TISSUES

Tissues

• are collections of specialized cells and the extracellular substances surrounding them.

extracellular matrix

composition of the noncellular substances surrounding the cells

Histology

is the microscopic study of tissues. Much information about a person’s health can be gained by examining tissues.

Biopsy

• is the process of removing tissue samples from patients surgically or with a needle for diagnostic purposes.

Autopsy

 is an examination of the organs of a dead body to determine the cause of death or to study the changes caused by a disease.

Embryonic development

Approximately 13 or 14 days after fertilization, the embryonic stem cells that give rise to a new individual form a slightly elongated disk consisting of two layers, the epiblast and the hypoblast. 

Cells of the epiblast then migrate between the two layers to form the three embryonic germ layers: the ectoderm, the mesoderm, and the endoderm.

Epithelial Cells

Covers and protects surfaces, both outside and inside the body.

Simple Squamous Epithelium

• Structure: single layer of flat, often hexagonal cells; the nuclei appear as bumps when viewed as a cross section because the cells are so flat. 

• Function: diffusion, filtration, some secretion, and some protection against friction. 

• Location: lining of blood vessels and the heart, lymphatic vessels and small ducts, alveoli of the lungs,

Simple Cuboidal Epithelium

• Structure: Single layer of cube-shaped cells; some cells have microvilli (kidney tubules) or cilia (terminal bronchioles of the lungs. 

• Function: secretion and absorption by cells of the kidney tubules; movement of particles embedded in mucus out of the terminal bronchioles by ciliated cells. 

• Location: kidney tubules,

Simple Columnar Epithelium

structure : single layer of tall, narrow cells; some cells have cilia or microvilli.

function : movement of particles out of the bronchioles of the lungs by ciliated cells. Partially responsible for the movement of oocytes through the uterine tubes by ciliated cells.

location : glands and some ducts, bronchioles of the lungs, auditory tubes, uterus, uterine tubes, stomach, intestines, gallbladder, bile ducts, ventricles of the brain.

Stratified Squamous Epithelium

• Structure: multiple layer of cells that are cube-shaped in the basal layer and progressively flattened toward the surface; the epithelium can be nonkeratinized and keratinized. 

Keratinized

The cytoplasm of cells at the surface is replaced by a protein called ‘’keratin’’, and the cells are dead.

Non-Keratinized

the surface retain a nucleus and cytoplasm. 

Stratified Cuboidal Epithelium

• Structure: multiple layers of somewhat cube-shaped cells. 

• function : secretion, absorption, protection against infection. 

• Location: sweat gland ducts, ovarian follicular cells, salivary gland ducts. 

Stratified Columnar Epithelium 

• Function: multiple layers of cells with tall, thin cells resting on layers of more cube-shaped cells; the cells are ciliated in the larynx. 

• Function: protection and secretion. 

• Location: mammary glands ducts, larynx, a portion of male urethra. 

Pseudostratified Columnar Epithelium

• Structure: single layer of cells; some cells are tall and thin and reach the free surface, and others do not; the nuclei of these cells are at different levels and appear stratified. The cells are almost always ciliated and are associated with goblet cells that secrete mucus onto the free surface. 

Transitional Epithelium

• Structure: stratified cells that appear cube-shaped when the organ or tube is not stretched and squamous when the organ or tube is stretched by fluid. The number of layers also decreases on stretch. 

• Functions: accommodate fluctuations in the volume of fluid in organs or tubes; protect against the caustic effects of urine. 

Tight Junctions

Structure: fusion of adjacent cell membranes. 

Function: create an impermeable barrier. 

Location: epithelial tissues (intestines) 

Adherens Junctions

Structure: Cadherins linked to actin cytoskeleton. 

Function: Provide mechanical attachment between cells. 

Location: Epithelial and endothelial tissues.

Desmosomes

Structure: Cadherins linked to intermediate filaments. 

Function: provide mechanical strength, prevent cells from pulling apart. 

Location: Skin, Cardiac muscle

Hemidesmosomes

Structure: link cells to the extracellular matrix. 

Function: anchor cells to the basal lamina. 

Location: epithelial cells 

Gap Junctions

Structure: connexons (protein channels) 

Function: allow direct passage of ions, nutrients, and signals. 

Location: cardiac and smooth muscle tissues. 

Glands

are secretory organs. Many glands are composed primarily of epithelium, with a

supporting network of connective tissue.

Exocrine Glands

maintains an open contact with the epithelium from which it developed, a duct is present. Glands with ducts. 

Endocrine Glands

Glands become separated from the epithelium of their origin and have no ducts. 

