Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

A HIERARCHICAL STEM-CELL SYSTEM: BLOOD CELL FORmation
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A Hierarchical Stem-Cell System: Blood Cell
Formation
The function of blood vessels is to carry blood, and it is to blood itself that we now
turn. Blood contains many types of cells, with functions that range from the transport
of oxygen to the production of antibodies. Some of these cells stay within the
vascular system, while others use the vascular system only as a means of transport
and perform their function elsewhere. All blood cells, however, have certain
similarities in their life history. They all have limited life-spans and are produced
throughout the life of the animal. Most remarkably, they are all generated ultimately
from a common stem cell, located (in adult humans) in the bone marrow.
This hematopoietic (blood-making) stem cell is thus multipotent, giving rise to all
the types of terminally differentiated blood cells as well as some other types of
cells, such as the osteoclasts in bone, as mentioned earlier. The hematopoietic
system is the most complex of the stem-cell systems in the mammalian body, and
it is exceptionally important in medical practice.
Red Blood Cells Are All Alike; White Blood Cells Can Be Grouped
in Three Main Classes
Blood cells can be classified as red or white. The red blood cells, or erythrocytes,
remain within the blood vessels and transport O 2 and CO 2 bound to hemoglobin.
The white blood cells, or leukocytes, combat infection and in some cases phagocytose
and digest debris. Leukocytes, unlike erythrocytes, must make their way
across the walls of small blood vessels and migrate into tissues to perform their
tasks. In addition, the blood contains large numbers of platelets, which are not
entire cells but small, detached cell fragments or “minicells” derived from the cortical
cytoplasm of large cells called megakaryocytes. Platelets adhere specifically
to the endothelial cell lining of damaged blood vessels, where they help to repair
breaches and aid in blood clotting.
All red blood cells belong in a single class, following the same developmental
trajectory as they mature, and the same is true of platelets; but there are many
distinct types of white blood cells. White blood cells are traditionally grouped into
three major categories—granulocytes, monocytes, and lymphocytes—based on
their appearance in the light microscope.
Granulocytes contain numerous lysosomes and secretory vesicles (or granules)
and are subdivided into three classes according to the morphology and
staining properties of these organelles (Figure 22–27). The differences in staining
reflect major differences of chemistry and function. Neutrophils (also called
polymorphonuclear leukocytes because of their multilobed nucleus) are the most
common type of granulocyte; they phagocytose and destroy microorganisms,
especially bacteria, and thus have a key role in innate immunity to bacterial infection,
as discussed in Chapter 24 (see Movie 16.1). Basophils secrete histamine
(and, in some species, serotonin) to help mediate inflammatory reactions; they
are closely related to mast cells, which reside in connective tissues but are also
generated from the hematopoietic stem cells. Eosinophils help to destroy parasites
and modulate allergic inflammatory responses.
Once they leave the bloodstream, monocytes (see Figure 22–27D) mature
into macrophages, which, together with neutrophils, are the main “professional
phagocytes” in the body. As discussed in Chapter 13, both types of phagocytic cells
contain specialized lysosomes that fuse with newly formed phagocytic vesicles
(phagosomes), exposing phagocytosed microorganisms to a barrage of enzymatically
produced, highly reactive molecules of superoxide (O 2 – ) and hypochlorite
(ClO – , the active ingredient in bleach), as well as to attack by a concentrated mixture
of lysosomal hydrolase enzymes that become activated in the phagosome.
Macrophages, however, are much larger and longer-lived than neutrophils. They
recognize and remove senescent, dead, and damaged cells in many tissues, and
they are unique in being able to ingest large microorganisms such as protozoa.