CL 5 Gametogenesis Till Implantation

Gametogenesis and Implantation Lecture Notes

Overview

Gametogenesis refers to the process of forming gametes (sperm and oocytes) which includes meiosis and cytodifferentiation necessary for maturation. This process spans from the formation of primordial germ cells to zygote implantation.

Objectives

1. Describe spermatogenesis and oogenesis.

2. Compare spermatogenesis and oogenesis.

3. Describe ovulation.

4. Explain capacitation and acrosome reaction of sperm.

5. Describe the phases of fertilization.

6. Explain cleavage of the zygote and implantation of the blastocyst.

Gametogenesis

Definition

Gametogenesis combines meiosis (to reduce chromosome numbers) and cytodifferentiation (to mature gametes).

Primordial Germ Cells (PGCs)

• Formed in the epiblast during week 2 of development.

• Move through primitive streak during gastrulation.

• Migrate to yolk sac, then to developing gonads, arriving by the end of the fifth week.

Meiotic Process

Meiosis Overview

• Results in genetic variability and haploid cells.

• Meiosis I produces secondary oocytes/secondary spermatocytes with 23 double-structured chromosomes.

• Meiosis II produces mature gametes: 23 single chromosomes (oocyte and sperm).

Spermatogenesis

• Begins at puberty, transforming spermatogonia into spermatozoa.

Stages of Development

1. Type A spermatogonia undergo mitosis to produce Type B spermatogonia.

2. Type B spermatogonia differentiate into primary spermatocytes.

3. Primary spermatocytes undergo meiosis I to form secondary spermatocytes.

4. Secondary spermatocytes undergo meiosis II to form spermatids.

5. Spermatids mature into spermatozoa through spermiogenesis, characterized by:

   • Formation of the acrosome.

   • Condensation of nucleus.

   • Development of a tail and neck and middle piece.

   • while shedding most cytoplasm.

Timeframe

• Total duration from spermatogonium to spermatozoon is approximately 74 days, producing about 300 million sperm daily.

Oogenesis

Oogenesis is a complex biological process responsible for the production of female gametes, known as oocytes. It commences during early developmental stages when primordial germ cells differentiate into oogonia, which then develop into primary oocytes. This process is essential for female fertility.

Timeline of Oocyte Development

• Initiation: Oogenesis begins in intrauterine life with the migration of primordial germ cells to the developing ovaries, where they differentiate into oogonia.

• Mitotic Divisions: The oogonia undergo a series of mitotic divisions, resulting in a peak population of approximately 7 million primary oocytes by around 5 months of gestation.

• Arrest in Meiosis: Upon birth, all primary oocytes are in a state of suspended animation, arrested in prophase of Meiosis I. They are encased in a layer of flat epithelial cells, forming what are known as primordial follicles. At this stage, estimates indicate there are around 600,000 to 800,000 primordial follicles present.

• Childhood Atrophy: Throughout childhood, many oocytes undergo a process called atresia, leading to a significant decrease in numbers. By the time puberty is reached, only about 40,000 primary oocytes remain viable within the ovaries.

• Follicular Development: Each menstrual cycle typically witnesses the growth of 15 to 20 primordial follicles; however, generally, only one of these follicles achieves full maturation and is released during ovulation. The remaining follicles that do not mature undergo atresia.

Maturation of Oocytes

• Ovulation Process: The maturation of oocytes culminates in ovulation, during which a secondary oocyte is released from the ovary. Notably, this secondary oocyte will only complete Meiosis II if fertilization by sperm occurs.

• Polar Body Formation: During this process of maturation, the first polar body is formed as a result of meiosis and contains minimal cytoplasmic material. This ensures that the mature oocyte retains the majority of the cytoplasm, which is crucial for supporting early embryonic development if fertilization takes place.

Ovulation

• Controlled by hormones from hypothalamus (GnRH) that stimulate pituitary to release FSH and LH, promoting maturation of the follicle and ovulation.

• Only one follicle fully matures and releases an egg; others degenerate.

Hormonal Influence

• FSH stimulates follicular maturation, while an LH surge induces final maturation and ovulation about 14 days before menstruation.

Fertilization

Phases of Fertilization

1. Capacitation – The sperm must undergo conditions in the female tract (removal of proteins) to fertilize the oocyte.

2. Acrosome Reaction – Ensures the sperm can penetrate the zona pellucida of the egg by releasing enzymes.

3. Fusion – Sperm and oocyte membranes fuse, allowing sperm entry.

Outcomes of Fertilization

• Restoration of diploid chromosome number.

• Sex determination (XX or XY).

• Initiation of cleavage and development.

Cleavage and Implantation

Cleavage Process

• Begins 30 hours post-fertilization, progressing from the two-cell stage to morula (16-cell) by day 3.

• The zygote undergoes rapid mitotic divisions, forming blastomeres that compact and start forming a blastocyst.

Implantation

• Occurs 5-7 days post-ovulation.

• The blastocyst implants into the uterine endometrium, typically at the anterior or posterior wall near the fundus.