CHAPTER 7
Interactions Between Cells and
Their Environment
Introduction
• Cells interact with extracellular material to
form defined tissues.
• These interactions are crucial to the
formation of epithelial tissue and
connective tissue, which are crucial for
various cellular activities.
Overview of cell organization into tissues
7.1 The Extracellular Space (1)
• The glycocalyx
(cell coat) is formed
from carbohydrate
projections form the
plasma membrane.
• Glycocalyx mediates
cell-cell and cellsubstratum
interactions.
The Extracellular Space (2)
• The extracellular
matrix (ECM) is an
organized network
beyond the plasma
membrane.
– It often plays a
regulatory role in
determining shape
and activities of the
cell.
The Extracellular Space (3)
• ECM (continued)
– The basement membrane (basal lamina) is a
continuous sheet that underlies epithelial
tissue and surrounds blood vessels.
• Helps maintain cells attached.
• Serves as substratum for cell migration.
• Forms a barrier to macromolecules.
The basement membrane
Organization of the ECM
The Extracellular Space (4)
• Collagens – fibrous glycoproteins found
only in the ECM.
– Collagen is the most abundant protein in the
human body.
– Provide high tensile strength.
– Each collage is restricted to particular
locations in the body.
– All collagens are a trimer of polypeptide
chains wound around each other.
The structure of collagen I
The Extracellular Space (5)
• Collagens (continued)
– Provide the insoluble
framework that
determines
mechanical properties
of the matrix.
– Abnormalities in
collagen formation
lead to serious
disorders.
The Extracellular Space (6)
• Not all collagens form
fibrils.
• Collagen type IV is
non-fibrillar, and is
restricted to the
basement membrane.
The Extracellular Space (7)
• Proteoglycans – protein-polysaccharide
complex, with a the core protein attached
to glycosaminoglycans (GAGs).
– Have a repeating disaccharide structure.
– Negatively charged GAGs attract lots of
cations, which in turn attract water forming a
porous, hydrated gel.
Structure of a proteoglycna complex
Structure of a proteoglycna complex
The Extracellular Space (8)
• Fibronectin (Fn) – a linear array of distinct
polypeptides giving it a modular structure.
– Each polypeptide is about 30 Fn modules.
– Fn modules are found in other proteins too.
– Fn has binding sites for other components of
the ECM.
– Fn guides migrating cells during
embryogenesis.
Structure of fibronectin
Structure of fibronectin
Cell migration during embryogenesis
The Extracellular Space (9)
• Laminins –
extracellular
glycoproteins
consisting of three
polypeptide chains
linked by disulfide
bonds.
– Help cell migration
during development.
– Components of
basement membranes.
The Extracellular Space (10)
• Dynamic Properties
– The ECM can be stretched during tension.
– ECM materials degraded by matrix
metalloproteinases (MMPs).
– MMPs possibly involved in tissue remodeling,
embryonic cell migration, wound healing , and
formation of blood vessels.
7.2 Interactions of Cells with
Extracellular Materials (1)
• Integrins – family of membrane proteins
composed of heterodimers with α and ß
subunits.
– Have a major role in integrating extracellular
and intracellular environments.
– Another role is adhesion of cells to their
substratum or other cells.
Model of integrin activation
Interactions of Cells with
Extracellular Materials (2)
• Integrins
(continued)
– Linkage between
integrins and their
ligands mediates
adhesion between
cells and their
environment.
– Binding of proteins
to integrins is
facilitated by
tripeptide RGD.
Interactions of Cells with
Extracellular Materials (3)
Interactions of Cells with
Extracellular Materials (4)
• Integrins (continued)
– Cytoplasmic domains of integrins contain
binding sites for a variety of cytoplasmic
proteins.
– Integrins make the connection between the
ECM and the cytoskeleton.
Interactions of Cells with
Extracellular Materials (5)
• Focal adhesions – scattered, discrete
sites for cell adhesion to their substratum
in vitro.
– They may act as a type of sensory structure.
– Are also implicated in cell locomotion.
Focal adhesions
Focal adhesions
Forces exerted by focal adhesions
Interactions of Cells with
Extracellular Materials (6)
• Hemidesmosomes – basal attachments
of epithelial cells to the basement
membrane in vivo.
– Contain a dense plaque with filaments
consisting of keratin.
– Keratin filaments are linked to the ECM by
membrane-spanning integrins.
Hemidesmosomes
7.3 Interaction of Cells with
Other Cells (1)
• Cells have surfacerecognition sites that
maintain organization
Interaction of Cells with Other
Cells (2)
• Selectins – family of
integral membrane
glycoproteins that
bind to sugars on the
surface of cells.
Interaction of Cells with Other
Cells (3)
• Selectins (continued)
– Contain a small cytoplasmic domain, a single
membrane-spanning domain, and a large
extracellular segment.
– Three types:
• E-selectin – on endothelial cells.
• P-selectin – on platelets and endothelial cells.
• L-selectin – on white blood cells.
Interaction of Cells with Other
Cells (4)
• Immunoglobulin
superfamily (IgSF) –
most proteins are
involved in immune
functions.
– Most IgSF molecules
mediate interaction of
lymphocytes with cells
required or immune
response.