Any species’ ability to reproduce is essential to its survival. While some organisms reproduce through asexual reproduction, humans and other mammals rely on sexual reproduction to keep their species alive. Specialised sex cells are formed to aid in transmitting genetic code from parents to offspring.

Gametogenesis is a biological process where the cells divide and develop the diploid or haploid precursor cells into mature haploid gametes. Gametogenesis happens either through the meiotic division of diploid gametocytes into different gametes or through mitosis, depending on the organism’s biological life cycle.

Gametogenesis

Different differentiation systems produce two physically distinct types of gametes (male and female) in mammals and higher plants.

In gonads, animals create gametes directly from diploid mother cells during meiosis (testis in males and ovaries in females). Gametogenesis differs between men and females of a sexually reproducing species:

• Spermatogenesis in male
• Oogenesis in female

 SPERMATOGENESIS

Beginning of spermatogenesis

Males begin producing sperm when they reach puberty, between 10 and 16 years. Biological males create a vast amount of sperm (200 million per day) daily. This increases the chances of sperm reaching the egg after ejaculation.

The whole process of spermatogenesis in humans is estimated to take about 74 days (according to tritium-labelled biopsies) to 120 days (according to DNA clock measurements). 

The male testes, specifically the seminiferous tubules, are where sperm is produced. To keep the tubules isolated from the systemic circulation, a blood-testis barrier arises in the testicles.

• Spermatogenesis is the process of spermatocytes (also known as sperms) developing and maturing. Male gametogenesis begins with the onset of puberty, unlike female gametogenesis. The pituitary gland releases both luteinising and follicle-stimulating hormones in response to gonadotropin-releasing hormone from the brain. Luteinising hormone acts on the testicular Leydig cells, which then secrete testosterone. The Sertoli cells of the testes will be stimulated by testosterone and follicle-stimulating hormone.
• The activation and mitotic division of the previously dormant spermatogonia within the seminiferous tubules of the testes is caused by the action of the Sertoli cells. They are transformed into primordial spermatocytes, undergoing the initial meiotic division.
• The secondary spermatocytes that result go through a second meiotic division, creating four haploid spermatids. Spermiogenesis is the process of transforming spherical spermatids into tadpole-like spermatocytes. The cell body lengthens, and the cytoplasmic volume shrinks throughout this process.
• The mature spermatocytes are made up of :

1. Head – The haploid nucleus and the acrosome, which contain the proteolytic enzymes required for fertilisation, are found in the head. It’s worth noting that the acrosome is a spermatid’s Golgi apparatus.
2. Neck – The neck forms a bridge between the head and the tail.
3. Tail – A tail aids motility (split into the primary, the middle, and the end parts). The mitochondria, which produce adenosine triphosphate (ATP) for cellular movement, are also housed here.

•  Spermiogenesis is the process through which the spermatids become spermatozoa or sperm. These mature spermatozoa are also known as sperm cells. Thus, the primary spermatocyte produces two cells; the secondary and secondary spermatocytes create four haploid spermatozoa cells due to their subdivision.

OOGENESIS

The differentiation of the ovum into a cell capable of continued development when fertilised is known as oogenesis, ovogenesis, or oogenesis. By maturation, it develops from the primary oocyte. The process of oogenesis begins during the embryonic stage.

Oogonia undergoes mitotic growth throughout the prenatal period (primordial oocytes). These cells grew larger after that and were identified as primary oocytes.

The female foetus begins gametogenesis by the fifth week of pregnancy; however, the cells are stopped in early prophase I. A basic squamous layer of follicular (granulosa) cells surrounds the primary oocytes. They produce an oocyte maturation inhibitor, which inhibits the primary oocyte from reaching the end of the meiosis cycle. They’re known as the primordial follicle when they’re all together. A female has roughly 2 million primordial follicles when she is born.

After the birth of the female, the ovaries will create no more primary oocytes. Most of these primordial follicles will have degenerated by the time adolescence arrives, leaving roughly 400,000 primary oocytes.  Only approximately 400 of these cells will develop over her reproductive lifetime (that is, from menarche to menopause).

 During the peripubertal stage, the primary oocyte continues to expand. Synapsis occurs, and tetrads form in those primary oocytes that continue to mature during the menstrual cycle, allowing for chromosomal crossing. Due to meiosis I, the primary oocyte has now evolved into the secondary oocyte. 

The haploid secondary oocyte enters meiosis II immediately after meiosis I. However, at the metaphase II stage, this activity is likewise suspended until fertilisation, if it occurs. The egg is destroyed and discharged (menstruation) if it is not fertilised, and the secondary oocyte does not complete meiosis II (and does not form an ovum). After meiosis II is finished, an ootid and another polar body are formed. The polar body is relatively small, and they disintegrate.

 The flat follicular cells evolve into cuboidal and subsequently columnar cells simultaneously. The cells also create zona pellucida, an amorphous, fenestrated glycoprotein material surrounding the primary oocyte. Just one primordial cell matures on average each month during the menstrual cycle.

Embryogenesis

• A multicellular embryo develops and forms from a unicellular zygote.
• It starts with the male and female gametes combining to form the zygote, a diploid cell.
• In humans, the embryonic phase lasts till 11 weeks, after which foetal development begins.
• Cleavage, gastrulation, neurulation, and organogenesis are the four stages of embryogenesis.

 Difference between gametogenesis and embryogenesis

Conclusion 

The formation of gametes from haploid progenitor cells is known as gametogenesis. In the male, the gametogenesis is called spermatogenesis, and in the female, it is called oogenesis. Spermatogenesis is the development of haploid sperm from the germ cells in the seminiferous tubules present in the testes. A mature sperm has a head, neck, and tail. Four haploid sperms are produced from a primary spermatocyte. The creation of female gametes (ova) within the ovaries is called oogenesis. A secondary oocyte matures to form the ootid. Only one ovum is produced, and the polar bodies made are disintegrated. Embryogenesis is the formation and growth of an embryo from the zygote.