Theories

Title By Description Created Last modifiedsort ascending
Chloroplasts Anna Guerrero Object is a digital image of a chloroplast. There are two boxes, one atop the other. In the top box is an image of a chloroplast, which is roughly ovoid. A scale bar indicates that the chloroplast is roughly 5 micrometers in length. The outer membrane is colored light green, and the inner membrane is a different shade of light green. The top right parts of the outer and inner membranes are cut away to reveal dozens thylakoids within, which are all dark green and look like tires. They are stacked on top of each other to form ten granums.Chloroplasts are the organelles in plant and algal cells that conduct photosynthesis. A single chloroplast has an outer membrane and an inner membrane, with an intermembrane space in between. Within the inner membrane, interconnected stacks of thylakoids, called granum, float in a protein rich fluid called the stroma. These thylakoid stacks contain chlorophyll, a pigment which converts sunlight into usable energy for plants and free oxygen from water. The stacks are sites of light reactions within a plant cell. 2017-02-06 3 July 2018
Beadle and Tatum's 1941 Experiments with Neurospora Revealed that Genes Produce Enzymes Amy Pribadi Object is a digital image that depicts four stages in Beadle and Tatum's Neurospora experiments. Each stage is depicted in a separate section, with different test tubes in each section.This illustration shows George Beadle and Edward Tatum's experiments with Neurospora crassa that indicated that single genes produce single enzymes. The pair conducted the experiments at Stanford University in Palo Alto, California. Enzymes are types of proteins that can catalyze reactions inside cells, reactions that produce a number of things, including nutrients that the cell needs. Neurospora crassa is a species of mold that grows on bread. 2016-10-12 3 July 2018
The Process of Gastrulation in Frog Embryos Chinami Michaels StageName: 12Illustration of the movement of the three hemispheres of cells, the animal cap (dark green) the marginal zone (lime green) and the ventral cap (yellow) during frog gastrulation. The external view column (images a.1-a.6) shows gastrulation as it occurs on the outside of the embryo. The cross-section view column (images b.1-b.6) shows the internal view of gastrulation. The cross-sections are through the middle of the embryo. 2013-12-13 3 July 2018
Mitochondria Anna Guerrero Object is a digital image of a mitochondrion. There are two boxes, one atop of the other. In the top box is the mitochondrion with a scale bar that indicates that the organelle is 1 micrometer in length. The image depicts the mitochondrion�s outer membrane, which is roughly ovoid in shape and is colored a transparent orange to reveal the inner membrane within, colored red. The top left quarter of the outer membrane and the inner membrane are cut away to reveal the cristae. In the bottom box is a round animal cell, colored teal.Mitochondria are organelles found in the cytoplasm of eukaryotic cells. They are composed of an outer membrane and an inner membrane. The outer membrane faces the cellular cytoplasm, while the inner membrane folds back on itself multiple times, forming inner folds, called cristae. The space between the two membrane layers is called the intermembrane space, and the space within the inner membrane is called the matrix. 2017-02-06 3 July 2018
Apoptosis in Embryonic Development Zane Bartlett 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. 2017-06-08 3 July 2018
Some of the Cells that Arise from Animal Gastrulas with Three Germ Layers Chinami Michaels Object is a digital image of an animal triploblastic gastrula. The image labels the three germ layers and some of the fully differentiated cell types that arise from those germ layers. The image shows the egg and sperm germ cells. It also shows and labels the epithelial cells, liver cells, and endothelial cells that develop from endoderm. It also shows the skeletal musclecells, osteoblast cells, cardiac muscle cells, and red blood cells that develop from the mesoderm. It also shows the hair cells, skin cells, and neuron cells that develop from the ectoderm.From a developing embryos three primary germ layers, ectoderm (green), mesoderm (pink) and endoderm (yellow), a variety of differentiated cell types and organ systems arise, far more than are shown here. The three primary germ layers are shown during the gastrula stage because they become distinct at the gastrula stage. The germ cells (blue) are pre- cursors to sperm and egg cells, and they are set aside early in development, and are thought to arise from the ectoderm. 2014-08-21 3 July 2018
