1

Muscle cell

Nerve cell

Figure

The trillions of cells in an adult human ultimately derive by mitosis from the original fertilized egg cell. As different genes are turned on or off in different cells, the characteristics of specific cell types emerge. (Relative cell sizes are not to scale.)

Shier-Butler-Lewis: I. Levels of Organization 3. Cells © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Cloning

The human body is built of more than 200 types of specialized, or differentiated, cell types. Once a cell activates certain subsets of the total genetic package present in all cells, there usually isn't any turning back. A nerve cell remains a nerve cell; an adipose cell stays an adipose cell. In contrast to a differentiated cell, a fertilized egg cell retains the potential to become any cell type — a little like a college student before declaring a major. Such a cell is called "pluripotent." Something about the cytoplasm of the cell keeps it in a state where it can follow any cell "fate."

What would happen if a nucleus from a differentiated cell was placed in a fertilized egg cell whose nucleus had been removed? Would the special egg cytoplasm literally turn back the developmental clock, returning the differentiated cell's nucleus to a pluripotent state, and possibly enabling it to specialize in a different way? In 1996, Scottish researchers did just that, in sheep. They took a cell from an adult sheep's breast and transferred its nucleus to a fertilized egg cell whose nucleus had been removed. The altered cell divided, and divided again, and when it was a ball of cells, it was implanted into an unrelated ewe. Development continued. On February 7, 1997, the result—a now-famous sheep named Dolly—graced the cover of Nature magazine and immediately triggered worldwide controversy (fig. 3A). Mice, pigs, and cows have since been cloned from the nuclei of adult cells. Cloning from fetal cell nuclei has been possible since the 1960s, mostly in amphibians.

Dolly is a clone of the sheep that donated the breast cell, which means that, except for the small amount of mitochondrial DNA, the two animals are genetically identical. Creating Dolly was quite a feat—it took 277 tries. Cloning other mammals has been equally challenging. Not only is it technically difficult, but newborn clones do not often fare well. Something about starting from a body (somatic) cell nucleus, rather than from a fertilized egg cell's nucleus, harms health.

Despite the difficulty of the procedure, and the fact that it had not been performed on humans, public reaction in the months following Dolly's debut was largely negative, as talk centered on cloning humans. Politicians called for "no-clone zones," editorials envisioned scenarios of farming replicas to harvest their organs for spare parts, and films and cartoons sensationalizing or poking fun at cloning flourished (fig. 3B).

Lost in the fear of cloning was the fact that restoring developmental potential to a specialized cell can have

Figure 3A

Dolly, a most unusual ewe.

medical applications. Cloning would enable rapid mass production of cows genetically altered to produce proteins that are of use to humans as drugs—such cows, for example, already make the human versions of an anticlotting drug that saves lives following heart attacks and strokes. Pigs genetically altered to have cell surfaces compatible with humans are being considered as organ donors for humans—cloning would scale-up that technology.

Cloning humans raises a broader, more philosophical issue. To what extent do genes determine who we are? That is, how identical are individuals who have the same genes? That question has already been answered by naturally occurring human clones—identical twins. Extensive analysis of identical twins who were separated at birth demonstrates that although they are physically alike and share many health characteristics and even some peculiar quirks, they also differ in many ways. The environment exerts powerful effects on who we are, contributing to characteristics such as personality traits that are harder to define biochemically. ■

Figure 3A

Dolly, a most unusual ewe.

Figure 3B

A view of cloning.

function of healthy tissue. A malignant, or cancerous, tumor looks quite different—it is invasive, extending into surrounding tissue. A growing malignant tumor may roughly resemble a crab with outreaching claws, which is where the name "cancer" comes from. Cancer cells, if not stopped, eventually reach the circulation and spread, or metastasize, to other sites. Table 3.5 lists characteristics of cancer cells, and figure 3.45 illustrates how cancer cells infiltrate healthy tissue.

Cancer is a collection of disorders distinguished by their site of origin and the affected cell type. Many cancers are treatable with surgery, radiation, chemicals (chemotherapy), or immune system substances (biologicals) used as drugs. Experimental gene therapy fights cancer by giving tumor cells surface molecules that attract an attack by the immune system.

At least two types of genes cause cancer. Oncogenes activate other genes that increase cell division rate. Tumor suppressor genes normally hold mitosis in check. When tumor suppressor genes are removed or otherwise inactivated, this lifts control of the cell cycle, and uncontrolled cell division leading to cancer results (fig. 3.46). Environmental factors, such as exposure to toxic chemicals or radiation, may induce cancer by altering (mutating) oncogenes and tumor suppressor genes in body (somatic) cells. Cancer may also be the consequence of a failure of normal cell death, resulting in overgrowth.

Normal anatomy and physiology—in other words, health—ultimately depend upon both the quality and quantity of the cells that comprise the human body. Understanding the cellular bases of disease suggests new diagnosis and treatment methods.

O How can too infrequent cell division affect health? ^9 How can too frequent cell division affect health?

^9 What is the difference between a benign and a cancerous tumor?

^9 What are two ways that genes cause cancer? 13 How can factors in the environment cause cancer?

Loss of cell cycle control

Heritability (a cancer cell divides to form more cancer cells) Transplantability (a cancer cell implanted into another individual will cause cancer to develop) Dedifferentiation (loss of specialized characteristics) Loss of contact inhibition

Ability to induce local blood vessel formation (angiogenesis) Invasiveness

Ability to metastasize (spread)

Loss of cell cycle control

Heritability (a cancer cell divides to form more cancer cells) Transplantability (a cancer cell implanted into another individual will cause cancer to develop) Dedifferentiation (loss of specialized characteristics) Loss of contact inhibition

Ability to induce local blood vessel formation (angiogenesis) Invasiveness

Ability to metastasize (spread)

Normal cells (with hairlike cilia)

Cancer cells

Figure

A cancer cell is rounder and less specialized than surrounding healthy cells. It secretes biochemicals that cut through nearby tissue and others that stimulate formation of blood vessels that nurture the tumor's growth (2,200x).

Normal cells (with hairlike cilia)

Cancer cells

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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