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.

The sex of a reptile embryo partly results from the production of sex hormones during development, and one process to produce those hormones depends on the temperature of the embryo's environment. The production of sex hormones can result solely from genetics or from genetics in combination with the influence of environmental factors. In genotypic sex determination, also called genetic or chromosomal sex determination, an organism's genes determine which hormones are produced. Non-genetic sex determination occurs when the sex of an organism can be altered during a sensitive period of development due to external factors such as temperature, humidity, or social interactions. Temperature-dependent sex determination (TSD), where the temperature of the embryo's environment influences its sex development, is a widespread non-genetic process of sex determination among vertebrates, including reptiles. All crocodilians, most turtles, many fish, and some lizards exhibit TSD.

Isotretinoin is a molecule and a byproduct (metabolite) of vitamin A, and in greater than normal amounts in pregnant women, it can cause fetal abnormalities including cleft lips, ear and eye defects, and mental retardation. Isotretinoin is commonly called by its trade name Accutane, and it's a chemical compound derived from vitamin A, or retinoic acid. Doctors prescribe isotretinoin to treat severe acne. For pregnant women, too much vitamin A or isotretinoin can also cause greater than normal rates of stillbirths and fetal disintegrations after the ninth week of gestation. Women who use isotretinoin during the first trimester of their pregnancies, even in small amounts, risk defects to their fetuses such as external ear malformations, cleft palates, undersized jaws (micrognathia), a variety of heart defects, buildups of fluids inside the skulls that leads to brain swelling (hydrocephalus), small heads and brains (microcephaly), and mental retardation.

Josef Warkany studied the environmental causes of birth defects in the United States in the twentieth century. Warkany was one of the first researchers to show that factors in the environment could cause birth defects, and he helped to develop guidelines for the field of teratology, the study of birth defects. Prior to Warkany’s work, scientists struggled to explain if or how environmental agents could cause birth defects. Warkany demonstrated that a deficiency or excess of vitamin A in maternal nutrition could cause birth defects. He also established that mercury in teething powders increased infant mortality rates. Warkany showed how substances outside the human body could adversely affect conception, growth, and development of the human fetus in utero.

The article Experimental Studies on Congenital Malformations was published in the Journal of Chronic Diseases in 1959. The author, James G. Wilson, studied embryos and birth defects at the University of Florida Medical School in Gainesville, Florida. In his article, Wilson reviewed experiments on birds and mammals from the previous forty years to provide general principles and guidelines in the study of birth defects and teratogens, which are things that cause birth defects. Those principles included what species are convenient for conducting teratological research, what principles act in human embryological and fetal development, and what agents impact those processes. Wilson's article was one of the first attempts in the twentieth century to synthesize basic research conducted in the field of teratology. The article helped to establish teratology as a field in medicine during the twentieth century.

Sidney Q. Cohlan studied birth defects in the US during the twentieth century. Cohlan helped to discover that if a pregnant woman ate too much vitamin A her fetus faced a higher than normal risk of teratogenic effects, such as cleft palate. A teratogen is a substance that causes malformation of a developing organism. Cohlan also identified the teratogenic effects of several other substances including a lack of normal magnesium and prenatal exposure to the antibiotic tetracycline. Cohlan's experiments with vitamins and other chemicals brought attention to how nutrition and environmental agents adversely affect human pregnancy outcomes.

Thalidomide is a sedative drug introduced to European markets on 1 October 1957 after extensive testing on rodent embryos to ensure its safety. Early laboratory tests in rodent populations showed that pregnant rodents could safely use it, so doctors prescribed Thalidomide to treat morning sickness in pregnant women. However, in humans Thalidomide interfered with embryonic and fetal development in ways not observed in rodent tests. Pregnant women who take Thalidomide are at grater than normal risk for spontaneous abortion and for giving birth to children with developmental anomalies such as shortened, absent, or extra limbs, as well as a variety of heart, ear, and internal organ defects. The failure of rodent models to inform scientists of Thalidomide's teratogenicity in humans ignited debate about the proper use of cross-species testing during drug development.

In Australia in the 1940s, Norman McAlister Gregg observed a connection between pregnant women who contracted the rubella virus, or German measles, and cataract formation in their children's eyes. Gregg published his findings in the 1941 article Congenital Cataract following German Measles in the Mother in Transactions of the Ophthalmological Society of Australia. In the article, Gregg analyzed seventy-eight cases of congenital cataracts and suggested that the mothers' environmental factors could cause birth defects, otherwise known as teratogenic effects. Gregg's paper on the teratogenic effects of an environmental agent, the rubella virus, changed the study of birth defects to include viruses as potential causes or teratogens.

Solomon A. Berson helped develop the radioimmunoassay (RIA) technique in the US during the twentieth century. Berson made many scientific contributions while working with research partner Rosalyn Yalow at the Bronx Veterans Administration (VA) hospital, in New York City, New York. In the more than twenty years that Berson and Yalow collaborated, they refined the procedures for tracing diagnostic biological compounds using isotope labels. In the late 1950s they developed the RIA based on the ability to trace the competition between and ligands, or small molecules that bind to specific sites of other biomolecules, and proteins for the same molecular binding site, a process called competitive binding. Scientists widely used Berson and Yalow's RIA, as these methods permit the use of a minimal sample of blood for accurate measurements of biological molecules such as hormones that cause the production of antibodies. Berson and Yalow's research has advanced the study of physiology, including that of the reproductive system, with particular applications to the diagnosis and treatment of infertility.

Advanced Cell Technology (ACT), a stem cell biotechnology company in Worcester, Massachusetts, showed the potential for cloning to contribute to conservation efforts. In 2000 ACT researchers in the United States cloned a gaur (Bos gaurus), an Asian ox with a then declining wild population. The researchers used cryopreserved gaur skin cells combined with an embryo of a domestic cow (Bos taurus). A domestic cow also served as the surrogate for the developing gaur clone. The successful procedure opened the opportunity to clone individuals from species for which there are few or zero live specimens. The official release of this experiment's data was published in the paper 'Cloning of an Endangered Species (Bos gaurus) Using Interspecies Nuclear Transfer,' in October 2000. In the article, the researchers presented data collected from several cloned fetuses that were aborted before the full term of 283 days. At the time of publication, the gaur bull fetus, named Noah at birth, had developed for greater than 180 days. Noah was born on 8 January 2001, but died two days later due to dysentery. The development, birth, and death of Noah became a platform for conservationists and ethicists to critique the role of cloning in society and as a method to conserve species.