Neurospora crassa Life Cycle Amy Pribadi Object is a digital image with two parts that together show the Neurospora life cycle. The left part shows the asexual reproductive cycle of the mold. The right part shows the sexual reproductive cycle of the mold.This diagram shows the life cycle of Neurospora crassa, a mold that grows on bread. N. crassa can reproduce through an asexual cycle or a sexual cycle. The asexual cycle (colored as a purple circle), begins in this figure with (1a) vegetative mycelium, which are strands of mature fungus. Some of the strands form bulbs (2a) in a process called conidiation. From those bulbs develop the conidia, which are spores. Next, (3a) a single conidium separates from its strand and elongates until it forms mycelium. 2016-10-12 3 July 2018
A Fate Map of the Chick Embryo Chinami Michaels StageName: 3A 3-D fate map of the chicken (Gallus gallus) embryo with the prospective point of ingression and yolk. The area where the primitive streak will form during gastrulation is shown. The anterior- posterior axis is shown by labeling the anterior and posterio ends (A) and (P). Different colors indicate prospective fates of different regions of the epiblast after gastrulation. 2014-02-26 3 July 2018
Paternal Sperm Telomere Elongation and Its Impact on Offspring Fitness Zane Bartlett, Joanna Yang Telomeres are structures at the ends of DNA strands that get longer in the DNA of sperm cells as males age. That phenomenon is different for most other types of cells, for which telomeres get shorter as organisms age. In 1992, scientists showed that telomere length (TL) in sperm increases with age in contrast to most cell of most other types. Telomeres are the protective caps at the end of DNA strands that preserve chromosomal integrity and contribute to DNA length and stability. 2017-02-07 3 July 2018
Frog Embryo in the Blastula Stage Chinami Michaels StageName: 8Illustration of the animal-vegetal gradient in Xenopus laevis ( African clawed frog) eggs after fertilization. During fertilization, the sperm s point of entry determines the future dorsal side (shaded) and ventral side (unshaded) of the embryo. The prospective ventral side of the embryo forms on the side where the sperm enters while the prospective dorsal side forms opposite the sperm s point of entry. 2013-12-16 3 July 2018
Jelly Fish and Green Fluorescent Protein Anna Guerrero Object is a digital image that represents green fluorescent protein at various levels of organization within an organism. On the left of the image is a blue circle, in which there is a jelly fish, with some of its parts aglow. From one such part, a zoom circle juts to the right, in which is represented a strand of DNA from the jelly fish. From that circle, a black arrow points to the right and to a new zoom circle, this one representing the primary amino acid sequence coded for by the DNA sequence and that eventually folds into the protein.The crystal jellyfish, Aequorea victoria, produces and emits light, called bioluminescence. Its DNA codes for sequence of 238 amino acids that forms a protein called Green Fluorescent Protein (GFP). FP is folded so that a part of the protein, called the chromophore, is located in the center of the protein. The chemical structure of the chromophore emits a green fluorescence when exposed to light in the range of blue to ultraviolet. 2017-02-06 3 July 2018
The Neuron Doctrine (1860-1895) Isra Mishqat The neuron doctrine is a concept formed during the turn of the twentieth century that describes the properties of neurons, the specialized cells that compose the nervous system. The neuron doctrine was one of two major theories on the composition of the nervous system at the time. Advocates of the neuron doctrine claimed that the nervous system was composed of discrete cellular units. Proponents of the alternative reticular theory, on the other hand, argued that the entire nervous system was a continuous network of cells, without gaps or synapses between the cells. 2017-06-15 3 July 2018
Trial of Madame Restell (Ann Lohman) for Abortion (1841) Rainey Horwitz In the spring of 1841, abortionist Ann Lohman, called Madame Restell, was convicted for crimes against one of her abortion clients, Maria Purdy. In a deathbed confession, Purdy admitted that she had received an abortion provided by Madame Restell, and she further claimed that the tuberculosis that she was dying from was a result of her abortion. Restell was charged with administering an illegal abortion in New York and her legal battles were heavily documented in the news. 2017-10-05 3 July 2018
