Page 127 - 86 human physiology part-2

Chapter 17

The term in vitro, from the Latin root, is used, because early biological experiments involving
cultivation of tissues outside the living organism from which they came, were carried out in glass
containers such as beakers, test tubes, or petri dishes.

While the overall live birth rate via IVF in the U.S. is about 27% per cycle (33% pregnancy rate),
the chances of a successful pregnancy via IVF vary widely based on the age of the woman (or, more
precisely, on the age of the eggs involved). Where the woman's own eggs are used as opposed to those
of a donor, for women under 35, the pregnancy rate is commonly approximately 43% per cycle (37%
live birth), while for women over 40, the rate falls drastically - to only 4% for women over 42. Other
factors that determine success rates include the quality of the eggs and sperm, the duration of the
infertility, the health of the uterus, and the medical expertise. It is a common practice for IVF
programmes to boost the pregnancy rate by placing multiple embryos during embryo transfer. A flip
side of this practice is a higher risk of multiple pregnancy, itself associated with obstetric
complications.

Embryo cryopreservation If multiple embryos are generated, patients may choose to freeze
embryos that are not transferred. Those embryos are placed in liquid nitrogen and can be preserved for
a long time. There are currently 500,000 frozen embryos in the United States. The advantage is that
patients who fail to conceive may become pregnant using such embryos without having to go through a
full IVF cycle. Or, if pregnancy occurred, they could return later for another pregnancy.

Embryonic stem cells

Pluripotent, embryonic stem cells originate as inner mass cells with in a blastocyst. The stem cells
can become any tissue in the body, excluding a placenta. Only the morula's cells are totipotent, able to
become all tissues and a placenta.

Embryonic celtic cell lines (ES cell lines) are cultures of cells derived from the epiblast tissue of
the inner cell mass (ICM) of a blastocyst. A blastocyst is an early stage embryo - approximately 4 to 5
days old in humans and consisting of 50-150 cells. ES cells are pluripotent, and give rise during
development to all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm. In
other words, they can develop into each of the more than 200 cell types of the adult body when given
sufficient and necessary stimulation for a specific cell type. They do not contribute to the extra-
embryonic membranes or the placenta. This means they can become any kind of human tissue (ie. heart
tissue, nerve tissue, etc.).

When given no stimuli for differentiation, ES cells will continue to divide in vitro and each
daughter cell will remain pluripotent. The pluripotency of ES cells has been rigorously demonstrated in
vitro and in vivo, thus they can be indeed classified as stem cells.

Because of their unique combined abilities of unlimited expansion and pluripotency, embryonic
stem cells are a potential source for regenerative medicine and tissue replacement after injury or
disease. To date, no approved medical treatments have been derived from embryonic stem cell
research. This is not surprising considering that many nations currently have moratoria (suspension of
practices) on either ES cell research or the production of new ES cell lines.

There exists a widespread controversy over stem cell research that emanates from the techniques
used in the creation and usage of stem cells. Embryonic stem cell research is particularly controversial

342 | Human Physiology