Embryo Project Encyclopedia Articles

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.

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.

In 1902, editors of the medical journal Alkaloidal Clinic Wallace C. Abbott and William Francis Waugh published Sexual Hygiene, a book about normal sexual physiology and behavior in Chicago, Illinois. Though the book includes a collection of passages from other books, articles, speeches, and documents surrounding sexual physiology and behavior, it does not include text regarding sexual hygiene. Rather, the book contains a preface and twenty-eight chapters on topics including masturbation, incomplete or delayed intercourse, and impotence, meaning the inability for a female to achieve an orgasm or a male to achieve an erection. Though physicians and those who served as experts in the book used seemingly factual scientific evidence to back up their claims, later scientific understanding of male and female physiology disproved many of those assertions. Sexual Hygiene is an early US discussion of sex by medical authorities that provides examples of historical medical misconceptions about sexual practices, physiology, gender roles, and context for understanding reproductive medicine during the early 1900s.

Haeckel believed that the development of an embryo revealed the adult stages of the organism’s ancestors. Haeckel represented this idea with drawings of vertebrate embryos at similar developmental stages. This is Haeckel’s embryo grid, the most common of all illustrations in biology textbooks. Yet, Haeckel’s embryo grids are much more complex than any textbook explanation. I examined 240 high school biology textbooks, from 1907 to 2010, for embryo grids. I coded and categorized the grids according to accompanying discussion of (a) embryonic similarities (b) recapitulation, (c) common ancestors, and (d) evolution.

Despite the minor differences in the inclusiveness of the word, there is a general assumption among the scientific community that the 'pursuit of knowledge' is the most fundamental element in defining the word 'science.' However, a closer examination of how science is being conducted in modern-day South Korea reveals a value system starkly different from the value of knowledge. By analyzing the political discourse of the South Korean policymakers, mass media, and government documents, this study examines the definition of science in South Korea.

The Hayflick Limit is a concept that helps to explain the
mechanisms behind cellular aging. The concept states that a normal human
cell can only replicate and divide forty to sixty times before it
cannot divide anymore, and will break down by programmed cell death
or apoptosis. The concept of the Hayflick Limit revised Alexis
Carrel's earlier theory, which stated that cells can replicate
themselves infinitely. Leonard Hayflick developed the concept while
at the Wistar Institute in Philadelphia,
Pennsylvania, in 1965. In his 1974 book Intrinsic
Mutagenesis, Frank Macfarlane Burnet named the concept after
Hayflick. The concept of the Hayflick Limit helped scientists study
the effects of cellular aging on human populations from embryonic
development to death, including the discovery of the effects of
shortening repetitive sequences of DNA, called telomeres, on the
ends of chromosomes. Elizabeth Blackburn, Jack Szostak and Carol
Greider received the Nobel Prize in Physiology or Medicine in 2009
for their work on genetic structures related to the Hayflick
Limit.

In 1976, midwife Ina May Gaskin published Spiritual Midwifery, with other editions published in 1980, 1990, and 2003. Spiritual Midwifery is a book about pregnancy, birth, and postpartum, or the time period after birth. During the 1970s, it was common for women to receive an epidural, a medication that reduces pain during labor, and for physicians to monitor a fetus’s heartbeat while separating women from their infants after birth. However, according to Gaskin, some women wanted to give birth outside of the hospital without medical interventions. Spiritual Midwifery is a collection of birth stories from women, information about pregnancy and giving birth, and instructions to midwives on how to care for women during childbirth and the period that followed. In Spiritual Midwifery, Gaskin introduced homebirth and midwifery to a broader audience, which helped repopularize midwifery in the US.