Nervous Tissues

Nervous tissues (nercvus tishcuz) are found in the brain, spinal cord, and peripheral nerves. The basic cells are called nerve cells, or neurons (nucronz), and they are among the more highly specialized body cells. Neurons sense certain types of changes in their surroundings and respond by transmitting nerve impulses along cellular processes to other neurons or to muscles or glands (fig. 5.31). As a result of the extremely complex patterns by which neurons connect with each other and with muscle and gland cells, they can coordinate, regulate, and integrate many body functions.

In addition to neurons, nervous tissue includes neuroglia (nu-rogie-ah). These cells support and bind the components of nervous tissue, carry on phagocytosis, and help supply nutrients to neurons by connecting them to blood vessels. They may also play a role in cell-to-cell communications. Nervous tissue is discussed in chapter 10.

Table 5.8 summarizes the general characteristics of muscle and nervous tissues. Clinical Application 5.2 discusses bioengineered tissues.

H Describe the general characteristics of nervous tissue.

Distinguish between neurons and neuroglial cells.

The cells of different tissues vary greatly in their abilities to divide. Epithelial cells of the skin and the inner lining of the digestive tract and the connective tissue cells that form blood cells in red bone marrow continuously divide. However, striated and cardiac muscle cells and nerve cells do not usually divide at all after differentiating (specializing).

Fibroblasts respond rapidly to injuries by increasing in numbers and increasing fiber production. They are often the principal agents of repair in tissues that have limited abilities to regenerate. For instance, cardiac muscle tissue typically degenerates in regions damaged by a heart attack. Fibroblasts then, over time, knit connective tissue that replaces the damaged cardiac muscle. A scar is born.

Tissue Engineering

If an automobile or appliance part is damaged or malfunctions, replacing it is fairly simple. Not so for the human body. To replace a human body part, biomedical engineers must first learn how to replicate the combination of cells, biochemicals, and intercellular materials that comprise tissues and organs. Then physicians must dampen the immune response sufficiently for the body to accept the replacement. A solution to the challenge of replacing body parts is tissue engineering, which combines synthetic materials with cells.

The basic recipe for a bioengi-neered tissue is to place cells in or on a scaffolding sculpted from a synthetic material that is accepted in the body. The cells secrete substances as they normally would, or they may be genetically altered to overproduce their natural secreted products or supply entirely different ones with therapeutic benefit, such as growth factors that might make the implant more acceptable to the body.

replacement blood vessels

The challenge of tissue engineering is to identify and reproduce the char acteristics that enable the part to function in the body. A stand-in blood vessel must:

• be strong enough to withstand the force of circulating blood and pressure from surrounding tissue;

• be smooth enough to prevent formation of blood clots;

• not evoke an immune response.

Several biotechnology companies have developed blood vessels by wrapping various tissues around synthetic tubes and surrounding the structure with collagen, the major protein of connective tissue. However, adding colla gen disturbs the fibroblasts that normally secrete it, resulting in a ragged tube. An innovative graduate student improved the approach by letting the engineered blood vessel secrete its own outer covering.

Nicolas L'Heureux knew that the blood vessel required an inner smooth lining of epithelial cells called endothelium; a middle layer of smooth muscle; and an outer layer of connective tissue. So he grew smooth muscle cells and fi-broblasts into sheets, then wrapped the sheets around a bio-degradable polymer tube to form a tubule. He seeded the inner surface of the muscle layer with endothelial cells, which divided and formed a one-cell-thick inner lining, just as in a natural blood vessel (fig. 5A). Then the fibroblasts secreted collagen, coating the vessel. L'Heureux obtained sleek blood vessel replacements that may one day help thousands of people who need grafts in their legs or new coronary arteries.

Nervous Tissue Micrograph

- Nucleus

Figure 5.31

A nerve cell with cellular processes extending into its surroundings (50x micrograph enlarged to 300x)

Cellular processes

Cytoplasm

- Nucleus

Cell membrane

Neuroglial cells

Figure 5.31

A nerve cell with cellular processes extending into its surroundings (50x micrograph enlarged to 300x)

Cellular processes

Cytoplasm

Cell membrane

Neuroglial cells

Tissue Mcgraw Hill

Shier-Butler-Lewis: I I. Levels of Organization I 5. Tissues I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition fBk

Fibroblasts - Smooth muscle cells

Endothelial cells

Endothelial cells fBk

Fibroblasts - Smooth muscle cells

Cardiac Muscle Blood Vessels
Collagen
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