Simple

Glands that have a single duct are called

Compound

glands with ducts that branch

Merocrine Gland

• Cells of the gland produce secretions by active transport or produce vesicles that contain secretory products, and the vesicles empty their contents into the duct through exocytosis. 

Apocrine Gland

Secretory products are stored in the cell near the lumen of the duct. A portion of the cell near the lumen containing secretory products is pinched off the cell and joins secretions produced by a merocine process.

Holocrine Gland

• Secretory products are stored in the cells of the gland. Entire cells are shed by the gland and become part of the secretion. The lost cells are replaced by other cells deeper in the gland. 

Connective Tissues

It is abundant—it makes up part of every organ in the body

Fibroblasts

• spindle-shaped cells with abundant cytoplasm

• produce extracellular matrix components

Macrophages

• Large, irregularly shaped cells with abundant lysosomes.

• phagocytosis of pathogens and cellular debris, immune response.

Mast Cells

Oval-shaped cells with granules containing histamine

release histamine and other mediators during allergic reactions.

Adipocytes

large cells with a central fat droplet.

Store energy as fat, insulate and cushion the body.

plasma cells

Oval cells with a prominent nucleus

produce antibodies for immune defense

Leukocytes

various shapes depending on type (lymphocytes, neutrophils)

immune response, fighting infections

chondrocytes

Rounded cells within lacunae in cartilage

produce and maintain the cartilaginous matrix

osteocytes

star-shaped cells located in lacunae in bone

maintain bone tissue, communicate with other bone cells

Mesenchymal cells

undifferentiated multipotent stem cells

differentiate into various connective tissue cell types

Ground Substance

Two types of large, nonfibrous molecules, called hyaluronic acid and proteoglycans, are part of the extracellular matrix.

These molecules constitute most of the ground substance of the matrix, the “shapeless” background against which the collagen fibers are seen through the microscope.

Hyaluronic acid

is a long, unbranched polysaccharide chain composed of repeating disaccharide units. It gives a very slippery quality to the fluids that contain it; for that reason, it is a good lubricant for joint

proteoglycan

monomer is a large molecule that consists of 80 to 100 polysaccharides, called glycosaminoglycans, such as chondroitin sulfate, each attached by one end to a protein core.

Adhesive molecules

trap large quantities of water which allows them to return to their original shape when compressed or deformed

Areolar Connective Tissue

Structure: cells (fibroblast, macrophages, and lymphocytes) within a fine network of mostly collagen fibers; often merges with denser connective tissues. 

Function: loose packing, support, and nourishment for the structure with which it is associated. 

location : widely distributed throughout the body; substance on which epithelial basement membranes rest. 

Adipose Tissue

Structure: little extracellular matrix surrounding cells; the adipocytes are so full of lipids that the cytoplasm is pushed to the periphery of the cell. 

Function: packing material, thermal insulation, energy storage, and protection of organs against injury from being bumped or jarred. 

Location: predominantly is subcutaneous areas, in mesenteries, in renal pelvis around kidneys, attached to the surface of the colon, in mammary glands.

Reticular Tissue

Structure: fine network of reticular fibers irregularly arranged. 

Function: provides a superstructure for lymphatic and hemopoietic tissues. 

Location: within the lymph nodes, spleen, bone marrow 

Dense Regular Collagenous Connective Tissue

Structure: matrix is composed of collagen fibers running in somewhat the same direction. 

Function: able to withstand great pulling forces exerted in the direction of fiber orientation, great tensile strength and stretch  resistance. 

Location: tendons (muscle to bone) and ligaments (bone to bone). 

Dense Regular Elastic Connective Tissue

Structure: matrix composed of regularly arranged collagen fibers and elastic fibers. 

Function: able to stretch and recoil like a rubber band, with strength in the direction of fiber orientation. 

Location: vocal folds and elastic ligaments between the vertebrae and along the dorsal aspect of the neck. 

Dense Irregular Collagenous Connective Tissue

Structure: matrix is composed of collagen fibers that run in all directions or in alternating planes of fibers orientated in a somewhat single direction. 

Function: tensile strength capable of withstanding stretching in all directions. 

Location: sheaths; most of the skin; organ capsules and septa; outer covering of the body tubes. 

Dense Irregular Elastic Connective Tissue

Structure: matrix composed of bundles and sheets of collagenous and elastic fibers oriented in multiple directions.

Functions: capable of strength, with stretching and recoil in several directions.

Location: elastic arteries.

Hyaline Cartilage

Structure: collagen fibers are small and evenly dispersed in the matrix, making the matrix appear transparent; the cartilage cells, or chondrocytes, are found in spaces, or lacunae, within the firm but flexible matrix.

Function: allows growth of long bones; provides rigidity with some flexibility in the trachea, bronchi, ribs, and nose.

