Chapter 5 - Tissues
Introduction and Epithelial Tissues
Background
• Tissues are layers or groups of similar cells
that have a common function.
• Tissues can be identified by variations in cell
size, shape, organization, and function.
• The study of tissues is called histology.
Types of Tissues
• The human body has 4 major types of tissues:
– Epithelial
– Connective
– Muscle
– Nervous
• These four types of tissue comprise organs.
Types of Tissues
Epithelial Tissues
• Found throughout the body.
– Cover the surface of the body
– Cover organs
– Form the inner lining of body cavities
– Line hollow organs
Epithelial Tissues
• Structure
– Always have a free (or apical) surface
• One side of the tissue is exposed to the outside or to an
open space on the inside
– The other surface (underside) is anchored to
connective tissue by a thin, nonliving layer call the
basement membrane.
– Lack blood vessels (avascular)
• Nutrients diffuse into the epithelial tissue from the
underlying connective tissue
• Cells are tightly packed, readily divide, and often have
desmosomes
Epithelial Tissues
• Functions
– Protection
• Cells divide easily to rapidly heal injuries
• Cells are tightly packed (often connected by
desmosomes…see p. 80) which allow the tissue to form
a protective barrier
– Secretion
– Absorption
– Excretion
Epithelial Tissues
• Classification
– Classified according to the shape and number of layers
of cells
• Squamous
– Composed of thin, flattened cells
• Cuboidal
– Composed of cube-like cells
• Columnar
– Composed of elongated cells
• Simple
– Composed of a single layer of cells
• Stratified
– Composed of two or more layers of cells
Epithelial Tissues
Simple Squamous Epithelium
• Consists of a single layer of thin, flattened cells
that fit tightly together and usually have broad
and thin nuclei.
• Found at sites of diffusion and filtration since
substances can easily pass through the tissue.
– Location examples: lines the air sacs (alveoli) of the
lungs and the insides of blood and lymph vessels,
forms the walls of capillaries, covers the membranes
that line body cavities
• Easily damaged because it is so thin.
Simple Squamous Epithelium
Simple Cuboidal Epithelium
• Consists of a single layer of cube-shaped cells
that usually have centrally located, spherical
nuclei.
• Locations:
– Lines the follicles of the thyroid gland, lines the kidney
tubules, lines the ducts of certain glands (such as the
salivary glands, pancreas, and liver), and covers the
ovaries.
• Simple cuboidal epithelium functions in tubular
secretion and reabsorption in the kidneys and
secreted glandular products in the glands.
Simple Cuboidal Epithelium
Simple Columnar Epithelium
• Consists of a single layer of elongated cells whose nuclei are
usually near the basement membrane.
• Some cells of simple columnar epithelium can be covered with
cilia or microvilli.
– Cilia are 7-10 µm in length and move constantly to aid in movement of
particles (such as an egg through the uterine tubes)
– Microvilli (tiny, cylindrical processes) are 0.5 to 1.0 µm in length and
increase the surface area of the cell membrane to aid in absorption
• Nonciliated simple columnar epitheleum lines the uterus and
portions of the digestive tract (such as stomach and small and
large intestines).
– This tissue functions as a protective layer (due to its elongated cells
which make the tissue thick), a secretor of digestive fluids, and an
absorber of nutrients from digested food
• Flask-shaped glandular cells called goblet cells are also scattered
among the simple columnar epithelium cells
– These cells secrete a protective fluid (mucus) onto the free surface of
the tissue.
Simple Columnar Epithelium
Microvilli
Pseudostratified Columnar Epithelium
• Consists of cells that appear to be layered (but
are really not) because the nuclei are at two or
more layers in a row of aligned cells.
• Cells have cilia which sweep away mucus that is
secreted from the goblet cells that are scattered
throughout the tissue.
– Psuedostratified columnar epithelium lines the
passages of the respiratory system, allowing the dust
and microorganisms that enter with the air to be
trapped in the mucus and swept out of the airways by
the cilia.
