Tissues

Tissues are closely associated cells that are similar in structure and perform a common function.

Principle Types of Tissue

There are four primary tissue

types that interweave to form the

fabric of the body.

Principle Types of Tissue1. Epithelial tissue:

Primary function is protection

2. Connective tissue:

Primary function is support

3. Muscle:

Primary function is movement

4. Nervous:

Primary function is control

Developmental Aspects of Tissue

Embryonic and Fetal Development

Primary Germ Layer Formation

Development of Tissues Through Adolescents

By the end of the second month of development, the primary tissues have developed and most organs are laid down. Most tissue cells, except neurons, continue to undergo mitosis until body size is achieved, after which only epithelial and connective tissues are routinely mitotic.

Effects of Aging on Tissue

With increasing age, epithelial sheets become thin and more easily breached. The amount of collagen in the body decreases, making tissue repair less efficient. Bone, muscle and nervous tissue begin to atrophy.

Epithelial Tissue

Epithelial tissue, or epithelium, is subdivided into two types:

1. membranous (covering or lining) epithelium and

2. glandular epithelium

Functions of Epithelial Tissue

1. Protection

2. Absorption

3. Filtration

4. Secretion

Glandular epithelium’s chief function is secretion.

Special Characteristics of Epithelia

1. Cellularity: composed almost entirely of cells

2. Specialized Contacts: fit close together to form continuous sheets; bound by tight junctions and desmosomes

3. Polarity: has one free (apical) surface; the portion exposed to the body exterior or the cavity of an internal organ

Special Characteristics of Epithelia

4. Avascular: it has no blood vessels within it; receives nourishment by diffusion of substances from blood vessels in the underlying connective tissue

5. Regenartion: able to replace cells rapidly by mitotic division

Special Characteristics of Epithelia

6. Basement Membrane: reinforces the epithelial sheets, helping it to resist stretching and tearing, defines the space that may be occupied by epithelial cells; made up of a thin supporting basal lamina (nonliving adhesive material) which separates epithelial tissue from the underlying connective tissue (secretes a similar extracellular material called reticular lamina); these two together for the basement membrane

Classification of Epithelia

Membranous Epithelium

Criteria for Classification • Cellular Shape: Irregularly polyhedral (many-sided) in

a cross section, differ in cell height; 4 common shapes are:

– Squamous: flattened and scale-like; nucleus is thin and flattened

– Cuboidal: as tall as they are wide; nucleus is spherical– Columnar: tall and column shaped; nucleus is elongated from

top to bottom– Pseudostratified columnar: one layer of oddly shaped

columnar cells that don’t always extend to the surface of the membrane; some nuclei are located toward the top and some toward the bottom

Membranous Epithelium

Criteria for Classification• Cellular Arrangement (layers)

a. simple epithelia: composed of a single layer of cells

b. stratified epithelia: multiple cell layers stacked on top of each other

c. transitional epithelia: differing cell shapes in a stratified, or layered sheet

Simple Epithelium

• Simple Squamous Epithelium: function is filtration and diffusion; single layer of thin flattened cells, like floor tiles; nucleus is broad and thin; lines air sacs of lungs, forms walls of capillaries, lines the inside of the blood and lymph vessels, and covers the surfaces of the pleura, pericardium, and peritoneum

Simple Epithelium

• Simple Cuboidal Epithelium: function is secretion and absorption; single layer of cube-

shaped cells; centrally located, spherical nucleus; covers the ovaries and lines most of the kidney tubules and ducts of certain glands such as salivary, thyroid, pancreas, and liver

Simple Epithelium

Simple Columnar Epithelium: specialized for protection, secretion, and absorption; single layer of elongated cells; nucleus is located at the same level near the basement membrane; lines the uterus and most organs of the digestive tract; flask-shaped glandular cells called “goblet cells” are scattered among columnar cells of this tissue and secrete a protective mucus onto the tissue’s surface

Simple Epithelium

Pseudostratified Columnar Epithelium: appear stratified or layered, but are not; possess cilia and goblet cells; one nucleus that is located at two or more levels within the cells; lines the passages of respiratory and reproductive systems

