tissue culture, the propagation of plants through the placement of small amounts of undifferentiated tissue or single cells in an artificial environment. The tissue is placed in a nutrient medium that favors the production of roots and shoots, and is later planted normally. By using tissue culture, the favorable qualities of plants can be precisely controlled, so that each plant is identical for the particular quality being sought, whether it be disease resistance or plant chemical production.

tissue, in biology, aggregation of cells that are similar in form and function and the intercellular substances produced by them. The fundamental tissues in animals are epithelial, nerve, connective, and muscle tissue; blood and lymph are commonly classed separately as vascular tissue. In the higher plants, there are four main types of tissue: (1) meristematic tissue (apical meristem and cambium), composed of cells that grow, divide, and differentiate into all the other cell types; (2) protective tissue (epidermis and cork), composed of thick-walled cells that cover roots, stem, and leaves; (3) fundamental tissues, consisting of cells that make up the bulk of the plant body, including parenchyma (thin-walled cells used for food storage), collenchyma (moderately thick-walled cells used for strength), and sclerenchyma (heavily thick-walled cells used for support in stems and roots); and (4) vascular tissue (xylem and phloem), specialized cells used for conduction. Organs are usually composed of several tissues. In many diseases there are apparent changes in tissue (see pathology). Histology is the study of the structure of tissues.

fetal tissue implant or fetal cell therapy, implantation of tissue from a fetus into a patient. In experimental procedures, fetal brain tissue has been implanted in the brains of patients with Parkinson's disease so that the fetal tissue will supply chemicals lacking in the diseased brain, but such therapy failed to show significant benefits in a controlled study. Because fetal cell therapy uses tissue from freshly aborted fetuses, the procedure is controversial. Both the Reagan and G. H. W. Bush administrations banned the use of federal funds for fetal-tissue research that used tissue from aborted fetuses. President Clinton lifted the ban early in his administration. The successful transplantation of eggs from fetal ovaries in experiments with mice led to the suggestion in 1994 that human fetal ovaries or eggs taken from such ovaries could be implanted in infertile women, a possibility that troubled many ethicists and others. Some fear the use of what they call "fetal farming," the conception in vitro or in vivo of embryos solely for their medical usefulness. Others point to the potential benefits in the treatment of Alzheimer's and Huntington's diseases, and possibly in cases of nerve injury.

connective tissue, supportive tissue widely distributed in the body, characterized by large amounts of intercellular substance and relatively few cells. The intercellular material, or matrix, is produced by the cells and gives the tissue its particular character. Connective tissue is diversified in function and may be divided into four categories according to the type of matrix. In connective tissue proper (which forms the framework for most organs) the matrix is soft. In cartilage it is firm but flexible. The intercellular substance of bone, which is high in mineral salts, is rigid. Blood and lymph have a fluid matrix. Three kinds of fibers generally form the supportive material in connective tissue proper. White, or collagenous, fibers vary in size and are composed of fine, parallel fibrils; reticular fibers are small, branching fibers that take on a meshlike pattern; yellow, or elastic, fibers are highly flexible and are capable of branching and anastomosing (or opening) directly into one another. Loose, or areolar, connective tissue is composed of all three of the above fibers; it supports most of the organs in the body and is widely distributed under the skin. The type of connective tissue that forms tendons, ligaments, and fascia is composed mainly of collagenous fibers. It is known as compact tissue. Reticular connective tissue forms the bone marrow and the framework for lymphoid tissue. Adipose, or fat, tissue serves as a cushion for various organs and as a fat reservoir. The colored area of the eye, or iris, is composed of pigmented connective tissue.

adipose tissue: see connective tissue.

Biological research method in which tissue fragments (a cell, a population of cells, or all or part of an organ) are sustained in an artificial environment for examination and manipulation of cell behaviour. It has been used to study normal and abnormal cell structure; biochemical, genetic, and reproductive activity; metabolism, functions, and aging and healing processes; and reactions to physical, chemical, and biological agents (e.g., drugs, viruses). A tiny sample of the tissue is spread on or in a culture medium of biological (e.g., blood serum or tissue extract), synthetic, or mixed origin having the appropriate nutrients, temperature, and pH for the cells being incubated. The results are observed with a microscope, sometimes after treatment (e.g., staining) to highlight particular features. Some viruses also grow in tissue cultures. Work with tissue cultures has helped identify infections, enzyme deficiencies, and chromosomal abnormalities; classify brain tumours; and formulate and test drugs and vaccines.

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or myeloid tissue

Soft, gelatinous tissue that fills bone cavities. Red bone marrow contains stem cells, progenitor cells, percursor cells, and functional blood cells (see reticuloendothelial system). Lymphocytes mature in the lymphoid organs (see lymphoid tissue). All other blood-cell formation occurs in red marrow, which also takes part in destruction of old erythrocytes (red blood cells). Yellow bone marrow mainly stores fats. Because the leukocytes (white blood cells) produced in bone marrow are involved in immune defenses, marrow transplants can treat some types of immunodeficiency. Radiation and some anticancer drugs can damage marrow and impair immunity. Bone-marrow examination helps diagnose diseases related to blood and blood-forming organs.

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Cells, tissues, and organs composing the immune system, including the bone marrow, thymus, spleen, and lymph nodes. The most highly organized components are the thymus and lymph nodes, and the least organized are the cells that wander in the loose connective-tissue spaces under membranes lining most body systems, where they can establish lymph nodules (local lymphocyte production centre) in response to antigens. The most common lymphoid tissue cell is the lymphocyte. Others are macrophages, which engulf foreign materials and probably alter them to initiate the immune response, and reticular cells, which produce and maintain thin networks of fibres as a framework for most lymphoid organs. Seealso immunity; lymphatic system.

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Tissue in the body that maintains the form of the body and its organs and provides cohesion and internal support, including bone, ligaments, tendons, cartilage, adipose tissue, and aponeuroses. Its major components are different kinds of cells and extracellular fibres and ground substance, which varies in consistency from thin gel to rigid structure. Various combinations of these elements make up the different kinds of connective tissue. Connective tissue diseases are either genetic disorders, attacking one of its elements (e.g., Marfan syndrome), or acquired inflammatory or immune-system diseases (e.g., rheumatoid arthritis, osteoarthritis, systemic lupus erythematosus, and rheumatic fever).

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or fatty tissue

Connective tissue consisting mainly of fat cells, specialized to synthesize and contain large globules of fat, within a structural network of fibres. It is found mainly under the skin but also in deposits between the muscles, in the intestines and in their membrane folds, around the heart, and elsewhere. The fat stored in this tissue comes from dietary fats or is produced in the body. It acts as a fuel reserve for times of starvation or great exertion, helps conserve body heat, and forms pads between organs.

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