|
Cells
Typical organelles in a a eukaryotic cell include the following:
Endosome Peroxisome Golgi Apparatus Cell
Membrane
Microfilament Endoplasmic Reticulum CAMs
Intermediate
filament Nucleus Microtubule Mitochondri Lysosome Basal Lamina
The basic structure of a eukaryotic cell differs from a
prokaryotic cell such as with bacteria.
A cell reproduces by duplicating its
contents and dividing in two. This cycle of duplication and division is known as
the cell cycle. Cells have evolved a complex
network of regulatory proteins known as cell cycle control that governs
progression through the cell cycle. The development of a multicellular organism
occurs by a process known as embryogenesis. Over 200 different cell types in the human
are known. The most numerous are the red blood
cells, then platelets and 3) white
blood cells or leukocytes. These cells form a variety of different
tissues. Blood can be separated in a centrifuge into a fluid and a cellular
fraction. The fluid fraction is the plasma which contains all of the
soluble small molecules and macromolecules of blood like fibrin and
other proteins required for the formation of blood clots. If blood or plasma is
allowed to clot, the fluid phase that remains is called serum
(the liquid, noncellular component of coagulated blood).
Antibodies reside in the serum. The cellular
fraction of blood contains 1)
red blood cells or erythrocytes, 2) white blood cells or leucocytes , 3) platelets and 4) plasma.
Peripheral blood mononuclear cells (PBMCs)
are a mixed population consisting of several cell types: CD4+ and CD8+
T-lymphocytes (70%), B-lympohocytes (15%), natural killer cells (10%), monocytes
(5%), and dendritic cells (<1%), each expressing a unique set of genes.
Isolation of PBMCs
Cells of the body are formed and
developed in a process referred to as hematopoiesis. Every mature blood
cell is derived from a common stem cell which is pluripotent
or able to differentiate along a number of pathways. Early in hematopoiesis, a pluripotent
stem cell differentiates along 1 of 2 pathways, giving rise to either a common
lymphoid progenitor cell or a common myeloid progenitor cell.
Myeloid stem cells generate progenitors of red blood cells (erythrocytes), many
of the various white blood cells (neutrophils,
eosinophils, basophils, monocytes, mast cells, dendritic cells) and
platelets.
In the adult, hemopoietic stem cells are
found mainly in bone marrow and they grow and mature on a meshwork of stromal
cells which include fat cells, endothelial cells, fibroblasts, and
macropages. Progenitor committment depends on various
growth factors called "cytokines". In the absence of infection, bone marrow stromal cells are the major source of hematopoietic cytokines. In the presence
of infection, cytokines produced by activated macrophages and
TH cells
induce additional hematopoietic activity resulting the the rapid expansion
of the WBC population that is necessary for fighting infection.
Various growth factors are required for
the survival, and maturation of hematopoietic cells in culture. These hematopoietic cytokines
include the colony-stimulating factors (CSFs) which induce the formation
of distinct hematopoietic cell lines. Another important cytokine is
erythropoietin (EPO) which is produced by the kidney and induces the
terminal development of erythrocytes and regulates the production of red blood
cells. The ability of a given cytokine to signal growth and differentiation is
dependent upon the presence of a receptor for that cytokine on the surface of
the target cell.
The regulation of hematopoiesis is quite complex. For example, the binding CSF to its receptor causes some of the receptors to be internalized by
the cells which serves to down modulate receptor expression by the cell. Steady
state hematopoisis is also regulated by cell apoptosis
or death. Abnormalities
in the expression of hematopoietic cytokines or their receptors may result in
some leukemias.
| 