Download 12 Specialised Cells

CELB30090
Advanced Cell Biology
Prof. Jeremy C. Simpson
Lecture 15
Specialised cell types
Today’s lecture ...
Introduction
Example 1 ‐ blood cells
Example 2 ‐ blood vessels / endothelial cells
Example 3 ‐ sensory cells
Example 4 ‐ neuronal cells
Introduction
‐ cell specialisation through differentiation allows multi‐cellular organisms to function more efficiently
‐ many cell types have distinct internal and external architectures suited to their function
‐ grouping of cells into organs also provides a mechanism through which efficiency is increased
Introduction
‐ cell specialisation is often achieved through specific rearrangements in internal organelles
eg: hepatocytes have an extensive endoplasmicreticulum network, and a high abundance of peroxisomes
eg: mast cells possess a huge volume of secretory granules, containing histamine ready for release
eg: glial cells such as Schwann cells have a modified plasma membrane, allowing them to wrap themselves around axons of neurons
Example 1 ‐ blood cells
‐ highly differentiated cell family, coming from a single common stem cell source in the bone marrow
‐ this differentiation has resulted in the various blood cells having a wide variety of roles
‐ red blood cells (erythrocytes) have a very uniform appearance, and have a specific role in transportation of O2 and CO2
‐ red blood cells lack a nucleus, and have a distinct biconcave shape maximising their surface area
‐ red blood cells remain within the blood vessels
‐ white blood cells (leucocytes) are highly diverse in form and function, and are able to pass from blood vessels into surrounding tissue
Example 1 ‐ blood cells
kill virus-infected cells
kill virus-infected cells and regulate other cells
antibody synthesis
phagocytosis and antigen presentation
phagocytosis
destroy larger parasites
release histamine
initiate clotting
O2 and CO2 transport
Example 1 ‐ blood cells
‐ damaged or inflamed tissue releases chemokines, which act as chemo‐attractants to white blood cells
‐ molecules termed ‘selectins’ on the endothelial cells in the capillary bind to oligosaccharides on the white blood cell, causing it to roll slowly along the capillary, increasing its adherence
‐ the white blood cell then activates an ‘integrin’ in its plasma membrane, enabling strong binding to a molecule termed ‘ICAM1’ in the membrane of the endothelial cell
‐ this stronger attachment allows the white blood cell to crawl out of the blood vessel into the surrounding tissue, attracted by the chemokines
Example 1 ‐ blood cells
MOVIE
MOVIE
Movies showing blood flow and white blood cells crawling from a capillary to a site of inflammation
Example 2 ‐ blood vessels / endothelial cells
‐ almost all tissues depend on a blood supply, and the blood supply depends on endothelial cells, which form the linings of the blood vessels
‐ endothelial cells have a remarkable capacity to adjust their number and arrangement to suit local requirements
‐ the largest blood vessels have a thick, tough wall of connective tissue and many layers of smooth muscle cells. The
wall is lined by a thin single sheet of endothelial cells, the endothelium, separated from the surrounding outer layers by a basal lamina
‐ endothelial cells arrange themselves in such a way that individual cells form the lumen of the capillaries Example 2 ‐ blood vessels / endothelial cells
‐ capillaries, and therefore the endothelial cells themselves, need to be able to respond to the changing blood supply needs of tissues
‐ new vessels originate as a capillary sprout from the side of an existing capillary or small venule
‐ at the tip of the sprout, leading the way, is an endothelial cell with a distinctive character ‐ the ‘tip cell’
‐ the tip cell’s most striking feature is that it puts out many
long filopodia
‐ the stalk cells become hollowed out to form a lumen
‐ growth factors act as guidance molecules, directing the formation of new capillaries and blood vessels
‐ understanding this process is a vital aspect of cancer biology
Example 3 ‐ sensory cells
‐ sensory epithelial cells are highly specialised cells used to detect smells, sound and light
‐ sensory epithelia are able to transduce signals to the nervous system
‐ in the nose, the sensory transducers are olfactory sensory neurons; in the ear, auditory hair cells; and in the eye, photoreceptors
‐ all of these cell types are either neurons or neuron‐like
‐ each carries at its apical end a specialised structure that detects the external stimulus and converts it to a change
in the membrane potential. At its basal end, each makes synapses with neurons that relay the sensory information to specific sites in the brain
Example 3 ‐ sensory cells
‐ the neural retina is the most complex of the sensory epithelia
‐ the neurons that transmit signals from the eye to the brain (called retinal ganglion cells) lie closest to the external world, so that the light, focused by the lens, must pass through them to reach the photoreceptor cells
‐ the photoreceptor cells are classified as rods or cones, according to their shape
‐ rods and cones contain different visual pigments ‐
photosensitive complexes of opsin protein with the light‐
absorbing small molecule retinal
‐ rods, whose visual pigment is called rhodopsin, are especially sensitive at low light levels
‐ cones (of which there are three types in humans, each with a different opsin, giving a different spectral response) detect colour and fine detail
Example 3 ‐ sensory cells
‐ the outer segment of a photoreceptor appears to be a modified cilium with a characteristic cilium‐like arrangement of microtubules in the region connecting the outer segment to the rest of the cell
‐ the remainder of the outer segment is almost entirely filled with a dense stack of membranes in which the photosensitive complexes are embedded
‐ in humans, photoreceptors are permanent cells that do not divide and are not replaced if destroyed
‐ the photosensitive molecules, however, are not permanent but are continually degraded and replaced
‐ pigment can be traced from the Golgi complex in the inner segment of the cell to the base of the stack of membranes in the outer segment. ‐ from here it is gradually displaced toward the tip as new material is fed into the base of the stack. Three to four new discs are formed per hour
‐ on reaching the tip of the outer segment, the pigment and the layers of membrane in which they are embedded are phagocytosed and by the cells of the pigment epithelium
Example 4 ‐ neuronal cells
‐ highly specialised cells involved in receiving, conduction and transmission of signals
‐ signals are conveyed from the cell body along the axon as a result of changes in the electrical potential across the neuron’s plasma membrane ‐ the plasma membrane of neurons contains a high density of sodium and potassium channels
‐ axons are wrapped in ‘myelin’ (80% lipid and 20% protein), which serves to protect them, and help propagate the electrical signal
‐ myelin is formed from specialised supporting cells called glial cells (Schwann cells and oligodendrocytes), that wrap their own plasma membrane around the axon
Example 4 ‐ neuronal cells
‐ the plasma membrane at the end of axons (the synapse) is also highly specialised
‐ it is important to maintain close proximity between the pre‐ and post‐synaptic membranes
‐ this is achieved through large assemblies of cell‐cell adhesion molecules, including cadherins, neuroligins and neurexins
‐ beneath the synaptic membrane a number of scaffold proteins help organise the adhesion molecules
‐ intracellular scaffold proteins consist of strings of protein binding domains, particularly ‘PDZ domains’ that are able to bind the C‐terminal tails of trans‐membrane proteins
Key take home point
Specialised cellular architecture has resulted in
multicellular organisms being able to carry out highly
complex and advanced processes