Stratified Epithelium: two or more cell layers; major function is protection because they are

considerably more durable

• Stratified Squamous Epithelium: many cell layers make this tissue thick; cells reproduce

in the deeper layer pushing older ones farther and farther outward; forms the outer layer of skin (epidermis), lines the mouth, throat, vagina, and anal canal

Stratified Epithelium

Stratified Cuboidal Epithelium: consist of two to three layers that line a lumen (space within a tubular structure); lines the larger ducts of the mammary glands, sweat glands, salivary glands, and pancreas; also lines developing ovarian follicles and seminiferous tubules

Stratified Epithelium

Stratified Columnar Epithelium: layers of cells; superficial cells are elongated, while the basal layers consist of cube-shaped cells; found in male urethra and vas deferens, and in parts of the pharynx

Stratified Epithelium

Stratified Transitional Epithelium: specialized to change in response to increased tension; forms the inner lining of urinary bladder and the passageways of the urinary system; the walls consist of several layers of cuboidal cells, when distended they flatten

Covering and Lining Epithelia• Endothelia: simple epithelial

sheet composed of a single layer of squamous cells attached to a basement membrane; provides a slick, friction-reducing lining in all hollow circulatory system organs (lymphatic vessels, blood vessels, and the heart; capillary walls consist only of endothelia because its extreme thinness encourages the exchange of nutrients and wastes across capillary walls

Covering and Lining Epithelia• Mucous Membrane: epithelial membranes that line body

cavities that open to the exterior, such as those of the digestive, respiratory, and urogenital tracts; they are “wet”, or moist membranes bathed by secretions or, in the case of the urinary mucosae, urine

• Cutaneous Membrane: your skin; exposed to air and is a dry membrane

• Serous Membrane: moist membranes found in closed ventral cavities; parietal layer that lines the cavity wall and visceral layer that covers the outer surface of the organs within the cavity; each layer secretes serous fluid which lubricates the facing surfaces of the parietal and visceral layers, so that they slide across each other; named according to site and specific organ association

Glandular Epithelium

Glandular Epithelia is made up of one or more cells that produce and secrete a particular product called a secretion (aqueous fluid, containing proteins). Secretion can refer to both the process of secretion formation and the release of the product of glandular activity. Glandular epithelium are classified according to the route of secretion and the general function of their products, and as unicellular or multicellular on the basis of their structure.

Glandular Epithelium

• Endocrine Glands: lose their ducts and are called ductless glands; produce hormones which they secrete directly into the extracellular space; not all endocrine glands are derivatives of epithelia

Glandular Epithelium• Exocrine Glands: more

numerous than endocrine glands; secrete their products through a duct onto body surfaces or into body cavities; include sweat glands, salivary glands, the liver (which secretes bile), the pancreas (which synthesizes digestive enzymes), mammary glands, mucous glands, and many others

Exocrine Glands

• Unicellular Glands: single cells interposed in epithelial tissue between cells with other functions; have no ducts; all such glands produce mucin ( a complex glycoprotein that dissolves in water and forms a slimy coating (mucus) that protects and lubricates surfaces; includes the goblet cells of the intestinal and respiratory mucosae as well as mucin-producing cells found in other body regions

Exocrine Glands

Multicellular Glands: classified according to the composition of their secretions and the method by which secretion occurs

1. Merocrine Glands: secrete watery, protein rich serous fluid by exocytosis ( salivary glands, pancreatic glands, certain sweat glands)

2. Apocrine Glands: lose small portions of their cytoplasm as fluid –filled packets (mammary glands, certain sweat glands)

3. Holocrine Glands: the whole cell lyses during secretion (sebaceous glands)

Exocrine Glands

Merocrine GlandApocrine Gland Holocrine Gland

Connective Tissue

Connective tissue is found throughout the body. It is the most abundant and widely distributed of all the tissues.

