BMSC11002 Lecture Notes - Lecture 4: Cytosol, Keratin, Adipocyte
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WEEK 4: TISSUE LEVEL OF ORGANISATION
4.1 FOUR TYPES: EPITHELIAL, CONNECTIVE, MUSCLE, NERVOUS
Body tissues can be classified into four basic types according to their structure and function:
• Epithelial tissue – covers body surfaces and lines hollow organs, body cavities and ducts.
o Also forms glands
o Allows body to interact with it internal and external environments
• Connective tissue- protects and supports the body and its organs
o Bind organs together, store energy reserves as fat, and help provide immunity against
disease-causing organism
• Muscle tissue – is composed of cells that can contract to generate the physical force needed to
move body structures and generate heat that warms the body
• Nervous tissue – detects the changes in a variety of conditions inside and outside the body and
responds by generating electrical signals called action potentials that activate muscular
contractions and glandular secretions
4.2 CELL JUNCTIONS HOLD CELLS TOGETHER TO FORM TISSUES
Cell junctions hold cells together by contact points between their plasm membranes
• Tight junctions – consist of weblike strands of transmembrane proteins that fuse together the
outer surface of adjacent plasma membranes to seal off passageways between adjacent cells
o Cells of the epithelial tissues that line the stomach, intestines and urinary bladder have
many tight junctions
o They form tight seals between adjacent cells, that prevent substances from passing
between cells and leaking out of these organs
• Desmosomes – contain transmembrane glycoproteins that extend into the intercellular space
between adjacent cell membranes, attaching cells to one another
o Plaque is a dense layer of proteins on the cytosol side of the plasma membrane,
attached to intermediate filaments of the cytoskeleton
▪ The intermediate filaments extend from the plaque on one side of the cell
o Contribute to the stability of cells and anchor adjacent cells within tissue
o Spot-wel-like junctions are common among skin cells, preventing them from separating
when skin is pulled, and muscle cells in the heart, keeping tissue from pulling apart
during a contraction
• Gap junctions – transmembrane proteins form tiny fluid-filled tunnels called connexons that
connect neighbouring
cells
o A very narrow
intercellular
gap separates
plasma
membranes of
gap junctions
o Ions and
small
molecules can
diffuse
through the
connexons from one cell to the next
o Allow the cells in a tissue to communicate with one another
o Also enable action potentials to spread rapidly among cells
4.3 EPITHELIAL TISSUE: COVERS, LINES OR SECRETES
• An epithelial tissue, consists of cells arranged in continuous sheets, in either single or multiple
layers
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• Cells are closely packed with little intercellular space between adjacent plasma membranes and
are held tightly together by many cell junctions
• Protective barrier
o Apical surface faces the body surface, a body cavity, or the lumen (interior space) of an
organ
▪ They may contain cilia or microvilli
o Lateral surfaces face adjacent cells on either side and contain cell junctions
o Basal surface is opposite to the apical surface and adheres to extracellular materials
o The basement membrane is a thin extracellular layer that commonly consists of the
basal lamina and reticular lamina
▪ Basal lamina contains mostly glycoproteins secreted by epithelial cells
▪ Reticular lamina is deep to the basal lamina and contains primarily collagen
proteins produced by connective tissue cells called fibroblasts
o The basement membrane attaches to and supports the overlying epithelial tissue and
anchors it to the underlying connective tissue
• Is avascular – it lacks it’s own blood vessels
• Classified into two types of epithelium:
1. Covering and lining epithelium – classified based on:
a. Arrangement of cells in layers
- Simple epithelium – single layer of cells that function in diffusion, osmosis,
filtration, secretion and absorption
o Secretion is the production and release of substances such as
mucus, swear or enzymes
o Absorption is the intake of substances, such as digested food from
the intestinal tract
- Pseudostratified epithelium – appears to have multiple layers of cells
because the cell nuclei lie at different levels and not all cells reach the
apical surface, but is actually simple because all cells rest on the basement
membrane
o Cells that do extend to the apical surface may contain cilia, others
(goblet cells) secrete mucus
- Stratified epithelium – consists of two or more layers of cells that protect
underlying tissues in locations where there is considerable wear and tear
b. Cell shapes
- Squamous cells – thin, which allows substances to pass rapidly through
them
- Cuboidal cells – as tall as they are wide and are shaped like cubes
o Have microvilli at their apical surfaces and function in either
secretion or absorption
- Columnar cells – much taller that they are wide and protect underlying
tissues
o Their apical surface may have cilia or microvilli, and they often are
specialised for secretion and absorption
- Transitional cells – change shape between squamous and cuboidal,
allowing organs to stretch in size, e.g. urinary bladder.
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Document Summary
4. 2 cell junctions hold cells together to form tissues. Ions and small molecules can diffuse through the connexons from one cell to the next: allow the cells in a tissue to communicate with one another, also enable action potentials to spread rapidly among cells. 4. 3 epithelial tissue: covers, lines or secretes: an epithelial tissue, consists of cells arranged in continuous sheets, in either single or multiple layers. Is avascular it lacks it"s own blood vessels: classified into two types of epithelium, covering and lining epithelium classified based on, arrangement of cells in layers. 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. Squamous cells thin, which allows substances to pass rapidly through them. Cuboidal cells as tall as they are wide and are shaped like cubes: have microvilli at their apical surfaces and function in either secretion or absorption.