Beadle's One Gene-One Enzyme Hypothesis Amy Pribadi Object is a digital image with two panes, one on top of the other, both of which picture the area within a cell between the nucleus and the cell membrane. The top pane represent three genes within the cell nucleus, each of which produces a distinct kind of enzyme outside of the nucleus. Those enzymes then function in three distinct kinds of metabolic reactions. The bottom pane represents the same situation, except the second gene is damaged by x-rays and can't produce its enzymes. As a result, two of the three metabolic reactions fail to happen.Between 1934 and 1945, George Beadle developed a hypothesis that each gene within the chromosomes of organisms each produced one enzyme. Enzymes are types of proteins that can catalyze reactions inside cells, and the figure shows that each enzyme controls a stage in a series of biochemical reactions. The top box in this figure represents a normal process of enzyme production and biochemical reactions, and the bottom box shows how Beadle's experiments affected the normal biochemical process. 2016-10-12 3 July 2018
The Blastoderm in Chicks During Early Gastrulation Chinami Michaels StageName: 4This image shows a chicken (Gallus gallus) embryo undergoing gastrulation in stage four (18-19 hrs after laying) according to the Hamburger-Hamilton staging series. At this point in time the chicken embryo is a blastoderm (shown in blue). The first magnification of the embryo shows that the blastoderm cell layers have thickened to form the primitive streak and Hensen's node. The primitive streak extends from the posterior (P) region to the anterior (A) region. The second rectangular magnification shows the blastoderm cross-sectioned through the primitive streak. 2014-02-26 3 July 2018
Human Papillomavirus (HPV) Strains 16 and 18 Grace Kim The Human Papillomavirus (HPV) strains 16 and 18 are the two most common HPV strains that lead to cases of genital cancer. HPV is the most commonly sexually transmitted disease, resulting in more than fourteen million cases per year in the United States alone. When left untreated, HPV leads to high risks of cervical, vaginal, vulvar, anal, and penile cancers. In 1983 and 1984 in Germany, physician Harald zur Hausen found that two HPV strains, HPV-16 and HPV-18, caused cervical cancer in women. In the early twenty first century, pharmaceutical companies Merck & Co. 2017-05-04 3 July 2018
Mechanism of Notch Signaling Cheryl Lancaster Object is a digital image of Notch signaling between a signaling cell and a receiving cell. Labels indicate the signaling and receiving cells, nuclear membrane, Notch receptor, the ligand, a protease, a transcription factor and a repressor. The image depicts three stages involved in Notch signaling, including the binding of the ligand with the receptor, the action of the protease, and the Notch intracellular domain fragment replacing the repressor in the nucleus.Mechanism of Notch Signaling: The image depicts a type of cell signaling, in which two animal cells interact and transmit a molecular signal from one to the other. The process results in the production of proteins, which influence the cells as they differentiate, move, and contribute to embryological development. In the membrane of the signaling cell, there is a ligand (represented by a green oval). The ligand functions to activate a change in a receptor molecule. In the receiving cell, there are receptors; in this case, Notch proteins (represented by orange forks). 2014-08-21 3 July 2018
DNA and X and Y Chromosomes Anna Guerrero Object is a digital image that represents how DNA partly constitutes a Y-chromosome. Image shows different parts of an unbroken strand that begins with the smallest parts on the left side of the image, and eventually forms the Y-chromosome on the right side of the image, so that the chromosome looks like a kite with a long tail. On the left side of the image, a DNA double helix is enlarged to reveal the paired nucleotides within. The width of the helix is 2 nanometers. As the helix continues to the right, it bends downwards, and it gets smaller and seemingly further way from the viewer.Y-chromosomes exist in the body cells of many kinds of male animals. Found in the nucleus of most living animal cells, the X and Y-chromosomes are condensed structures made of DNA wrapped around proteins called histones. The individual histones bunch into groups that the coiled DNA wraps around called a nucleosome, which are roughly 10 nano-meters (nm) across. The histones bunch together to form a helical fiber (30 nm) that spins into a supercoil (200 nm). During much of a cell's life, DNA exists in the 200 nm supercoil phase. 2017-02-06 3 July 2018
