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In animals 
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.
|brain||neurons and glial cells|
|lungs||Lung parenchyma in its strictest sense refers solely to alveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles. However, the term is often used loosely to refer to any form of lung tissue, also including bronchioles, bronchi, blood vessels and lung interstitium. Source: Medical Dictionary - 'Parenchyma Of Lung' In turn citing: Stedman's Medical Dictionary. 2006|
|ovary||Follicles with egg cells|
|pancreas||Islets of Langerhans and Pancreatic acini|
|spleen||white pulp and red pulp|
|placenta||placental villi, including the fetal vessels, and the maternal intervillous space; non-parenchyma comprises chorionic and decidual plates, fetal vessels of diameter >0.1 cm and intercotyledonary septa (Aherne, W. & Dunnill, M. S. (1966) Quantitative aspects of placental structure. J Path Bacteriol 91 123-139)|
In plants 
Parenchyma cells are thin-walled cells of the ground tissue that make up the bulk of most nonwoody structures, although sometimes they have lignified cell walls. Parenchyma cells in between the epidermis and pericycle in a root or shoot constitute the cortex, and tissue specialised for food storage commonly is parenchyma. They generally constitute "filler" tissue 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).
- Storage of starch, protein, fats and oils and water in roots, tubers (e.g. potato), seed endosperm (e.g. cereals) and cotyledons (e.g. pulses and groundnut)
- Secretion (e.g. hydathodes, nectaries and cells lining the inside of resin ducts)
- Wound repair and the potential for renewed meristematic activity
- Other specialized functions such as aeration (aerenchyma) and support
The form of parenchyma cells varies with their function. In the epidermis of higher plants, only the guard cells have chloroplasts, but in the spongy mesophyll of a non-specialised leaf, most parenchyma cells exposed to light are full of chloroplasts. The mesophyll 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 with various functions, such as photosynthesis or food storage.