Download Chapter 3 Cell Types

Chapter 3
Cell Types
3.1 A Generic Cell
There are over 200 different cell types in the human body, each with its own specialty,
shape, and mechanical properties. Although we cannot learn about all of them, it is useful
to recognize some features common to most if not all of them. Thus the generic cell is
born:
Bao &
Suresh
Figure 3.1
The cartoon-like drawing shows the basic features common to most cells. The
components shown enable the cell to perform many functions: the synthesis, sorting,
storage and transport of molecules; storage and expression of genetic information; the
recognition, transmission and transduction of signals; and the powering of molecular
motors and machines1. The cell also converts energy from one form to another and
responds to external environments by continually altering its structure9. Most living cells
sense, support and/or generate forces. For example, muscle cells generate contractile
forces when excited, performing many essential functions of the body1. Three types of
muscle cells, skeletal, heart and smooth, have different mechanical structures and
behaviour. Endothelial cells can recognize the magnitude, mode (steady or pulsatile),
type (laminar or turbulent) and duration of applied shear flow34, 35, and respond
accordingly, maintaining healthy endothelium or leading to vascular diseases including
thrombosis and atherosclerosis36. Fibroblast cells 'crawl' like an inchworm by pulling the
cell body forward using contractile forces11. The various chemical constituents and
organelles will be discussed in the following chapter.
The diversity of cells in the human body is astounding. Cells range from the red blood
cell, which is little more than a sack of hemoglobin, to nerve cells 1 meter in length, but a
million times smaller in width, with intricate tree-like branches at both ends. We only
outline a few of them here.
3.2 RBCs
The best cell type to begin with is the red blood cell (RBC), since it is at the foundation
of cytomechanics. It is also arguably the simplest mammalian cell because it is relatively
devoid of organelles, including the nucleus and its size stays in a tight range of 7.0–8.5
m. The RBC speeds around the circulation for one purpose only: to exchange gases. It
must withstand large shear and deformations as it bumps into obstacles, gets exposed to
variable osmotic pressures, and gets squeezed into tiny < 3 m wide capillaries. Each
RBC easily deforms elastically over 100% many times a minute they are whisked through
narrow tunnels. Few man-made objects can come close to this ability. This amazing
malleability of RBCs has played a major role in the development of the field of
cytomechanics, since it sparked to curiosity of its founder, Y.C. Fung.
Two interrelated structural features of the RBC underly its mechanical adaptability. The
first is its shape under normal conditions. The biconcave disc shape is a brilliant design,
since it provides a low bending stiffness in the center, and a reservoir to allow swelling
without increase in surface area. These properties allow it to easily squeeze through
narrow capillaries, and to swell in hypotonic environments without breaking, as depicted
below. Since the plasma membrane of RBCs undergoes lysis at an area expansion of
only 3%, if the cell did not have the concavity, acting as an expandable reservoir, the
cells would quickly explode when exposed to even slight hypotonicity.
Figure 3.2
RBC Squeezing
through a capillary
RBC in isotonic
media
RBC in
moderate
hypotonicity
Higher
hypotonicity
What feature enables the above structure and behaviour? The key to it lies in the CSK of
RBCs, which is deficient in actin, but rich in spectrin - a simpler and more flexible
polymer. Actin in the CSK of the RBC is used to cross-link the long spectrin filaments
RBCs sometimes aggregate in the blood stream to form ‘roulleau’ as shown below:
Figure 3.3
3.3 WBCs
White Blood cells, or leukocytes are a group of circulating cells whose job is responding
to immune needs and inflammation. As they speed through the circulatory system, WBCs
are incredibly capable of finding the trouble spot infection site, stopping, then attaching
themselves to the endothelial cells lining the vessel, and then move through them into the
body tissues to attack.
WBCs must undergo not only the same passive deformations as RBCs, but must also
actively change shape as they perform phagocytosis , i.e. envelope foreign or inflamed
objects. Their shapes, however, are not like that of RBCs, because they are generally
spherical in overall outline. How then do WBCs achieve their flexibility and motility? In
fact WBCs use an entirely different strategy to acccomplish the same feats of
deformability and volume expansion without surface area change. The answer is that
WBCs can deform and change volume without undue stress on their membranes, because
their plasma membranes and CSK are extensively folded. These folds consist of CSK
curvatures and membrane microvilli; these serve the same role as the RBC concavities:
they are expandable reservoirs. WBCs expansion-shrinkage behaviour is much the same
as the Hoberman sphere. At low magnification, WBCs appear smooth, but folds and
microvilli can be seen at higher magnification as depicted below.
Figure 3.4
Outline of WBC
Microvilli
3.4 Endothelial Cells
Endothelial Cells (ECs) line the walls of blood vessels, and are directly exposed to
flowing blood, as depicted schematically and photographically below.
Figure 3.5
ECs with black
nucleus (Fung)
Inside a blood vessel
looking at ECs bulging
3.5 Myocytes
Myocytes, or muscle cells, are the primary motor cells, and hence have the highest
content of actin. The picture below shows myocytes cultured with fibroblasts that have
been stained for actin. Myocytes are brilliantly stained (white) showing an abundance of
actin, while fibroblasts (left portion) are much darker.
Figure 3.6
3.6 Exercises
1. Describe two types of transduction that a cell can perform.
2. What types of forces would be experienced by ECs? Draw a sketch resembling a free
body diagram.
3. State 2 structural adaptations of RBCs that relate to its function.
4. Compare the way in which red and white cells adapt to different environments in the
blood stream.