Human Papillomavirus (HPV) Strains 16 and 18 Grace Kim The Human Papillomavirus (HPV) strains 16 and 18 are the two most common HPV strains that lead to cases of genital cancer. HPV is the most commonly sexually transmitted disease, resulting in more than fourteen million cases per year in the United States alone. When left untreated, HPV leads to high risks of cervical, vaginal, vulvar, anal, and penile cancers. In 1983 and 1984 in Germany, physician Harald zur Hausen found that two HPV strains, HPV-16 and HPV-18, caused cervical cancer in women. In the early twenty first century, pharmaceutical companies Merck & Co. 2017-07-19 3 July 2018
Fruit Fly Life Cycle Amy Pribadi Object is a digital image of fruit flies, showing how they develop through stages of egg, larva, pupa, and adult. The image has a magnification box on parts of the larvae. The box displays imaginal disc, which eventually develop into the adult body parts.Fruit flies of the species Drosophila melanogaster develop from eggs to adults in eight to ten days at 25 degrees Celsius. They develop through four primary stages: egg, larva, pupa, and adult. When in the wild, female flies lay their fertilized eggs in rotting fruit or other decomposing material that can serve as food for the larvae. In the lab, fruit flies lay their fertilized eggs in a mixture of agar, molasses, cornmeal, and yeast. After roughly a day, each egg hatches into a larva. 2016-10-11 3 July 2018
Estrogen Brendan Van Iten This image contains three line-angle formulas of three estrogen molecules: estriol on the left, estradiol in the center, and estrone on the right. The molecules line-angle formulas contain wedged and dashed bonds for three-dimensional geometry or stereochemistry.The figure depicts three different molecular structures of estrogen found in mammals’ that differ by the arrangement of bonds and side groups. The molecular structures of the three estrogen molecules differ by the arrangement of chemical bonds and side groups attached to the core steroid structure, cholesterol, which contains three cyclohexane rings and one cyclopentane ring. 2017-05-18 3 July 2018
"Testing the Kin Selection Theory: Who Controls the Investments?" from The Ants (1990), by Bert Hölldobler and Edward O. Wilson Kelle Dhein In “Testing the Kin Selection Theory: Who Controls the Investments?” Bert Hölldobler and Edward Osborne Wilson discussed the predictive power of kin selection theory, a theory about the evolution of social behaviors. As part of Hölldobler's and Wilson's 1990 book titled The Ants, Hölldobler and Wilson compared predictions about the reproductive practices of ants to data about the reproductive practices of ants. They showed that the data generally supported the expected behaviors proposed by kin selection theory. 2017-04-18 3 July 2018
Julia Barlow Platt's Embryological Observations on Salamanders' Cartilage (1893) Karina Ramirez 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. 2017-03-06 3 July 2018
The Development of the Neural Crest and the Migration of Neural Crest Cells (NCCs) in the Embryos of Various Vertebrates Brian K. Hall, M. Elizabeth Barnes Object is a digitized image line drawings that depict the formation of neural crest cells in vertebrates. Image has six sub images, which depict six stages, labeled (a) through (f), of the neural plate as it folds to form the neural tube, neural crest cells, and a sheet of extoderm. Arrows between the six subimages indicate developmental processes for different taxa. The path from (a) to (b) to (c) represents the process in rats. The path (a) to (b) to (d) to (f) represents the process in birds. The path (a) to (b) to (e) to (f) represents the process in amphibians.This diagram shows how NCCs migrate differently in rats, birds and amphibians. The arrows represent both chronology of NCCs migration and the differential paths that NCCs follow in different classes of animals. The solid black portion of each illustration represents the neural crest, and the large black dots in (c) and in (f) represent the neural crest cells. The speckled sections that at first form a basin in (a) and then close to form a tube in (f) represent the neural ectoderm. The solid white portions represent the epidermal ectoderm. 2014-08-21 3 July 2018
