Chapter 4 Objectives

1. Define tissue, organ, endocrine gland, exocrine gland.

2. Describe the functions of each of the 4 primary tissue types.

3. Describe the special characteristics and classification of epithelial tissue. For each of the subtypes of epithelial tissue describe the structure, location, and functions. Contrast merocrine, apocrine, and holocrine glands.

4. Describe the characteristics of connective tissue. For each subtype of connective tissue describe the structure, location, and functions.

5. Contrast cutaneous and mucous membranes.

6. Describe the 3 types of muscle by structure, function, and location.

7. Describe the structure and function of nervous tissue.

 

TISSUE: THE LIVING FABRIC

In multicellular animals, individual cells are specialized with each type performing specific functions that help maintain homeostasis.Cell specialization allows the various parts of the body to function in very sophisticated ways. However, this division of labor is not without certain hazards. When a particular cell group is indispensable, its loss can severely disable or even destroy the body.

Groups of closely associated cells that are similar in structure and perform a common function are called tissues. Four primary tissue types form the basis of the body: epithelial - covering; connective - support; muscle - movement; nervous - control

Tissues are orgaruzed into organs. Most organs contain representatives of all four tissue types. The arrangement of tissues determines the organ's structure and capabilities.

Epithelial Tissue Occurs in the body as coverings of the outside of organs, linings of the inside of organs, and glands. The covering and lining epithelium is found on all free surfaces of the body. For example the outer layer of skin, dipping into and lining the open cavities of the digestive tract and respiratory systems, lining blood vessels and the heart, and covering the walls and organs of the closed ventral body cavity. Glandular epithelium fashions the glands of the body.

Epithelium is highly specialized to accomplish many functions: protection, absorption, filtration, excretion, and secretion

Specialized Characteristics of Epithelium

1. Cellularity: composed almost entirely of cells. The cells are close and only a small amount of extracellular material lies in the narrow spaces between them.

2. Specialized contacts: fit closely together to form continuous sheets and are bound together at many points by lateral contacts, including tight junctions and desmosomes.

3. Polarity: epithelium always has one free surface (apical), which is exposed to the body exterior or the cavity of an internal organ. Some exposed plasma membrane surfaces are smooth and slick; others exhibit cell surface modification such as microvilli or cilia.

4. Avascularity: epithelium may be well supplied by nerve fibers, but is avascular. The cells are nourished by substances diffusing from blood vessels in the underlying connective tissue.

5. Basement Membrane: The lower (basal) surface rests on thin supporting basal lamina, which separates it from the underlying connective tissue. The basal lamina is a nonliving, adhesive material formed largely of glycoproteins. The connective tissue cells below the basal lamina secrete a similar extracellular material containing fine collagenous fibers. The basal lamina and the reticular lamina together form the basement membrane, which reinforces the epithelial sheet, helping it to resist stretching and tearing, and it also defines the space that may be occupied by the epithelial cells.

6. Regeneration: epithelium has a high regenerative capacity as long as the cells receive adequate nutrition.

Classification of Epithelia The many types are identified structurally according to two criteria: number of cell layers and shape of the cells

Number of layers: two major varieties

1. Simple epithelia composed of a single layer of cells; found where absorption and filtration occur.

2. Stratified epithelia consists of layers of cells stacked one on top of the other; common in areas of high abrasion where protection is important.

Shape of the cells: All cells have 6 somewhat irregular sides (polyhedral) which allows them to be closely packed. They vary in cell volume and consequently in cell height. On the basis of height; three cell types: 1. Squamous cells are flattened and scalelike. 2. Cuboidal cells are approximately as tall as they are wide. 3. Columnar cells are tall and column shaped.

The nucleus of a squamous cells is disc shaped; that of a cuboidal cell is spherical; and a columnar cell nucleus is elongated and is usually located close to the cell base.

Maior Classification of Simple Epithelia:

1. simple squamous
2. simple cuboidal
3. simple columnar
4. pseudostratified-highly modified

Maior Classification of Stratified Epithelia:

1. stratified squamous
2. stratified cuboidal
3. stratified columnar
4. transitional epitheliums

In stratified epithelia, cell shape varies in the different layers. Thus stratified epithelia are named according to the shape of the cells at the free surface (also called the apical surface).

Simple Epithelia

Most concerned with absorption, secretion, and filtration.Very thin.

Simple Squamous

Flattened laterally, sparse cytoplasm, thin, often permeable, close-fitting cells resemble a tiled floor. Found where filtration or the exchange of substances by rapid diffusion is a priority. Two simple squamous epithelia have special names: endothelium - slick, friction reducing lining of vessels and heart; mesothelium - found in serous membranes lining body cavity

Simple Cuboidal Epithelium

Single layer; Functions in secretion and absorption. In glands, it forms the secretory portion and the ducts that deliver secretions to their destinations. Located in: kidney tubules, ducts, secretory portion of small glands

Simple Columnar

Single layer of tall, closely packed cells. Line the digestive tract from stomach to the rectum. Most associated with absorption and secretion

Digestive tract lining has two distinct modifications: microvilli - increase surface area for absorption goblet cells - secrete mucus.

