Embryonic Development Review

Fertilization and Zygote Formation

  • The Fertilization Process

    • Sperm travels through the female reproductive tract to encounter the ovum.

    • Capacitation: This is the final maturation stage of the sperm, occurring within the female tract, which prepares the sperm for fertilization.

    • Acrosome Reaction: Triggered upon contact with the ovum, enzymes are released to penetrate the zona pellucida.

    • Fusion and Meiosis: The fusion of the sperm and ovum triggers the completion of meiosis II in the egg.

  • Zygote Formation

    • The zygote represents the first single-cell stage of a new organism.

    • Shortly after fusion, the first mitotic divisions begin, signaling the transition from a single cell to a multi-cellular embryo.

Implantation and Hormonal Support

  • Stages of Implantation

    • Implantation typically initiates around Day 7 post-fertilization when the blastocyst attaches to the endometrium.

    • Apposition: The initial, loose alignment of the blastocyst with the uterine lining.

    • Adhesion: Firm contact is established by trophoblasts. This stage involves the differentiation into the cytotrophoblast and the syncytiotrophoblast.

    • Invasion: Occurring between Days 8 and 10, the blastocyst burrows deeper into the endometrium to establish a connection with the maternal blood supply.

  • Hormonal Support

    • The syncytiotrophoblast secretes Human Chorionic Gonadotropin (hCG).

    • hCG acts to maintain the corpus luteum, ensuring continued progesterone production to support the pregnancy until the placenta is fully functional.

Differentiation of Embryonic Stages

  1. Zygote

    • The initial diploid cell resulting from the fusion of gametes.

  2. Blastocyst

    • A hollow sphere of cells produced after cleavage. It consists of an inner cell mass (which becomes the embryo) and a trophoblast outer layer (which becomes part of the placenta).

  3. Gastrulation

    • A critical process that transforms the blastocyst into a structure with three distinct germ layers:

      • Ectoderm: Develops into the skin and the nervous system.

      • Mesoderm: Develops into muscles, bones, kidneys, and the circulatory system.

      • Endoderm: Develops into the lungs and the gastrointestinal tract.

Placenta and Umbilical Cord

  • Placental Formation and Function

    • The placenta serves as the vital interface for nutrient and waste exchange between the fetus and the mother.

    • It serves as a selective barrier, while also acting as an endocrine organ.

  • Umbilical Cord Structure

    • The cord contains 2 umbilical arteries and 1 umbilical vein.

    • Transport: The vein carries oxygenated blood to the fetus, while the arteries return deoxygenated blood and waste to the placenta.

Neurulation and the Neural Tube

  • Neurulation Process

    • This involves the formation of the neural plate, followed by its folding into the neural groove and finally the closure of the neural tube.

  • Significance

    • Proper closure of the anterior and posterior neuropores is essential. Failure of the neural tube to close correctly leads to significant defects, such as anencephaly or spina bifida.

Organ System Development

  • Heart Formation

    • A primitive heart tube forms during weeks 3 to 4. This is followed by complex atrial and ventricular septation processes.

  • Kidney Development

    • Occurs in three consecutive stages: Pronephros, Mesonephros, and finally the Metanephros (the permanent kidney).

    • Development relies on reciprocal induction for the proper formation of nephrons.

Placental Hormones Regulating Pregnancy

  • hCG: Sustains early pregnancy by maintaining the corpus luteum.

  • hPL (Human Placental Lactogen): Regulates maternal metabolism to ensure nutrient availability for the fetus.

  • Estrogen and Progesterone: Support the uterine environment and prevent premature contractions.

  • Relaxin: Softens pelvic ligaments and helps the body prepare for labor.