What is the Difference Between Synergid and Egg Cell

The main difference between synergid and egg cell is that synergid is a type of supporting cell in the embryo sac whereas egg cell is the female gamete. Furthermore, synergid works together with the egg cell and central cell to accomplish double fertilization by attracting and accepting the pollen tube. 

Synergid and egg cell are two types of cells which occur inside the embryo sac of most flowering plants. Moreover, two synergids occur in the embryo sac at the micropylar end while a single egg cell occurs per embryo sac.

Key Areas Covered

1. What is the Synergid
     – Definition, Features, Function
2. What is the Egg Cell
     – Definition, Features, Function
3. What are the Similarities Between Synergid and Egg Cell
     – Outline of Common Features
4. What is the Difference Between Synergid and Egg Cell
     – Comparison of Key Differences

Key Terms

Double Fertilization, Egg Cell, Embryo Sac, Fertilization, Flowering Plants, Supporting, Synergid

What is the Synergid

A synergid is one of the two symmetrical cells that accompany the egg cell inside the embryo sac of flowering plants. The main function of these cells is to work together with the egg cell and the central cell to accomplish double fertilization. Here, the term synergid means ‘working together’ in Greek. More importantly, synergid cells play a key role in attracting and accepting the pollen tube.

Figure 1: Cells in the Embryo Sac

However, since synergid cells are one of the components of the embryo sac, these cells are haploid. They form as a result of the free nuclear division. Also, these cells occur at the micropylar end of the embryo sac. In addition, synergid cells contain Filliform apparatus, special cellular thickenings at the tip of the cells, guiding the pollen tube. Along with the egg cells, the two synergid cells form the egg apparatus. Ultimately, with the formation of the primary endosperm nucleus in double fertilization, these cells degenerate.

What is the Egg Cell

The egg cell is the female gamete present inside the embryo sac when considering flowering plants. Here, ovule is the structure which gives rise to the female reproductive cells. The female gametophyte or the embryo sac occurs inside the ovule. Also, the nucellus and the integuments are the other two structures in the ovule. The small opening in the integuments is the micropyle through which the pollen tube enters the female gametophyte. Significantly, the megaspore mother cell, which is one of the cells inside the nucellus, undergoes meiosis to form four haploid cells. One of these four cells becomes the megaspore and undergoes mitosis to form eight haploid cells, which are collectively known as the embryo sac. One of the cells of the embryo sac near the micropyle develops into the egg cell.

Figure 2: Double Fertilization

Especially, flowering plants undergo double fertilization in which one of the male gamete cell, which comes through the pollen tube, fertilizes the egg cell while the second male gamete cell fertilizes the two polar nuclei. Here, the fertilization of the egg cell forms the zygote, which further develops into the embryo. On the other hand, the fertilized two polar nuclei form the endosperm nucleus, which is triploid, and upon cell division, forms the endosperm.

Similarities Between Synergid and Egg Cell

• Synergid and egg cell are two types of cell in the embryo sac of flowering plants.
• Both are haploid cells.
• Besides, both have a function in fertilization and forming the seed.

Difference Between Synergid and Egg Cell

Definition

Synergid refers to one of two small, short-lived nuclei lying near the egg in the mature embryo sac of a flowering plant while the egg cell refers to the female reproductive cell (gamete) in oogamous organisms. Thus this is the main difference between synergid and egg cell.

Importance

Moreover, synergid is a type of supporting cell to the egg cell while egg cell is the female gamete which undergoes fertilization.

Number per Embryo Sac

Two synergids occur per embryo sac while a single egg cell occurs per embryo sac. Hence, this is an important difference between synergid and egg cell.

Function

Their function is another major difference between synergid and egg cell. Synergids work together with the egg cell and central cell to accomplish double fertilization by attracting and accepting the pollen tube while egg cell undergoes fertilization.

Conclusion

Synergid is one of the two supporting cells in the embryo sac of most of the flowering plants. The main function of them is to facilitate double fertilization. On the other hand, the egg cell is the female gamete inside the embryo sac, undergoing fertilization. The main difference between synergid and egg cell is their function.

References:

1. Higashiyama, The synergid cell: attractor and acceptor of the pollen tube for double fertilization, T. J Plant Res (2002) 115: 0149. https://doi.org/10.1007/s102650200020

Image Courtesy:

1. “Angiosperm embryo sac diagram” By BlueRidgeKitties (CC BY 2.0) via Flickr
2. ” Figure 32 02 07″ By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia