Meristem cells are a group of cells that reside at the shoot and root tips of plants. As undifferentiated (or slightly differentiated cells) they are considered as stem cells given that they are the origin of many of the cells that go on to rapidly differentiate/specialize and form various parts of the plant.
While meristem cells share a number of characteristics with stem cells of animals, one of the main differences between the two lies in the fact that meristem cells can be restored and continue dividing which in turn allows indeterminate growth in plants (as long as the necessary resourses are available).
Collectively, meristem cells make up three types of meristematic tissues that include:
* The word meristem comes from the Greek word "Meristos" meaning divisible.
Meristem cells are classified based on their origin and location in the plant. For this reason, there are two major classifications. Different types of these cells have various characteristics and functions that contribute to the growth and development of the plant.
Also known as the primordial meristem or the procambrian meristem, these are the earliest and youngest meristem cells that originate from the embryo. As such, promeristem cells form the first meristematic tissues in the root tips and shoot tip of the plant.
In addition to being very few in number (given that they originate from the embryo and are found in very young plants), these cells are undifferentiated. However, they ultimately go through cell division to produce primary meristem cells.
Primary meristematic cells arise from the promeristem and make up the apical tip (present at the shoot tip) as well as the root promeristem. They are therefore more in numbers, as compared to the promeristematic cells and play an important role as the origin of primary tissues (primary growth).
* With actively dividing cells, the primary meristem tissue produces the cortex, epidermis, pith as well as the leaves of a plant.
These cells are produced by primary meristematic cells that are located in the shoot apices and plant roots. Unlike the previous types of cells, secondary meristematic cells are produced once the plant has already started developing. They are involved in the production of secondary tissues including vascular bundles (xylem and phloem).
* Another group of cells known as tertiary meristematic cells in the tertiary meristem have been shown to develop from parenchymatic tissues. These cells reside in the cortex and vascular tissue of plants.
* In plants, primary and secondary meristematic cells contribute to primary and secondary growth of the plant. Apical meristematic cells in the roots and shoot contribute to the primary growth that results in the plant growing longer while the vascular cambium and cork cambium that make up the lateral meristems contribute to the secondary growth (making the plant wider).
Cells of the apical meristem are located at the growing points of the plant. As such, they are present at the shoot, roots as well as branches of the plant. In these locations, they contribute to the length of the plant.
During division, cells of the apical meristem produce new meristematic cells that reside in the shoot tip and roots. Some of the new cells, however, differentiate to produce specialized cells that form different tissues of the plant.
Using cell to cell interactions as well as hormones that act as positional cues, cells of the apical meristem are not only capable of specializing to specific functions (thus forming specific tissues) but also settling in certain parts of the plant. Through the positional cues, certain genes are activated or inhibited thus regulating the differentiation pattern.
The apical meristem is divided in to (SAM) shoot apical meristem (cells located at the tip of branches and plant tip) and the (RAM) root apical meristem where cells are located at the tip of each root.
The primary meristem is basal to the shoot apical meristem (SAM) and is composed of cells that are considered to be in their embryonic stage.
These meristematic cells are divided into the following parts:
The differentiation of the protoderm is one of the major events of embryo development. Here, cells of the protodermal cell layer start differentiating following the polarity of the apical meristem and before the meristems form at the opposite ends of the embryo.
This differentiation results in the production of epidermal cells and consequently the epidermis. In the roots, some of the cells elongate (elongation of the cell walls) and form the root hairs. Although the cells formed here have a thin cell wall, they contain cellulose and pectic substances that help protect cells of the roots.
In addition, protoderm cells continually differentiate to produce new epidermal cells of the roots that ensure that new root hairs continue to be formed given that they have a short lifespan (a few days).
In the roots, protoderm plays an important role in the formation of root hairs that are involved in the absorption of nutrients and water in their environment. The epidermis (which is, for the most part, a single cell layer) also covers all organs in the stem of plants thereby acting like a protective layer.
Although cells of the ground meristem are a type of primary meristem, as is the case with protoderm cells and cells of the primary procambial, they are segregated and thus set apart from the other cells. Division of these cells results in the production of the cortex, pith as well as a number of other related tissues.
