Uploaded by Valentine Lutanda


H.Z. Mwale
• 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.
• Covering,lining and protecting surfaces (e.g
• 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
– “columnar” - taller than they are wide
where diffusion is important
where tissues are involved in
secretion and absorption:
Simple squamous
• Lines surfaces involved in
passive transport (diffusion ) of
gases or fluids (capillaries).
• Lining
(endothelium), serous lining of
cavities; pleura, pericardium
Stratified: regenerate from below
Epithelial surface features
• Lateral surface
– Adhesion proteins
– Cell junctions
• Basal surface
– Basal lamina: noncellular sheet of protein
together with reticular fibers form basement
• 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
• Flagellum
– Long isolated cilium
– Only found as sperm
in human
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
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
• 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.