The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour".
In cancer, the parenchyma refers to the actual mutant cells of a single lineage, whereas the stroma is the surrounding connective tissue and associated cells that support it.
Early in development the mammalian embryo has three distinct layers: ectoderm (external layer), endoderm (internal layer) and in between those two layers the middle layer or mesoderm. The parenchyma of most organs is of ectodermal (brain, skin) or endodermal origin (lungs, gastrointestinal tract, liver, pancreas). The parenchyma of a few organs (spleen, kidneys, heart) is of mesodermal origin. The stroma of all organs is of mesodermal origin.
Examples include:
Organ | Parenchyma |
brain | |
heart | |
kidney | |
liver | |
lungs | |
pancreas | >Islets of Langerhans and Pancreatic acini |
spleen |
Parenchyma cells are thin-walled cells of the ground tissue that make up the bulk of most nonwoody structures, yet sometimes their cell walls can be lignified. Parenchyma cells in between the epidermis and pericycle in a root or shoot constitute the cortex, and are used for storage of food. They are mainly present in the soft areas of the stems, leaves, root, flowers, fruits etc. Parenchyma cells within the center of the root or shoot constitute the pith. Parenchyma cells in the ovary constitutes the nucellus and are brick-like in formation. Parenchyma cells in the leaf constitute the mesophyll; they are responsible for photosynthesis and they allow for the interchange of gases.
Parenchyma is the most common and versatile ground tissue. It forms, for example, the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma cells are living cells and may remain meristematic at maturity, meaning that they are capable of cell division. They have thin but flexible cellulose cell walls, and are generally polygonal when close-packed, but approximately spherical when isolated from their neighbours. They have large central vacuoles, which allows the cells to store and regulate ions, waste products and water.
Parenchyma cells have a variety of functions: In leaves, they form the mesophyll and are responsible for photosynthesis and the exchange of gases, parenchyma cells in the mesophyll of leaves are a specialized parenchymatous tissue known as chlorenchyma (parenchyma with chloroplasts).
The form of parenchyma cells varies with their function. The epidermal parenchyma cells of a leaf are barrel shaped in cross section, but have a variety of outline shapes ranging from simple polygons to strongly branched and interlocked shapes resembling the pieces of a jigsaw puzzle, as in the leaves of Arabidopsis thaliana. In the epidermis of higher plants, only the guard cells have chloroplasts. This tissue serves as a barrier wall and protects the internal tissues from injury. In the spongy mesophyll of a leaf, parenchyma cells range from near-spherical and loosely arranged with large intercellular spaces to branched or stellate, mutually interconnected with their neighbours at the ends of the arms to form a three-dimensional network, as in the red kidney bean Phaseolus vulgaris and other mesophytes. These cells, with the epidermal guard cells of the stoma, form a system of air spaces and chambers that regulate the exchange of gases. They usually contain plastids.
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