Gametogenesis in females happens in ovaries and the formation of gametes in the ovaries is termed as oogenesis.
Oogenesis in females begins before they are even born. It starts at the 6-8th week of the fetus development. At this stage, the primordial (primitive) germ cells starts differentiating into oogonium within the ovaries. They start dividing vigoursly, at around 20th week of fetus development, the amount of oogonium present will be around 6-7 million in number. After 20th week, the oogonium starts degenerating and by the time of birth around 2 millions of oogonium will be found in the fetal ovaries. [1]
Oogonium, are diploid (2n) stem cells that divide mitotically to produce millions of germ cells. Even before birth most of the cells degenerate in process known as atresia. The oogonium that is survived starts differentiating, where only few of them forms primary oocytes. So at the time of puberty only around 60,0000 to 80,000 primary oocytes are formed and will be in arrested phase of meiosis I prophase (diplotene stage). During this arrested phase each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a primordial follicle.
Oogenesis Process
Out of 60,000 to 80,000 primary oocytes at puberty only around 400 will mature and ovulate during the women’s reproductive lifetime. Each month after puberty until menopause, gonadotropins (FSH & LH) are secreted by the anterior pituitary gland which further stimulates the development of several primordial follicles. Out of which only one will typically reach the maturity needed for ovulation and develops into primary follicles.
The primary follicle which was arrested at meiosis I prophase (diplotene stage) starts dividing and forms secondary follicle. The secondary follicle eventually becomes larger turning into mature (graafian) follicle.[2]
Just before ovulation the diploid primary oocyte completes meiosis I producing 2 haploid (ncells of unequal size each with 23 chromosomes. The smaller cell produced by meiosis I is called the first polar body, is essentially a packet of discarded nuclear material. The largest cell known as secondary oocyte receives most of the cytoplasm. After the formation of secondary oocyte it begins meiosis II, but will be arrested at the metaphase II stage.
The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process known as ovulation. At ovulation the secondary oocytes are swept into the uterine tube, if fertilization doesn’t occur the cell degenerates. If the sperm is present in the uterine tube and one penetrates the secondary oocyte, the arrested meiosis II resumes. The secondary oocyte splits into two haploid cells again of unequal size. The larger cell is the ovum or mature egg. The smaller one is the secondary polar body. The nuclei of the sperm cell and the ovum then unites forming a diploid zygote. [2]
References :
1. Principles of Anatomy And Physiology 12th edition By Gerard J. Tortora and Bryan Derrickson
2. Langman’s Medical Embryology 12th edition by T.W Sadler
Gametogenesis in male happens in testis. In humans, spermatogenesis takes 65 to 75 days. It begins with the spermatogonia, which contain the diploid (2n) number of chromosomes. Spermatogonia which are present in seminiferous tubule keeps dividing, where some remains as precursor stem cells and some differentiate into primary spermatocytes. Primary spermatocytes like sprematogonia are diploid (2n) cells where they have 46 chromosomes.
After each spermatocytes replicates its DNA through mitosis, the meiosis begins and at the end of meiosis I formation of secondary spermatocytes happen where each spermatocyte has haploid (n) 23 chromosomes. Each chromosome within a secondary sprematocyte however is made up of two chromatids (two copies of the DNA) still attached by a centromere. No replication of DNA occurs in the secondary spermatocytes.
Spermatogensis Process
In meiosis II, the chromosomes line up in a single line along the metaphase plate, and the two chromatids of each chromosome separates. The four haploid cells resulting from meiosis II are called spermatids. A single primary spermatocyte, therefore produces four spermatids via two rounds of cell division (meiosis I & meiosis II).[2]
The final stage of spermatogenesis is the development of haploid spematids into spermatozoa which is known as spermiogeneis. This process involves an extensive remodelling of the spermatids without further division. Each spermatid gradually differentiates into a minute motile spermatozoan with a head, middle piece and tail. The head consists of haploid nucleus and an acrosome, a cap like vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization. The middle piece is packed with mitochondria which provide energy for the movement of the sperm. The tail helps in movement of the sperm. [2]
References:
1. Principles of Anatomy And Physiology 12th edition By Gerard J. Tortora and Bryan Derrickson
2. Langman’s Medical Embryology 12th edition by T.W Sadler
Gamete Intrafallopian Transfer (GIFT) is an assisted reproductive technique were the eggs are retrieved from the ovaries just like an IVF procedure. Then the Gametes (Eggs and Sperms) are placed into Fallopian tube with a help of a catheter using a surgical procedure called laparoscopy(as shown in the figure below) where the egg gets fertilized.[1]
Zygote Intrafallopian Transfer (ZIFT) is an assisted reproductive technique were the eggs are retrieved from the ovaries just like an IVF procedure. These retrieved eggs are then mixed with sperms in the laboratory. Then the fertilized eggs (zygotes) are placed into fallopian tube with a help of a catheter using a surgical procedure called laparoscopy(as shown in the figure below). The zygote should eventually be implanted in the uterus and develop into a fetus. [1]
In vitro fertilization is an assisted reproductive technique were the male sperms and the female eggs are fertilized in a laboratory. The fertilized egg is developed into embryo stage which is then placed into the woman’s uterus with a help of a catheter.[1]
Hormones control the human body’s reproductive system. The two hormones secreted by the pituitary glands are follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are secreted in response to the release of GnRH from the hypothalamus. FSH travels to the ovaries and stimulates a group of follicles to grow. These primordial follicles develop into primary follicles and then secondary follicles.
