Développement embryonnaire Semaines 2 à 4

Embryonic Development Weeks 2, 3, and 4

• Week 2 (Implantation): Implantation begins in the first week, concluding in the second. Two independent structures, the blastocyst and the endometrium, are involved. A precise synchronization is crucial for successful implantation. The trophoblast, a key component of the blastocyst, synthesizes adhesion molecules (integrins and cadherins) and proteolytic enzymes. These facilitate blastocyst attachment and penetration into the endometrium. Following implantation, human chorionic gonadotropin (hCG) maintains the corpus luteum, ensuring progesterone production for continued pregnancy. hCG levels are measured in pregnancy tests.

Implantation Process

• Initiation: The first week of development involves the interaction of blastocyst and endometrium.
• Sequence: Apposition (contact), adhesion (binding), and invasion (penetration) stages occur sequentially.
• Cellular Changes: Cytotrophoblasts (outer trophoblastic cells) and syncytiotrophoblasts (inner trophoblastic cells with no clear cell boundaries) are formed during invasion. These cells play crucial roles in the development of the placenta.
• Enzyme Action: Trophoblastic cells secrete enzymes to degrade the extracellular matrix of the endometrium, aiding in the blastocyst's embedding.

Week 3 Development

• Primitive Streak: The early structure of the embryo is a flattening. It is bilayered. A visible thickening called the primitive streak appears on the dorsal surface of the embryo. This streak becomes the key organizer in the development of the embryo's body plan. Crucial to later development.
• Germ Layers: Three primary germ layers (ectoderm, mesoderm, and endoderm) differentiate and form cell types crucial to later organogenesis.
• Mesoderm Formation: The formation of the mesoderm, a critical middle layer, begins as cells migrate and organize between the ectoderm and endoderm.

Week 4 Development

• Gastrulation: A process of cell movement and differentiation that creates the three germ layers.
• Axis Formation: The primitive streak helps to establish the anterior-posterior (head-tail) axis of the embryo.
• Notochord Development: The notochord, a rod-like structure, forms from the mesoderm. It's crucial for initiating further development and serving as a signaling center for the development of the nervous system.
• Neural Plate: Induction by the notochord leads to the formation of the neural plate, which will eventually develop into the central nervous system.
• Body Segmentation: The mesoderm starts to separate into blocks called somites, forming the basis for the segmented body plan.