Biomedical Engineering Reference
In-Depth Information
bone marrow accumulates to a staggering 10
16
cells. This cell number is several hundred
times greater than the total number of cells that are in the body at any given time. Similarly,
the intestinal epithelium, the body's second most prolific tissue, produces about 5
10
14
cells over a lifetime—ten times the total number of cells in the human body.
Tissues have their own characteristic turnover rates (Table 6.3). Bone marrow is the most
proliferative tissue in the body, followed by the lining of the small intestine, and then by
the epidermis. The turnover rate of these two tissues is on the order of a few days; that
for the epidermis is in terms of weeks. The turnover in quiescent tissues is on the order
of months to even years. For example, the turnover of the liver of rodents is estimated to
be about one year; the turnover rate in the livers of humans is also slow. Even the tissue
of the central nervous system is now known to turn over, though at a very slow rate,
through the action of stem cells in compartments that line the ventricles of the brain.
Tissue Genesis
The preceding overview relates mainly to steady-state tissues that develop and regener-
ate through stem cell compartments and maturing lineages of cells. This steady state is
achieved gradually during embryonic development and the process of organogenesis,
which is quite complicated, as exemplified by hematopoiesis or the formation of blood
cells. During vertebrate ontogeny, hematopoiesis sequentially occupies the yolk sac, fetal
liver, spleen, and bone marrow. Variations in this pattern exist among vertebrate species.
The earliest identification of hematopoietic cells is their assignment to the progeny of the
C4 blastomere in the 32-cell embryo. The blastula grows to about 1,000 cells (ten doublings)
and assumes a spherical shape. Then, the blastula undergoes gastrulation, not unlike
TABLE 6.3
Cell Renewable Rates in Tissues
Tissue
Species
Turnover Time (days)
Erythropoiesis
Rat
2.5
Myelopoiesis
Rat
1.4
Hematopoiesis
Human
2.5
Small intestinal epithelium
Human
4-6
Rat
1-2
Epidermis
Human
7-100
Corneal epithelium
Human
7
Lymphatic cells
Rat (thymus)
7
Rat (spleen)
15
Epithelial cells
Rat (vagina)
3.9
Human (cervix)
5.7
Spermatogonia
Human
74
Renal interstitial cells
Mouse
165
Hepatic cells
Rat
400-500
