Saturday 21 January 2012
PARENCHYMA
The parenchyma are the functional parts of an organ in the body. This is in contrast to
the stroma, which refers to the structural tissue of organs,
namely, the connective
tissues. 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 neurons and glial cells heart myocyte kidney nephron liver hepatocyte 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 pancreas Islets of Langerhans and Pancreatic acini spleen white pulp and red pulp In plants Further information: Ground tissue#Parenchyma 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.[2] 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 [3], 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. 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 [3]. In the spongy mesophyll of a leaf, parenchyma cells range from
near-spherical and loosely
arranged with large intercellular spaces[3] 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 .[4] 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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