Human Growth & Development - Extra-embryonic Tissues
Learning Outcomes
- Understand the difference between embryonic and extraembryonic tissues
- Describe the formation of the extraembryonic membranes
- Embryonic Tissues: Tissues contributing to structures retained in the fetus (i.e., become part of the born individual).
- Extraembryonic Tissues: Tissues primarily giving rise to support structures for the embryo during its development (i.e., not retained in the individual after birth).
- Amnion: Surrounds the embryo, creating the amniotic cavity.
- Yolk Sac: Provides nutrients and supports early blood cell formation.
- Chorion: Involved in gas exchange and nutrient transfer.
- Allantois: Vestigial in humans; stores nitrogenous wastes in birds/reptiles.
- Enable vertebrates to reproduce on land, independent of aquatic environments.
- Ensure nutrient supply and gas exchange to developing embryo.
- Prevent dehydration of embryo.
Key Phases of Embryonic Development (Week 1)
- Fertilization: Sperm and egg fusion forms a zygote.
- Cleavage: Zygote undergoes multiple cell divisions forming a blastocyst.
- Blastocyst Formation: Comprises an outer trophoblast layer and an inner cell mass.
- Implantation: Blastocyst attaches to uterine lining.
Details: Day 8 Development
- Blastocyst Embedding: Partially embedded into endometrium.
- Differentiation of Trophoblast:
- Cytotrophoblast: Inner layer, mononucleated.
- Syncytiotrophoblast: Outer layer, multinucleated, erodes maternal tissue leading to formation of lacunae filled with maternal blood.
- Inner Cell Mass Differentiates:
- Hypoblast: Cuboidal cells facing blastocyst cavity.
- Epiblast: Columnar cells facing the amniotic cavity.
- Amniotic Cavity: Cavity within epiblast, lined by epiblast and amnioblasts forming the amnion.
- Blastocyst Embedment: Trophoblast reaches lacunar stage.
- Development: Vacuoles in Syncytiotrophoblast fuse to form lacunae.
- Exocelomatic Membrane Formation: Formed from flattened hypoblast cells, lining the exocelomatic cavity/primitive yolk sac.
Day 11/12: Uteroplacental Circulation
- Complete Embedding: Blastocyst is fully embedded in endometrial stroma.
- Syncytiotrophoblast Invasion: Penetrates deeper, eroding maternal capillaries, allowing maternal blood to flow into trophoblastic system.
- Cells appear between cytotrophoblast and outer yolk sac surface, forming extraembryonic mesoderm as loose connective tissue, filling the space externally and internally.
- Cavity Development: Large cavities in extraembryonic mesoderm merge to form the extraembryonic coelom/chorionic cavity, surrounding yolk sac and amniotic cavity, except where connected by the body stalk.
- Structures:
- Somatopleuric Mesoderm: Lining cytotrophoblast and amniotic cavity.
- Splanchnopleuric Mesoderm: Lining covering yolk sac.
- Proliferation of Cytotrophoblast: Cells proliferate and infiltrate Syncytiotrophoblasts to form primary villi.
- Additional Hypoblast Cell Migration: Forms a new cavity (secondary yolk sac) inside the exocoelomic cavity.
- Importance: Vital for metabolising and transporting nutrients to the embryo, and as a site for primitive blood cell production (haematopoiesis).
Day 13: Chorionic Cavity & Plate
- Expansion: Extraembryonic coelom enlarges to form chorionic cavity.
- Chorionic Plate: Lined by extraembryonic mesoderm and forms the umbilical cord from the connecting stalk.
Allantois Functionality
- Function in Humans: Vestigial structure with limited function.
- Function in Other Species: Serves as a storage organ for nitrogenous wastes in birds and reptiles.
Summary of Week 2 Development
- Trophoblast Differentiation:
- Into Cytotrophoblasts and Syncytiotrophoblasts.
- Inner Cell Mass Differentiation:
- Into Epiblast and Hypoblast.
- Cavities Formation:
- Amniotic and yolk sac cavities.
- Mesoderm Division:
- Into somatopleuric and splanchnopleuric primary mesoderm.