Two types of EPITHELIAL TISSUE (EPITHELIUM)
: 1) covering and lining epithelium, and 2) glandular epithelium. COVERING AND LINING EPITHELIUM forms the outer layer of the skin
and the outer covering of some internal organs. It lines body cavities (for example, the abdominal cavity), the inside walls of the hollow organs of the digestive,
respiratory, urinary and reproductive tracts, and the inside walls of ducts and blood vessels. GLANDULAR EPITHELIUM makes up the secretory portion of
CHARACTERISTICS OF EPITHELIUM
1. Epithelium consists of closely packed cells that are in continuous sheets arranged in single or multiple layers. The only extracellular material is a thin layer of
INTERSTITIAL FLUID between the cells.
2. Epithelium is supported by underlying connective tissue, which holds the epithelium in place and keeps it from being stretched or torn. The attachment between the epithelium and connective tissue is an extracellular protein structure called the BASEMENT MEMBRANE. The basement membrane reinforces the sheet of epithelium tissue and holds it in place over the connective tissue.
3. The epithelium always has one free, exposed surface called the APICAL SURFACE. Sometimes cilia or microvilli are found at the apical surface of epithelial cells. The epithelium also has a BASAL SURFACE that is attached to the basement membrane.
4. Epithelium is AVASCULAR (it has no blood vessels). The epithelial cells get their oxygen and nutrients from blood vessels in the underlying connective tissue.
5. Epithelium is INNERVATED (it has a nerve supply).
6. Epithelium has a high regenerative capacity, since its apical surface is often exposed to a certain amount of wear and tear (friction, bacteria, acids, smoke,
CLASSIFICATION OF COVERING AND LINING EPITHELIUM
Epithelial cells fit closely together in continuous sheets that are either in a single layer or are multi-layered. The covering and lining epithelia are classified
1. Epithelium that is composed of a single layer of cells is called SIMPLE EPITHELIUM.
2. STRATIFIED EPITHELIUM consists of 2 or more cell layers stacked on top of each other.
The covering and lining epithelia are also classified according to the shape of the cells. Notice that the shape of the nucleus conforms to that of the cell.
1. Epithelial cells that are flattened and scale-like are called SQUAMOUS cells. The nucleus of a squamous cell is disc-shaped.
2. Epithelial cells that are as tall as they are wide are called CUBOIDAL. The nucleus of a cuboidal cell is spherical.
3. COLUMNAR cells are column-shaped & look like a rectangle set on its end. A columnar cell nucleus is elongated from top to bottom and is usually located close
to the cell base.
Simple epithelium is a thin tissue, so it is usually found in areas where there is minimal abrasion (wear and tear). SIMPLE EPITHELIUM is usually involved in the functions of ABSORPTION, SECRETION, and FILTRATION. SECRETION is the production and release of a useful substance by cells. An example would be
mucus secreted by epithelial cells that line the respiratory, digestive, urinary, and reproductive tracts. ABSORPTION is the uptake of useful substances by cells. For example, epithelial cells that line the digestive tract absorb digested nutrients and then pass those nutrients into the bloodstream. FILTRATION refers to the
movement of a fluid through a filter to produce a filtrate. An example of filtration occurs in the kidneys, where blood is filtered to produce urine (the filtrate).
Urine contains excess water, excess salts, and waste products that have been filtered from the blood.
SIMPLE SQUAMOUS EPITHELIUM
is a single layer of flat cells. It resembles a tile floor and is the thinnest kind of epithelium. A good example of
simple squamous epithelium is in the walls of the lung's air sacs where gas exchange occurs between the lungs and the blood.
SIMPLE COLUMNAR EPITHELIUM
is a single layer of tall, rectangular cells, lined up like soldiers in a row. There are 2 types of simple columnar
epithelium: ciliated and nonciliated (without cilia).
NONCILIATED SIMPLE COLUMNAR EPITHELIUM
lines the inside wall of the stomach and intestines, and is involved in the SECRETION of
digestive enzymes and the ABSORPTION of nutrients. Interspersed among the columnar cells are modified columnar cells called GOBLET CELLS
that secrete mucus. Notice in the diagram the MICROVILLI at the apical surface of the epithelial cells. What is their purpose?
CILIATED SIMPLE COLUMNAR EPITHELIUM
The Fallopian tubes in women are lined with CILIATED SIMPLE COLUMNAR EPITHELIUM. The ovulated egg moves into a Fallopian tube, where
the cilia wave in unison, propelling the egg toward the uterus. Notice the presence of mucus-secreting goblet cells.