Location: Growing long bones, cartilage rings of the respiratory system, costal cartilage of ribs, nasal cartilages, articulating surface of bones, embryoning skeleton.

Fibrocartilage

Structure: collagen fibers similar to those in hyaline cartilage; the fibers are more numerous than in other cartilages and are arranged in thick bundles.

Function: somewhat flexible and capable of withstanding considerable pressure; connects structures subjected to great pressure.

Location: intervertebral disks, symphysis pubis articular disks.

Elastic Cartilage

Structure: similar to hyaline cartilage, but matrix also contains elastic fibers.

Function: provides rigidity with even more flexibility than hyaline cartilage because elastic fibers return to their original shape after being stretched.

Location: external ears, epiglottis, auditory tubes.

Spongy Bones

structure : latticelike network of scaffolding characterized by trabeculae with large spaces between them filled with hemopoietic tissue; the osteocytes, or bone cells are located within lacunae in the trabeculae. 

Function: acts as scaffolding to provide strength and support without greater weight of compact bone. 

Location: in the interior of the bones of the skull, vertebrae, sternum, and pelvis; in the ends of the long bones. 

Compact Bone

Structure: hard, bony matrix predominates; many osteocytes are located within lacunae that are distributed in a circular fashion around the central canals; small passageways connect adjacent lacunae. 

Function: provides great strength and support; forms a solid outer shell on bones that keeps them from being easily broken or punctured. 

Location: outer portions of all bones, the shafts of long bones. 

Blood

Function: transport oxygen, carbon dioxide, hormones, waste products, and other substances; protects the body from infections and is involved in temperature regulation. 

Location: within the blood vessels; white blood cells frequently leave the blood vessels and enter the interstitial spaces. 

Bone marrow

Structure: reticular framework with numerous blood-forming cells (red marrow) 

Function: produces new blood cells (red marrow) and stores lipids (yellow marrow). 

Location: within marrow cavities of bone; two types: 

• red marrow (hemopoietic, or blood forming tissue) in the ends of long bones and in short, flat, and irregularly shaped bones. 

• Yellow Marrow: mostly adipose tissue, in the shafts of long bones. 

Muscle Tissues

it contracts, or shortens, with a force and therefore is responsible for movement.

Skeletal Muscle

Structure: skeletal muscle cells or fibers appear striated (banded); cells are large, long, and cylindrical, with many nuclei located at the periphery. 

Function: moves the body; is under voluntary (conscious) control. 

Location: attached to the bone or other connective tissue. 

Cardiac Muscle

Structure: are cylindrical and striated and have a single, centrally located nucleus; they are branched and connected to one another by intercalated disks, which contain gap junctions. 

Functions: pumps the blood; is under involuntary control 

Location: in the heart. 

Smooth Muscle

Structure: smooth muscle cells are tapered at each end, are not striated, and have a single nucleus. 

Function: regulates the size of organs, forces fluid through tubes, controls the amount of light entering the eye, and produces ‘’goose flesh’’ in the skin; is under involuntary control. 

Location: in hollow organs, such as the stomach and small and large intestines. 

Nervous Tissue

is found in the brain, spinal cord, and nerves and is characterized by the ability to conduct electrical signals called

action potentials.

Neurons

are the conducting cells of nervous tissue. Just as an electrical wiring system transports electricity throughout a house, neurons transport electrical signals throughout the body

Dendrites

• usually receive action potentials. They are much shorter than axons and have multiple branches at their ends.

Axon

• usually conducts action potentials away from the cell body. Axons can be much longer than dendrites, and they have a constant diameter along their entire length

Neuroglia

are the support cells of the brain, spinal cord, and peripheral nerves.

Multipolar Neuron

Structure: the neuron consists of dendrites, a cell body, and a long axon, neuroglia, or support cells, surrounding the neurons. 

Function: neurons transmit information in the form of action potentials, store ‘’information’’, and integrate and evaluate data; neuroglia support, protect, and form specialized sheaths around axons. 

Location: in the brain, spinal cord, ganglia. 

Pseudo-Unipolar Neuron 

Structure: the neuron consists of a cell body with one axon. 

function : conducts action potentials from the periphery to the brain or spinal cord. 

Location: in ganglia outside the brain and spinal cord. 

Mucous Membrane

• epithelial layer over a connective tissue layer

• secretes mucus, protects, and lubricates surfaces.

Serous Membrane

• simple squamous epithelium over connective tissue

• produces serous fluid, reduces friction between organs.

Cutaneous Membrane

• stratified squamous epithelium over connective tissue

• protects body from external environment, prevents water loss.

Synovial Membrane

Connective tissue layer, lacks epithelium

secretes synovial fluid, lubricates and nourishes joint surfaces.