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Parenchymal tissue is the functional tissue in most plants and animals. The word parenchyma is derived from the Greek word “Parenkhien” meaning “beside” and “enkhuma” meaning “content of a vessel”. It is a type of tissue composed of cells that is responsible for essential functions in the body.
What is Parenchyma?
The parenchyma is considered simple permanent ground tissue in plant biology or botany. It forms the bulk of the plant tissues that are commonly found in the soft parts such as leaves, fruit pulp, and other organs of a plant. Let’s understand the functions of parenchyma in plants as well as in and animals. So, plants chiefly use parenchyma tissues for food storage and photosynthesis. In animals, photosynthesis did not occur thus they are responsible for detoxification and filtration of the toxins in the liver and kidneys respectively. It is found in invertebrates as spongy connective tissue.
Plants are chiefly made up of three types of cells: Parenchyma, Sclerenchyma, and collenchyma. Parenchymal tissues are found as ground tissues in plants.
• Parenchyma- They are metabolically active cells composed of a thin cell wall.
• Collenchyma- They have thicker cell walls than parenchyma.
• Sclerenchyma- sclerenchyma tissues are composed of dead cells and have thick cell walls.
Features of Plant Parenchyma
• They undergo cell division and divide themselves at maturity. Thus, they play important role in tissue regeneration and tissue repair.
• The reproductive cells (spores, gametes) are composed of parenchymatous cells.
• A zygote is also a type of totipotent parenchymatous cell. The ability of any cell to develop into a complete plant is called totipotence.
• Parenchyma is found as a mass of the cells in the pith and cortex of root and stem, fleshy part of succulents, and the endosperm of the seed.
• Functions of parenchymatous tissues in plants are assimilation, respiration, excretion, and radial transport of solute and water.
Structure of Plant Parenchymal Cells
• They are the living cells having a prominent nucleus and protoplast.
• The symmetry of cells can be isodiametric or polygonal or polyhedral.
• The shape varies from ovum, round to elongate with no intercellular space, or sometimes has large intercellular space.
• The cell wall is made up of only the primary cell wall that is composed of cellulose.
• The plasmodesmata connect the plant parenchymal tissues.
• They contain vacuoles for osmoregulation and to store anthocyanin or tannins.
• The parenchymal cells of leaves contain numerous chloroplast and cells of stem and flower are rich in chromoplasts.
• Parenchymal cells provide turgidity to the plant and also helps to provide mechanical strength.
Types of Plant Parenchyma
Based on the functions or shape of the parenchymal cells, they are categorized into the following types:
i. Chlorenchyma: The mesophyll part of the leaves is chiefly composed of chlorenchyma cells. It is also a type of parenchyma having chloroplast within them. They are loosely packed and have large intercellular spaces between them. The large number of chloroplast present in these cells provide the green color to the stem and the aerial root.
ii. Aerenchyma: The aerenchyma cells are mainly found in aquatic plants and provide buoyancy to these plants. The parenchymal cells of these plants have large intercellular space and contain air pockets or air cavities in the spaces. Aerenchyma store essential gases (oxygen and carbon dioxide), which is further utilized by the plant.
iii. Prosenchyma: The vascular tissues of the plants are composed of prosenchyma cells. These cells have a specific shape of the pointed end and elongated narrow cell body.
iv. Vascular Parenchyma: Vascular parenchyma is also found in the vascular tissues of the plant and provides nutrients to the xylem and phloem. For example- The xylem is composed of xylem parenchyma and the phloem has phloem parenchyma. The ergastic material such as resins, tannins are also stored in the vascular parenchyma. Axial parenchyma is an example of such parenchyma tissues.
v. Medullary Parenchym: a As the name suggests, the medullary rays of the primary vascular tissue are made up of medullary parenchyma. They have a thin cell wall and radially elongated cells that regulate the radial distribution of water and nutrients in the plant.
vi. Conjunctive Parenchyma: The conjunctive tissues of the stele of the plant roots are made up of conjunctive parenchyma. The conjunctive tissues in monocotyledons convert into sclerenchymatous tissues upon maturity.
vii. Armed Parenchyma: They are star-shaped parenchymatous cells. The mesophyll of the gymnosperms is made up of armed parenchyma.
