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.

Tooth enamel contains relics of its formation process, in the form of microstructures, which indicate the incremental way in which it forms. These microstructures, called cross-striations and striae of Retzius, develop as enamel-forming cells called ameloblasts, whcih cyclically deposit enamel on developing teeth in accordance with two different biological clocks. Cross-striations result from a twenty-four hour cycle, called a Circadian rhythm, in the enamel deposition process, while striae of Retzius have a longer periodicity. Unlike other tissues, enamel does not remodel after it forms, leaving those microstructures intact after deposition. Cross-striations and striae of Retzius thus provide evidence of the timing and processes of tooth development, and they indicate how organisms in a lineage differently grow and develop across generations. Researchers have examined those microstructures to investigate human evolution.

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)."

Matthew Kaufman was a professor of anatomy at the University of Edinburgh, in Edinburgh, UK, who specialized in mouse anatomy, development, and embryology during the late twentieth century. According to the The Herald, he was the first, alongside his colleague Martin Evans, to isolate and culture embryonic stem cells. Researchers initially called those cells Evans-Kaufman cells. In 1992, Kaufman published The Atlas of Mouse Development, a book that included photographs of mice development and mice organs over time. Kaufman also wrote books about UK medical history, phrenology, or the study of craniums as an indicator of character or mental ability, and medical teaching in the eighteenth and nineteenth centuries. Kaufman’s anatomical records and experiments in mouse development contributed to genetic engineering, embryology, and anatomy.

John Hunter studied human reproductive anatomy, and in eighteenth century England, performed one of the earliest described cases of artificial insemination. Hunter dissected thousands of animals and human cadavers to study the structures and functions of organ systems. Much of his anatomical studies focused on the circulatory, digestive, and reproductive systems. He helped to describe the exchange of blood between pregnant women and their fetuses. Hunter also housed various natural collections, as well as thousands of preserved specimens from greater than thirty years of anatomy work. Hunter's work developed practices in reproductive and reparative surgery and furthered the study of human anatomy and physiology.

Isidore Geoffroy Saint-Hilaire studied anatomy and congenital abnormalities in humans and other animals in nineteenth century France. Under the tutelage of his father, Etienne Geoffroy Saint-Hilaire, Isidore compiled and built on his father's studies of individuals with developmental malformations, then called monstrosities. In 1832, Isidore published Histoire generale et particuliere des anomalies de l'organisation chez l'homme et les animaux (General and Particular History of Structural Monstrosities in Man and Animals), in which he defined the term teratology as the study of birth defects and deformities. Isidore Geoffroy Saint-Hilaire established teratology as a legitimate branch of scientific study.

Andreas Vesalius, also called Andries van Wesel, studied anatomy during the sixteenth century in Europe. Throughout his career, Vesalius thoroughly dissected numerous human cadavers, and took detailed notes and drawings of his research. Compiling his research, Vesalius published an anatomy work titled De humani corporis fabrica libri septem (On the fabric of the human body in seven books). The Fabrica included illustrations of dissected men, women, and uteruses with intact fetuses. Vesalius was one of the first physicians to accurately record and draw what was inside of the human body based on his findings from human autopsies, which led to improved understanding of the human body and enhanced surgery techniques.

In 2012, a team of scientists across the US conducted an experiment to find the mechanism that allowed a group of flatworms, planarians, to regenerate any body part. The group included Danielle Wenemoser, Sylvain Lapan, Alex Wilkinson, George Bell, and Peter Reddien. They aimed to identify genes that are expressed by planarians in response to wounds that initiated a regenerative mechanism. The researchers determined several genes as important for tissue regeneration. The investigation helped scientists explain how regeneration is initiated and describe the overall regenerative mechanism of whole organisms.

Regeneration is a fascinating phenomenon. The fact that many organisms have the capacity to regenerate lost parts and even remake complete copies of themselves is difficult to fathom; so difficult, in fact, that for a very long time people were reluctant to believe regeneration actually took place. It seemed unbelievable that some organisms could re-grow lost limbs, organs, and other body parts. If only we could do the same! Unfortunately, our regenerative capacities are limited to hair, nails, and skin, while the liver and a few other tissues display more restricted regenerative abilities. What if we could grow back lost limbs, or damaged organs? This question has inspired many stories, dating back to Greek mythology, wherein Prometheus was doomed to regenerate his liver after it had been devoured by birds. Regeneration has permeated many imaginations; it has appeared in many literary and religious texts, and has also provoked much interest from the scientific community.