TISSUES
A tissue is a group of cells that usually have a common embryonic origin and function together to carry
out specialized activities.
Histology (histo= tissue; -logy= study of) is the science that deals with the study of tissues.
A pathologist is a physician who specializes in laboratory studies of cells and tissues to help other
physicians make accurate diagnoses.
Types of Tissues in Body:
Body tissues can be classified into four basic types according to structure and function:
1. Epithelial tissue covers body surfaces and lines hollow organs, body cavities, and ducts. It also forms
glands.
2. Connective tissue protects and supports the body and its organs. Various types of connective tissue
bind organs together, store energy reserves as fat, and help provide immunity to disease-causing
organisms.
3. Muscular tissue generates the physical force needed to make body structures move and generates
body heat.
4. Nervous tissue detects changes in a variety of conditions inside and outside the body and responds by
generating action potentials (nerve impulses) that activate muscular contractions and glandular
secretions.
1. EPITHELIAL TISSUE:
An epithelial tissue or epithelium (plural is epithelia) consists of cells arranged in continuous sheets, in
either single or multiple layers. Because the cells are closely packed and are held tightly together by
many cell junctions, there is little intercellular space between adjacent plasma membranes.
Epithelial tissue forms coverings and linings throughout the body. It is never covered by another tissue,
so it always has a free surface.
Epithelial tissues have three major functions:
➢ Selective barriers that limit or aid the transfer of substances into and out of the body;
➢ Secretory surfaces that release products produced by the cells onto their free surfaces
➢ Protective surfaces that resist the abrasive influences of the environment.
1
,Structure of Epithelial Tissue:
➢ The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen
(interior space) of an internal organ, or a tubular duct that receives cell secretions. Apical surfaces
may contain cilia or microvilli.
➢ The lateral surfaces of an epithelial cell face the adjacent cells on either side.
➢ The basal surface of an epithelial cell is opposite the apical surface, and the basal surfaces of the
deepest layer of cells adhere to extracellular materials.
➢ The basement membrane is a thin extracellular layer that commonly consists of two layers, the
basal lamina and reticular lamina.
➢ The basal lamina (thin layer) is closer to and secreted by the epithelial cells. It contains proteins
such as laminin and collagen, as well as glycoproteins and proteoglycans.
➢ The reticular lamina is closer to the underlying connective tissue and contains proteins such as
collagen produced by connective tissue cells called fibroblasts.
➢ The basement membrane functions as a point of attachment and support for the overlying
epithelial tissue.
➢ Epithelial tissue has its own nerve supply, but is avascular; it lacks its own blood supply.
Epithelial tissue may be divided into two types.
(1) Covering and lining epithelium forms the outer covering of the skin and some internal organs. It
also forms the inner lining of blood vessels, ducts, and body cavities, and the interior of the respiratory,
digestive, urinary, and reproductive systems.
(2) Glandular epithelium makes up the secreting portion of glands such as the thyroid gland, adrenal
glands, and sweat glands.
2
, ❖ Covering and lining epithelium:
The types of covering and lining epithelial tissue are classified according to two characteristics: the
arrangement of cells into layers & the shapes of the cells.
Arrangement of cells in layers: The cells are arranged in one or more layers depending on the
functions the epithelium performs:
a. Simple epithelium is a single layer of cells that functions in diffusion, osmosis, filtration, secretion, or
absorption. Secretion is the production and release of substances such as mucus, sweat, or enzymes.
Absorption is the intake of fluids or other substances such as digested food from the intestinal tract.
b. Pseudostratified epithelium (pseudo- false) appears to have multiple layers of cells because the cell
nuclei lie at different levels and not all cells reach the apical surface. Cells that do extend to the apical
surface may contain cilia; others (goblet cells) secrete mucus. Pseudostratified epithelium is actually a
simple epithelium because all its cells rest on the basement membrane.
c. Stratified epithelium consists of two or more layers of cells that protect underlying tissues in
locations where there is considerable wear and tear.
Cell shapes: The cells vary in shape depending on their function.
a. Squamous cells are arranged like floor tiles and are thin, which allows for the rapid passage of
substances.
b. Cuboidal cells are as tall as they are wide and are shaped like cubes or hexagons. They may have
microvilli at their apical surface and function in either secretion or absorption.
c. Columnar cells are much taller than they are wide, like columns, and protect underlying tissues. Their
apical surfaces may have cilia or microvilli, and they often are specialized for secretion and absorption.
d. Transitional cells change shape, from flat to cuboidal and back, as organs such as the urinary bladder
stretch (distend) to a larger size and then collapse to a smaller size.
