Embryo Development in Applied Animal Reproduction

Embryo Development Notes

Lecture Objectives

  • Understand the process of embryo development from zygote to implantation in the uterus.

  • Recognize the importance of hatching from the zona pellucida as a critical developmental step.

  • Define various stages of embryo development.

  • Summarize the creation and formation of the placenta.

Key Terminology

  • Zygote: The fertilized egg cell that is the first stage after sperm fertilizes the egg.

  • Perivitelline Space: The space surrounding the oocyte that plays a role in fertilization.

  • Syngamy: The fusion of male and female pronuclei.

  • Embryo: Early stage of development post-fertilization before the fetus stage.

  • Fetus: The later stage in development after the embryo.

  • Conceptus: The embryo and all associated membranes or tissues (e.g., placenta).

  • Pronucleii: The nucleus of sperm and egg just before they fuse.

  • Ootid: A brief stage before the zygote forms as the gametes come together.

  • Morula: A solid ball of cells (around 16 cells) resulting from cleavage of the zygote.

  • Blastomeres: Individual cells formed during the cleavage of the zygote.

  • Blastocyst: A hollow ball of cells formed after cleavage that will implant into the uterine wall.

  • Trophoblast: The outer layer of the blastocyst that helps in implantation and forms part of the placenta.

  • ICM (Inner Cell Mass): The mass of cells inside the blastocyst that will develop into the embryo.

Steps of Embryo Development

  • Within Zona Pellucida: Embryo develops while contained within the zona pellucida (protective layer).

  • Hatching of Blastocyst: Critical stage where the blastocyst escapes the zona pellucida for implantation.

  • Maternal Recognition of Pregnancy: The embryo signals its presence to prevent menstruation.

  • Formation of Extraembryonic Membranes: Include the placenta, which consists of:

    • Amnion: Provides a cushioning fluid-filled sac.

    • Chorion: Involved in gas exchange and nutrient transfer.

    • Allantois: Involved in waste disposal and gas exchange.

Morula Stage

  • The embryo is now a cluster of cells, resembling a grape cluster.

  • Cells differentiate into:

    • Inner Cell Mass: Forms the embryo, connected by gap junctions allowing communication.

    • Outer Cell Mass: Buddles into the trophoblast, creating tight junctions affecting permeability and helping form a blastocoele (fluid-filled cavity).

Timing of Embryo Development by Species

  • Bitch: 3-7 days for 2-cell, up to 15 days for hatching.

  • Cow: 24 hours for 2-cell, 9-11 days for hatching.

  • Ewe: 24 hours for 2-cell, 7-8 days for hatching.

  • Mare: same as ewe for 2-cell, 7-8 days for hatching.

  • Queen: 5 days for 2-cell, 10-12 days for hatching.

  • Sow: 14-16 hours for 2-cell, 6 days for hatching.

  • Human: 24 hours for 2-cell, 5-6 days for hatching.

Hatching Process

  • Growth and Fluid Accumulation: The blastocyst grows and accumulates fluid.

  • Enzyme Production: Necessary for breaking down the zona pellucida during hatching.

  • Contraction of Blastocyst: Assists in the physical release from the zona pellucida.

Pre-Implantation Journey

  • Once in the uterus, the embryo relies on:

    • Adequate luteal function (maintenance of pregnancy).

    • Progesterone synthesis (ensures uterine receptivity).

    • Uterus responsiveness to progesterone (support embryo development).

Development of Extraembryonic Membranes

  • Yolk Sac: First structure formed; provides initial nutrient supply.

  • Chorion: Surrounds the embryo and yolk sac; involved in implantation and exchange.

  • Amnion: Provides a protective cushioning environment.

  • Allantois: Handles waste and gas exchange; forms part of the umbilical cord.

  • Development involves interactions between various layers, including trophoblast and mesoderm.

Conclusion

  • Successful hatching and implantation are critical for embryo development.

  • The development of extraembryonic membranes plays a significant role in supporting the embryo's growth and preparing for uterine attachment.

Further Questions

  • Encourage clarification on any points related to embryonic development and implantation processes.