How big is a human cell?
A human is an assortment of 1013-1014 cells (BNID 102390), plus an
enormous complement of 1014 allied microbes, (BNID 102392). The
identities of these 1013 cells are shared among more than 200 different
cell types (BNID 103626, 106155) which perform a staggering variety of
different functions. One of the ways we can recognize these diverse cell
types is on the basis of their very different shapes (though in some cases,
such as the different types of white blood cells, the distinctions are more
subtle and often reflected in molecular signatures). For example, the
leukocytes of the immune system are approximately spherical in shape
while adherent tissue cells on a microscope slide resemble a fried egg
with the nucleus analogous to the yolk. By way of contrast, the neurons
connecting our brains to our legs can reach lengths of over a meter
(BNID 104901) but with a width of only about 10 m.
Mature female oocytes are among the largest cell types with a ≈120 m
diameter. Other large cell types include muscle fiber cells that merge
together to form syncytia where multiple nuclei reside in one cell and
Figure
1:
Dividing
HeLa cells as
seen by a
scanning
electron
micrograph
(colored).
Magnification
: x2600 when
printed at 10
centimetres
wide. (i.e. the
cells
would
have
a
diameter of
about
15
micron (RM)).
Caption:
Dividing
HeLa
Coloured
(SEM)
of
occurs
after
nuclei.
The two daughter cells
transient
HeLa cells
cells.
structure
continuously
immortal
nuclear
and
formed
cultured
so
undergoing
division
cell
thrive
cytokinesis
(mitosis),
are still
from
line
in
the
scanning
(cell
which
micrograph
division).
Cytokinesis
produces
two
daughter
connected by a midbody,
microtubules.
of
electron
human
laboratory.
HeLa
cancer
They
biological and medical research.
From: http://www.sciencephoto.com/media/137829/view#
are
cells
cells,
are
which
widely
used
a
a
are
in
megakaryocytes, bone marrow cells responsible for the production of
blood platelets. Both of these cell types can reach 40 m in diameter
(BNID 105114). Red blood cells, also known as erythrocytes, are some of
the smallest of human cells. These cells have a characteristic biconcave
disk shape with a depression where the nucleus was lost in maturation
and have a corresponding diameter of 7-8 m (BNID 100509) and a
volume of ≈100 m3 (BNID 101711, 101713).
Certain human cell lines have been domesticated as laboratory
workhorses. Perhaps the most familiar of all are the so-called HeLa cells
(see Fig. 1), immortal cancer cell lines that divide indefinitely, alleviating
the need to sacrifice primary animal tissue for experiments. These cells
lines have been used for studies such as the molecular basis of signal
transduction and the cell cycle. In these cell types, the cell volumes are
captured by a “rule of thumb” value of 2000 m3 with a range of 5004000 m3 (BNID 100434), with these numbers coming from studies of
cell lines such as the HeLa cells introduced above. HeLa cells adhere to
the extracellular matrix and like many other cell types on a microscope
slide spread thinly to a diameter of ≈40 m (BNID 103718, 105877,
105878) but only a few m in height. When grown to confluence they
press on each other to compact the diameter to ≈20 m such that in one
of the wells of a 96 multiwell plate they create a monolayer of ≈100,000
cells. Tighter bounds were put on the variability of a given cell type by a
careful analysis of a mouse lymphocyte cell line as shown in fig. 2. The
distribution is centered at about 1000 m3 with a variance of about 300
m3. To put these cellular sizes in perspective, if we think of E. coli as
having the size of a human being, then a HeLa cell is about the size of a
blue whale.
Our examination of the sizes of different cell types will serve as a
jumping off point for developing intuition for a variety of other biological
numbers we will encounter throughout the book. For example, when
thinking about diffusion we will interest ourselves in the time scale for
Figure 2:
Distribution of
cell sizes for
L1210, a mouse
lymphoblast cell
line. The cell
volumes are
reported in units
of fL (1 fL = 1
m3). Figure
from Tzur et al.
Science 2010
particular molecules to traverse a given cell type and this result depends
in turn upon the size of those cells.