Embryo Project Encyclopedia Articles

Endoderm is one of the germ layers-- aggregates of cells that organize early during embryonic life and from which all organs and tissues develop. All animals, with the exception of sponges, form either two or three germ layers through a process known as gastrulation. During gastrulation, a ball of cells transforms into a two-layered embryo made of an inner layer of endoderm and an outer layer of ectoderm. In more complex organisms, like vertebrates, these two primary germ layers interact to give rise to a third germ layer, called mesoderm. Regardless of the presence of two or three layers, endoderm is always the inner-most layer. Endoderm forms the epithelium-- a type of tissue in which the cells are tightly linked together to form sheets-- that lines the primitive gut. From this epithelial lining of the primitive gut, organs like the digestive tract, liver, pancreas, and lungs develop.

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 Cell-Theory was written by Thomas Henry Huxley in Britain and published in 1853 by The British and Foreign Medico-Chirurgical Review. The twenty-two page article reviews twelve works on cell theory, including those in Germany by Caspar Friedrich Wolff in the eighteenth century and by Karl Ernst von Baer in the nineteenth century. Huxley spends much of The Cell-Theory on a cell theory proposed in the late 1830s by Matthias Schleiden and Theodor Schwann in Germany. Schleiden and Schwann maintained that the cell was the most fundamental unit of life and that the nucleus was the most significant cellular component. Huxley, instead, promoted an epigenetic theory of the cell, for which properties of life emerge from the outer cytoplasm, cell membrane, and wall (the periplast), as opposed to the inner contents of the cell, including the nucleus (the endoplast). Huxley's arguments in The Cell-Theory influenced future scientists about the role of epigenetic processes in embryology and development.

Samuel Randall Detwiler was an embryologist who studied neural development in embryos and vertebrate retinas. He discovered evidence for the relationship between somites and spinal ganglia, that transplanted limbs can be controlled by foreign ganglia, and the plasticity of ganglia in response to limb transplantations. He also extensively studied vertebrate retinas during and after embryonic development. Detwiler's work established many principles studied in later limb transplantation experiments and was identified by Viktor Hamburger as an important bridge between his and Ross Granville Harrison's research.

Rudolf Carl Virchow lived in nineteenth century Prussia, now Germany, and proposed that omnis cellula e cellula, which translates to each cell comes from another cell, and which became and fundamental concept for cell theory. He helped found two fields, cellular pathology and comparative pathology, and he contributed to many others. Ultimately Virchow argued that disease is caused by changes in normal cells, also known as cellular pathology.

Matthias Jacob Schleiden helped develop the cell theory in Germany during the nineteenth century. Schleiden studied cells as the common element among all plants and animals. Schleiden contributed to the field of embryology through his introduction of the Zeiss microscope lens and via his work with cells and cell theory as an organizing principle of biology.

In 2011, Sonja Vernes and Simon Fisher performed a series of experiments to determine which developmental processes are controlled by the mouse protein Foxp2. Previous research showed that altering the Foxp2 protein changed how neurons grew, so Vernes and Fisher hypothesized that Foxp2 would affect gene networks that involved in the development of neurons, or nerve cells. Their results confirmed that Foxp2 affected the development of gene networks involved in the growth of neurons, as well as networks that are involved in cell specialization and cell communication. The researchers determined that Foxp2 is important for a variety of developmental processes such as motor control, language acquisition, and cognition.

Camillo Golgi studied the central nervous system during the late nineteenth and early twentieth centuries in Italy, and he developed a staining technique to visualize brain cells. Called the black reaction, Golgi’s staining technique enabled him to see the cellular structure of brain cells, called neurons, with much greater precision. Golgi also used the black reaction to identify structures within animal cells like the internal reticular apparatus that stores, packs, and modifies proteins, later named the Golgi apparatus in his honor. Golgi, along with Santiago Ramón y Cajal, received the Nobel Peace Prize in 1906 for their independent work on the structure of the nervous system. Golgi’s discovery of the black reaction enabled other scientists to better study the structure of the nervous system and its development.

Scientists use cerebral organoids, which are artificially produced miniature organs that represent embryonic or fetal brains and have many properties similar to them, to help them study developmental disorders like microcephaly. In human embryos, cerebral tissue in the form of neuroectoderm appears within the first nine weeks of human development, and it gives rise to the brain and spinal cord. In the twenty-first century, Juergen Knoblich and Madeleine Lancaster at the Institute of Molecular Biotechnology in Vienna, Austria, grew cerebral organoids from pluripotent stem cells as a model to study developmental disorders in embryonic and fetal brains. One such disorder is microcephaly, a condition in which brain size and the number of neurons in the brain are abnormally small. Scientists use cerebral organoids, which they've grown in labs, because they provide a manipulable model for studying how neural cells migrate during development, the timing of neural development, and how genetic errors can result in developmental disorders.

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.