Connective Tissue

Chief subclasses include:

1. Connective tissue proper

2. Cartilage

3. Bone

4. Blood

Connective Tissue

Major functions include:

1. Binding

2. Support

3. Protection

4. Insulation

5. Transportation of substances within the body

Common Characteristics of Connective Tissue

Common Origin: arise from mesenchyme (an embryonic tissue derived from the mesoderm germ layer

Degrees of Vascularity: cartilage is avascular; dense connective tissue poorly vascularized; other types have a rich supply of blood vessels

Matrix: composed largely of nonliving extracellular matrix, which separates the living cells of the tissue; makes connective tissue able to bear weight, withstand great tension, and endure abuses such as physical trauma and abrasions

Structural Elements of Connective Tissue

Connective tissues are placed into different categories or types according to the structural characteristics of the intercellular material. The consistency of the intercellular material varies from fluid to semisolid to solid.

Fibers• Collagenous: thick threads made from

molecules of the protein collagen; grouped in long, parallel bundles; they are flexible but only slightly elastic; they resist considerable pulling force (tendons that connect muscle to bone); tissues containing abundant fibers are called dense tissue which appear white and are referred to as white fibers.

• Elastic: composed of a protein called elastin; fibers branch, forming complex networks; stretch easily and can resume their original lengths and shapes (vocal chords); yellow fibers

• Reticular: very thin collagenous fibers; highly branched and form delicate supporting networks in a variety of tissues

Cells

Resident Cells1. Fibroblasts: most common; large and usually star-shaped; produce fibers by secreting proteins into the matrix around them2. Mast Cells: large and widely distributed; located near blood vessels; release heparin, which prevents blood clotting, and histamine, which promotes reactions associated with inflammation and allergies

Wandering Cells1. Macrophages: almost as numerous as fibroblasts; specialized to carry on phagocytosis; move about and function as scavengers and defense cells that clear foreign particles from tissue

Types of Connective Tissue

1. Embryonic Connective Tissue: Mesenchyme

a. Mesenchymal Tissue: first definitive tissue formed from the mesoderm germ layer; arises during

the early weeks of development and eventually differentiates into all other connective tissue; star-shaped cells and a fluid ground substance containing fine fibrilsb. Mucous Connective Tissue: temporary tissue; derived from mesenchyme and similar to it; appears In the fetus in very limited amounts (ex. Wharton’s jelly, which supports the umbilical cord)

Types of Connective Tissue

2. Connective Tissue Proper: Loose Fibrous

Areolar: soft and pliable; serves as a type of universal packing material between other tissues; the most widely distributed in the body; separates muscles, wraps small blood vessels and nerves, surrounds glands, and forms the subcutaneous tissue; present in all mucous membranes

Loose FibrousAdipose: (fat); develops when

certain cells store fat in droplets within their cytoplasm and enlarge; when such cells are so numerous that they crowd other cell types, adipose tissue results; cushions joints and some organs (kidneys), insulates beneath the skin, stores energy in fat molecules; located beneath the the skin, inspaces between muscles, around kidneys, behind eyeballs, in certain abdominal membranes, on surface of the heart, and around certain joints

Loose Fibrous

Reticular: delicate network of interwoven reticular fibers; limited to certain sites such as: forms the stroma, or internal supporting framework of lymph nodes, spleen, bone marrow, and liver; some are fibroblast-like, others differentiate into phagocytic macrophages

Dense Fibrous

Regular: primarily parallel collagen fibers, a few elastin fibers, and major cell type is the fibroblast; located in the tendons, most ligaments, and aponeuroses (sheet-like tendon that separates muscle from muscle); attaches muscle to bone or to muscles, bone to bone; withstands great tensil stress when pulling force is applied in one direction

Dense Fibrous

Irregular: primarily irregularly arranged collagen fibers, some elastin, major cell type is fibroblast; located in the dermis of the skin, submucosa of the digestive tract, fibrous capsules of organs and of joints, fascia (white sheets that surround the muscle); able to withstand tension exerted in many directions; provides structural strength