Based on the shape, the parenchyma can be classified into the following types: Angular parenchyma and Circular parenchyma.
Parenchyma Function in Plants
• The ground tissue in plants is made up of parenchyma tissues.
• Parenchyma is responsible for the storage of food & nutrients like starch, hormones, proteins, etc.
• The photosynthetic parts such as leaves, stems are also made up of parenchyma cells. Here, the photosynthetic parenchyma helps in receiving maximum sunlight. The intercellular space of these cells facilitates the exchange of gases and water.
• The guard cells in leaves regulate the exchange of gases.
• They are responsible for the storage of water in xerophytes.
• Play an important role in growth and development as well as tissue repair.
• Help in transportation through phloem and xylem tissues.
• The totipotent parenchyma can develop into different plant organs and meristematic cells.
Parenchyma in Animals
All the organs in humans, as well as other animals, have the primary composition of parenchyma cells and stromal cells. The parenchyma cells have a major role in the functionality of the organ. The stromal cells or collectively defined as stroma, are the supportive cells in an organ. The examples of the organ parenchyma include:
i. Salivary Gland Parenchyma: present in the secretory and ductal salivary gland of animals and produce serous products.
ii. Brain Parenchyma: the brain cells, neurons, glial tissues are also made up of parenchymatous cells. The blood-brain barrier is made up of parenchyma cells that make the brain impervious. The parenchyma cells are also a part of the central nervous system and microglial cells. The microglial cells help in the homeostasis of brain physiology.
iii. Pulmonary Parenchyma or Lung Parenchyma: The parenchymal lung cells form the pulmonary alveoli, alveolar ducts, and respiratory bronchioles of the lungs. These cells are also responsible for gaseous exchange in animals. The pulmonary parenchymal cells are tensile and have strength, mechanical properties that increase the efficiency of the gaseous exchange in the lungs.
iv. Liver Parenchyma or Hepatic Parenchyma: The hepatocytes constitute 80% of the liver that are the parenchymatous cells. They are the main functional unit of the liver and perform detoxification, metabolism, and synthesis of proteins. The dysfunction of these parenchyma results in steatosis and liver cirrhosis disease.
v. Kidney Parenchyma or Renal Parenchyma: The renal medulla of the kidney has the chief composition of parenchymatous cells. There are numerous nephrons present in the renal cortex whereas the renal medulla is made up of renal tubules and ducts. The nephrons are the main filtration unit while the ducts are responsible for collecting the filtrate.
Functions of Parenchyma in Animals
• The nervous system has parenchymatous cells that involve in the process of acuity, information storage, and processing.
• Gaseous exchange in respiration is regulated by parenchymal cells of the lungs.
• Generate immune system.
• The hormones are secreted by secretory parenchymal cells present in glands such as the pancreas, adrenal gland, etc.
• Helps is blood filtration by renal cortex or nephrons.
• Liver parenchymal cells eliminate toxins from the body.
A tumor can be classified into two interrelated sections-
i. Parenchyma: The neoplastic cells of the tumor are parenchymal cells.
ii. Stroma: The stroma of stromal cells from the extracellular matrix of the tumor that is non-malignant. Stroma is mainly composed of two types of cells, Resident ( cancer stem cells, endothelial cells, cancer-associated cells), and non-resident cells (T cells and B cells, myeloid-derived suppressor cells, and tumor-associated macrophages)
The acoelometes are the category of invertebrates that consist of flatworms. These organisms do not possess a body cavity and all the physiological functions of the body are organized by tissues. They consist of three body layers- ectoderm, endoderm, and mesoderm. The outermost layer is called the ectoderm that is followed by the middle layer called the mesoderm and the innermost layer is the endoderm. The mesoderm layer is made up of mesenchymal cells and parenchymal cells. All three layers enclose the digestive system of the organisms of acoelometes. Here, the parenchymal cells help in the storage of nutrients, skeletal support, motility, regenerative cells reservoir, and transportation of the material.