Parenchyma
n., plural: parenchymata or parenchymas
[pəˈɹɛŋ.kɪm.ə]
Definition: a type of tissue that is an essential component of an organ
Table of Contents
Parenchyma Definition
What does parenchyma mean? Let’s define the word “parenchyma”. Most of the functional tissues in plants and animals are parenchyma tissues. The word parenchyma came from the Greek parénkhyma, or from “parenkhein”, meaning “beside”, “to pour in” whereas énkhuma means “content of a vessel”. Parenchyma is a type of tissue consists of cells that carry out an essential function. In botany (plant biology), parenchyma is the simple permanent ground tissues that form the bulk of the plant tissues, such as the soft part of leaves, fruit pulp, and other plant organs. Now, what is the main function of parenchyma in plants? Parenchyma is the tissue that is chiefly used by plants for storage and photosynthesis.
We, too, have parenchyma. Our parenchyma tissues though are not involved in photosynthesis. Instead, they are involved in detoxification (in the liver) and filtration of the toxins (in the kidneys). Interestingly, in invertebrates like flatworms, their parenchyma tissue is a spongy connective tissue.
Let’s take each type of parenchymal cell one by one.
Plant Parenchyma
The fundamental (or ground tissues) of plants are made up of three types of plant cells: parenchyma, sclerenchyma, and collenchyma.
- Parenchyma – composed of living cells with a thin cell wall; metabolically active
- Collenchyma – composed of living cells with a thicker cell wall than parenchyma
- Sclerenchyma – composed of dead cells with a thick cell wall (due to an additional cell wall layer referred to as “secondary wall”); primarily for structural support
Read more: Parenchyma, Collenchyma, Sclerenchyma – Biology Tutorial
Characteristic features of the plant parenchymal cells
- Parenchyma cells are living tissues with the capability to undergo cell division at maturity. Thus, providing the required assistance in tissue regeneration and tissue repair.
- The primary composition of the reproductive cells (spores, gametes) is parenchymatous.
- Each parenchymatous cell of the zygote is totipotent, meaning it has the ability to develop into a complete plant.
- Continuous mass or the bulk of the cells in the pith and cortex of root and stem, mesophyll of leaves, fleshy part of the succulents, and the endosperm of seed is parenchymatous.
- Essential activities like storage, secretion, photosynthesis, assimilation, respiration, excretion, and radial transport of solute and water are carried out by parenchymatous tissues.
Structure of plant parenchymal cells
- Parenchymal cells in plants are living cells with a prominent nucleus and protoplast
- Parenchymal cells in plants can be isodiametric or polyhedral or polygonal or oval or round or elongated in shape
- Parenchymal cells can be tightly packed with no intercellular spaces or maybe loosely packed with large intercellular space
- The cell wall of the plant parenchymal cells is thin. It is made up of one layer (“primary wall”) that is composed of cellulose and hemicellulose
- Plant parenchyma tissues are connected via plasmodesmata
- Plant parenchymal cells possess many small vacuoles, which upon maturation may merge to form a large central vacuole. Vacuoles are important in osmoregulation (water concentration regulation). They may also store anthocyanin or tannins.
- Parenchymal cells that accumulate or act as storage houses possess thick xyloglucan walls. The sugar stored in these cells acts as an energy source during germination, resulting in the thinning of the walls.
- Some of the parenchymal cells that are present in the stem or flowers possess numerous chromoplasts. In green leaves and stems, the parenchymal cells are rich in the chloroplast, which is a plant organelle responsible for photosynthesis.
- Some parenchyma may have a thick lignified cell wall, which, in this case, could be difficult to identify from sclerenchyma
- Parenchymal cells also help to provide turgidity to the plant, thus providing the required mechanical strength
- Secretory plant parenchymal cells are rich in Golgi bodies, ribosomes, and a highly established endoplasmic reticulum
Now, let’s find out how many types of parenchyma are there.
Types of plant parenchyma
Parenchymal cells in plants are classified on the basis of function and shape. Functionally, parenchymal cells in plants can be categorized into the following types:
1. Chlorenchyma
Chlorenchyma is present in the mesophyll part of the leaves. These parenchymal cells have chloroplast in them. Additionally, these cells are also found in the young plant cortex of the stem of the plant. These cells are loosely packed with multiple intercellular spaces. Chlorenchymal cells provide the green color to the stem and the aerial roots of the plants. These parenchymal cells carry out the function of manufacturing the food by the process of photosynthesis.
