BASIC TISSUE TYPES
H.Z. Mwale
Tissues
• Definition: A tissue is an aggregate of cells
and extracellular matrix organized to function
in a collective manner.
• Four basic types of tissue…function
– Epithelium…covering
– Connective tissue…support
– Muscle tissue…movement
– Nervous tissue…control
Epithelial Tissue
• Epithelium: sheet of cells that covers the body surface
or lines a body cavity; also form most of the body’s
glands. Epithelial cells also form special nerve end
structures that are sensory.
Functions:
• Covering,lining and protecting surfaces (e.g
epidermis)
• Absorption (eg, the intestinal lining)
• Secrection (eg, parenchymal cells of glands)
Special characteristics of epithelia
• Cellularity
 Cells are packed tightly together, with very little
extracellular material.
 Cells are joined by special junctions
• Polarity
– apical surface facing a space
– basal surface contacting the connective tissue
– Lateral surfaces
• Avascular but innervated
– Receives nutrients from underlying connective tissue where
capillaries lie
• Regeneration (lost cells are quickly replaced by mitosis)
Classification of epithelia
• According to number of cell layers
– “simple” - one cell layer
– “stratified” – 2 or more layers of cells
• According to shape
– “squamous” – thin cells
– “cuboidal” – cell width and thickness roughly
similar
– “columnar” - taller than they are wide
where diffusion is important
where tissues are involved in
secretion and absorption:
Simple
Simple squamous
• Lines surfaces involved in
passive transport (diffusion ) of
gases or fluids (capillaries).
• Lining
of
vessels
(endothelium), serous lining of
cavities; pleura, pericardium
and
peritonium
cavities
(mesothelium).
Stratified: regenerate from below
Rare…
Rare…
Epithelial surface features
• Lateral surface
– Adhesion proteins
– Cell junctions
• Basal surface
– Basal lamina: noncellular sheet of protein
together with reticular fibers form basement
membrane
• Apical surface
Cell Junctions
• Zonula occludens (Tight junctions)
– Seal the space between epithelial cells so that no ion or water can flow between them.
• Zonula Adherens
– Physically bind the cells together in a zone near the apical pole.
• Macula Adherens (Desmosomes)
– Physically bind the cells together in smaller areas resembling spots.
– Anchoring junctions
– Filaments anchor to the opposite side
• Gap junctions (Nexus)
– Connect cells together via protein lined channels that provide a means of communication by
permitting ions to flow from one cell to another.
Cell surface specialisation
• Microvilli – maximize surface area
– Fingerlike extensions of the plasma membrane of
apical epithelial cells
– On moist and mucus secreting epithelium
– Longest on epithelia that absorb nutrients (small
intestine) or transport ions (kidneys)
• Cilia
– Whiplike motile
extentions of the
apical surface
membranes
• Flagellum
– Long isolated cilium
– Only found as sperm
in human
Glands
•
•
•
•
Epithelial cells form them
Production & secretion of needed substances
Are aqueous (water-based) products
The protein product is made in rough ER, packed
into secretory granules by Golgi apparatus,
released from the cell by exocytosis
Classification of glands
• By where they release their product
– Exocrine: external secretion onto body surfaces
(skin) or into body cavities
– Endocrine: secrete messenger molecules
(hormones) which are carried by blood to target
organs; “ductless” glands
• By whether they are unicellular or
multicellular
Exocrine glands
unicellular or multicellular
Unicellular: goblet cell
scattered within epithelial lining
of intestines and
respiratory tubes
Product: mucin
mucus is mucin & water
Examples of exocrine gland products
•
•
•
•
•
•
•
Many types of mucus secreting glands
Sweat glands of skin
sebaceous glands of skin
Salivary glands of mouth
Liver (bile)
Pancreas (digestive enzymes)
Mammary glands (milk)
Endocrine glands
• Ductless glands
• Release hormones into extracellular space
– Hormones are messenger molecules
• Hormones enter blood and travel to specific
target organs
Medical Application
• Under certain abnormal conditions, one type of epithelial tissue may
undergo transformation into another type.
• This reversible process is called metaplasia
• In heavy cigarette smokers, the ciliated pseudostratified epithelium lining
the bronchi can be transformed into stratified squamous epithelium.
• In individuals with chronic vitamin A deficiency, epithelial tissues of the
type found in the bronchi and urinary bladder are gradually replaced by
stratified squamous epithelium.
• Metaplasia is not restricted to epithelial tissue; it also occurs in
connective tissue.
Medical Application
• Both benign and malignant tumors can arise from most types of
epithelial cells.
• A carcinoma is a malignant tumor of epithelial cell origin.
• Malignant tumors derived from glandular epithelial tissue are usually
called adenocarcinomas; these are by far the most common tumors in
adults.
• In children up to age 10 years, most tumors develop are from (decreasing
order); hematopoietic organs, nerve tissues, connective tissues, and
epithelial tissues.
• This proportion gradually changes, and after age 45 years, more than
90% of all tumors that develop are of epithelial origin.
END OF LECTURE