel.

COMMON CHARACTERISTICS OF CONNECTIVE TISSUE

• Common origin. All connective tissues arise from mesenchyme (an embryonic tissue) and hence have a common kinship
• Degrees of vascularity. Connective tissues run the entire gamut of vascularity.

Cartilage is avascular; dense connective tissue is poorly vascularized; and the other types of connective tissue have a rich supply of blood vessels.

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• Extracellular matrix. Whereas all other primary tissues are composed mainly of cells, connective tissues are largely nonliving extracellular matrix , which separates, often widely, the living cells of the tissue.

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Because of its matrix, connective tissue is able to bear weight, withstand great tension, and endure abuses, such as physical trauma and abrasion, that no other tissue would be able to tolerate.

STRUCTURAL ELEMENTS OF CONNECTIVE TISSUE

• Connective tissues have three main elements: ground substance, fibers, and cells.

Ground substance and fibers make up the extracellular matrix.

GROUND SUBSTANCE

Is the unstructured material that fills the space between the cells and contains the fibers.

It is composed of interstitial (tissue) fluid, cell adhesion proteins, and proteoglycans.

Cell adhesion proteins,serve mainly as a connective tissue glue that allows connective tissue cells to attach themselves to matrix elements.

FIBERS

The fibers of connective tissue provide support.

Three types of fibers are found in connective tissue matrix: collagen, elastic, and reticular fibers.

Of these, collagen fibers are by far the strongest and most abundant

Collagen fibers are constructed primarily of the fibrous protein collagen.

Collagen molecules are secreted into the extracellular space, where they assemble spontaneously into cross-linked fibrils, which in turn are bundled together into the thick collagen fibers.

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Because of the cross-linking of their fibrils, collagen fibers are extremely tough and provide high tensile strength to the matrix.

Indeed, stress tests show that collagen fibers are stronger than steel fibers of the same size!

Elastic fibers are long, thin fibers that form branching networks in the extracellular matrix.

These fibers contain a rubberlike protein, elastin, that allows them to stretch and recoil like rubber bands.

Connective tissue can stretch only so much before its thick, ropelike collagen fibers become taut.

Reticular fibers are short, fine, collagenous fibers and are continuous with collagen fibers.

They branch extensively, forming delicate networks surround small blood vessels and support the soft tissue of organs.

CELLS

Each major class of connective tissue has a fundamental cell type that exists in immature and mature forms.

The undifferentiated cells, indicated by the suffix blast , are actively mitotic cells that secrete the ground substance and the fibers characteristic of their particular matrix.

The primary blast cell types by connective tissue class are

• Connective tissue proper: fibroblast
• Cartilage: chondroblast
• 3)Bone: osteoblast
• Blood: hematopoietic stem cell.

TYPES OF CONNECTIVE TISSUE

• All classes of connective tissue consist of living cells surrounded by a matrix.

Their major differences reflect cell type, and fiber types and relative amounts.

Mature connective tissues arise from a common embryonic tissue, called mesenchyme, derived from embryonic mesoderm.

CONNECTIVE TISSUE PROPER

• Has two subclasses:
• The loose connective tissues (areolar, adipose, and reticular)
• Dense connective tissues (dense regular, dense irregular, and elastic).

Except for bone, cartilage, and blood, all mature connective tissues belong to this class.

AREOLAR CONNECTIVE TISSUE

The functions of areolar connective tissue, shared by some but not all connective tissues, include:

• Supporting and binding other tissues
• Holding body fluids
• Defending against infection

(4) Storing nutrients as fat (in fat cells).

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The most obvious structural feature of this tissue is the loose arrangement of its fibers; hence, its classification as a loose connective tissue is fitting.

Because of its loose nature, areolar connective tissue provides a reservoir of water and salts for surrounding body tissues, always holding approximately as much fluid as there is in the entire bloodstream.

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When a body region is inflamed, the areolar tissue in the area soaks up excess fluids like a sponge, and the affected area swells and becomes puffy, a condition called edema

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Areolar connective tissue is the most widely distributed connective tissue in the body and it serves as a kind of universal packing material between other tissues.

It binds body parts together while allowing them to move freely over one another; wraps small blood vessels and nerves; surrounds glands; and forms the subcutaneous tissue, which cushions and attaches the skin to underlying structures.

ADIPOSE (FAT) TISSUE

Is similar to areolar tissue in structure and function but its nutrient-storing ability is much greater.

Consequently, adipocytes , commonly called adipose or fat cells, predominate and account for 90% of this tissue’s mass.

The matrix is scanty and the cells are packed closely together, giving a chicken wire appearance to the tissue.

Adipose tissue is richly vascularized, indicating its high metabolic activity.

Adipose tissue is certainly abundant: It constitutes 18% of an average person’s body weight.

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Adipose tissue may develop almost anywhere areolar tissue is plentiful, but it usually accumulates in subcutaneous tissue, where it also acts as a shock absorber, as insulation, and as an energy storage site.

Because fat is a poor conductor of heat, it helps prevent heat loss from the body.

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Other sites where fat accumulates include surrounding the kidneys, behind the eyeballs, and at genetically determined fat depots such as the abdomen and hips.

RETICULAR CONNECTIVE TISSUE

Resembles areolar connective tissue, but the only fibers in its matrix are reticular fibers, which form a delicate network along which fibroblasts called reticular cells lie scattered.

