Compensation and Other Benefits

Pay scales for educators vary depending upon the grade level taught and the institution at which they teach. At the K-12 level, teachers can earn from $21,000 to $80,000, depending on years of experience and the school district in which they teach. At two-year colleges, starting salaries average about $35,000, but the upper salary limit is about the same as for K-12. Four-year colleges and universities typically offer starting salaries of about $40,000,

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and can rise to more than $100,000 for full professors whose research and publications have earned them renown in their fields.

Educators typically are free of teaching duties during the summer months, but those who work at the college level are expected to conduct research and publish even when classes are not in session. College-level teachers are also often eligible for sabbaticals, during which teaching duties are not required, but they are expected to use this time for a research project. A love of teaching and learning is needed for educators at all levels, but each student age group makes different demands on the teacher. see also College Professor.

Linnea Fletcher

Embryonic Stem Cells

Mammalian embryonic stem (ES) cells have the special property of being able to differentiate into virtually every cell type. Because ES cells can be genetically manipulated in vitro and can be transplanted into embryos and adults, they are a powerful tool in biological experiments and hold promise for future medical therapies.

The ability to differentiate into all cell types, a property known as pluripotency, arises from the fact that ES cells are isolated from in vitro outgrowths of early stage embryos (in the mouse, at three and one-half days, at the blastocyst stage). These outgrowths are cultured in specialized conditions—often in the presence of support cells, called feeder cells, which do not proliferate, and specific growth factors. The ES cells proliferate rapidly in culture, and clonal (identical) populations can readily be initiated from single cells.

Until 1998 the only mammalian embryonic stem cells isolated were those from the mouse. The first human embryonic stem cells were isolated in 1998 from embryos created through in vitro fertilization. In the United States, government funding for research on human ES cells was restricted in 2002 to a small set of existing human ES cell lines. Only privately funded research remained able to use other sources of cells.

In the following years, most publicized research on human ES cells will concentrate on manipulating cells in culture, attempting to understand the signals that cells require to proliferate and differentiate into various adult cell types, such as nerve or muscle cells.

In 2001 there was a highly publicized report of human cloning using human ES cells, but the resulting embryos did not progress past the twelve-cell stage, and there was no reliable evidence that the ES cells ever contributed to the development of the embryos. Nonetheless, most researchers thought obstacles would likely be overcome, and that the use of ES cells for therapy or cloning would become technically possible.

Mouse ES cell research is much further advanced. Research in the mouse has included injection of ES cells into blastocysts, an early stage in embryo development. Once injected, ES cells fully participate in embryonic development and form part of every tissue in the embryo. The resulting mouse is called a chimera, since it usually contains a mixture of cells derived from in vitro "in glass"; in lab apparatus, rather than within a living organism blastocyst early stage of embryonic development

Figure 1. Mouse embryonic stem cells in culture.

Figure 1. Mouse embryonic stem cells in culture.

both the ES cells and the host blastocyst. An important feature of a chimera is that its ES cells will contribute to forming the germ cells, which eventually form the gametes of the animals. Consequently, mating of the chimera with recipient females allows the (ES cell derived) sperm to deliver their genetic material into an egg, and subsequent offspring will carry genes from the ES cell. Thus genetic manipulations made in ES cells in culture can be transmitted through chimeras to intact animals.

As with other cell lines grown in vitro, it is possible to add new genes, termed "transgenes," or to modify existing genes in ES cells using a technique known as gene targeting. Gene targeting involves using specialized vectors carriers DNA vectors that share substantial DNA sequence similarity with the gene that needs to be modified. When introduced into cultured ES cells, the genetargeting vector undergoes "homologous recombination," a process in which the vector is integrated into the existing chromosomal gene and leaves the ES cells genetically modified.

Removing a gene product is often referred to as creating a "knockout," since it prevents a protein from being expressed. Modifying a specific gene

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