The Formation of Reticular Theory Isra Mishqat In the nineteenth century, reticular theory aimed to describe the properties of neurons, the specialized cells which make up the nervous system, but was later disconfirmed by evidence. Reticular theory stated that the nervous system was composed of a continuous network of specialized cells without gaps (synapses), and was first proposed by researcher Joseph von Gerlach in Germany in 1871. 2017-06-19 3 July 2018
The Hedgehog Signaling Pathway in Vertebrates  Dorothy Regan Haskett The hedgehog signaling pathway is a mechanism that regulates cell growth and differentiation during embryonic development, called embryogenesis, in animals. The hedgehog signaling pathway works both between cells and within individual cells. 2016-06-27 3 July 2018
Estrogen and the Menstrual Cycle in Humans Brendan Van Iten Estrogen is the primary sex hormone in women and it functions during the reproductive menstrual cycle. Women have three major types of estrogen: estrone, estradiol, and estriol, which bind to and activate receptors within the body. Researchers discovered the three types of estrogen over a period of seven years, contributing to more detailed descriptions of the menstrual cycle. Each type of estrogen molecule contains a slightly different arrangement or number of atoms that in turn causes some of the estrogens to be more active than others. 2016-06-22 3 July 2018
ABO Blood Type Identification and Forensic Science (1900-1960) Corey Harbison The use of blood in forensic analysis is a method for identifying individuals suspected of committing some kinds of crimes. Paul Uhlenhuth and Karl Landsteiner, two scientists working separately in Germany in the early twentieth century, showed that there are differences in blood between individuals. Uhlenhuth developed a technique to identify the existence of antibodies, and Landsteiner and his students showed that humans had distinctly different blood types called A, B, AB, and O. 2016-06-02 3 July 2018
Interspecies SCNT-derived Humanesque Blastocysts Sarah Taddeo, Jason S. Robert, Nicole Diehnelt Since the 1950s, scientists have developed interspecies blastocysts in laboratory settings, but not until the 1990s did proposals emerge to engineer interspecies blastocysts that contained human genetic or cellular material. Even if these embryos were not permitted to mature to fetal stages, their ethical and political status became debated within nations attempting to use them for research. 2017-06-23 3 July 2018
Twilight Sleep Jessica Pollesche Twilight Sleep (Dammerschlaf) was a form of childbirth first used in the early twentieth century in Germany in which drugs caused women in labor to enter a state of sleep prior to giving birth and awake from childbirth with no recollection of the procedure. Prior to the early twentieth century, childbirth was performed at home and women did not have anesthetics to alleviate the pain of childbirth. In 1906, obstetricians Bernhardt Kronig and Karl Gauss developed the twilight sleep method in 1906 to relieve the pain of 2018-05-16 16 May 2018
The Debate over DNA Replication Before the Meselson-Stahl Experiment (1953–1957) Victoria Hernandez Between 1953 and 1957, before the Meselson-Stahl experiment verified semi-conservative replication of DNA, scientists debated how DNA replicated. In 1953, James Watson and Francis Crick proposed that DNA was composed of two helical strands that wound together in a coil. Their model suggested a replication mechanism, later termed semi-conservative replication, in which parental DNA strands separated and served as templates for the replication of new daughter strands. 2018-01-03 3 January 2018
Hermaphrodites and the Medical Invention of Sex (1998), by Alice Domurat Dreger Mary Drago Hermaphrodites and the Medical Invention of Sex, by historian of science Alice Domurat Dreger, was published in 1998 by Harvard University Press. In the book, Dreger describes how many doctors and scientists treated human hermaphrodites from the late nineteenth century to the early twentieth century. She states that during this time period, many physicians and scientists struggled to determine the nature sex, and to support a classification of sex as male or female, many physicians and scientists resorted to viewing a person's gonads for identification of his or her sex. 2014-04-09 4 October 2016
