Difference Between Microspore and Pollen Grain

The main difference between microspore and pollen grain is that the microspore is a small spore in land plants, which develops into the male gametophyte whereas the pollen grain is a fine granule in seed plants, consisting of a reduced male gametophyte. Furthermore, microspore is a unicellular structure while pollen grain is a multicellular structure.

Microspore and pollen grain are two structures in male sexual structures of plants. They develop the male gametophyte.

Key Areas Covered

1. What is Microspore
     – Definition, Structure, Function
2. What is Pollen Grain
     – Definition, Structure, Function
3. What are the Similarities Between Microspore and Pollen Grain
     – Outline of Key Features
4. What is the Difference Between Microspore and Pollen Grain
     – Comparison of Key Differences

Key Terms

Land Plants, Male Gametophyte, Microspore, Pollen Grain, Seed Plants

What Microspore

Microspore is one of the two types of spores produced by heterosporous land plants. It is small and the large type of spore is the megaspore. Microspore develops into the male gametophyte while megaspore develops into the female gametophyte. Hence, microspores and megaspores link the sporophyte stage to the gametophyte stage in plants which undergo alteration of generations. Further, male gametophyte produces male gametes while female gametophyte produces female gametes.

Figure 1: Microspores (Small Red Cell) and Megaspores (Large Red cells) of Spikemoss

Diploid microsporocytes in microsporangium give rise to haploid microspores by undergoing meiosis. Microsporangia occur in modified leaves called microsporophylls. This process is known as microsporogenesis. Three layers that cover the microspore are perispore (the outermost layer), exospore (the middle layer), and endospore (the inner layer). Perispore shows the highest thickness among the three. The development of microspores can be seen in ferns, spikemosses, and quillworts. Microspore in seed plants develops into a pollen grain. 

What is Pollen Grain

Pollen grain is the reduced, male gametophyte in seed plants, angiosperms, as well as, gymnosperms. This means, microspore of these plants further develops into the pollen grain. In gymnosperms, clusters of 50 or more microstrobili develop in a pollen cone at the tips of the lower branches of the tree. The microsporophylls of gymnosperms contain a pair of microsporangia, which comprise of microsporocytes. The microspores developed through meiosis of microsporocytes then develop into pollen grains in a process called microgametogenesis. Each pollen gain consists of four cells and a pair of external air sacs.

Figure 2: Germinating Pollen Grains

In angiosperms, the mass of cells that give rise to pollen grains is organized into four sacs in the anther. It is made up of microsporocytes, which undergo meiosis to produce microspores. These microspores then undergo mitosis and a double layer is formed surrounding the microspore, producing the pollen grain. Since the pollen grain contains mast cells, which are haploid, it is called the male gametophyte. Upon pollination, it germinates to form the two male gamete cells.

Similarities Between Microspore and Pollen Grain

• Microspore and pollen grain are two structures produced by diploid microsporocytes through meiosis.
• They are produced by heterosporous plants.
• They represent the male part during sexual reproduction.
• Their formation occurs through a process known as microsporogenesis.
• Both are haploid and tiny structures.

Difference Between Microspore and Pollen Grain

Definition

Microspore refers to any of the spores in heterosporous plants that give rise to male gametophytes, which are generally smaller than megaspores whereas pollen grain refers to a microscopic body that contains the male reproductive cell of a plant. This explains the main difference between microspore and pollen grain.

Occurrence

Microspore occurs in seedless vascular plants such as ferns and spike mosses while pollen grain occurs in seed plants.

Unicellular/Multicellular

The respective structure is another difference between microspore and pollen grain. Microspore is a unicellular structure while pollen grain is a multicellular structure.

The development cycle is another important difference between microspore and pollen grain.

Formation

The formation of a microspore is known as microsporogenesis while pollen grain undergoes both microsporogenesis and microgametogenesis.

Meiosis/Mitosis

While microspore only undergoes meiosis, pollen grain undergoes both meiosis and mitosis.

Male Gametophyte

Microspore develops into the male gametophyte later while pollen grain has already developed into the male gametophyte.

Nutrients

Furthermore, microspore contains a lesser amount of nutrients while pollen grain contains a larger amount of nutrients.

Conclusion

Microspore is a small spore, which develops the male gametophyte. It occurs in seedless, vascular plants like ferns. In seed plants, microspores further develop into the male gametophyte, forming the pollen grain. The main difference between microspore and pollen grain is the development and structure.

Reference:

1. “Microspore.” Biology Online, Available Here
2. “Pollen – Definition, Structure and Formation.” Biology Dictionary, Biology Dictionary, 20 Aug. 2017, Available Here

Image Courtesy:

1. “Selaginella heterospores” By AerobicFox – Own work (CC0) via Commons Wikimedia  
2. “Germination of pollen (255 27) Total preparation (pollen tube visible)” By Doc. RNDr. Josef Reischig, CSc. – Author’s archive (CC BY-SA 3.0) via Commons Wikimedia