2. Aerenchyma
These parenchymal cells are characteristically found in aquatic plants wherein they are involved in providing buoyancy to the plants. These parenchymal cells are loosely packed cells with large intercellular spaces wherein the air cavities or the air pockets are present, which provide the required buoyancy to the plant afloat. The gases (oxygen or carbon dioxide) stored in the aerenchyma can be utilized by the plant also.
3. Prosenchyma
These types of parenchymal cells are usually found in the vascular tissues of plants. These cells characteristically possess pointed end and are elongated narrow cells.
4· Vascular parenchyma
Vascular parenchymal cells present in the vascular tissues of the plants provide nutrients to the vascular tissues. Examples are xylem parenchyma in xylem tissue and phloem parenchyma in phloem tissue. Some parenchymal cells are also involved in the storage of the ergastic material, like resins, tannins to name a few. Examples of such parenchyma tissue involved in storing ergastic material are axial parenchyma and ray parenchyma in wood. Parenchymatous cells of the old xylem cells (which are referred to collectively as heartwood) synthesize tylose that blocks the xylem cavity thereby preventing water transportation through them.
5. Medullary parenchyma
As its name suggests, the medullary parenchyma consists of parenchymal cells found in the medullary ray of the primary vascular tissue of the stem. These parenchymatous cells are thin-walled radially elongated cells that carry out the radial distribution of water and nutrients to the plants. These cells also function as storage of starch grains.
6· Conjunctive parenchyma
These parenchymatous cells are part of the conjunctive tissue of the stele of the plant roots. In monocotyledons, upon maturation, these conjunctive tissues are converted into sclerenchymatous cells.
7. Armed parenchyma
These are the star-shaped parenchymatous cells found in the mesophyll part of the gymnosperms (e.g. in pine tree leaves).
Classification of plant parenchyma based on the shape of the cells:
- Angular parenchyma: These are polygonal cells with angular ends that are packed tightly without any intercellular spaces (Figure).
- Circular parenchyma: As the name suggests, these are the circular cells that are loosely packed thus having multiple intercellular spaces between them (figure).
Parenchyma function in plants
The function of parenchyma in plants are as follows:
- Parenchymal cells are part of the ground tissues of the plants. Ground tissues are the tissues apart from dermal and vascular tissues.
- Storage of food & nutrients like starch, hormones, proteins, etc. is the primary function of parenchyma in plants.
- In the leaves and cortex of the young stem, parenchyma cells are associated with the manufacturing of food by photosynthesis. Such photosynthetic parenchyma or the mesophyll cells can be found just under the epidermis, in order to receive maximum sunlight. The mesophyll is subdivided into two types: palisade and spongy mesophyll. Palisade mesophyll is located near to the upper epidermis of the leaves, where light exposure is higher, while the spongy mesophyll is found on the lower side of the leaves (Figure). The palisade layer or the palisade mesophyll parenchymatous cells are tightly packed and possess a higher density of chloroplasts. This ensures high photosynthetic activity. In contrast, the spongy mesophyll parenchymatous cells are loosely packed with intercellular spaces. These intercellular spaces facilitate the exchange of gases and water.
- In leaves, guard cells for gaseous exchange
- Parenchymal cells act as a store for water in xerophytes
- Helps in providing structure to the plant by giving a mechanical rigidity to it
- Play role in growth and development of the plant
- Metabolic activities are carried out in parenchyma
- Wound regeneration, healing, and repair
- Parenchymal cells also specialize in providing buoyancy to certain aquatic plants
- Storage of the ergastic substances like resins etc
- Parenchymal cells of the xylem and phloem help in the transportation of water and nutrients throughout the plant.
- These cells have the capability to differentiate into secondary meristematic cells (for example, cork cambium). It is important to note here, that all the meristematic cells are the parenchymal cells.
Parenchyma in Animals
The functional cells of the organs are parenchymal while the neoplastic cells of the tumor are also parenchymatous.
Organ parenchyma
Essentially, any organ is made up of parenchymal cells and stromal cells wherein the functionality of the organ is due to the parenchymal cells. A biological organ is made up of multiple tissues. One may ask to define a tissue — a tissue is a collection of similar cells that carry out a specific function.
The primary cell of any organ that provides them the functionality are the parenchymal cells. While the supportive cells in an organ are collectively defined as stroma (or stromal cells).
Some of the examples of the organ parenchyma are as follows:
- Salivary gland parenchyma: parotid, the secretory and ductal epithelium of the salivary gland that produces serous products is parenchymatous.
