Biomedical Engineering Reference
In-Depth Information
DNA synthesis, and protein content all change in a progressive and coordinated fashion.
The differentiation from a pronormoblast (earlier precursor stage) to a fully mature enucleated
erythrocyte takes about 180 hours, or about one week. The replication activity is the highest
at the preprogenitor and progenitor stages, but once the precursor stage is reached, replication
activity ceases sharply. This information can be used to derive and solve equations that describe
the process of erythropoeisis.
Experimental Observations of Differentiation
The process of differentiation can be observed directly using fluorescent surface markers
and/or light microscopy for morphological observation. Ultrastructural changes are also
used to define the stage of differentiation.
A flow cytometer is often used to monitor the process of cellular differentiation. The
basis for this approach is the fact that characteristic surface proteins are found on cells at
different stages of differentiation. These surface markers can be used as binding sites for
fluorescently conjugated monoclonal antibodies. The flow cytometer can be used to trace
the expression of several surface markers, and by performing such studies over time, it is
possible to track the differentiated state of the cells and cell population.
For example, erythropoiesis can be traced based on expression of the transferrin recep-
tor (CD71) and glycophorin-A. The latter is an erythroid-specific surface protein that
is highly negatively charged and serves to prevent red cell aggregation in dense red cell
suspensions. The transferring receptor plays a critical role during the stages in which iron
is sequestered in hemoglobin. The measurement of this process is shown in Figure 6.15.
DESCRIBING THE KINETICS OF CELL DIFFERENTIATION
The process of differentiation is a slow one, often taking days or weeks to complete. The
kinetics of this complex process can be described mathematically using two different
approaches:
A. Compartmental models.
The traditional approach to describing cell growth and differen-
tiation is to use compartmental models. The differentiation process involves a series of
changes in cell phenotype and morphology, typically becoming more pronounced at the
latter stages of the process.
X
!
X
!
X
! ......:
X
i
! ......
X
n
!
turnover
ð
6
:
1
Þ
0
1
2
where
can be as high as 16 to 18.
In the use of compartmental models in the past, the transition from one stage to the next
was assumed to represent cell division. Thus, these models coupled the dual process of
cell differentiation with cell replication. Mathematically, this model is described by a set
of ordinary differential equations as
n
dX
i
dt
¼
2
k
i
1
X
i
1
k
i
X
i
ð
6
:
2
Þ