Follicle stimulating hormone (FSH) is one of the gonadotrophic hormones. FSH is one of the hormones essential to pubertal development and the function of women’s ovaries and men’s testes. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg from one follicle at ovulation.[1]
Luteinizing hormone (LH) is produced and released in the anterior pituitary gland. It controls the function of ovaries in females. Two weeks into a woman’s cycle, a surge in luteinizing hormone causes the ovaries to release an egg during ovulation. If fertilization occurs, luteinizing hormone will stimulate the corpus luteum, which produces progesterone to sustain the pregnancy.[3]
During the time of puberty in females, out of 60,000 to 80,000 available primary oocytes, only 400 of them gets a chance to form a mature oocyte. Only one of the mature oocyte out of the 400 will be released every month which can get fertilized when it comes in contact with the sperm. [2]
How does all this happen?
The hypothalamus produces a hormone called the gonadotropin-releasing hormone or GnRH. GnRH stimulates the anterior lobe of the pituitary to secrete follicle-stimulating hormone (FSH).FSH travels to the ovaries and stimulates a group of follicles to grow, were one of them will survive and become a mature follicle, the rest die.
FSH triggers the stimulation of the follicles
FSH stimulates estrogen production. The increasing level of estrogen acts on the hypothalamus and the anterior pituitary to increase the level of GnRH and induce the production of another hormone, luteinizing hormone (LH). [3]
FSH triggers the production of estrogen
A surge in LH secretion triggers ovulation – the release of the egg from the follicle and the ovary.
Release of mature oocyte
Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the Fallopian tube. The fertilized egg immediately begins the process of development while travelling toward the uterus.
References:
Textbook of medical physiology, 11th edition , Arthur C. Guyton and John E. Hall.
Super-Ovulation:
Naturally, in each ovarian cycle one follicle becomes mature and gets ready for fertilization. Now women who have infertility problem and is going to undergo IVF, FSH are given as subcutaneous (under the skin) injection that will regulate ovulation, the growth and development of eggs in the ovaries.[4]
Administration of FSH for Ovarian follicle stimulation
Trans-vaginal Ultrasound:
After FSH treatment, ultrasound scans are needed to monitor the response of the follicles growth in the ovaries. The growth of the follicles is assessed by observing their increase in size using a trans-vaginal ultrasound.[4]
Trans-vaginal Ultrasound
If the follicles seen on the scan is in the range of 16 mm to 20 mm in size, then the trigger shot is given as mentioned in the next step. This hormone initiates the final maturation and release of the eggs. This mimics the LH surge that stimulates ovulation during normal cycle.
Taking a hCG shot:
The next step in IVF treatment is triggering the oocyte for the last stage of maturation, before retrieval. This last growth is triggered with an injection. This is also called the “hCG (human Chorionic Gonadotrophin) Trigger shot”.[4]
hCG trigger shot
The injection is given when the follicles have grown in range of 16 to 20 mm in size. This shot is typically a one-time injection.
Going for the Gold- Retrieval of Eggs:
About 34 to 36 hours after the “trigger shot” is received, the egg retrieval or ovum pick up will take place.
Expecting couples should be familiar with a wide range of topics related to pregnancy, childbirth, and parenting. Some common topics that expecting couples may want to learn about include:
Prenatal care: This includes regular check-ups with a healthcare provider, as well as recommended tests and screenings during pregnancy.
Nutrition and exercise during pregnancy: It is important for expecting mothers to maintain a healthy diet and engage in regular physical activity to support the health and development of the baby.
Labor and delivery: Expecting couples should learn about the different stages of labor and what to expect during childbirth. They may also want to consider their options for pain management during labor.
Postpartum care: After the baby is born, couples should be aware of the physical and emotional changes that can occur, as well as how to care for the newborn.
Breastfeeding: Many couples may choose to breastfeed their newborn, and it is important to learn about the benefits of breastfeeding and how to properly latch and feed the baby.
Infant care: Expecting couples should also be familiar with basic infant care tasks, such as changing diapers, bathing the baby, and feeding.
Parenting styles and philosophies: Couples may want to consider their approaches to parenting and the values and beliefs that they want to instill in their child.
It is also important for expecting couples to seek out reliable sources of information and to discuss any questions or concerns with a healthcare provider.
DISCLAIMER: The information provided in here is for general educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for any questions you may have regarding a medical condition. Do not rely on the information provided here for decision-making or self-treatment. If you are experiencing a medical emergency, seek immediate medical attention.
