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Embryonic Development: The Third Week
Overview of Embryonic Layers
- The formation of the three embryonic layers is crucial as they give rise to all body tissues and organs, occurring during the third week of gestation.
- This video addresses significant phenomena and changes in embryonic development during this critical period. If you missed earlier videos, links are provided for easy access.
Recap of Second Week Components
- At the end of the second week, key components included the epiblast and hypoblast forming a bilaminar germinative disc, with amniotic cavity and definitive yolk sac present.
- Gastrulation is highlighted as the most relevant event in the third week, establishing three germ layers: ectoderm, mesoderm, and endoderm.
Understanding Gastrulation
- The trilaminar embryonic disc is formed through two cuts separating it at different levels (amniotic cavity and yolk sac). A dorsal view reveals structures like the buccal membrane that will form the oral cavity.
- The primitive line's appearance on the epiblast surface marks the beginning of gastrulation, establishing embryo polarity with cranial and caudal ends identified.
Cell Migration During Gastrulation
- Epiblast cells migrate towards the primitive line; upon reaching it, they detach and slide beneath in a process called invagination, leading to endoderm formation by displacing hypoblast cells.
- Cells between newly formed endoderm and remaining epiblastic cells constitute intraembryonic mesoderm; those left in epiblast become ectoderm. This completes trilaminar germ disc formation.
Formation of Notochord
- Cells invaginate at the primitive node moving cranially along midline to form notochordal plaque from prenotochordal cells intercalating with hypoblast cells. This leads to definitive notochord formation essential for neural tube induction (neurulation).
- As hypoblast is replaced by endoderm cells, notochord forms a solid cord extending from prechord plate to primitive node—serving as an axis for spinal column development.
Membrane Development & Growth Direction
- Oropharyngeal membrane forms cranially while cloacal membrane develops caudally; both consist solely of endoderm and ectoderm layers—important developmental aspects noted here.
- A diverticulum known as allantois extends into fixation pedicle when cloacal membrane establishes posterior region of yolk sac; growth occurs cephalocaudally due to cell migration from primitive line until fourth week gestation concludes.
Development of Chorionic Villi
Development of the Placenta: Key Stages
Structure and Function of Villi
- A secondary villus consists of an outer layer of syncytiotrophoblast, followed by cytotrophoblast, with a mesoderm nucleus at its center. By the end of the third week, mesodermal cells differentiate into blood cells and small blood vessels, forming the villous capillary system.
- The capillaries within tertiary villi connect to the embryonic heart, supplying nutrients and oxygen as the heart begins to beat in the fourth week.
Developmental Changes in Trophoblast
- At day 19 or 20, the embryo attaches to its trophoblastic capsule via a fixation pedicle, which later develops into the umbilical cord connecting placenta and embryo.
- The evolution of chorionic villi is illustrated through primary, secondary, and tertiary stages. Stem or anchor villi extend from the chorionic plate to decidua basalis.
Types of Villi and Maternal Exchange
- Free or branched villi grow from lateral areas of anchor villi; these are crucial for material exchange between maternal blood and embryo.