Difference Between Secondary Oocyte and Ovum

Main Difference – Secondary Oocyte vs Ovum

Secondary oocyte and ovum are two stages of the differentiation of female gametes from the primary oocyte in a process called oogenesis. Oogenesis occurs during the sexual reproduction in mammals. During the fetal development, a finite number of primordial follicles are developed from the germinal epithelium of the ovary. These primordial follicles are transformed into primary oocytes in a process called oocytogenesis The main difference between secondary oocyte and ovum is that secondary oocyte is produced by the meiosis 1 of the primary oocyte whereas ovum is produced by the maturation of the ootid. The ootid is produced by the meiosis 2 of the secondary oocyte. The ovum is considered as the mature female gamete.   

Key Areas Covered

1. What is Secondary Oocyte
      – Definition, Development Process, Characteristics
2. What is Ovum
      – Definition, Development Process, Characteristics
3. What are the Similarities Between Secondary Oocyte and Ovum
     – Outline of Common Features
4. What is the Difference Between Secondary Oocyte and Ovum
      – Comparison of Key Differences

Key Terms: Meiosis 1, Meiosis 2, Oogenesis, Oogonia, Ootid, Ovary, Ovum, Polar Body, Primary Oocyte, Secondary Oocyte

What is Secondary Oocyte

Secondary oocyte is the oocyte formed after the completion of the meiosis 1 that gives rise to the ootid and ovum upon the completion of meiosis 2. The oognia are the cells in the ovary that give rise to the primary oocytes during oogenesis. Oogenesis is the differentiation process of oogonia into mature ova in humans. Oogonia are diploid cells, and they divide by mitosis to produce primary oocytes. Generally, around 7 million of primary oocytes are produced during the fetal development stage. Approximately 1-2 million primary oocytes are present in the ovary at birth. Those primary oocytes undergo meiosis 1. The meiosis 1 of the primary oocytes is arrested at the diplotene stage of the prophase 1. This meiosis 1 continues after puberty. The resumption of the meiosis 1 occurs under the influence of luteinizing hormone (LH). The meiosis 1 produces a haploid cell that contains two chromatids. It is called the secondary oocyte.

Figure 1: Oogenesis 

The meiosis 1 also produces the first polar body (1PB). This polar body is a cytoplasmic exclusion body, which contains excess DNA. After the completion of meiosis 1, the secondary oocyte commences meiosis 2. However, meiosis 2 is also arrested at metaphase 2 until the fertilization of the female gamete by a male gamete. The process of oogenesis is shown in figure 1. 

What is Ovum

Ovum is the mature female gamete in mammals, which gives rise to the embryo after fertilization. The secondary oocyte, which is arrested at the metaphase 2 of the meiosis 2 resumes meiosis 2 after fertilization by a sperm. The completion of meiosis 2 produces one ootid and another polar body. This polar body also contains excess DNA produced by the fertilization. The ootid matures into the ovum. The maturation of the ootid occurs under the influence of the follicle stimulating hormone (FSH).

Figure 2: An ovum

The outer layer of the ovum is enclosed by a follicle cell layer. The nucleus of the ovum is enclosed by the cell plasma, which is also called the yolk. The yolk contains essential nutrients for the growth of the ovum. An ovum is shown in figure 2. 

Similarities Between Secondary Oocyte and Ovum

• Both secondary oocyte and ovum are stages of the differentiation of primary oocyte to a mature female gamete.
• Both secondary oocyte and ovum are produced inside the ovary after puberty.
• Both secondary oocyte and ovum are haploid.

Difference Between Secondary Oocyte and Ovum

Definition

Secondary Oocyte: Secondary oocyte is the resulting oocyte when meiosis 1 is completed and gives rise to ootid and ovum upon the completion of meiosis 2.

Ovum: Ovum is the mature female gamete in mammals, which gives rise to the embryo after fertilization.

Origin

Secondary Oocyte: A secondary oocyte is produced from a primary oocyte by undergoing meiosis 1.

Ovum: An ovum is produced when the ootid matures. 

Composition

Secondary Oocyte: Each chromosome of the secondary oocyte consists of two chromatids.

Ovum: Each chromosome of the ovum consists of a single chromatid.

Develop into

Secondary Oocyte: A secondary oocyte develops into the ovum by undergoing meiosis 2.

Ovum: An ovum develops into the zygote after fertilization.

Differentiation

Secondary Oocyte: Secondary oocyte is a less differentiated female gamete.

Ovum: The ovum is the differentiated female gamete.

Conclusion

Secondary oocyte and ovum are two stages of the female reproductive cells in mammals. The differentiation of the primary oocyte into the ovum is called the oogenesis. The primary oocyte undergoes meiosis 1 to produce a secondary oocyte and a polar body. The secondary oocyte undergoes meiosis 2 to produce the ootid and another polar body. The ootid matures to produce the ovum, which produces the embryo after the fertilization of the ovum by a sperm. This is the difference between secondary oocyte and ovum.

Image Courtesy:

1. “Gray5” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body Bartleby.com: Gray’s Anatomy, Plate 5 (Public Domain) via Commons Wikimedia
2. “Ovum in Cumulus Oophorus, Human Ovary (6264021209)” By Ed Uthman from Houston, TX, USA – Ovum in Cumulus Oophorus, Human OvaryUploaded by CFCF (CC BY 2.0) via Commons Wikimedia

Reference:

1. Gilbert, Scott F. “Oogenesis.” Developmental Biology. 6th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here. Accessed 25 Sept. 2017.
2. “Ovum.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 22 Nov. 2010, Available here. Accessed 25 Sept. 2017.

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