Key Differences Between Spermatogenesis and Oogenesis
What is gametogenesis or define gametogenesis?
Gametogenesis could be defined as the process in which a haploid cell is formed from a diploid cell through meiotic cell division and cell differentiation. In higher plants and animals, two distinct types of gametes are observed via these processes.
In animals, a tissue is dedicated to forming gametes and is called a germline and individual cells in a germline are called germ cells. In animals, these germ cells undergo meiosis to produce gamete cells which directly develop into gametes.
In plants, diploid cells undergo meiosis to produce haploid spores which develop into haploid cells and gives rise to the haploid generation called gametophyte.
In this topic, we will learn about gametogenesis in humans which takes place in two processes. These are
Spermatogenesis
Oogenesis
We will discuss each process in brief.
Spermatogenesis process
The human male starts producing sperms when they reach puberty which is when they are around 10-16 years old and the process continues till death. Approximately 200million sperms are produced each day which increases the likelihood of a sperm reaching the egg.
Sperm production takes place in the testes of the human male, in the seminiferous tubules to be more specific. The seminiferous tubules are separated from the systematic circulation by the blood-testis barrier.
Blood-testes barrier: This barrier is formed by Sertoli cells and prevents hormones and constituents of the human circulatory system from affecting the developing sperm. It also prevents the immune system from recognising the sperm as a foreign object. (The sperm is genetically different from the male it can be recognised as an antigen) The Sertoli cells also play a role in the development of the spermatozoa.
Spermatogonia: It is the initial pool of diploid cells that divide via mitosis to produce identical cells. One cell will go under meiosis to form a sperm cell and the other cell will be used to replenish the spermatogonia. The cells which replenish the spermatogonia are called A1 spermatogonia.
The type B spermatogonia duplicate via mitosis many times to form identical diploid cells which are linked by cytoplasm bridges and they are called spermatocytes. The primary spermatocytes undergo meiosis I to produce two secondary spermatocytes. Then the secondary spermatocytes undergo meiosis II to produce four haploid cells called spermatids.
Then the cytoplasmic bridge breaks down and the spermatids are released into the lumen of the seminiferous tubule in a process called spermiation. Then they mature into spermatozoa in a process called spermiogenesis and travel along the seminiferous tubules till they arrive at the epididymis.
The sperm travels to the rete testes after they travel from the seminiferous tubule. The rete testes act to concentrate the sperm as it removes excess fluid. Before the sperm moves to the epididymis, it undergoes the final stages of maturation.
The entire process of spermatogenesis takes around 70 days and new groups of spermatogonia develop every 16 days. It is a continuous process as multiple spermatogonic processes take place simultaneously.
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Oogenesis process
Oogenesis in human females begins before birth in the foetus. Primordial germ cells in the yolk sack of the embryo move to colonise the cortex of the primordial gonad and duplicate by mitosis and reach around 7 million in numbers by mid-gestation which is approximately 20 weeks. After reaching the peak, cell death takes place and around 2 million cells begin meiosis I before birth. These cells are known as primary oocytes. Which means, a human female is born with 2 million primary oocytes arrested in meiosis I. These oocytes are arranged in the gonads in clusters and they are surrounded by epithelial cells which form primordial follicles.
Once puberty begins at the age of 15-20 for human females, a number of primary oocytes begin to mature each month and they undergo three stages to attain full maturation.
Pre-antral stage: The primary oocytes grow although they are arrested in meiosis I. The follicular cells grow and proliferate to form a stratified cuboidal epithelium. The calls are called granulosa cells and secrete glycoproteins to form the zona pellucida around the primary oocyte cells. The surrounding connective tissue cells differentiate to become the theca folliculin which responses to LH and can secrete androgens under the hormone’s influence.
Antral stage: Fluid-filled spaces form between granulosa cells. Eventually, these spaces combine to form the antrum which is a central fluid-filled space. The follicles are now known as secondary follicles. During the menstrual cycle each month, one of these secondary follicles becomes dominant and develops further under the influence of oestrogen, LH and FSH.
Pre-ovulatory stage: This stage is induced by a surge in LH and meiosis I is completed. Two haploid cells of different sizes are formed in the follicle. One daughter cell receives less cytoplasm and develops into a polar body. The other haploid cell is called the secondary oocyte. Both cells undergo meiosis II and the polar body will be replicated to produce two polar bodies whilst the growth of the secondary oocyte is arrested in the metaphase of meiosis II approximately 3 hours before ovulation.
Ovulation
In this stage, the follicle is fully grown and mature and it is called a Graafian follicle. A surge in LH increases collagenase activity which weakens the follicular wall. Furthermore, muscular contractions of the ovarian wall result in the release of ovum from the ovary and travel to the fallopian tube.
