Embryo Project Encyclopedia Articles

Telomeres are sequences of DNA on the ends of chromosomes that protect chromosomes from sticking to each other or tangling, which could cause irregularities in normal DNA functions. As cells replicate, telomeres shorten at the end of chromosomes, which correlates to senescence or cellular aging. Integral to this process is telomerase, which is an enzyme that repairs telomeres and is present in various cells in the human body, especially during human growth and development. Telomeres and telomerase are required for normal human embryonic development because they protect DNA as it completes multiple rounds of replication.

Mesoderm is one of the three germ layers, groups of cells that interact early during the embryonic life of animals and from which organs and tissues form. As organs form, a process called organogenesis, mesoderm interacts with endoderm and ectoderm to give rise to the digestive tract, the heart and skeletal muscles, red blood cells, and the tubules of the kidneys, as well as a type of connective tissue called mesenchyme. All animals that have only one plane of symmetry through the body, called bilateral symmetry, form three germ layers. Animals that have only two germ layers develop open digestive cavities. In contrast, the evolutionary development of the mesoderm allowed in animals the formation of internal organs such as stomachs and intestines (viscera).

Carol Widney Greider studied telomeres and telomerase in the US at the turn of the twenty-first century. She worked primarily at the University of California, Berkeley in Berkeley, California.
She received the Nobel Prize in Physiology or Medicine in 2009, along with Elizabeth Blackburn and Jack Szostak, for their research on telomeres and telomerase. Telomeres are repetitive sequences of
DNA at the ends of chromosomes that protect chromosomes from tangling, and they provide some protection from mutations. Greider also studied telomerase, an enzyme that repairs telomeres. Without telomeres, chromosomes are subject to mutations that can lead to
cell death, and without telomerase, cells might not reproduce fast enough during embryonic development. Greider's research on telomeres helped scientists explain how chromosomes function within cells.

The Y-chromosome is one of a pair of chromosomes that determine the genetic sex of individuals in mammals, some insects, and some plants. In the nineteenth and twentieth centuries, the development of new microscopic and molecular techniques, including DNA sequencing, enabled scientists to confirm the hypothesis that chromosomes determine the sex of developing organisms. In an adult organism, the genes on the Y-chromosome help produce the male gamete, the sperm cell. Beginning in the 1980s, many studies of human populations used the Y-chromosome gene sequences to trace paternal lineages. In mammals, the Y-chromosomes contain the master-switch gene for sex determination, called the sex-determining region Y, or the SRY gene in humans. In most normal cases, if a fertilized egg cell, called a zygote, has the SRY gene, the zygote develops into an embryos that has male sex traits. If the zygote lacks the SRY gene or if the SRY gene is defective, the zygote develops into an embryo that has female sex traits.

Barbara McClintock conducted experiments on corn (Zea mays) in the United States in the mid-twentieth century to study the structure and function of the chromosomes in the cells. McClintock researched how genes combined in corn and proposed mechanisms for how those interactions are regulated. McClintock received the Nobel Prize in Physiology or Medicine in 1983, the first woman to win the prize without sharing it. McClintock won the award for her introduction of the concept of transposons, also called jumping genes. McClintock conceptualized some genetic material as not static in structure and order, but as subject to re-arrangement and may be altered during development.

In 1893, Julia Barlow Platt published her research on the origins of cartilage in the developing head of the common mudpuppy (Necturus maculosus) embryo. The mudpuppy is an aquatic salamander commonly used by embryologists because its large embryonic cells and nuclei are easy to see. Platt followed the paths of cells in developing mudpuppy embryos to see how embryonic cells migrated during the formation of the head. With her research, Platt challenged then current theories about germ layers, the types of cells in an early embryo that develop into adult cells. In most organisms' development, three types of germ layers are responsible for the formation of tissues and organs. The outermost layer is called ectoderm, the middle layer mesoderm, and the innermost layer endoderm, although Platt called it entoderm. Platt's research provided a basis for scientists to clarify the destination or function of the germ layers in vertebrates' development.

Curt Jacob Stern studied radiation and chromosomes in humans and fruit flies in the United States during the twentieth century. He researched the mechanisms of inheritance and of mitosis, or the process in which the chromosomes in the nucleus of a single cell, called the parent cell, split into identical sets and yield two cells, called daughter cells. Stern worked on the Drosophila melanogaster fruit fly, and he provided early evidence that chromosomes exchange genetic material during cellular reproduction. During World War II, he provided evidence for the harmful effects of radiation on developing organisms. That research showed that mutations can cause problems in developing fetuses and can lead to cancer. He helped explain how genetic material transmits from parent to progeny, and how it functions in developing organisms.

Kurt Benirschke studied cells, placentas, and endangered species in Germany and the US during the twentieth century. Benirschke was professor at the University of California in San Diego, California, and a director of the research department at the San Diego Zoo in San Diego, California. He also helped form the research department of the San Diego Zoo and its sister organization, the Center for Reproduction of Endangered Species. Benirschke contributed to the field of embryology through his work on human and animal reproduction, including work on human placentas and birth defects, through work on the structure of chromosomes, and through work on the reproduction and conservation of endangered species.

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.

Theophilus Shickel Painter studied the structure and
function of chromosomes in the US during in the early to mid-twentieth century. Painter worked at
the University of Texas at Austin in Austin, Texas. In the 1920s
and 1930s, Painter studied the chromosomes of the salivary gland
giant chromosomes of the fruit fly (Drosophila
melanogaster), with Hermann J. Muller. Muller and Painter
studied the ability of X-rays to cause changes in the chromosomes
of fruit flies. Painter also studied chromosomes in mammals.
He investigated the development of the male gamete, a process
called spermatogenesis, in several invertebrates and vertebrates,
including mammals. In addition, Painter studied the role the
Y-chromosome plays in the determination and development of the male
embryo. Painter's research concluded that egg cells fertilized by
sperm cell bearing an X-chromosome resulted in a female embryo,
whereas egg cells fertilized by a sperm cell carrying a
Y-chromosome resulted in a male embryo. Painter's work with
chromosomes helped other researchers determine that X- and
Y-chromosomes are responsible for sex determination.