Types, Characteristics, Functions, Vs Stem Cells
Definition: What are Meristem Cells?
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:
- Apical meristems
- Lateral meristems
- Basal/Intercalary meristems (present in some plants)
* The word meristem comes from the Greek word "Meristos" meaning divisible.
Types and Characteristics
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.
Classification based on Origin
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.
Secondary Meristem Cells
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).
Classification based on their Position/Location
Apical Meristematic Cells
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.
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