PSEUDOSTRATIFIED COLUMNAR EPITHELIUM
consists of one layer of columnar cells, though the tissue appears multilayered when
observed under the microscope. All of its columnar cells are attached at their bases to the basement membrane, but some cells are shorter than others. Also, the nuclei of the cells are at various levels. All of this gives the appearance of multi-layers, when there is really only one layer. Pseudostratified
columnar epithelium lines most of the upper respiratory tract. Here it is CILIATED and contains GOBLET CELLS, which produce mucus. Inhaled
foreign particles are trapped in the mucus. The cilia wave in unison, propelling the mucus to the throat, where it is swallowed. This keeps dust, etc. from
entering the lungs
consists of at least 2 layers of cells. It is typically found in high abrasion areas, where PROTECTION is important. Since it is
multilayered, it is more durable, can withstand abrasion, and can protect underlying tissues from wear and tear.
STRATIFIED SQUAMOUS EPITHELIUM
is the most widespread epithelium. It is composed of several layers of cells, which makes it very durable.
The basal layer of cells are columnar, while the apical cells are flat, squamous cells. Since this epithelium is found in places where the most wear and tear occur, the outer apical cells are constantly being rubbed away and replaced from below by cell division of the basal columnar cells.
STRATIFIED SQUAMOUS EPITHELIUM forms the outer layer (epidermis) of the skin. It also covers the tongue and lines the mouth, throat,
esophagus, anus, & vagina. It protects underlying tissues from abrasion, dehydration, chemicals, and bacterial invasion.
STRATIFIED CUBOIDAL EPITHELIUM
is a relatively uncommon type of epithelium. It is found in the ducts of sweat glands, mammary glands and salivary glands. It consists of 2 or more layers. Its function is protection of underlying tissues from glandular secretions.
STRATIFIED COLUMNAR EPITHELIUM
is also uncommon. Its deeper layers consist of small cells that vary in shape, but the apical cells are columnar. Its functions are protection and secretion
forms the lining of the urinary bladder. When the bladder is empty the cells in the apical layer are round and dome-like. When the bladder is filled with urine, the apical cells become flattened. The ability of transitional cells to change their shape allows the
bladder to be stretched by urine without rupturing.
Glandular epithelium is tissue that makes up the secretory portion of glands. The function of glandular epithelium is SECRETION. Secretions from glands include
hormones, digestive enzymes, mucus, sweat, oil, and milk. Glands are classified as EXOCRINE GLANDS or ENDOCRINE GLANDS.
secrete their products into ducts or tubes that empty onto a body surface or into a body cavity.
They include lacrimal (tear) glands, sweat and oil glands in the skin, salivary glands in the mouth, the liver (which secretes bile), the pancreas (which secretes
digestive enzymes), mammary glands (which secrete milk), and many others.
differ from exocrine glands in 2 ways: 1) Endocrine glands secrete their product into the bloodstream, not into
ducts; and 2) Endocrine glands secrete HORMONES. Hormones are "chemical messengers" that the blood carries to specific "target cells". The hormone signals
the target cell to perform a physiological response. For example, after a meal when blood glucose levels are high, the pancreas secretes insulin into the bloodstream.
Insulin stimulates the body's cells to take in glucose. This helps lower blood glucose levels back to normal.
Endocrine glands include the pituitary, thyroid, parathyroid, adrenal glands, pancreas, testes, and ovaries. Some glands perform both endocrine and exocrine
functions. For example, the pancreas is both an endocrine gland (secreting the hormone insulin into the bloodstream) and an exocrine gland (secreting digestive enzymes into the small intestine). Endocrine glands will be discussed later when studying the endocrine system.
Connective tissue is the most abundant and widespread tissue in the body. Examples of connective tissue include tendons, ligaments, fat, cartilage, blood, & bone.
Connective tissue PROTECTS, SUPPORTS and BINDS ORGANS TOGETHER. But you will soon learn that it has many other functions as well.
Unlike epithelial tissue, connective tissue is usually highly vascular (there are exceptions). And it usually has a nerve supply.
The connective tissue cells are usually widely scattered. The cells in connective tissue are separated by considerable extracellular material called the MATRIX. This
matrix consists of GROUND SUBSTANCE and PROTEIN FIBERS. The ground substance may be fluid, gel or solid, and contributes to the matrix consistency.
Read about the connective tissue matrix and the different kinds of ground substance in your book on page 71.
