Embryo Project Encyclopedia

Leonard Hayflick studied the processes by which cells age during the twentieth and twenty-first centuries in the United States. In 1961 at the Wistar Institute in the US, Hayflick researched a phenomenon later called the Hayflick Limit, or the claim that normal human cells can only divide forty to sixty times before they cannot divide any further. Researchers later found that the cause of the Hayflick Limit is the shortening of telomeres, or portions of DNA at the ends of chromosomes that slowly degrade as cells replicate. Hayflick used his research on normal embryonic cells to develop a vaccine for polio, and from HayflickÕs published directions, scientists developed vaccines for rubella, rabies, adenovirus, measles, chickenpox and shingles.

The "Humanae Vitae," meaning "Of Human Life" and subtitled "On the Regulation of Birth," was an encyclical promulgated in Rome, Italy, on 25 July 1968 by Pope Paul VI. This encyclical defended and reiterated the Roman Catholic Church's stance on family planning and reproductive issues such as abortion, sterilization, and contraception. The document continues to have a controversial reputation today, as its statements regarding birth control strike many Catholics as unreasonable.

The principal work of St. Thomas Aquinas, the Summa Theologica is divided into three parts and is designed to instruct both beginners and experts in all matters of Christian Truth. It discusses topics central to Christian morality, ethics, law, and the life of Christ, providing philosophical and theological solutions to common arguments and questions surrounding the Christian faith. The views presented in this body of writing are currently upheld in large part by the modern doctrines of the Roman Catholic Church. Interesting references to and insights on ensoulment and embryology, as well as other topics discussed in Summa Theologica, indicate a strong Aristotelian and Augustinian influence.

Karl Wilhelm Theodor Richard von Hertwig is an important figure in the history of embryology for his contributions of artificial hybridization of sea urchin eggs and the formulation of his coelom theory. He was born 23 September 1850 in Friedelberg, Germany, to Elise Trapp and Carl Hertwig. Richard and his older brother Oscar began their studies at Jena under the direction of Ernst Haeckel from 1868 to 1871. In 1872 Hertwig became a lecturer in zoology at Jena while Oscar lectured in anatomy and embryology. As both brothers advanced in their respective fields, Hertwig left Jena to become a professor at Königsberg. In 1883 he was professor at Bonn and in 1885 in Munich, where he stayed until his retirement in 1925. Hertwig married Julia Braun in 1887 and had two sons and one daughter. He remained very active his entire life, outliving his brother Oscar by fifteen years.

Chorionic villus sampling (CVS) is a test used for prenatal diagnosis. Safe to perform at an earlier stage in pregnancy than amniocentesis, CVS is another invasive prenatal diagnostic test that can be performed as early as ten weeks after the woman's last menstrual cycle. While this test does carry some risks, it is generally very effective at predicting heritable diseases during or soon after the embryonic stage of development.

Leonard Hayflick in the US during the early 1960s showed that normal populations of embryonic cells divide a finite number of times. He published his results as 'The Limited In Vitro Lifetime of Human Diploid Cell Strains' in 1964. Hayflick performed the experiment with WI-38 fetal lung cells, named after the Wistar Institute, in Philadelphia, Pennsylvania, where Hayflick worked. Frank MacFarlane Burnet, later called the limit in capacity for cellular division the Hayflick Limit in 1974. In the experiment, Hayflick refuted Alexis Carrel's hypothesis that cells could be transplanted and multiplied indefinitely from a single parent cell line.

Telomeres are structures at the ends of DNA strands that get longer in the DNA of sperm cells as males age. That phenomenon is different for most other types of cells, for which telomeres get shorter as organisms age. In 1992, scientists showed that telomere length (TL) in sperm increases with age in contrast to most cell of most other types. Telomeres are the protective caps at the end of DNA strands that preserve chromosomal integrity and contribute to DNA length and stability. In most cells, telomeres shorten with each cell division due to incomplete replication, though the enzyme telomerase functions in some cell lines that undergo repetitive divisions to replenish any lost length and to prevent degradation. Cells, and therefore organisms, with short telomeres are more susceptible to mutations and genetic diseases. While TL increases in a subset of sperm cells and longer telomeres may prevent early disintegration of DNA, it may also prevent natural mechanisms of apoptosis, or cell death, from occurring in abnormal sperm.

Apoptosis, or programmed cell death, is a mechanism in embryonic development that occurs naturally in organisms. Apoptosis is a different process from cell necrosis, which is uncontrolled cell death usually after infection or specific trauma. As cells rapidly proliferate during development, some of them undergo apoptosis, which is necessary for many stages in development, including neural development, reduction in egg cells (oocytes) at birth, as well as the shaping of fingers and vestigial organs in humans and other animals. Sydney Brenner, H. Robert Horvitz, and John E. Sulston received the Nobel Prize in Physiology or Medicine in 2002 for their work on the genetic regulation of organ development and programmed cell death. Research on cell lineages before and after embryonic development may lead to new ways to reduce or promote cell death, which can be important in preventing diseases such as Alzheimer's or cancer.

In the second half of the
twentieth century, scientists learned how to clone organisms in some
species of mammals. Scientists have applied somatic cell nuclear transfer to clone human and
mammalian embryos as a means to produce stem cells for laboratory
and medical use. Somatic cell nuclear transfer (SCNT) is a technology applied in cloning, stem cell
research and regenerative medicine. Somatic cells are cells that
have gone through the differentiation process and are not germ
cells. Somatic cells donate their nuclei, which scientists
transplant into eggs after removing their nucleuses (enucleated eggs).
Therefore, in SCNT, scientists replace the nucleus in an egg cell
with the nucleus from a somatic cell.

"Casti Connubii," a papal encyclical given by Pope Pius XI on 31 December 1930, served primarily as a reaffirmation and expansion of the issues discussed in Arcanum, an encyclical written by Pope Leo XIII. It was released to address new threats to marriage and conjugal unity, and indeed is translated "On Christian Marriage" or "On Chastity in Marriage." The document explores the meaning of Christian marriage and emphasizes its threefold purpose as borrowed from St. Augustine: to produce offspring, to grow in conjugal faith, and to show benefit from the sacrament. It begins by exploring the nature of marriage, followed by a discussion of its advantages for individuals and societies, erroneous but common beliefs about marriage, threats to pure marriage, and finally how to address them. Included in the threats to pure marriage is that of the growing popularity of contraception and abortive procedures, at which point Pope Pius XI elaborates on the Church' s statement that life begins at conception.