Located in: digestive tract, gallbladder, excretory ducts of some glands, uterine tubes, some regions of the uterus

Pseudostratified Eoithelium

All of its cells rest on the basement membrane.Some cells are shorter than others and may not reach the surface. Nuclei vary in shape and are located at different levels above the basement membrane giving the false impression that several layers of cells are present. Functions in secretion and absorption

Location of nonciliated type - ducts of large glands, part of male urethra Location of ciliated type - trachea, most of upper respiratory tract

Stratified Epithelia

Consists of two or more layers of cells. More durable. Function in protection.

Stratified Squamous - Most widespread. Composed of several layers. Free surface cells are squamous, deeper layers are cuboidal.Found in areas subjected to wear and tear; surface cells constantly being rubbed off.Forms the external part of the skin.Extend a short distance into every body opening directly continuous with the skin.Covers the tongue, lines the mouth, pharynx, esophagus, anal canal, and vagina. Outer layer (of epidermis) is keratinized -- contains the waterproofing protein (keratin).

Stratified Columnar - Rare. Free surface cells are columnar, deeper layers are small and vary in size.

Stratified Cuboidal Generally forned of only two layers.Limited distribution in the body. Primarily found in the ducts of sweat glands and other larger glands.

Transitional Enithelium - Forms the lining of the urinary organs which are subjected to stretching and varying internal pressure.Basal layer cells are cuboidal or columnar.Apical cells vary in appearance, depending on the degree of distension of the organ. The ability of the cells to slide past one another and change their shape accommodates the flow of a greater volume of urine.

Glandular Epithelia - A gland consists of one or more cells that make and secrete a particular product called a secretion. It is an aqueous fluid containing proteins. Glands can be classified as endocrine or exocrine, depending on their route of secretion.

Endocrine Glands - Ductless glands. Produce regulatory chemicals called hormones. Secrete directly into extracellular spaces and thence into blood.

Exocrine Glands - More numerous and diverse Secrete products through ducts onto the body surface or into body cavities. Include sweat and oil glands, salivary glands, liver, pancreas, mammary and mucus glands.

Unicellular Exocrine Glands: Single cells interposed in an epithelium between cells with other functions. In humans, all such glands produce mucin (forms mucus when dissolved in water). The only important unicellular glands in humans are goblet cells.

Multicellular Exocrine Glands

The glands can be described according to their structure of their secretory parts: tubular - secretory cells forming a tube, alveolar - secretory cells forming small flasklike sacs, tubuloalveolar - contain both tubular and alveolar units

Note: "acinar" used interchangeably with "alveolar"

Mode of Secretion

Most exocrine glands are merocrine - secrete their products by exocytosis shortly after the products are produced. Examples are the pancreas and most sweat and salivary glandsSecretory cells of holocrine glands accumulate their products within them until they rupture. Cells are replaced by division of underlying cells. Examples are the sebaceous oil glands. Apocrine glands accumulate their products just beneath the free surface. Eventually the apex of the cell pinches off end the secretion is released. The cell repairs itself and the process is repeated again and again.

 

Connective Tissue Found everywhere in the body. The most abundant and widely distributed of the primary tissues, but the amount in any particular organ varies. The skin consists primarily of connective tissue, while the brain contains very little. Connective tissue has many forms and functions. Its chief subclasses are: connective tissue proper, cartilage, bone, blood

Its major fimctions include: binding and support, protection, insulation, and transportation

Common Characteristics of Connective Tissue: Connective tissues have certain properties in common that set them apart from other primary tissues.

1. Common Origin: arises from mesenchyme - embryonic tissue derived from the mesoderm layer.

2. Degrees of Vascularity: runs the entire gamut of vascularity. Cartilage is avascular. Dense connective tissue is poorly vascularized. The other types have a rich supply of blood vessels.

3. Extracellular Matrix: connective tissues are composed largely of nonliving extracellular matrix, which separates the living cells of the tissue. Because of the matrix, connective tissue is able to bear weight, withstand great tension, and endure abuses.

Structural Elements:

Three types of elements make up connective tissue: ground substance, fibers, cells

The ground substance and fibers make up the extracellular matrix.

The matrix can be delicate and fragile to form a soft packing around an organ, or it can form ropes of incredible strength.

Ground substance: Amorphous material that fills the space between the cells and contains the fibers.Functions as a molecular sieve, or medium through which nutrients and other dissolved substances can diffuse between the blood capillaries and the cells.

