Search

Displaying 1 - 25 of 34 items.

Pages

The Hedgehog Signaling Pathway in Vertebrates 

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.

Format: Articles

Subject: Theories

The Role of the Notch signaling pathway in Somitogenesis

Among other functions, the Notch signaling pathway contributes to the development of somites in animals. It involves a cell signaling mechanism with a wide range of functions, including cellular differentiation, and the formation of the embryonic structures (embryogenesis). All multicellular animals use Notch signaling, which is involved in the development, maintenance, and regeneration of a range of tissues. The Notch signaling pathways spans two cells, and consists of receptor proteins, which cross one cell's membrane and interacts with proteins on adjacent cells, called ligands.

Format: Articles

Subject: Theories, Processes

The Role of the Notch Signaling Pathway in Myogenesis

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.

Format: Articles

Subject: Theories, Processes

The Source-Sink Model

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.

Format: Articles

Subject: Theories

Endothelium

The endothelium is the layer of cells lining the blood vessels in animals. It weighs more than one kilogram in adult humans, and it covers a surface area of 4000 to 7000 square meters. The endothelium is the cellular interface between the circulating blood and underlying tissue. As the medium between these two sets of tissues, endothelium is part of many normal and disease processes throughout the body.

Format: Articles

Subject: Processes, Theories

“A Two-Factor Hypothesis of Freezing Injury: Evidence from Chinese Hamster Tissue-Culture Cells” (1972), by Peter Mazur, Stanley Leibo, and Ernest Chu

In 1972, Peter Mazur, Stanley Leibo, and Ernest Chu published, “A Two-Factor Hypothesis of Freezing Injury: Evidence from Chinese Hamster Tissue-culture Cells,” hereafter, “A Two-Factor Hypothesis of Freezing Injury,” in the journal, Experimental Cell Research. In the article, the authors uncover that exposure to high salt concentrations and the formation of ice crystals within cells are two factors that can harm cells during cryopreservation. Cryopreservation is the freezing of cells to preserve them for storage, study, or later use.

Format: Articles

Subject: Publications, Theories

Purkinje Cells

Purkinje cells, also called Purkinje neurons, are neurons in vertebrate animals located in the cerebellar cortex of the brain. Purkinje cell bodies are shaped like a flask and have many threadlike extensions called dendrites, which receive impulses from other neurons called granule cells. Each cell also has a single projection called an axon, which transmits impulses to the part of the brain that controls movement, the cerebellum. Purkinje cells are inhibitory neurons: they secrete neurotransmitters that bind to receptors that inhibit or reduce the firing of other neurons.

Format: Articles

Subject: Theories

Syncytial Theory

The syncytial theory of neural development was proposed by Victor Hensen in 1864 to explain the growth and differentiation of the nervous system. This theory has since been discredited, although it held a significant following at the turn of the twentieth century. Neural development was well studied but poorly understood, so Hensen proposed a simple model of development. The syncytial theory predicted that the nervous system was composed of many neurons with shared cytoplasm.

Format: Articles

Subject: Theories

The French Flag Model

The French flag model represents how embryonic cells receive and respond to genetic information and subsequently differentiate into patterns. Created by Lewis Wolpert in the late 1960s, the model uses the French tricolor flag as visual representation to explain how embryonic cells can interpret genetic code to create the same pattern even when certain pieces of the embryo are removed. Wolpert's model has provided crucial theoretical framework for investigating universal mechanisms of pattern formation during development.

Format: Articles

Subject: Processes, Theories

Molecular Epigenetics and Development: Histone Conformations, DNA Methylation and Genomic Imprinting

Introduced by Conrad Hal Waddington in 1942, the concept of epigenetics gave scientists a new paradigm of thought concerning embryonic development, and since then has been widely applied, for instance to inheritable diseases, molecular technologies, and indeed the human genome as a whole. A genome contains an embedded intricate coding template that provides a means of genetic expression from the initial steps of embryonic development until the death of the organism. Within the genome there are two prominent components: coding (exons) and non-coding (introns) sequences.

Format: Articles

Subject: Theories

The Formation of Reticular Theory

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.

Format: Articles

Subject: Theories

Mitochondria

All cells that have a nucleus, including plant, animal, fungal cells, and most single-celled protists, also have mitochondria. Mitochondria are particles called organelles found outside the nucleus in a cell's cytoplasm. The main function of mitochondria is to supply energy to the cell, and therefore to the organism. The theory for how mitochondria evolved, proposed by Lynn Margulis in the twentieth century, is that they were once free-living organisms.

