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Kharkov National Medical
University
Department of histology, cytology and
embryology
Lecture 3
Tissues
1. Epithelial tissue
Definition
• Tissues -- groups of cells organised
to perform one or more functions.
• Note!!(but do not write) Cell is the basic
structural unit of the body. But in the
multicellular organism there are tissues
that are responsible for maintaining body
functions due to collaborative efforts of
their individual cells.
4 basic types of tissues:
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Epithelial Tissue (Epithelium).
Connective tissue.
Muscular Tissue (Muscle).
Nervous tissue (Nerve).
Epithelial Tissue
- covers body surfaces, - - lines body cavities, and
- - - forms glands.
There are 2 main types of ET:
• Covering and lining (covers outer body surfaces
(skin) and inner body surfaces (lumen of hollow
organs (intestine) and cavities (peritoneal,
pericardial, pleural), blood vessels, ducts, tubules).
They separate the underlying connective tissue
from the external environment or environment
of internal cavities
• Glandular - forms glands
Main function :
• Epithelium creates a selective barrier
between the organism and its external
environment.
• This barrier facilitates or inhibit the passage of
specific substances.
• Any substances that enter the body as a
metabolites or discharges from the body as a
waste products must pass through the
epithelial cell, not between them.
Other functions
• protection
• diffusion or absorption or excretion =
exchange
• transport (along free surface)
• secretion
• sensation.
Characteristics
• 1. There is no extracellular substance between
the cells,
• cells are tightly apposed in sheets and strongly
attached to one other by means of special
junctions.
Characteristics.
• 2. Their basal surface is attached to an
underlying basement membrane –
extracellular protein-polysaccharide-rich
layer.
• 3. polarity - cells have 2 surfaces :
the apical or free side (surface) - is the side
towards the lumen or outside medium
the basal side (surface) - is the side closest
to the basement membrane
Epithelial Tissues and Their
Basement Membrane
The Polarity of Epithelial Cells
Characteristics.
• 4. Epithelia are avascular :
• epithelium does not contain blood vessels
but is nourished by diffusion of substances
from capillaries of underlying connective
tissue.
• 5. It is frequently mitotically active
Morphological classification
of Epithelial Tissues
is based on combination of the number of
cell layers (first name) and the shape of
surface cell (second name).
E. can be called simple or stratified on the
basis of the number of cell layers which
comprise it:
• Epithelium may be simple, when it is one
cell layer thick and stratified when it is
two or more cell layer thick.
The second name describes the shape
of the cells:
• Squamous, where the width of the
cell is greater than its height.
• Cuboidal, where the width, depth
and height are approximately the
same.
• Columnar, where the height
appreciably exceeded the width.
Simple Squamous Epithelium
• Single layer of flat cells (very thin)
– lines blood vessels (endothelium), closed body cavities
(mesothelium), alveoli in the lungs
– Function: controls diffusion, osmosis and filtration
Simple Cuboidal Epithelium
• Single layer of cube-shaped cells, lines tubules and
ducts
Simple Columnar
• Lines stomach, intestine. Usuallly has microvilli =
finger-like projections of the cell membrane
• Function – absorption.
Pseudostratified Columnar
• Single cell layer
• All cells attach to
basement membrane
but not all reach free
surface
• Nuclei at varying depths
Stratified squamous
Epithelium
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•
•
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Several cell layers thick,
Surface cells are flat
2 types:
Keratinized = surface
cells are dead and filled
with keratin
– Example - Skin
• Nonkeratinized = no
keratin in moist, living
cells at apical surface
– Example - Cornea
Stratified Cuboidal Epithelium
• Surface cells
cuboidal
Lines sweat gland
ducts
Stratified Columnar Epithelium
• Surface cells are columnar
• Lines very large ducts of gland
Transitional Epithelium
• Multilayered
• Surface cells vary in shape from round to flat if
stretched
• Lines hollow organs of the urinary tract that
expand from within
!! The morphology of an epithelium often
correlates with its function:
• - Epithelia involved in secretion or
absorption are typically simple.
• -- Stratification of the epithelium usually
correlates with impermeability and
protection
• --- The height of the cell often reflects
the level of secretory or absorptive
activity.
GLANDULAR EPITHELIA
• Secretion is the main function
of the glandular epithelia.
• Secretion – cyclic process.
4 phases:
• 1. diffusion of metabolites into the cell
• 2. synthesis proper
• 3. releasing of substances from the cell
• 4. cell restoration
There are two types of glands in the body:
exocrine and endocrine.
• Exocrine glands secrete
through ducts or directly
onto the surface (skin or
cavity of inner organs).
• Without ducts - surface
mucous cells of the
stomach;
• With ducts - most
exocrine glands; Ex.:
salivary glands.
• Endocrine secrete
hormones into the
bloodstream.
Classification of glands by cell
number
• Unicellular gland
Ex.: Goblet cell secrete mucus and lubricate
small and large intestine, respiratory tract
• Multicellular gland
2 portions of gland:
Parenchyma and Stroma
• Parenchyma - the epithelial, secreting cells
of the gland which perform the gland's
essential function.
• Stroma - supporting cellular framework of
connective tissue which contains blood
vessels and nerves and gives structure to
the gland.
Classification of Exocrine Glands
by type of secretion
• 1. Mucous - viscous, slimy mucus
Ex.: glands of esophagus, Brunner's
glands of duodenum
• 2. Serous - watery serum
Produce wide variety of proteins
(enzymes).
Ex.: exocrine pancreas, parotid salivary
glands, uterine glands.
Classification of Exocrine Glands
by type of secretion
• 3. Mixed (Sero-mucous)
Ex.: Submandibular and sublingual
salivary glands; mixed glands of the
nasal cavity, paranasal sinuses,
nasopharynx, larynx, trachea and
bronchi.
Secretion mechanism – 3 types
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•
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Merocrine (= eccrine)
Apocrine
Holocrine
Merocrine (eccrine)
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( (note!) This is the most common type of
secretion mechanism).
Secretory granules are formed in the
cells and accumulated in the apex.
The granules fuse with the apical plasma
membrane and are secreted into the
lumen of the gland by exocytosis.
Apocrine
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(ex.: lactating mammary gland (fat droplets))
- secretion of product plus a small portion of
pinched off apical cytoplasm, enclosed by a
membrane derived from the plasma
membrane.
Holocrine
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- ex: Sebaceous glands of the skin
the cell get filled with its secretory product
while the nucleus becomes smaller.
maturing cells filled with lipid move to the
center, then toward the opening of the acinus.
the cell dies and the entire cell is released and
disintegrates, releasing its product.
Mechanisms of Glandular Secretion
A Structural Classification of
Exocrine Glands
• By branching of the duct exocrine glands
may be: simple or compound
• By the shape of secretory ends – tubular
or alveolar (acinar)
• By branching of secretory ends exocrine
glands may be: branched or non-branched
A Structural Classification of Exocrine Glands