Should any particular type(s) of human bodily material be singled out as 'special' in some way?
There are several types of human bodily material that should be regarded as special. Three examples are gametes, stem cells and mitochondria.
Gametes (sperm and eggs), embryos and reproductive tissues are special, inasmuch as they can lead to conception - in other words, to the creation of an individual whose genetic complement is partially derived from the gamete donor. This genetic connection between individuals is of potential medical, emotional and legal significance.
Genetic connections are medically significant, because characteristics (including disorders) can be transmitted via genetic inheritance. Genetic connections are emotionally significant, because many people ascribe an automatic emotional significance to their genetic relatives (although this is not always the case and it may not be as common a sentiment in future, so such significance should not be taken as axiomatic). Genetic connections are legally significant, because following the removal in 2005 of entitlement to donor anonymity in the UK, individuals conceived through gamete donation can receive identifiable information about the donor once aged 18 and may try to contact the donor.
Stem cells are special, inasmuch as their potency is different from that of other cells in the body. A cell's potency is its potential to differentiate into different types of cells. Stem cells are typically totipotent (meaning that they can develop into an entire organism) or pluripotent (meaning that they can give rise to any fetal or adult cell type). By contrast, other cells in the body are typically multipotent or oligopotent (meaning that they can give rise to a limited number of cell types), or else unipotent (meaning that they can give rise to one cell type).
This, together with the fact that it is not always possible to obtain stem cells from adults (they must sometimes be obtained from embryos, fetuses, umbilical cords or amniotic fluid), means that stem cells may pose different challenges and require different considerations from other types of human bodily material.
Mitochondria are special because they contain genetic information in the form of DNA. This mitochondrial DNA is involved in processes that are related to, but distinct from, the processes associated with the more familiar DNA that is contained within the nucleus of human cells. Evolutionarily, mitochondria are enslaved primitive bacteria with their own genetic code. They replicate independently of the nucleus, and carry no genetic information that defines any specifically human attribute.
Like nuclear DNA, mitochondrial DNA is involved in processes of genetic inheritance. But unlike nuclear DNA, which contains genes derived from both of a person's genetic parents, mitochondrial DNA is derived only from a person's mother (except in very rare instances - see the journal article 'Paternal Inheritance of mitochondrial DNA', Marianne Schwartz and John Vissing, New England Journal of Medicine, 22 August 2002).
The number of genes contained within mitochondria is far smaller than the number of genes contained within the cell nucleus. Nonetheless, abnormalities in mitochondrial DNA can lead to debilitating and sometimes fatal disorders, such as mitochondrial myopathy. The transmission of such disorders from women to their offspring has eluded biomedical remedy until recently.
A pioneering technique known as pronuclear transfer (see the journal article 'Pronuclear Transfer in Human Embryos to Prevent Transmission of Mitochondrial DNA Disease', Mary Herbert, Douglass Turnbull et al, Nature, 6 May 2010) involves fertilising the egg of a woman who has abnormal mitochondrial DNA, and then transferring the nucleus of the resulting zygote into the egg of a donor who has normal mitochondrial DNA. This achievement elicited some ethical concerns, from people who claimed that the resulting (healthy) child had 'three parents'.
These concerns are unfounded. Since mitochondria carry no genetic information that defines any specifically human attribute, it is as erroneous (if not more so) to say that a child following pronuclear transfer has 'three parents' as it would be to say that the recipient of a donated organ has 'four parents'.
Nonetheless, misapprehensions about mitochondria are noteworthy in that they illustrate the importance of making a correct distinction between mitochondrial DNA and nuclear DNA. Most people's assumptions about DNA, genes and inheritance in humans is restricted to nuclear DNA, whereas mitochondrial DNA requires more flexible thinking.