Although reticular fibers are widely distributed in the body, reticular tissue is limited to certain sites.

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It forms a labyrinth-like stroma , or internal framework, that can support many free blood cells (largely lymphocytes) in lymph nodes, the spleen, and bone marrow

DENSE REGULAR CONNECTIVE TISSUE

Is one variety of the dense connective tissues, all of which have fibers as their predominant element.

For this reason, the dense connective tissues are often referred to as fibrous connective tissues.

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Dense regular connective tissue contains closely packed bundles of collagen fibers running in the same direction, parallel to the direction of pull.

This results in white, flexible structures with great resistance to tension (pulling forces) where the tension is exerted in a single direction.

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This allows the tissue to stretch a little, but once the fibers are straightened out by a pulling force, there is no further give to this tissue.

Unlike our model (areolar) connective tissue, this tissue has few cells other than fibroblasts and it is poorly vascularized.

DENSE IRREGULAR CONNECTIVE TISSUE

Has the same structural elements as the regular variety.

However, the bundles of collagen fibers are much thicker and they are arranged irregularly; that is, they run in more than one plane

This type of tissue forms sheets in body areas where tension is exerted from many different directions.

It is found in the skin as the leathery dermis, and it forms fibrous joint capsules and the fibrous coverings that surround some organs (kidneys, bones, cartilages, muscles, and nerves).

CARTILAGE

Which stands up to both tension and compression, has qualities intermediate between dense connective tissue and bone.

• It is tough but flexible, providing a resilient rigidity to the structures it supports.

Cartilage lacks nerve fibers and is avascular.

It receives its nutrients by diffusion from blood vessels located in the connective tissue membrane surrounding it.

Its ground substance contains large amounts of the chrondroitin sulfate and hyaluronic acid, firmly bound collagen fibers, and is quite firm.

Cartilage matrix also contains an exceptional amount of tissue fluid; in fact, cartilage is up to 80% water!

The movement of tissue fluid in its matrix enables cartilage to rebound after being compressed and also helps to nourish the cartilage cells.

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Chondroblasts, the predominant cell type in growing cartilage, produce new matrix until the skeleton stops growing at the end of adolescence.

The firmness of the cartilage matrix prevents the cells from becoming widely separated, so chondrocytes, or mature cartilage cells, are typically found in small groups within cavities called lacunae.

There are three varieties of cartilage:

• Hyaline cartilage
• Elastic cartilage
• Fibrocartilage

Each dominated by a particular fiber type.

HYALINE CARTILAGE

Is the most abundant cartilage type in the body.

Although it contains large numbers of collagen fibers, they are not apparent and the matrix appears amorphous and glassy blue-white when viewed by the unaided eye.

Chondrocytes account for only 1–10% of the cartilage volume.

Hyaline cartilage provides firm support with some pliability.

It covers the ends of long bones as articular cartilage, providing springy pads that absorb compression at joints.

Hyaline cartilage also supports the tip of the nose, connects the ribs to the sternum, and supports most of the respiratory system passages.

Most of the embryonic skeleton is formed of hyaline cartilage before bone is formed.

Skeletal hyaline cartilage persists during childhood as the epiphyseal plates, actively growing regions near the end of long bones that provide for continued growth in length.

ELASTIC CARTILAGE

Is nearly identical to hyaline cartilage. However, there are many more elastin fibers in elastic cartilage.

Found where strength and exceptional stretchability are needed, elastic cartilage forms the “skeletons” of the external ear and the epiglottis.

FIBROCARTILAGE

Is a perfect structural intermediate between hyaline cartilage and dense regular connective tissues.

Its rows of chondrocytes (a cartilage feature) alternate with rows of thick collagen fibers.

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Because it is compressible and resists tension well, fibrocartilage is found where strong support and the ability to withstand heavy pressure are required.

For example, the intervertebral discs and the spongy cartilages of the knee (menisci) are fibrocartilage structures.

BONE (OSSEOUS TISSUE)

Because of its rocklike hardness, bone, or osseous tissue , has an exceptional ability to support and protect body structures.

Bones of the skeleton also provide cavities for fat storage and synthesis of blood cells.

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Bone matrix is similar to that of cartilage but is harder and more rigid because, in addition to its more abundant collagen fibers, bone has an added matrix element—inorganic calcium salts.

Osteoblasts produce the organic portion of the matrix; then bone salts are deposited on and between the fibers.

Mature bone cells, or osteocytes, reside in the lacunae within the matrix they have made.

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In cross section, bone tissue is seen as closely packed structural units called osteons formed of concentric rings of bony matrix (lamellae) surrounding central canals containing the blood vessels and nerves serving the bone.

Unlike cartilage, the next firmest connective tissue, bone is well supplied by invading blood vessels.

BLOOD

• Blood, the fluid within blood vessels, is the most atypical connective tissue.

It does not connect things or give mechanical support It is classified as a connective tissue because it develops from mesenchyme and consists of blood cells, surrounded by a nonliving fluid matrix called blood plasma

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The fibers of blood are soluble protein molecules that precipitate, forming large visible fiberlike structures during blood clotting.

Blood functions as the transport vehicle for the cardiovascular system, carrying nutrients, wastes, respiratory gases, and many other substances throughout the body.