Fertilisation
The secondary oocyte will only complete meiosis II on fertilisation producing a third polar body and fertilised egg. If fertilisation does not occur, the oocyte degenerates 24 hours after ovulation.
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FAQs on Gametogenesis: Spermatogenesis and Oogenesis Made Simple
1. What is gametogenesis, and what are its two main types?
Gametogenesis is the biological process of forming specialised reproductive cells called gametes from precursor germ cells. This process involves meiotic divisions to produce haploid cells. The two main types of gametogenesis are spermatogenesis (the formation of sperm in males) and oogenesis (the formation of ova or eggs in females).
2. What are the main differences between spermatogenesis and oogenesis?
The primary differences between spermatogenesis and oogenesis lie in their outcome, timing, and cellular division process. The key distinctions include:
- Outcome: Spermatogenesis produces four small, motile sperm cells (spermatozoa) from one primary spermatocyte. In contrast, oogenesis produces only one large, non-motile ovum and two or three smaller polar bodies.
- Timing: In males, spermatogenesis begins at puberty and is a continuous process. In females, oogenesis starts during the foetal stage, pauses, and then continues in cycles from puberty until menopause.
- Cytokinesis: The division of cytoplasm is equal in spermatogenesis. In oogenesis, it is unequal, ensuring that the single resulting ovum receives the majority of the cytoplasm and nutrients needed for early development.
3. What are the three key stages of gametogenesis?
Gametogenesis generally proceeds through three distinct stages:
- Multiplication Phase: Primordial germ cells undergo repeated mitotic divisions to increase their numbers, forming spermatogonia (in males) or oogonia (in females).
- Growth Phase: The spermatogonia or oogonia grow in size by accumulating cytoplasm and nutrients, transforming into primary spermatocytes or primary oocytes. This is the longest phase.
- Maturation Phase: The primary gametocytes undergo two successive meiotic divisions (Meiosis I and Meiosis II) to produce haploid spermatids or a haploid ovum.
4. What is the role of key hormones like GnRH, LH, and FSH in regulating gametogenesis?
Hormonal control is crucial for gametogenesis and is regulated by the hypothalamic-pituitary-gonadal axis. Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to release two essential hormones:
- Follicle-Stimulating Hormone (FSH): In males, FSH acts on Sertoli cells to support spermatogenesis. In females, it stimulates the growth and development of ovarian follicles.
- Luteinizing Hormone (LH): In males, LH acts on Leydig cells to stimulate testosterone production, which is vital for sperm maturation. In females, a mid-cycle LH surge is the primary trigger for ovulation.
5. Why does oogenesis produce only one viable ovum and multiple polar bodies, unlike spermatogenesis which produces four sperm cells?
This difference is a result of unequal cytokinesis (cytoplasm division) during the meiotic divisions in oogenesis. The primary goal is to create one large, nutrient-rich egg cell capable of supporting early embryonic development post-fertilisation. By funnelling most of the cytoplasm and organelles into one daughter cell (the ovum) and creating small, non-functional polar bodies with the remaining genetic material, the process ensures the ovum has maximum resources for survival. In contrast, sperm cells only need to be motile and deliver DNA, so equal division is more efficient for producing large numbers.
6. How does the timing of gamete formation differ throughout the lifespan of human males and females?
The timing of gamete formation is fundamentally different between the sexes. In human females, oogenesis begins during the foetal development stage, where all primary oocytes are formed and then arrested in Prophase I of meiosis. This process only resumes at puberty, with one oocyte maturing per menstrual cycle until menopause. In human males, spermatogenesis does not begin until puberty. From that point on, it is a continuous and prolific process that produces millions of sperm daily throughout most of their adult life.
7. What is the importance of the acrosome in a sperm cell, and how is it formed?
The acrosome is a cap-like organelle covering the anterior part of the sperm's head. Its primary importance is to contain powerful hydrolytic enzymes, such as hyaluronidase and acrosin. During fertilisation, these enzymes are released in the acrosome reaction to digest the ovum's protective outer layers (the corona radiata and zona pellucida), allowing the sperm to penetrate and fertilise the egg. The acrosome is formed during spermiogenesis (the maturation of a spermatid into a spermatozoon) from the Golgi apparatus of the developing spermatid.
8. What is capacitation and why is it essential for fertilisation?
Capacitation is the final step in the maturation of spermatozoa, which occurs inside the female reproductive tract. It involves a series of physiological changes, including the removal of certain inhibitory glycoproteins and cholesterol from the sperm's head. This process is essential because it destabilises the acrosomal membrane, making the sperm hyperactive (improving its motility) and enabling it to undergo the acrosome reaction upon contact with the egg. Without capacitation, a sperm cannot fertilise an oocyte.