Dense Fibrous

Elastic: predominant fiber type is elastin; located in the walls of the aorta, some parts of the trachea and bronchi, forms the vocal cords and the ligamenta flava connecting the vertebrae; yields easily to a pulling force or pressure and then recoils to its original length

Cartilage

partly rigid, partly flexible connective tissue; provides support frameworks and attachements; protects underlying tissues and forms structural models for many developing bones; composed of collagenous fibers embedded in a gel-like ground substance; chondrocytes (cartilage cells) occupy small chambers called lacunae and are completely surrounded by matrix; avascular and devoid of nerve fibers; different types of matrix distinguish three types of cartilage

Cartilage

Hyaline: most common type; very fine collagenous fibers in its matrix and looks somewhat like white plastic; found on the ends of bones in many joints, in the soft part of the nose, and in supporting rings of the respiratory passages; important in development of most bones

Cartilage

Fibrocartilage: very tough tissue containing many collagenous fibers; shock absorber for structures subjected to pressure; forms pads (intervertebral diswks) between the individual parts of the backbone, cushions bones in the knees and in the pelvic girdle

Cartilage

Elastic: dense network of elastic fibers; more flexible than hyaline cartilage; provides framework for the external ears and parts of the larynx

Bone (Osseous Tissue)

hard, calcified matrix containing many collagen fibers; very well vascularized; supports and protects, provides levers for muscles to act on, stores calcium and other minerals and fat, marrow inside bones is site for blood cell formation (hematopoiesis); contains osteocytes (bone forming cells)

Blood (Vascular Tissue)

red and white blood cells in a fluid matrix (plasma); contained within blood vessels; transport of respiratory gases, nutrients, wastes, and other substances

Muscle Tissue

• highly cellular

• well vascularized

• elongated shape

• specialized myofilaments, composed of contractile proteins actin and myosin

• responsible for most types of body movement

Muscle Tissue

Skeletal Muscle: long, cylindrical, multinucleated cells with obvious striations;m located in skeletal muscles attached to bones or occasionally to skin; voluntary control; functions in locomotion, manipulation of environment, facial expressions

Muscle Tissue

Cardiac Muscle: branching, striated generally uninucleated cells that fit together tightly at unique junctions called intercalated discs; located only in the walls of the heart; propels blood into circulation as it contracts; involuntary control

Muscle Tissue

Smooth Muscle: spindle-shaped cells with central nuclei; cells arranged closely to form sheets; no striations; mostly in the walls of hollow organs; propels substances or objects (foodstuff, urine, a baby) along internal passages; involuntary control

Nervous Tissue

makes up the nervous system: the brain, spinal cord, and nerves that conduct impulses to and from the various body organs; branching cells, cytoplasmic extensions, that may be quite long, extend from the nucleus-containing cell body; transmits electrical signals from sensory receptors and to effectors (muscles and glands) which control their activity; types of cells:

Nervous Tissuetypes of cells:

Neurons: highly specialized cells that generate and conduct nerve impulses; branching cells cytoplasmic extensions allow conduction of electrical impulses over substantial distances

Nervous Tissuetypes of cells:

Neuroglial Cells: nonconducting cells that support, insulate, and protect the delicate neurons

Tissue Repair

The skin and mucosae, the ciliary activity of epithelial cells lining the respiratory tract, and strong acid (chemical barrier) produced by stomach glands represent three defenses exerted at the local tissue level.

Tissue Repair

When injury does occur it stimulates the body’s inflammatory and immune responses.

Replacement of destroyed tissue by proliferation of the same kind of cells is called regeneration.

Tissue Repair

Proliferation of fibrous connective tissue, that is scar tissue, is called fibrosis.

Tissue Repair

Organization is the process during which the temporary blood clot is replaced by the ingrowth of granulation tissue (a delicate pink tissue composed of several elements:

1. extremely permeable capillaries bud from undamaged capillaries and enter the damaged area; laying down a new capillary bed

2. scattered macrophages and large, immature fibroblast synthesize new collagen fibers to permanently bridge the gap

3. macrophages digest and remove the original blood clot4. secretes bacterium-inhibiting substances5. granulation tissue becomes scar tissue