Dinosaur Egg Parataxonomy Paige Madison Dinosaur egg parataxonomy is a classification system that organizes dinosaur eggs by descriptive features such as shape, size, and shell thickness. Though egg parataxonomy originated in the nineteenth century, Zi-Kui Zhao from Beijing, China, developed a modern parataxonomic system in the late twentieth century. Zhao's system, published in 1975, enabled scientists to organize egg specimens according to observable features, and to communicate their findings. 2015-03-23 4 October 2016
The Discovery of The Dikika Baby Fossil as Evidence for Australopithecine Growth and Development Paige Madison When scientists discovered a 3.3 million-year-old skeleton of a child of the human lineage (hominin) in 2000, in the village of Hadar, Ethiopia, they were able to study growth and development of Australopithecus afarensis, an extinct hominin species. The team of researchers, led by Zeresenay Alemseged of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, named the fossil DIK 1-1 and nicknamed it Dikika baby after the Dikika research site. The Dikika fossil 2015-02-02 4 October 2016
Ovism Cera R. Lawrence Ovism was one of two models of preformationism, a theory of generation prevalent in the late seventeenth through the end of the eighteenth century. Contrary to the competing theory of epigenesis (gradual emergence of form), preformationism held that the unborn offspring existed fully formed in the eggs or sperm of its parents prior to conception. The ovist model held that the maternal egg was the location of this preformed embryo, while the other preformationism model known as spermism preferred the paternal germ cell, as the name implies. 2008-08-13 22 June 2016
The Y-Chromosome in Animals Dorothy R. Haskett 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. 2015-05-28 2 June 2015
Study of Fossilized Massospondylus Dinosaur Embryos from South Africa (1978-2012) Paige Madison In 1978, James Kitching discovered two dinosaur embryos in a road-cut talus at Roodraai (Red Bend) in Golden Gate Highlands National Park, South Africa. Kitching assigned the fossilized embryos to the species of long necked herbivores Massospondylus carinatus (longer vertebra) from the Early Jurassic period, between 200 and 183 million years ago. The embryos were partially visible but surrounded by eggshell and rock, called matrix. Kitching said that the eggs were too delicate to remove from the matrix without damage. 2015-03-31 31 March 2015
The Inheritance of Acquired Characteristics (1924), by Paul Kammerer Federica Turriziani Colonna The Inheritance of Acquired Characteristics is a book published in 1924, written by Paul Kammerer, who studied developmental biology in Vienna, Austria, in the early twentieth century. The Inheritance of Acquired Characteristics summarizes Kammerer's experiments, and explains their significance. In his book, Kammerer aims to explain how offspring inherit traits from their parents. Some scholars criticized Kammerer's reports and interpretations, arguing that they were inaccurate and misleading, while others supported Kammerer's work. 2015-03-31 31 March 2015
Telomerase in Human Development Zane Bartlett Telomerase is an enzyme that regulates the lengths of telomeres in the cells of many organisms, and in humans it begins to function int the early stages of embryonic development. Telomeres are repetitive sequences of DNA on the ends of chromosomes that protect chromosomes from sticking to each other or tangling. In 1989, Gregg Morin found that telomerase was present in human cells. In 1996, Woodring Wright and his team examined human embryonic cells and found that telomerase was active in them. Scientists manipulate telomerase in cells to give cells the capacity to replicate infinitely. 2015-03-23 23 March 2015
Fetus in Fetu Corinne DeRuiter Fetus in fetu is a rare variety of parasitic twins , where the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. In the late eighteenth century, German anatomist Johann Friedrich Meckel was the first to described fetus in fetu, which translates to “fetus within fetus.” Fetus in fetu is thought to result from the unequal division of the totipotent inner cell mass , the mass of cells that is the ancestral precursor to all cells in the body. 2012-05-30 3 March 2015
The Source-Sink Model Jack Resnik The source-sink model, first proposed by biologist Francis Crick in 1970, is a theoretical system for how morphogens distribute themselves across small fields of early embryonic cells. A morphogen is a substance that determines the fate and phenotype of a group of cells through a concentration gradient of itself across that group. Crick’s theory has been experimentally confirmed with several morphogens, most notably with the protein bicoid , the first discovered morphogen. The model provides a theoretical structure for the understanding of some features of early embryonic development. 2012-05-07 27 February 2015