Fibers: Three types of fibers found in the matrix: collagen, elastic, reticular

Collagen fibers are extremely tough and provide high tensile strength. When fresh, they have a glistening white appearance (white fibers).

Elastic fibers are formed largely from another fibrous protein, elastin. Found where greater elasticity is needed, as in the skin, lungs, and blood vessels. Fresh elastic fibers appear yellow.

Reticular fibers are fine collagenous fibers, extensively branched, delicate networks that surround small blood vessels and support the soft tissue or organs. They are particularly abundant at junctions between connective tissue and other tissue types.

Cells Each major class of connective tissue has a fundamental cell type that exists in immature and mature forms. The undifferentiated cells are actively mitotic cells that secrete both the ground substance and the fibers characteristic of their particular matrix.

The primary blast cell types by connective tissue class are: 1. connective tissue proper -- fibroblasts 2. cartilage -- chondroblast 3. bone -- osteoblast 4. blood - hemocytoblast

Once thee matrix has been synthesized, the blast cells assume their less active, mature mode. The mature cells are responsible for maintaining the matrix. If the matrix is damaged, the mature cells can revert to their more active state to make repairs and regenerate the matrix.

Types of Connective Tissue All classes of connective tissue consist of living cells surrounded by a matrix. Major differences reflect cell type, fiber type, and proportion of the matrix contributed by the fibers. These three factors determine not only major connective tissue classes but also subclasses & types.

Connective Tissue Proper: Two subclasses: 1. loose connective tissue (areolar, adipose, reticular) 2. dense connective tissue (dense regular, dense irregular, elastic)

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

Areolar: Semifluid ground substance. All three fiber types are loosely dispersed. Cell types - fibroblasts, macrophages, mast cells, some white blood cells. Serves as a kind of universal packing material between other tissues.

Adipose: Very cellular. Very little matrix seen. Richly vascularized. Located under the skin, around the kidneys and eyeballs, in bones, within abdomen and breasts. Acts as a shock absorber and as insulation, helps prevent heat loss.

Reticular: Limited to certain sites in the body - lymphoid organs, bone marrow, spleen. Forms the soft internal skeleton supporting other cell types.

Dense regular connective tissue: Flexible tissue with great resistance to pulling forces. Found where tension is always exerted in a single direction.Forms tendons and ligaments.

Dense irregular tissue: Forms sheets in body areas where tension is exerted from many different directions.Found in the skin as the dermis, forms fibrous joint capsules and fibrous coverings that surround some organs (testes, kidneys, bones, nerve).

Elastic connective tissue: Found in the walls of the aorta, parts of the tracheas & bronchi, forms vocal cords and ligaments connecting the vertebrae.

Cartilage: Qualities intermediate between dense connective tissue and bone. Tough yet flexible. Avascular. Devoid of nerve fibers.Firm matrix prevents cell movement.

Bone: Has a rocklike hardness and an exceptional ability to support and protect softer tissues. Provides cavities for fat storage and synthesis of blood cells. Has an added matrix element - calcium salts

Blood: Considered a connective tissue because it consists of blood cells, surrounded by a nonliving fluid matrix called plasma. The fibers are soluble protein molecules (clotting proteins).

Epithelial Membranes Continuous multicellular sheet composed of at least two primary tissue types: an epithelium bound to a discrete underlying connective tissue layer.

Cutaneous Membranes: Skin. An organ consisting of a keratinized squamous epithelium firmly attached to a thick layer of dense irregular connective tissue (dermis). Exposed to the air and is a dry membrane.

Mucous Membranes: Line the body cavities that are open to the exterior (G.I., respiratory, urogenital tracts).Wet or moist membranes bathed by secretions. Mucosa refers to the location of the epithelial membrane, not its cell composition. Mucous membranes are often adapted for absorption and secretion.

Muscles

Highly cellular, well-vascularized tissues that are responsible for most types of body movements. Muscle cells possess myofilaments that promote movement or contractions. Three kinds of muscle tissue: skeletal, smooth, cardiac

Skeletal: Packaged by connective tissue sheets into organs. Attached to the bones of the skeleton. Form the flesh of the body. Muscle cells are long, cylindrical cells containing many nuclei. Banded (striated) appearance reflects the alignment of the myofilaments.

Cardiac: Occurs in the walls of the heart, it is found nowhere else in the body. Cardiac muscle cells are uninucleated, striated, and the branching cells fit together tightly at unique junctions called intercalated discs.

Smooth: No externally visible striations can be seen. Muscle cells are spindle shaped and contain one centrally located nucleus. Occurs in the walls of hollow organs. Generally act to propel substances through the hollow organ by alternately contracting and relaxing. Under involuntary control.

Nervous Tissue

Makes up the central nervous system (brain, cspinal cord, nerves). Nerves conduct impulses to and from various body organs. Composed of two major cell types: neurons and supporting cells