Thursday, September 07, 2006

Definition of blastocyst

In the press release fiasco accompanying the ACT report in the embryonic stem cell area, one notes that different people are defining/using the term "blastocyst" differently.

Merely for information purposes:

AND FETUSES, 40 Wake Forest L. Rev. 159 (2005):

Although the term "embryo" is the most commonly used label, the
appropriate scientific term for the conceptus (fertilized egg) at the
earliest stage of development is "blastocyst." So-called "extra-corporeal
embryos" - those that are outside the womb - are actually blastocysts. A
blastocyst is a multi-celled organism (a group of cells around a fluid filled cavity
called the morula) that forms four days after fertilization. Fertilization is the
event that begins with the sperm entering the egg and concludes when the
genetic material is combined to form the zygote, or the single-celled organism,
immediately after the egg and sperm have joined. The cells in question
are undifferentiated. In other words, they are able to form into any of the
cells in the body, and each one of the cells can be separated from the unit and
divide to form another blastocyst. This process, called "twinning," would result
in two (or more) genetically identical individuals. "Embryo" is the term given
to the entity at approximately two weeks after fertilization, which coincides
with the formation of the primitive streak (which will eventually develop into
the neural system). Occasionally, the term "preembryo" is used to refer to the
developing entity during the two-week stage prior to the formation of the
primitive streak. Despite the scientific definitions, the common usage of the term
"embryo" has been extended to cover the many frozen blastocysts currently in
storage, as well as those at almost all stages of development following fertilization. For simplicity, this Article refers to all of the entities in question
prior to the fetal stage as embryos. A "fetus" is the label given to the entity from
eight weeks after fertilization until birth, at which point it is referred to
as a "baby" or "neonate." I do not believe that the use of the terms (or of
proposed different terms) necessarily alters the debate. Certainly there are
misleading terms, including, for example, "developing baby," which imply that the
entity in question is closer in attributes to a child than to original gametes
(that is, sperm and egg). But it does not appear to make much difference in
people's ethical analysis whether the entity is called a blastocyst, preembryo,
or an embryo.

from Robert L. Stenger, 2 J. Health & Biomed. L. 33 (2006)

By the fifth and sixth days the hollow ball of cells (called a blastocyst) contains cells which are totipotent, that is, wholly undifferentiated and capable of becoming
any of the specialized cells in the human body.

Separately, I discuss some issues in 88 JPTOS 239 (2006).


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