3 types of PROTEIN FIBERS are embedded in the EXTRACELLULAR MATRIX 1. COLLAGEN FIBERS 2. ELASTIC FIBERS 3. RETICULAR FIBERS
are made up of the fibrous protein COLLAGEN. Collagen fibers are extremely tough and provide high tensile strength (resistance to a
pulling force) to the tissue. In living tissue, they appear glistening white and are called "white fibers". Collagen fibers often occur in collagen bundles (see Fig. 3.6).
are formed from a rubber-like protein, ELASTIN. Elastic fibers are able to stretch and recoil like a rubber band, so they provide strength and
elasticity to connective tissue. The presence of elastin in the connective tissue matrix gives it a rubbery or resilient quality. Elastic fibers are found where greater
elasticity is needed, as in the skin and lungs. Living elastic fibers appear yellow and are called "yellow fibers".
consist of fine COLLAGEN fibers that form delicate branching fiber networks. They provide support and strength.
EMBRYONIC CONNECTIVE TISSUE
is found primarily in the embryo or fetus. An EMBRYO is the developing human from conception
through the first 2 months of pregnancy. The FETUS is the developing human from 3 months to birth,
is the embryonic connective tissue that develops into the "mature" connective tissues described below. Mesenchyme is composed of a semifluid ground substance that contains reticular fibers. Embedded in this matrix are star-shaped embryonic mesenchymal cells. Mesenchymal
cells are embryonic stem cells that arise during the early weeks of embryological development and eventually differentiate (specialize) into all other
connective tissue cells.
MATURE CONNECTIVE TISSUE
Mesenchyme develops into mature connective tissue, which is present in the newborn baby and on into adulthood. Mature connective tissue is subdivided into:
LOOSE CONNECTIVE TISSUE, DENSE CONNECTIVE TISSUE, CARTILAGE, BONE TISSUE, and VASCULAR (BLOOD) TISSUE.
AREOLAR CONNECTIVE TISSUE
is one of the most widespread connective tissues in the body. The extracellular matrix consists of a semifluid ground substance that contains all 3 fiber types. Within the matrix are several kinds of cells. The most abundant cells are the FIBROBLASTS, which are large, flat, spindle-shaped cells that produce the extracellular matrix. Also present are MACROPHAGES,
white blood cells (WBC's) that engulf bacteria and dead cell debris. Other types of WBC's (mast cells, plasma cells) are also found in areolar connective tissue. Areolar connective tissue is soft and pliable. It wraps and cushions organs, blood vessels and nerves. It also forms the subcutaneous layer
below the skin.
contains large numbers of ADIPOCYTES (fat cells), which develop from fibroblasts. Adipose tissue is found wherever areolar connective tissue is found. It accumulates in the subcutaneous layer below the skin, where it acts as a shock absorber and insulates against heat loss. Other sites of fat accumulation include the abdomen (primarily in men), hips and thighs (primarily in women),bone marrow, around the kidneys and heart, behind the eyeballs, and around joints. Adipose tissue is also a major energy reserve for the body.
RETICULAR CONNECTIVE TISSUE
consists of a network of interlacing reticular fibers. It helps form a delicate supporting framework (STROMA) for many soft organs, including the liver, spleen and lymph nodes.
DENSE CONNECTIVE TISSUE
is different from loose connective tissue because its fibers are more numerous and densely packed in its matrix,
and there are fewer fibroblasts and other cells. It includes DENSE REGULAR CONNECTIVE TISSUE, DENSE IRREGULAR CONNECTIVE
TISSUE, and ELASTIC CONNECTIVE TISSUE.
DENSE REGULAR CONNECTIVE TISSUE
contains bundles of collagen fibers arranged parallel to each other. This allows the connective tissue to resist tension (pulling) in one direction. Dense regular connective tissue makes up the tendons, which are cords that attach muscles to bones. It also makes up most ligaments, which are cords that attach bones together at the joints.
DENSE IRREGULAR CONNECTIVE TISSUE
contains randomly arranged collagen fibers and it resists tension in all directions. It is found in the dermis (deep layer) of the skin. It also forms fibrous capsules around joints and organs such as the kidneys, testes, liver, and
ELASTIC CONNECTIVE TISSUE
contains mostly elastic fibers. This tissue can be stretched and it will snap back in place. It forms the vocal cords, ligaments between vertebrae, and is found in lung tissue.
