Biomedical Engineering Reference
In-Depth Information
some of the medical and scientific indications, adult stem cells are much closer to
therapeutic applications; embryonic cells still have a variety of obstacles that need
to be overcome. Moreover, embryo cells can be a bit too flexible, differentiating
into all kinds of tissue, both desirable and not. When injected under the skin of cer-
tain mice, for example, they grow into teratomas, tumors consisting of numerous
tissue types, from gut to skin to teeth. Injected adult stem cells are better behaved,
growing into other tissues only after the application of appropriate growth factors or
other external cues. This need for external cues is ideal since doctors want to be able
to tightly control results [
2,
3
] .
Overview
James Thomson (1988) with colleagues reported methods for deriving and main-
taining human embryonic stem (ES) cells from the inner cell mass of human blasto-
cysts that were produced through in vitro fertilization (IVF) and donated for research
purposes [
4
]. John Gearhar's group reported at the same time the derivation of cells
that were identified as embryonic germ (EG) cells. The cells were cultured from
primordial germ cells obtained from the gonadal ridge and mesenchyma of 5- to
9-week fetal tissue that resulted from elective abortions [
5
] .
The methods were developed by two teams for culturing human ES and EG cells
by drawing on a host of animal studies based in the past: derivations of pluripotent
mouse ES cells from blastocysts (Andrews PW, personal communication) [
6
] ,
reports of the derivation of EG cells [
7,
8
], experiments with stem cells derived from
mouse teratocarcinomas [
9-
11
], the derivation and culture of ES cells from the
blastocysts of rhesus monkeys [
4
] and marmosets [
12
], and methods used by IVF
clinics to prepare human embryos for transplanting into the uterus to produce a live
birth [
13,
14
] .
Since the first isolation of embryonic stem cells from human blastocysts in 1994
(Andrews PW, personal communication; Bongso A, personal communication) [
7-
14
], techniques for deriving and culturing human ES cells have been refined [
15,
16
]. The ability to isolate human ES cells from blastocysts and grow them in culture
seems to depend in large part on the integrity and condition of the blastocyst from
which the cells are derived. In general, blastocysts with a large and distinct inner
cell mass tend to yield ES cultures most efficiently [
13
] .
Blastocyst In Vitro
After a human oocyte is fertilized in vitro by a sperm cell, the cascade of events
occur in fertilized egg (zygote) according to a fairly predictable timeline (Bongso A,
personal communication) [
15-
19
]. A normal day-5 human embryo in vitro consists
of 200-250 cells, when blastocysts are used to derive ES cell cultures. Most of the cells
