Embryo Project Encyclopedia Articles

Conrad Hal Waddington's "Experiments on Embryonic Induction III," published in 1934 in the Journal of Experimental Biology, describes the discovery that the primitive streak induces the mammalian embryo. Waddington's hypothesis was that a transplanted primitive streak could induce neural tissue in the ectoderm of the rabbit embryo. The primitive streak defines the axis of an embryo and is capable of inducing the differentiation of various tissues in a developing embryo during gastrulation. In this experiment Waddington was, in fact, able to induce neural differentiation. Waddington noted that the tissue is "competent"; for a chick organizer, and by deduction a mammalian organizer must exist. Competence refers to a cell's ability to respond to an inducing signal, which is temporally limited to certain developmental stages. Waddington's initial work laid the foundation for many decades of research to follow, including further experiments by Waddington with the mammalian organizer.

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.

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.

Frederik Ruysch, working in the Netherlands, introduced the term epithelia in the third volume of his Thesaurus Anatomicus in 1703. Ruysch created the term from the Greek epi, which means on top of, and thele, which means nipple, to describe the type of tissue he found when dissecting the lip of a cadaver. In the mid nineteenth century, anatomist Albrecht von Haller adopted the word epithelium, designating Ruysch's original terminology as the plural version. In modern science, epithelium is a type of animal tissue in which cells are packed into neatly arranged sheets. The epithelial cells lie proximate to each other and attach to a thin, fibrous sheet called a basement membrane. Epithelia line the surfaces of cavities and structures throughout the body, and also form glands. Although they lack blood vessels, epithelia contain nerves and can function to receive sensation, absorb, protect, and secrete, depending on which part of the body the epithelia line. During development, epithelia act in conjunction with another tissue type, mesenchyme, to form nearly every organ in the body, from hair and teeth to the digestive tract. Epithelia are an essential part of embryonic development and the maintenance and function of the body throughout life.

Hilde Proscholdt Mangold was a doctoral student at the Zoological Institute at the University of Freiburg in Freiburg, Germany, from 1920-1923. Mangold conducted research for her dissertation 'On the Induction of Embryonic Primordia by Implantation of Organizers from Different Species' ('Ueber Induktion von Embryonanlagen durch Implantation artfremder Organisatoren'), under the guidance of Hans Spemann, a professor of zoology at the University of Freiburg. The dissertation was the culmination of five experiments on three species of newt embryos, of the genus Triton (presently, Triturus), performed during the summers of 1921 and 1922, which resulted in a confirmation of Spemann's organizer concept. Spemann and Mangold published the dissertation in a 1924 edition of Roux's Archives for Microscopic Anatomy and Developmental Mechanics (Roux's Archiv fur Mikroskopische Anatomie und Entwicklungsmechanik)."

In 1962 researcher John Bertrand Gurdon at the University of Oxford in Oxford, England, conducted a series of experiments on the developmental capacity of nuclei taken from intestinal epithelium cells of feeding tadpoles. In the experiments, Gurdon conducted nuclear transplantation, or cloning, of differentiated cells, or cells that have already specialized to become one cell type or another, in tadpoles. Gurdon's experiment showed that differentiated adult cells could be induced to an undifferentiated state, where they could once again become multiple cell types. Gurdon's experiment disproved the theory that differentiated cells could not be undifferentiated or dedifferentiated into a new type of differentiated cell. Gurdon's experiment demonstrated nuclear transplantation, also called cloning, using differentiated cells.

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.

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.

Torsten Nils Wiesel studied visual information processing and development in the US during the twentieth century. He performed multiple experiments on cats in which he sewed one of their eyes shut and monitored the response of the cat’s visual system after opening the sutured eye. For his work on visual processing, Wiesel received the Nobel Prize in Physiology or Medicine in 1981 along with David Hubel and Roger Sperry. Wiesel determined the critical period during which the visual system of a mammal develops and studied how impairment at that stage of development can cause permanent damage to the neural pathways of the eye, allowing later researchers and surgeons to study the treatment of congenital vision disorders.