Understanding:
• Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation
Gametogenesis is the process by which diploid precursor cells undergo meiotic division to become haploid gametes [sex
cells]
- In males, this process is called spermatogenesis and produce spermatozoa [sperm]
- In females, this process is called oogenesis and produce ova [eggs]
The process of gametogenesis occurs in the gonads and involves the following steps:
- Multiple mitotic divisions and cell growth of precursor germ cells
- Two meiotic divisions [meiosis I and II] to produce haploid daughter cells
- Differentiation of the haploid daughter cells to produce functional gametes
Spermatogenesis
- Spermatogenesis describes the producton of spermatozoa [sperm] in the seminiferous tubules of the testes
- The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis
- These cells [spermatogonia] then undergo a period of cell growth, becoming spermatocytes
- The spermatocytes undergo two meiotic divisions to form four haploid daughter cells [spermatids]
- The spermatids then undertake a process of differentiation in order to become functional sperm cells [spermatozoa]
Oogenesis
- Oogenesis describes the production of female gametes [ova] within the ovaries [and, to a lesser extent, the oviduct]
- The process begins during foetal development, when a large number of primordial cells are formed by mitosis [~40,000]
- These cells [oogonia] undergo cell growth until they are large enough to undergo meiosis [becoming primary oocytes]
- The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles
- The primary oocytes remain arrested in prophase I until puberty, when a girl begins her menstrual cycle
- Each month, hormones [FSH] will trigger the continued division of some of the primary oocytes
- These cells will complete the first meiotic division to form two cells of unequal size
- One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body
- The polar body remains trapped within the follicle until it eventually degenerates
- The secondary oocyte begins the second meiotic division but is arrested in metaphase II
- The secondary oocyte is released from the ovary [ovulation] and enters into the oviduct [or fallopian tube]
- The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte
- If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body [the first polar body may also undergo a second division to form a third polar body]
- Once meiosis II is complete the mature egg forms a ovum, before fusing
its nucleus with the sperm nucleus to form a zygote
Formation of Polar Bodies
Understanding:
• Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts
of cytoplasm
While the processes of spermatogenesis and oogenesis follow the same basic pathway, several key differences exist:
1. Number of cells produced
- In spermatogenesis, the cells divide equally during meiosis to produce four functional gametes
- In oogenesis, the cells do not divide equally and as a result only one functional gamete is formed [plus 2 – 3 polar bodies]
2. Size of cells produced
- In spermatogenesis, the cells that are formed following differentiation are all of equal size with equal amounts of cytoplasm
- In oogenesis, one daughter cell [the ovum] retains all of the cytoplasm, while the other daughter cells form polar bodies
- The polar bodies remain trapped within the surrounding layer of follicle cells until they eventually degenerate
3. Timing of the process
- In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death
- In oogenesis, the production of gametes is a staggered and finite process:
- It begins before birth [prenatally] with the formation of a fixed number of primary oocytes [~40,000]
- It continues with the onset of puberty according to a monthly menstrual cycle
- It ends when hormonal changes prevent the further continuance of the menstrual cycle [menopause]
Summary of the Differences between Spermatogenesis and Oogenesis
Gametogenesis Comparison
Which cell is formed as a result of meiosis egg?
Meiosis produces our sex cells or gametes? [eggs in females and sperm in males].
Why is only one egg produced in meiosis?
The primary oocyte undergoes the first meiotic division, resulting in the formation of a secondary oocyte [haploid]. A second meiosis division occurs in secondary oocytes, resulting in a haploid ovum. As a result of meiosis, one egg is produced, which contains the most important nutrients, cytoplasm, and organelles.
How many eggs form in meiosis what else forms and why?
Just one egg is produced from the four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg in Figure below.
How is egg cell formed?
The process of producing eggs in the ovary is called oogenesis. Oogenesis begins before birth but is not finished until after puberty. If the secondary oocyte is fertilized by a sperm as it is passing through the fallopian tube, it completes meiosis and forms a mature egg and another polar body.