Human Evolution Inferred from Tooth Growth and Development Kate MacCord To study human evolution, researchers sometimes use microstructures found in human teeth and their knowledge of the processes by which those structures grow. Human fetusus begin to develop teeth in utero. As teeth grow, they form a hard outer substance, called enamel, through a process called amelogenesis. During amelogenesis, incremental layers of enamel form in a Circadian rhythm. This rhythmic deposition leaves the enamel with microstructures, called cross-striations and striae of Retzius, which have a regular periodicity. 2013-03-28 26 February 2015
Wilhelm Johannsen's Genotype-Phenotype Distinction B. R. Erick Peirson Wilhelm Johannsen in Denmark first proposed the distinction between genotype and phenotype in the study of heredity in 1909. This distinction is between the hereditary dispositions of organisms (their genotypes) and the ways in which those dispositions manifest themselves in the physical characteristics of those organisms (their phenotypes). This distinction was an outgrowth of Johannsen's experiments concerning heritable variation in plants, and it influenced his pure line theory of heredity. 2012-12-07 26 February 2015
Richard Woltereck's Concept of Reaktionsnorm B. R. Erick Peirson Richard Woltereck first described the concept of Reaktionsnorm (norm of reaction) in his 1909 paper 'Weitere experimentelle Untersuchungen uber Art-veranderung, speziell uber das Wesen quantitativer Artunterschiede bei Daphniden' ('Further investigations of type variation, specifically concerning the nature of quantitative differences between varieties of Daphnia'). This concept refers to the ways in which the environment can alter the development of an organism, and its adult characteristics. 2012-09-06 26 February 2015
Epithelium Kate MacCord 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. 2012-10-17 26 February 2015
Telomeres and Telomerase in Cellular Aging (Senescence) Zane Bartlett 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. 2015-02-11 19 February 2015
The Meckel-Serres Conception of Recapitulation Lindsey O'Connell Johann Friedrich Meckel and Antoine Etienne Reynaud Augustin Serres developed in the early 1800s the basic principles of what later became called the Meckel-Serres Law. Meckel and Serres both argued that fetal deformities result when development prematurely stops, and they argued that these arrests characterized lower life forms, through which higher order organisms progress during normal development. The concept that the embryos of higher order organisms progress through successive stages in which they resemble lower level forms is called recapitulation. 2013-07-10 17 February 2015
The Role of the Notch Signaling Pathway in Myogenesis Justin M. Wolter Among other functions, the Notch signaling pathway forestalls the process of myogenesis in animals. The Notch signaling pathway is a pathway in animals by which two adjacent cells within an organism use a protein named Notch to mechanically interact with each other. Myogenesis is the formation of muscle that occurs throughout an animal's development, from embryo to the end of life. The cellular precursors of skeletal muscle originate in somites that form along the dorsal side of the organism. 2013-07-26 17 February 2015
Germ Layers Kate MacCord A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues. All animals, except perhaps sponges, form two or three germ layers. The germ layers develop early in embryonic life, through the process of gastrulation. During gastrulation, a hollow cluster of cells called a blastula reorganizes into two primary germ layers: an inner layer, called endoderm, and an outer layer, called ectoderm. 2013-09-17 14 February 2015
The Germ-Plasm: a Theory of Heredity (1893), by August Weismann Yawen Zou Friedrich Leopold August Weismann published Das Keimplasma: eine Theorie der Vererbung (The Germ-Plasm: a Theory of Heredity, hereafter The Germ-Plasm) while working at the University of Freiburg in Freiburg, Germany in 1892. William N. Parker, a professor in the University College of South Wales and Monmouthshire in Cardiff, UK, translated The Germ-Plasm into English in 1893. In The Germ-Plasm, Weismann proposed a theory of heredity based on the concept of the 2015-01-26 26 January 2015

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