🐣Developmental Biology Unit 2 – Gametogenesis and Fertilization

Key Concepts and Terminology

• Gametogenesis process of forming haploid gametes (sperm and egg cells) from diploid germ cells through meiosis and cellular differentiation
• Spermatogenesis production of male gametes (sperm) in the testes, starting with spermatogonia and ending with mature spermatozoa
• Oogenesis production of female gametes (eggs) in the ovaries, starting with oogonia and ending with mature ova
• Meiosis specialized cell division that reduces the chromosome number by half, resulting in haploid daughter cells
  • Meiosis I first division, separates homologous chromosomes
  • Meiosis II second division, separates sister chromatids
• Fertilization fusion of male and female gametes to form a diploid zygote, restoring the full complement of chromosomes
• Zygote diploid cell resulting from the union of sperm and egg, marking the beginning of embryonic development

Stages of Gametogenesis

• Proliferation mitotic divisions of primordial germ cells (PGCs) to increase their number and form a pool of stem cells
• Growth period of cell enlargement and accumulation of nutrients, preparing the cells for meiosis
• Maturation meiotic divisions (meiosis I and II) that reduce the chromosome number and produce haploid gametes
• Differentiation process by which the haploid cells develop into mature, functional gametes with specialized structures and capabilities
  • Spermiogenesis transformation of round spermatids into elongated, motile spermatozoa
  • Oocyte maturation final stages of oocyte development, including meiotic arrest and cytoplasmic changes

Spermatogenesis: Process and Outcomes

• Occurs continuously in the seminiferous tubules of the testes, starting at puberty and continuing throughout adult life
• Spermatogonia diploid stem cells that undergo mitotic divisions to maintain their population and produce primary spermatocytes
• Primary spermatocytes enter meiosis I, resulting in two haploid secondary spermatocytes
• Secondary spermatocytes quickly undergo meiosis II, producing four haploid spermatids
• Spermiogenesis transformation of spermatids into mature spermatozoa, involving nuclear condensation, acrosome formation, and flagellum development
• Spermatozoa released into the lumen of the seminiferous tubules and transported to the epididymis for storage and further maturation
• Each spermatogonium can theoretically produce up to 16 spermatozoa (4 per meiotic division)

Oogenesis: Process and Outcomes

• Occurs in the ovaries, with most of the process completed before birth
• Oogonia diploid stem cells that undergo mitotic divisions to form a pool of primary oocytes
• Primary oocytes enter meiosis I during fetal development but arrest at prophase I (dictyate stage) until puberty
• At each menstrual cycle, a cohort of primary oocytes resumes meiosis I, forming a secondary oocyte and the first polar body
• Secondary oocyte begins meiosis II but arrests at metaphase II until fertilization occurs
• Meiotic divisions in oogenesis are asymmetric, with the oocyte retaining most of the cytoplasm and organelles
  • Polar bodies small, non-functional cells that degenerate
• Mature oocyte (ovum) is released from the ovary during ovulation, ready for fertilization
• Each oogonium can produce only one mature ovum, with the other meiotic products forming polar bodies

Meiosis in Gametogenesis

• Reduces the chromosome number from diploid (2n) to haploid (n), ensuring that the zygote has the correct number of chromosomes after fertilization
• Promotes genetic diversity through independent assortment and crossing over (recombination) during prophase I
• Meiosis I separates homologous chromosomes, resulting in two haploid daughter cells with unique combinations of maternal and paternal chromosomes
• Meiosis II separates sister chromatids, producing four haploid cells with single copies of each chromosome
• Errors in meiosis can lead to aneuploidy (abnormal chromosome number) in the resulting gametes and offspring (Down syndrome, Turner syndrome)

Fertilization: Steps and Mechanisms

• Occurs in the ampulla of the fallopian tube, where the sperm encounters the ovulated oocyte
• Capacitation process by which sperm become competent for fertilization, involving changes in the sperm membrane and motility
• Acrosome reaction exocytosis of enzymes from the sperm head, enabling the sperm to penetrate the oocyte's protective layers (cumulus cells and zona pellucida)
• Fusion of sperm and oocyte membranes, allowing the sperm nucleus and other components to enter the oocyte cytoplasm
• Cortical reaction release of enzymes from the oocyte that modify the zona pellucida, preventing polyspermy (entry of multiple sperm)
• Completion of meiosis II in the oocyte, producing the female pronucleus and the second polar body
• Fusion of male and female pronuclei, restoring the diploid chromosome number and forming the zygote

Zygote Formation and Early Development

• Zygote the diploid cell resulting from the union of sperm and egg, marking the beginning of a new individual
• Cleavage rapid mitotic divisions of the zygote without significant cell growth, producing a solid ball of smaller cells (blastomeres)
  • Cleavage patterns vary among species (holoblastic vs. meroblastic, radial vs. spiral)
• Morula 16-cell stage, resembling a mulberry, with the first signs of cellular differentiation
• Blastocyst hollow ball of cells, consisting of an inner cell mass (embryoblast) and an outer layer (trophoblast)
  • Inner cell mass gives rise to the embryo proper
  • Trophoblast contributes to the development of extraembryonic membranes and the placenta
• Implantation attachment of the blastocyst to the uterine wall, initiating the process of placentation and pregnancy

Clinical and Ethical Considerations

• Infertility inability to conceive after 12 months of regular, unprotected intercourse; may be caused by factors affecting gametogenesis or fertilization
• Assisted reproductive technologies (ART) medical interventions that assist with conception, such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), and gamete donation
• Preimplantation genetic testing (PGT) analysis of embryos created through IVF for genetic disorders or chromosomal abnormalities before transfer to the uterus
• Gamete and embryo cryopreservation freezing and storage of sperm, oocytes, or embryos for future use, allowing for fertility preservation and delayed childbearing
• Ethical considerations surrounding ART include access and affordability, embryo status and personhood, selective reproduction, and the welfare of children born through these technologies
• Gametogenesis and fertilization research has implications for understanding infertility, developing contraceptives, and advancing regenerative medicine and stem cell therapies