3
A tissue is a group of cells that usually have a common embryonic origin and function together to carry
out specialized activities.
Histology (histo= tissue; -logy= study of) is the science that deals with the study of tissues.
A pathologist is a physician who specializes in laboratory studies of cells and tissues to help other
physicians make accurate diagnoses.
Types of Tissues in Body:
Body tissues can be classified into four basic types according to structure and function:
1. Epithelial tissue covers body surfaces and lines hollow organs, body cavities, and ducts. It also forms
glands.
2. Connective tissue protects and supports the body and its organs. Various types of connective tissue
bind organs together, store energy reserves as fat, and help provide immunity to disease-causing
organisms.
3. Muscular tissue generates the physical force needed to make body structures move and generates
body heat.
4. Nervous tissue detects changes in a variety of conditions inside and outside the body and responds by
generating action potentials (nerve impulses) that activate muscular contractions and glandular
secretions.
1. EPITHELIAL TISSUE:
An epithelial tissue or epithelium (plural is epithelia) consists of cells arranged in continuous sheets, in
either single or multiple layers. Because the cells are closely packed and are held tightly together by
many cell junctions, there is little intercellular space between adjacent plasma membranes.
Epithelial tissue forms coverings and linings throughout the body. It is never covered by another tissue,
so it always has a free surface.
Epithelial tissues have three major functions:
➢ Selective barriers that limit or aid the transfer of substances into and out of the body;
➢ Secretory surfaces that release products produced by the cells onto their free surfaces
➢ Protective surfaces that resist the abrasive influences of the environment.
1
,Structure of Epithelial Tissue:
➢ The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen
(interior space) of an internal organ, or a tubular duct that receives cell secretions. Apical surfaces
may contain cilia or microvilli.
➢ The lateral surfaces of an epithelial cell face the adjacent cells on either side.
➢ The basal surface of an epithelial cell is opposite the apical surface, and the basal surfaces of the
deepest layer of cells adhere to extracellular materials.
➢ The basement membrane is a thin extracellular layer that commonly consists of two layers, the
basal lamina and reticular lamina.
➢ The basal lamina (thin layer) is closer to and secreted by the epithelial cells. It contains proteins
such as laminin and collagen, as well as glycoproteins and proteoglycans.
➢ The reticular lamina is closer to the underlying connective tissue and contains proteins such as
collagen produced by connective tissue cells called fibroblasts.
➢ The basement membrane functions as a point of attachment and support for the overlying
epithelial tissue.
➢ Epithelial tissue has its own nerve supply, but is avascular; it lacks its own blood supply.
Epithelial tissue may be divided into two types.
(1) Covering and lining epithelium forms the outer covering of the skin and some internal organs. It
also forms the inner lining of blood vessels, ducts, and body cavities, and the interior of the respiratory,
digestive, urinary, and reproductive systems.
(2) Glandular epithelium makes up the secreting portion of glands such as the thyroid gland, adrenal
glands, and sweat glands.
2
, ❖ Covering and lining epithelium:
The types of covering and lining epithelial tissue are classified according to two characteristics: the
arrangement of cells into layers & the shapes of the cells.
Arrangement of cells in layers: The cells are arranged in one or more layers depending on the
functions the epithelium performs:
a. Simple epithelium is a single layer of cells that functions in diffusion, osmosis, filtration, secretion, or
absorption. Secretion is the production and release of substances such as mucus, sweat, or enzymes.
Absorption is the intake of fluids or other substances such as digested food from the intestinal tract.
b. Pseudostratified epithelium (pseudo- false) appears to have multiple layers of cells because the cell
nuclei lie at different levels and not all cells reach the apical surface. Cells that do extend to the apical
surface may contain cilia; others (goblet cells) secrete mucus. Pseudostratified epithelium is actually a
simple epithelium because all its cells rest on the basement membrane.
c. Stratified epithelium consists of two or more layers of cells that protect underlying tissues in
locations where there is considerable wear and tear.
Cell shapes: The cells vary in shape depending on their function.
a. Squamous cells are arranged like floor tiles and are thin, which allows for the rapid passage of
substances.
b. Cuboidal cells are as tall as they are wide and are shaped like cubes or hexagons. They may have
microvilli at their apical surface and function in either secretion or absorption.
c. Columnar cells are much taller than they are wide, like columns, and protect underlying tissues. Their
apical surfaces may have cilia or microvilli, and they often are specialized for secretion and absorption.
d. Transitional cells change shape, from flat to cuboidal and back, as organs such as the urinary bladder
stretch (distend) to a larger size and then collapse to a smaller size.
3