Cells of the ground meristem, therefore, contribute to the growth and development of the plant through the formation of such parts as the root cortex. The endodermis, located in the inner layer of the cortex helps regulate the accumulation of minerals in the roots and thus to other parts of the plant.
As the origin of ground tissue systems, cells of the ground meristem are involved in the production of three main types of cells that include:
Parenchyma - Although they may take up a variety of shapes, most parenchyma cells are spherical or elongated in shape. They may also have two cell walls (a thin primary wall as well as a secondary wall) consisting of a polylayer known as lignin.
These cells play a number of important roles related to metabolism (e.g. respiration, secretion, and storage among others). For instance, while the pith is the site of storage for most plants, the mesophyll in leaves is the site of photosynthesis.
* Parenchyma cells are also capable of being reprogrammed to form different types of cells when need be.
Collenchyma - Cells of the collenchyma cells serve to provide support to young stems and leaves when they differentiate. In young stems, they can be found beneath the epidermis (as the outer cells of cortex).
With regards to structure, collenchyma cells are elongated whose walls consist of cellulose and pectin. Thickening observed at the corner of their cell walls contributes to their supportive function.
Sclerenchyma - Like collenchyma cells, cells that make up the sclerenchyma also serve to provide supportive functions. As such, they also have a thicker secondary cell wall that consists of lignin.
They include fibers that are elongated cells which act like strengthening cables and sclereids of varying shapes that either exist as solitary cells or in clusters. Compared to the fibers, sclereids have thicker cell walls that help support the weight of plant organs.
* Cells of the Sclerenchyma die when they mature which causes them to lose their flexibility. As a result, they cannot provide the flexible support observed in collenchyma cells.
The procambium is composed of elongated cells that are characterized by a large nucleus, dense cytoplasm, and proplastids. Differentiation of these cells results in the production of the vascular tissue of the plant (primary xylem and phloem in plants). Various types of vessels develop at different stages and serve different functions.
Protoxylem - Involved in the transportation of water as the plant roots elongate. The presence of annular rings in the protoxylem also provide supportive function.
Metaxylem - Found in elongated parts of the roots and are involved in the exchange of water and minerals.
Phloem cells - Located between the arms of the protoxylem and form the phloem. They are involved in the transportation of food material during the adult life of the plant.
Cells of the intercalary meristem are located between the segments of non-meristematic organs (e.g. above nodes, the base of young leaves, etc). These cells are said to originate from primary meristematic cells and tend to lose their meristematic nature as they gradually become permanent. These cells are found in monocot plants such as grass (bamboo etc).
These cells are located parallel to the sides of organs (lateral side/along the stem) where they contribute to the girth/thickness of the plant. Cells that make up the lateral meristem are characterized by their rectangular shape that develops along one plane.
* Secondary tissue produced by cells of the lateral meristem include the vascular cambium and cork cambium in the epidermis.
* Cells of the cork cambium replace those of the epidermis in the stem as the plant matures. They are characterized by a rectangular shape, vacuoles in their protoplasm as well as tannins.
* Meristematic cells that do not differentiate into adult cells are known as initiating cells/meristematic initials.
As in animals, stem cells in plants are located in stem cell niches known as meristems (protomeristem and primary meristem). Here, plant cells that are regarded as stem cells are undifferentiated and located in the very tips of the plant.
Compared to stem cells in animals, studies have shown that stem cells in the meristem can be restored even in cases where they have been removed. In the event that favorable conditions are provided, this allows the plant to continue growing.
* Here, it is worth noting that plant stems are located in the meristem tissue at the growth points of the plant.
· Meristem cells can be restored - One of the most beneficial adaptations of is that they can be repeatedly restored. According to studies, meristem cells can arise from differentiated cells. This is made possible by the feedback principle between stem cells and actively dividing cells where a protein synthesized by the stem cells help restore meristem.
· Reprogramming - Types of meristematic cells can be reprogrammed to form different types of plant cells. In particular, this has been shown to be the case in tissue injury. Here, reprogramming allows specific types of cells to be produced to repair the injury.
· Small, polygonal or spherical in shape - This characteristic is important given that it allows for a large number of meristematic cells to be closely packed.
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