Pseudostratified Columnar Epithelium
Stratified Squamous Epithelium
• Consists of many layers of cells which make the tissue
thick.
• Cells nearest the free surface are flattened whereas
deeper cells are cuboidal or columnar.
– This occurs because the deeper cells divide and then are
pushed outward where they become flattened.
• Locations:
– Epidermis (outer layer of the skin)
• In these layers the cells obtain a protein called keratin as they age
and are pushed outward.
• This process (called keratonization) results in dead cells that serve
as a dry, tough, protective layer that prevents water and other
materials from escaping from underlying tissues and blocks
chemicals and microorganisms from entering.
– Oral cavity, esophagus, vagina, and anal canal
• In these tissues, the outer cells remain living, keeping the tissue
soft and moist.
Stratified Squamous Epithelium
Stratified Cuboidal Epithelium
• Consists of two or three layers of cuboidal cells
that form the lining of a lumen (which is a space
within a tubular structure such as a blood vessel
or intestine).
– The layers provide more protection than a single layer.
• Locations:
– Lines the larger ducts of the mammary glands, sweat
glands, salivary glands, and pancreas as well as lines
developing ovarian follicles and seminiferous tubules.
Stratified Cuboidal Epithelium
Stratified Columnar Epithelium
• Consists of several layers of cells.
– The cells at the surface are elongated whereas the
cells at the base are cube-shaped.
• Found in part of the male urethra and parts of
the pharynx.
Stratified Columnar Epithelium
Transitional Epithelium
• Consists of tissue that is specialized to change in
response to increases in tension.
• Lines the urinary bladder, the ureters, and part of the
urethra.
– When the wall of one of these organs contracts, the
transitional epithelium consists of several layers of
cuboidal cells.
– When one of these organs is distended, the tissue
stretches and appears to contain only a few layers of cells.
• Along with providing an expandable lining, transitional
epithelium also forms a barrier that helps to prevent
the contents of the urinary tract from diffusing back
into the internal environment.
Transitional Epithelium
Glandular Epithelium
• Consists of cells that are specialized to
produce and secrete substances into ducts or
body fluids.
• Cells are found within columnar or cuboidal
epithelium and constitute a gland.
– Exocrine glands secrete their products into ducts
that open onto surfaces (such as skin or the lining
of the digestive tract). (Have ducts)
– Endocrine glands secrete their products into tissue
fluid or blood. (Do not have ducts)
Glandular Epithelium
• Structures of exocrine glands:
– Unicellular glands
• Consist of a single epithelial cell (such as a mucous-secreting
goblet cell)
– Multicellular glands
• Consist of many cells and can be divided into simple and
compound glands
– A simple gland communicates with the surface by means of a
duct that does not branch before reaching the glandular cells or
secretory portion.
– A compound gland has a duct that branches repeatedly before
reaching the secretory position
Glandular Epithelium
• Simple and compound glands can further be classified in
different manners:
– According to the shapes of their secretory portions.
• Tubular glands consist of epithelial-lined tubes
• Alveolar glands have terminal portions that form saclike dilations
• Branching and coiling of the secretory portions may also occur
– According to the way products are secreted
• Merocrine glands release fluid products by exocytosis
– Most common type of exocrine secretory cell
– Examples: pancreas, sweat glands
– Further divided into serous cells and mucous cells
» Serous cells (which are common in the linings of body cavities) secrete serous
fluid which is watery and has a high concentration of enzymes
» Mucous cells secrete a thicker fluid rich in the glycoprotein mucin called
mucous (which is abundantly secreted from the inner linings of the digestive
and respiratory systems)
• Apocrine glands lose small portions of their glandular cell bodies during
secretion
• Examples: mammary glands, ceruminous glands
• Holocrine glands release entire cells
• Examples: sebaceaus glands
Structures of Exocrine Glands
Types of Glandular Secretions