Format: Articles

Subject: Organisms, Theories

“Prenatal Stress, Glucocorticoids and the Programming of the Brain” (2001), by Leonie Welberg and Jonathan Seckl

In 2001, researchers Leonie Welberg and Jonathan Seckl published the literature review “Prenatal Stress, Glucocorticoids, and the Programming of the Brain,” in which they report on the effects of prenatal stress on the development of the fetal brain. The fetus experiences prenatal stress while in the womb, or in utero. In discussing the effects of prenatal stress, the authors describe prenatal programming, which is when early environmental experiences permanently alter biological structure and function throughout life.

Format: Articles

Subject: Publications, Theories

ABO Blood Type Identification and Forensic Science (1900-1960)

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.

Format: Articles

Subject: Theories, Legal, Technologies

Mitochondrial DNA (mtDNA)

Mitochondrial DNA (mtDNA) is located outside the nucleus in the liquid portion of the cell (cytoplasm) inside cellular organelles called Mitochondria. Mitochondria are located in all complex or eukaryotic cells, including plant, animal, fungi, and single celled protists, which contain their own mtDNA genome. In animals with a backbone, or vertebrates, mtDNA is a double stranded, circular molecule that forms a circular genome, which ranges in size from sixteen to eighteen kilo-base pairs, depending on species. Each mitochondrion in a cell can have multiple copies of the mtDNA genome.

Format: Articles

Subject: Theories

Estrogen and the Menstrual Cycle in Humans

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.

Format: Articles

Subject: Theories, Reproduction

The Debate over DNA Replication Before the Meselson-Stahl Experiment (1953–1957)

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.

Format: Articles

Subject: Theories

Somatic Cell Nuclear Transfer in Mammals (1938-2013)

In the second half of the
twentieth century, scientists learned how to clone organisms in some
species of mammals. Scientists have applied somatic cell nuclear transfer to clone human and
mammalian embryos as a means to produce stem cells for laboratory
and medical use. Somatic cell nuclear transfer (SCNT) is a technology applied in cloning, stem cell
research and regenerative medicine. Somatic cells are cells that
have gone through the differentiation process and are not germ
cells. Somatic cells donate their nuclei, which scientists

Format: Articles

Subject: Theories, Technologies, Processes

George W. Beadle's One Gene-One Enzyme Hypothesis

The one gene-one enzyme hypothesis, proposed by George Wells Beadle in the US in 1941, is the theory that each gene directly produces a single enzyme, which consequently affects an individual step in a metabolic pathway. In 1941, Beadle demonstrated that one gene in a fruit fly controlled a single, specific chemical reaction in the fruit fly, which one enzyme controlled.

Format: Articles

Subject: Theories

Human Evolution Inferred from Tooth Growth and Development

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.

Format: Articles

Subject: Theories

"Evolution and Tinkering" (1977), by Francois Jacob

In his essay Evolution and Tinkering, published in
Science in 1977, Francois Jacob argued that a common analogy
between the process of evolution by natural selection and the
methods of engineering is problematic. Instead, he proposed to
describe the process of evolution with the concept of
bricolage (tinkering). In this essay, Jacob did not deny the
importance of the mechanism of natural selection in shaping complex
adaptations. Instead, he maintained that the cumulative effects of

Format: Articles

Subject: Publications, Theories

"Mitochondrial DNA and Human Evolution" (1987), by Rebecca Louise Cann, Mark Stoneking, and Allan Charles Wilson

In 1987 Rebecca Louise Cann, Mark Stoneking, and Allan Charles Wilson published Mitochondrial DNA and Human Evolution in the journal Nature. The authors compared mitochondrial DNA from different human populations worldwide, and from those comparisons they argued that all human populations had a common ancestor in Africa around 200,000 years ago. Mitochondria DNA (mtDNA) is a small circular genome found in the subcellular organelles, called mitochondria.

Format: Articles

Subject: Publications, Theories

Apoptosis in Embryonic 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.

Format: Articles

Subject: Theories

Endometriosis

Endometriosis is a medical condition that involves abnormal growths of tissue resembling the endometrium, which is the tissue that lines the inside of the uterus. Those growths, called endometrial lesions, typically form outside the uterus, but can spread to other reproductive organs such as ovaries and fallopian tubes. Endometrial lesions swell and bleed during menstruation, which can cause painful and heavy menstruation, as well as infertility.

Format: Articles

Subject: Disorders, Reproduction, Theories

The Y-Chromosome in Animals

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.

Format: Articles

Subject: Reproduction, Theories

Pages