Early Embryogenesis

Embryogenesis Overview

• Embryogenesis involves forming body structures and organs through organogenesis.
• Cell division (proliferation) and cell differentiation (specialization) are critical processes in embryogenesis.
• A wide variety of cell types and extracellular products are created during this stage.

Cell Specialization

• Selective gene expression dictates cell differentiation by producing specific proteins and suppressing others.
• A cell's genetic expression is influenced by its genetic history (commitment lineage) and its current cellular environment (intercellular communications).

Role of Proteins and Peptides

• Different proteins and peptides determine cell specialization and functions:
  • Structural components (e.g., cytoskeleton, extracellular structures)
  • Enzymes regulating cell metabolism
  • Secretory products (e.g., hormones, digestive enzymes)
  • Channels and pumps for molecular transport
  • Receptors for cellular communication

Periods of Development

• Embryonic Period: Lasts from fertilization to the early stages of organ development (approximately 30 days in dogs, cats, sheep, and pigs; nearly 60 days in horses, cattle, and humans).
• Fetal Period: Occurs from the end of the embryonic phase to parturition, during which organs grow and start functioning.

Fertilization Process

• Fertilization is the fusion of a haploid oocyte and a haploid spermatozoon, resulting in a diploid zygote, a pluripotent cell.
• Occurs in the uterine tube near the ovary; required species recognition prevents fusion with foreign sperm.
• Sperm attaches to a glycoprotein on the zona pellucida of the oocyte, triggering the acrosomal reaction.
• Enzymatic degradation of the zona pellucida allows sperm penetration and completion of oocyte meiosis.
• Male and female pronuclei unite, lose nuclear membranes, and mitosis initiates 12 hours post-fusion.

Oocyte Development

• Oocytes, surrounded by zona pellucida and corona radiata, mature selectively in response to FSH from the pituitary.
• Germ cells (oogonia) produce primary oocytes via mitosis during embryonic development.
• Primary oocytes initiate Meiosis I and remain arrested until ovulation.
• Secondary oocytes complete Meiosis II upon fertilization; if unfertilized, they degenerate.

Spermatozoa Development

• Spermatozoa are produced in large quantities (hundreds of millions per ejaculation).
• Movement occurs via contractions in the female genital tract.
• Spermatogonia become primary spermatocytes through repeated mitosis after puberty.
• Primary spermatocytes undergo Meiosis I, resulting in secondary spermatocytes, which complete Meiosis II to form spermatids.
• Spermatids mature into spermatozoa (spermiogenesis) and undergo capacitation to enable oocyte interaction.

Cleavage Stage

• Cleavage begins as the zygote undergoes mitotic divisions, producing smaller cells called blastomeres.
• The process progresses from zygote to morula and ends at the blastocyst stage.
• The first cleavage division happens 1 to 5 days post-ovulation, dividing every 12 hours.
• The initial eight blastomeres in mammals are undifferentiated with identical potential before differentiating into inner and outer cells.

This quiz explores the intricate process of early embryogenesis, focusing on the formation of body structures and organs through cell division and differentiation. Gain insights into how selective gene expression leads to the specialization of various cell types in the developing embryo.