- Brain parenchyma: brain cells, neurons, and glial cells are parenchymatous as well. Brain parenchymal cells constitute the blood-brain barrier that makes the brain impervious to any external substance. Microglial cells are the macrophages of the central nervous system that are essentially parenchymatous. These microglial cells are essential for the homeostasis of brain physiology. Hemorrhage (the bleeding in the brain parenchyma) that is usually seen in stroke patients is termed “intraparenchymal hemorrhage”.
- Pulmonary parenchyma or lung parenchyma: Pulmonary alveoli, alveolar ducts, and respiratory bronchioles are made up of parenchymal lung cells. These pulmonary parenchymal cells are involved in the gaseous exchange. The thin-walled pulmonary alveoli provide enormous surface area thus making them efficient in carrying out the respiratory gaseous exchange. The architecture of the pulmonary parenchymal cells, tensile strength or pre-stress, and the mechanical properties of the pulmonary parenchyma determine the efficiency of the lung gaseous exchange.
- Liver parenchyma or Hepatic parenchyma: hepatocytes are the parenchyma cells in the liver. They constitute 80% of the liver. Hepatocytes are the functional unit of the liver that carries out detoxification, metabolism, and synthesis of proteins. Hepatic steatosis and liver cirrhosis are some of the liver parenchymal diseases.
- Kidney parenchyma or renal parenchyma: renal cortex and the renal medulla constitute the renal parenchyma — the functional part of the kidney. The renal cortex is made up of numerous nephrons whereas the renal medulla is formed by the renal tubules and ducts. Nephrons are the filtration unit of the kidneys; the renal tubules or ducts are responsible for collecting the filtrate that has to be eventually eliminated from the body.
The function of parenchyma in animals
- Parenchymal cells of the nervous system are involved in the process of acuity, thinking along with information storage and processing
- Parenchymal cells of the pulmonary system (lungs) are involved in the process of gaseous exchange in respiration
- Lymphatic system parenchymal cells are required for the generation of immune cells
- Secretory parenchymal cells are involved in the production of hormones from the glands such as the pancreas, adrenal gland, reproductive organs
- Renal parenchymal cells carry out the function of filtration of blood
- Toxins are eliminated or broken down by the liver parenchymal cells
Tumor parenchyma
A solid tumor has two discrete interrelated sections
- Parenchyma– Parenchymal cells are the neoplastic cells in a solid tumor
- Stroma– Non-malignant and extracellular matrix constitute the stroma. Stroma is made up of resident (cancer-associated fibroblasts, endothelial cells, and pericytes, cancer stem cells, mesenchymal cells) and non-resident (T cells and B cells, natural killer cells, myeloid-derived suppressor cells, and tumor-associated macrophages) types of cells.
The stromal cells work in conjunction with the parenchymal cells. Parenchymal cells are responsible for the progression and the invasiveness of the tumor, i.e., malignancy.
Invertebrate parenchyma
Flatworms are the invertebrates that fall under the category of acoelomates. Acoelomates are organisms that do not possess a body cavity and tissues are organized to carry out all the physiological functions of the body. In acoelomates, the body layers — ectoderm, endoderm, and mesoderm — enclose the gut of the organism. The ectoderm is the outermost layer while the endoderm layer is the innermost layer enclosing the gut or the digestive tract. The middle layer, the mesoderm, is made up of the mesenchymal cells and the parenchymal cells (Figure 6). The muscle fibers and loose tissue occupy the space between the body wall (ectoderm) and the digestive tract are parenchymatous cells present in the mesodermal layer. These parenchymal cells in the acoelomates provide storage of nutrients, skeletal support, motility, regenerative cells reservoir, and transportation of the material.
Try to answer the quiz below to check what you have learned so far about parenchyma.
References
- Suki, B., Stamenović, D., & Hubmayr, R. (2011). Lung parenchymal mechanics. Comprehensive Physiology, 1(3), 1317–1351. https://doi.org/10.1002/cphy.c100033
- Connolly JL, Schnitt SJ, Wang HH, et al. (2003). Tumor Structure and Tumor Stroma Generation. In: Kufe DW, Pollock RE, Weichselbaum RR, et al., editors. Holland-Frei Cancer Medicine. 6th edition. Hamilton (ON): BC Decker. Available from: https://www.ncbi.nlm.nih.gov/books/NBK13447/
- Nobel P. S. (2006). Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus. Annals of botany, 97(3), 469–474. https://doi.org/10.1093/aob/mcj054
- He, X. Q., Suzuki, K., Kitamura, S., Lin, J. X., Cui, K. M., & Itoh, T. (2002). Toward understanding the different function of two types of parenchyma cells in bamboo culms. Plant & cell physiology, 43(2), 186–195. https://doi.org/10.1093/pcp/pcf027
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