Final answer:
The sample size for fetal cell transplantation in HD is small due to the complex ethical issues, the need for precise tissue matching due to MHC polygeny and polymorphism, and the high expense of such cutting-edge treatments.
Step-by-step explanation:
Reasons for Small Sample Sizes in Fetal Cell Transplantation for Huntington's Disease (HD)
The sample sizes for fetal cell transplantation for HD are typically small due to the complexity and ethical considerations involved in such procedures. This type of treatment involves transplanting fetal cells into the brain to replace the damaged cells caused by the disease. There are inherent risks and ethical dilemmas associated with using fetal cells for transplantation, which limits the number of available transplants and participants, making large-scale studies difficult. Additionally, matching the tissue types of the donor and recipient to prevent rejection can be extremely challenging due to MHC polygeny and MHC polymorphism, resulting in a need for precise matching that further reduces the pool of suitable donors and recipients.
Moreover, cutting-edge treatments like this are often very expensive and may not be covered by insurance, making it inaccessible for many patients which contributes to smaller sample sizes. Intensive research and the evolution of IVF technology also impact the availability and ethical considerations of using fetal cells. As a consequence, only a very small percentage of cases use these advanced methodologies, and any related studies are likely to involve small groups of participants.