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October, 2006
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Death: Irreversible cessation of all bodily functions, manifested by absence of spontaneous breathing and total loss of cardiovascular and cerebral functions.

Adult Stem Cells

JoVE 10810

Stem cells are undifferentiated cells that divide and produce more stem cells or progenitor cells that differentiate into mature, specialized cell types. All the cells in the body are generated from stem cells in the early embryo, but small populations of stem cells are also present in many adult tissues including the bone marrow, brain, skin, and gut. These adult stem cells typically produce the various cell types found in that tissue—to replace cells that are damaged or to continuously renew the tissue. The epithelium lining the small intestine is continuously renewed by adult stem cells. It is the most rapidly replaced tissue in the human body, with most cells being replaced within 3-5 days. The intestinal epithelium consists of thousands of villi that protrude into the interior of the small intestine—increasing its surface area to aid in the absorption of nutrients. Intestinal stem cells are located at the base of invaginations called crypts that lie between the villi. They divide to produce new stem cells, as well as daughter cells (called transit amplifying cells) that divide rapidly, move up the villi and differentiate into all the cell types in the intestinal epithelium, including absorptive, goblet, enteroendocrine, and Paneth cells. These mature cells continue to move up the villi as they carry out their functions, except Paneth cell

 Core: Biology

Negative Regulator Molecules

JoVE 10764

Positive regulators allow a cell to advance through cell cycle checkpoints. Negative regulators have an equally important role as they terminate a cell’s progression through the cell cycle—or pause it—until the cell meets specific criteria.

Three of the best-understood negative regulators are p53, p21, and retinoblastoma protein (Rb). The regulatory roles of each of these proteins were discovered after faulty copies were found in cells with uncontrolled replication (i.e., cancer). These proteins exert most of their regulatory effects at the G1 checkpoint early in the cell cycle. P53 strongly influences a cell’s commitment to divide. It responds to DNA damage by discontinuing the cell cycle and summoning enzymes to repair the damage. If the DNA damage is irreparable, p53 can prevent the cell from proceeding through the cell cycle by inducing apoptosis, or cell death. An increase in p53 triggers the production of p21. P21 prevents the cell from transitioning from the G1 to the S phase of the cell cycle by binding to CDK/cyclin complexes, inhibiting their positive regulatory actions. Rb negatively regulates the cell cycle by acting on different positive regulators, mainly in response to cell size. Active (dephosphorylated) Rb binds to transcription factors, preventing them from initiating gene tran

 Core: Biology

Electrical Safety

JoVE 10364

Robert M. Rioux & Suprita Jharimune, Pennsylvania State University, University Park, PA

Among the many hazards present in the laboratory, electrical hazards are one of the most common we must be cognizant of since most of the laboratory equipment we use requires electricity for operation. Improper handling or operation of electrical…

 Lab Safety

Invertebrate Lifespan Quantification

JoVE 5338

Many animals naturally stop growing upon reaching adulthood, after which they undergo aging or "senescence" until dying. The amount of time between an organism\'s birth and death is called its lifespan, which can be influenced by various biological and environmental factors. By exposing organisms to different growth conditions, scientists can better understand the factors affecting lifespan.…

 Developmental Biology

An Introduction to Saccharomyces cerevisiae

JoVE 5081

Saccharomyces cerevisiae (commonly known as baker’s yeast) is a single-celled eukaryote that is frequently used in scientific research. S. cerevisiae is an attractive model organism due to the fact that its genome has been sequenced, its genetics are easily manipulated, and it is very easy to maintain in the lab. Because many yeast proteins are similar in sequence and function…

 Biology I

Histological Sample Preparation for Light Microscopy

JoVE 5039

Histology is the study of cells and tissues, which is typically aided by the use of a light microscope. The preparation of histological samples can vary greatly based on the inherent properties of the samples such as size and hardness as well as expected post-processing which includes planned staining techniques or other down-stream applications. As described in this video, specimen…

 General Laboratory Techniques

Framing Effects

JoVE 11051

Information is everywhere and its presentation—such as how and when items are presented—can impact our perceptions and decisions surrounding the info. This broad concept umbrellas framing effects—influences that occur due to the way information is framed in its appearance, whether it’s purely the order or the specific wording of a…

 Core: Psychology


JoVE 10993

When organisms require the same limited resources within an environment, they may have to compete for them. Competition is a net-negative interaction. Even if two competing individuals or populations do not interact directly, the overall fitness of both competitors is lowered as a result of not having full access to the limited resource.

Intraspecific competition, which occurs between individuals of the same species, serves as a natural mechanism for regulating population size. Too much population growth can lead to crowding and diminished resources. Stronger members of the population may outcompete weaker individuals for resources, leading to reduced reproduction or death for the weaker individuals and keeping the population size in check. Competitive exclusion may occur as a consequence of competition between species, where one is better suited to use a resource and forces out the other, but this is not the only possible outcome when a resource is not abundant. Organisms can also find ways to share limited resources. Competing populations may engage in resource partitioning, dividing the resource in a spatial manner by keeping to non-overlapping territories or using the resource at different times of the day. Alternatively, one population might differentiate its niche so that it no longer has to compete. Many similar species of anole lizard coexis

 Core: Biology

Population Growth

JoVE 10943

Population size is dynamic, increasing with birth rates and immigration, and decreasing with death rates and emigration. In ideal conditions with unlimited resources, populations can increase exponentially, which plots as a J-shaped growth rate curve of population size against time. This type of curve is characteristic of newly-introduced invasive species, or populations that have suffered catastrophic declines and are rebounding. However, realistic environmental conditions limit the number of individuals that can occupy a habitat. This limit is known as the habitat’s carrying capacity. Because of carrying capacities, population growth is generally better represented by S-shaped (logistic) curves. The population initially increases exponentially until the carrying capacity is reached, at which point resource limitations cause growth to level off or fluctuate around the carrying capacity, producing an S-shaped curve. The per capita rate of population increase, r, is the change in population size (calculated as the present population size minus the initial population size) divided by the initial population size. When there are no environmental limits and immigration and emigration are assumed to be equal, the population can grow at its maximal rate, known as its biotic potential, or rmax. Therefore, the per capita rate of increase under

 Core: Biology

Energy Budgets

JoVE 10942

Organisms must balance energy intake with the energy required for growth, maintenance and reproduction. These trade-offs result in a variety of survivorship and reproductive strategies, including semelparity and iteroparity. Semelparous species, like annual plants, have only one reproductive episode in their lifetimes and consequently have short lifespans. Iteroparous species, by contrast, have many reproductive events during their lifetimes but have relatively few offspring. These two strategies are not mutually exclusive but represent two extremes on a continuum of possible reproductive strategies. During its lifetime, an organism has a limited amount of energy and resources available to it and must allocate the energy to growth, reproduction, and maintenance. Energy used for reproduction cannot be used for growth and vice versa. This creates a trade-off among fecundity, growth, and survivorship that is reflected by a variety of reproductive strategies. Two primary reproductive strategies are semelparity and iteroparity. However, rather than being strictly semelparous or iteroparous, many organisms lie somewhere on a continuum between the two reproductive strategies. A truly semelparous species allocates all available resources to reproduction at the expense of lifespan, reproducing only once before death, but producing many offspring. Semelparous or

 Core: Biology
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