While dense connective tissues resist tension (pulling forces), CARTILAGE is able to resist COMPRESSION (pushing forces). It is a firm, but flexible tissue that consists of a dense network of collagen and elastic fibers embedded in a rubbery ground substance (chondroitin sulfate). The collagen gives cartilage its strength, while the chondroitin sulfate gives cartilage resilience (ability to return to its original shape after being compressed). The cells found in mature cartilage are called CHONDROCYTES. The chondrocytes live in spaces within the firm matrix called LACUNAE. Cartilage is AVASCULAR (without blood vessels) and has no nerves. There are 3 types of cartilage:
also known as gristle, is a bluish-white, shiny cartilage. It is the most abundant cartilage found in the body. It covers the end of long bones (leg bones), forms the tip of the nose and the cartilage of the ribs. Most of the embryonic skeleton is hyalinecartilage. Hyaline cartilage functions in providing flexibility and support. In joints, it absorbs shock and reduces friction between bones.
contains thick bundles of collagen fibers that give this cartilage a fibrous appearance. It is a durable tissue adapted to withstand more compression than hyaline cartilage. It is found in high pressure areas of the body. For example, it forms the intervertebral discs, which provide resilient cushions between the vertebrae (bones of the back).
has elastic fibers in it, so it has a yellowish color. It maintains the shape of structures while allowing elasticity. It is the cartilage found in the outer ear.
BONE (OSSEOUS) TISSUE
is hard connective tissue. The extracellular matrix contains deposits of inorganic mineral salts (mostly calcium
phosphate and calcium carbonate) and collagen fibers. The cells that are found in bone tissue are called OSTEOCYTES. The osteocytes reside in small spaces (LACUNAE) within the hard matrix of the bone. Unlike cartilage, bone is VASCULAR and has a nerve supply.
One function of bone is to support and protect softer internal tissues of the body. Bones also act as levers that muscles pull on to create movement.
BLOOD (VASCULAR) TISSUE
is a liquid connective tissue. It consists of a liquid matrix called PLASMA. Suspended in the matrix are the
FORMED ELEMENTS, which are the red blood cells, white blood cells and platelets. Red blood cells function in oxygen transport to body cells, white blood cells function in fighting disease and platelets are involved in blood clotting. Another liquid connective tissue is LYMPH, which is found in lymphatic vessels. We will go into more detail about blood and lymph when we study the cardiovascular & lymphatic systems.
An EPITHELIAL MEMBRANE consists of an epithelial tissue layer and an underlying connective tissue layer. The 3 primary types of epithelial tissue membrane are the MUCOUS, SEROUS, and CUTANEOUS MEMBRANES.
MUCOUS MEMBRANES (MUCOSA)
line body cavities that open directly to the outside of the body. These include the digestive, respiratory, urinary, and reproductive tracts. Mucosa consists of various kinds of epithelial tissue over a layer of AREOLAR CONNECTIVE TISSUE. Mucous membranes are moistened by MUCUS secreted by GOBLET CELLS.
SEROUS MEMBRANES (SEROSA)
line closed body cavities and cover the outside surface of organs in these cavities. The serous membrane consists of SIMPLE SQUAMOUS EPITHELIUM over a thin layer of AREOLAR CONNECTIVE TISSUE. Serous membranes are kept moistened by SEROUS FLUID.
The CUTANEOUS MEMBRANE
is the skin (FIG. 5.1, page 122). It consists of STRATIFIED SQUAMOUS EPITHELIUM (called the EPIDERMIS) firmly attached to a thick connective tissue layer (called the DERMIS). The dermis is mostly DENSE IRREGULAR CONNECTIVE TISSUE. Unlike other epithelial membranes, the cutaneous membrane is exposed to air, so it is a dry membrane. We'll discuss it later when studying the integumentary system.
lines the cavities of freely-movable joints (like the knee joint).
Synovial membranes consist of AREOLAR CONNECTIVE TISSUE that contains elastic fibers and some ADIPOSE TISSUE. Synovial membranes secrete synovial fluid, which lubricates the cartilage at joints.
Muscle tissue consists of cells (called MUSCLE FIBERS) that are contractile. When the muscle cell contracts, it shortens its length, producing motion. Muscle tissue is classified into 3 types:
SKELETAL MUSCLE TISSUE
is attached to bones and forms the "flesh" of the body. As muscles contract, they pull on bones, making them move.
Skeletal muscle is a voluntary type of muscle tissue.
CARDIAC MUSCLE TISSUE
makes up the heart wall. Contractions of the cardiac muscle causes the heart to beat, which propels blood through the
blood vessels. It is an involuntary type of muscle tissue.
SMOOTH MUSCLE TISSUE
is found in the walls of blood vessels, digestive organs, urinary organs, and reproductive organs. Smooth muscle is
involuntary, and it is used to propel substances through these organs. For example, rhythmic contractions of smooth muscle in the walls of the digestive tract propel food from one end of the digestive tract to the other.
Nervous tissue makes up the tissue of the brain, spinal cord, and nerves. It consists of specialized cells called NEURONS and NEUROGLIA. NEURONS are highly specialized nerve cells that generate and conduct electrical nerve impulses to other cells. They provide our brain with sensory information and allow us to stimulate muscle cells to contract. NEUROGLIA are cells that support and protect the neurons.