EMBRYONIC STEM CELLS PROPERTIES BASIC INFORMATION

In the movie “Star Trek IV: The Voyage Home,” the crew of the Enterprise travels back in time to 20th-century Earth, and Dr. McCoy helps a woman undergoing dialysis. As she’s being wheeled down a hospital corridor, she joyfully tells everyone, “Doctor gave me a pill, and I grew a new kidney!”

Okay, that kind of biological miracle is still the stuff of science fiction. But embryonic stem cells hold enormous potential for giant leaps in medical treatments because of their unique properties:

✓ They can divide and grow more or less indefinitely.
✓ They can develop into any cell type found in the adult body.
✓ They have an incredibly lengthy shelf life, so they can be stored for very long periods without losing their potency.

Growing and growing and growing . . .
Left to their own devices, cells in the blastocyst eventually differentiate, or begin developing special characteristics of job-specific cells. Some become neurons, for example — the brain cells that control thought and movement.

Others spin off into red or white blood cells, or heart muscle cells, or liver cells, and so on. DNA, which provides the blueprints for cell development, and RNA, the user’s manual stating which part of the blueprint to follow, determine which job each cell takes on.

In the lab, though, scientists can delay the reading and implementation of the genetic instructions that tell cells to specialize by isolating the inner cells from blastocysts, transferring them to Petri dishes, and feeding them a mixture of appropriate nutrients (called a growth medium or culture medium) so they can
continue to grow.

Under the correct conditions, embryonic stem cells read the genetic instructions for self-renewal and continue to grow until they’re exposed to signals that tell them to read the genetic instructions for specialization.


Cells like to touch other cells, so scientists usually line the Petri dish with alayer of other cells — most commonly embryonic skin cells from mice that have been treated so they won’t grow. The cells in this layer are called feeder cells.

The inner cells from a blastocyst are placed on top of the feeder cells, and a growth medium (a broth rich in the nutrients the cells need to thrive) is added. Depending on what’s in the growth medium, these inner cells can continue dividing into more embryonic stem cells, or they can begin differentiating into specific categories or types of cells.

One of the risks associated with using mouse feeder cells to grow human embryonic stem cells is that the human cells may be infected with viruses or contaminated with other unwanted material from the mouse cells. Researchers have come up with other ways to grow embryonic stem cells, but only time will tell us whether these alternative methods are as useful and reliable as using mouse cells.

Assuming the relocation of the blastocyst’s inner cell mass into a Petri dish is successful — and that’s not always the case — the embryonic stem cells grow until they crowd the dish. Then they’re removed from the original dish, divided up, and placed into several new dishes with fresh growth media.

This process is called subculturing, and each round of subculturing is called a passage. Scientists can separate, freeze, and store batches of cells at any stage of the subculturing process. They also can ship them to other researchers after they’ve verified that the cells are stable and usable.

It takes at least six months and several passages to create an embryonic stem cell line — that is, millions of cells derived from the original inner cell mass of the blastocyst that meet two critical conditions:

✓ They retain their ability to grow into any kind of cell in the adult body.
✓ They appear to have no genetic defects.

Scientists use a method called karyotyping to make sure that stem cells have the correct number of chromosomes. Normal human cells have a total of 46 chromosomes, paired in sets of two; you inherit one of each set, or 23 chromosomes, from each parent.

Embryonic stem cells’ ability to grow and divide practically indefinitely under the right conditions is an important property for medical research and developing useful treatments. It takes millions of cells to conduct reliable experiments, and one of the biggest eventual challenges in in using stem cells to treat illness is creating enough of them to do the job.

FEATURES OF PERIODIC TABLE OF ELEMENTS

Guide In Using The Periodic Table of Elements.


In the periodic table, elements are listed according to increasing atomic number starting at the upper left and arranged in a series of horizontal rows. This arrangement places similar elements in vertical groups, or families.

For example, sodium and potassium are found together in a group labeled 1 (called the alkali metals). We should expect other members of the group, such as cesium and rubidium, to have properties similar to sodium and potassium. Chlorine is found at the other end of the table in a group labeled 17.

Some of the groups are given distinctive names, mostly related to an important property of the elements in the group. For example, the group 17 elements are called the halogens, a term derived from Greek, meaning salt former.


Each element is listed in the periodic table by placing its symbol in the middle of a box in the table. The atomic number (Z) of the element is shown above the symbol, and the weighted-average atomic mass of the element is shown below its symbol. Some periodic tables provide other information, such as density and melting point, but the atomic number and atomic mass are generally sufficient for our needs.

Elements with atomic masses in parentheses, such as plutonium, Pu (244), are produced synthetically, and the number shown is the mass number of the most stable isotope. It is customary also to divide the elements into two broad categories metals and nonmetals. In figure below, colored backgrounds are used to distinguish the metals (tan) from the nonmetals (blue and pink).

Except for mercury, a liquid, metals are solids at room temperature. They are generally malleable (capable of being flattened into thin sheets), ductile (capable of being drawn into fine wires), and good conductors of heat and electricity, and have a lustrous or shiny appearance.

The properties of nonmetals are generally opposite those of metals; for example, nonmetals are poor conductors of heat and electricity. Several of the nonmetals, such as nitrogen, oxygen, and chlorine, are gases at room temperature.

Some, such as silicon and sulfur, are brittle solids. One bromine is a liquid. Two other highlighted categories in the figure are a special group of nonmetals known as the noble gases (pink), and a small group of elements, often called metalloids (green), that have some metallic and some nonmetallic properties.

The horizontal rows of the table are called periods. (The periods are numbered at the extreme left in the periodic table inside the front cover.) The first period of the table consists of just two elements, hydrogen and helium.

This is followed by two periods of eight elements each, lithium through neon and sodium through argon. The fourth and fifth periods contain 18 elements each, ranging from potassium through krypton and from rubidium through xenon.

The sixth period is a long one of 32 members. To fit this period in a table that is held to a maximum width of 18 members, 15 members of the period are placed at the bottom of the periodic table. This series of 15 elements start with lanthanum and these elements are called the lanthanides.

The seventh and final period is incomplete (some members are yet to be discovered), but it is known to be a long one. A15-member series is also extracted from the seventh period and placed at the bottom of the table. Because the elements in this series start with actinium they are called the actinides.

The labeling of the groups of the periodic table has been a matter of some debate among chemists. The 1-18 numbering system used is the one most recently adopted. Group labels previously used in the United States
consisted of a letter and a number, closely following the method adopted by Mendeleev, the developer of the periodic table.

As seen in the figure, the A groups 1 and 2 are separated from the remaining Agroups (3 to 8) by B groups 1 through 8. The International Union of Pure and Applied Chemistry (IUPAC) recommended the simple 1 to 18 numbering scheme in order to avoid confusion between the American number and letter system and that used in Europe, where some of the Aand B designations were switched! Currently, the IUPAC system is officially recommended by the American Chemical Society (ACS) and chemical societies in other nations. Because both numbering systems are in use, we show both in the figure and in the periodic table inside the front cover.