Research and Therapy Developments 
125 
that they can be used in future treatments for devastating neu- 
rological diseases such as Alzheimer Disease and PD. 
4. Adult Stem Cells from Other Sources. 
Prentice [see (4)] has summarized a large amount of recent 
information on preparations of stem cells isolated from amni- 
otic fluid , peripheral blood, umbilical cord blood, umbilical 
cord, brain tissue, muscle, liver, pancreas , cornea , salivary 
gland, skin, tendon, heart, cartilage, thymus , dental pulp , and 
adipose tissue . Studies of many of the stem cell preparations 
from these sources are just getting started, and further work is 
needed to determine their biological properties and their relat- 
edness to other stem cell types. In some cases, the long-term 
expandability in vitro of these stem cells has not been demon- 
strated. Yet, the demonstration that they can be isolated from 
such tissue compartments in animals should spur the search 
for similar human stem cell types. 
As Prentice also reports,^ many attempts have already been 
made using various preparations of adult stem cells to influ- 
ence or alter the course of diseases in animal models. Despite 
the fact that the stem cell preparations used are not well char- 
acterized, and reproducible results have yet to be obtained, 
preliminary findings are sometimes encouraging. It is of course 
not yet clear whether the injected cells are functioning as stem 
cells, fusing with existing host cells, or stimulating the influx 
of the host’s own stem cells into the target tissue.* But, if re- 
produced, these preliminary findings may point the way to fu- 
ture therapies, even in the absence of precise knowledge of 
the mechanism(s) of cellular action. 
B, Human Embryonic Stem Cells 
1. Human Embryonic Stem Cells (ESCs). 
Human embryonic stem cells have been isolated from the 
inner cell masses of blastocyst-stage human embryos in multi- 
In a recent review article on adult stem cell plasticity, Raff (see (8)] dis- 
cusses the phenomenon of spontaneous cell fusion masquerading as cell 
plasticity. 
PRE -PUBLICATION VERSION 
