TERCERA SEMANA DE DESARROLLO EMBRIONARIO, GASTRULACIÓN Y NOTOCORDA.

Summary

The video reviews key events of the third week of embryonic development, especially gastrulation and formation of the notochord. It begins with the bilaminar germ disc from the second week, consisting of the epiblast and hypoblast, alongside the amniotic cavity, definitive yolk sac, chorionic cavity, connecting stalk, and trophoblast.

Orientation and anatomical terms

• Dorsal view: Toward the embryo’s future back and the amniotic cavity.
• Ventral view: Toward the future belly and the definitive yolk sac.
• Sagittal section: A cut through the body’s midline.
• Lateral view: A view from the embryo’s right or left side.
• Cephalic (cranial) end: The future head.
• Caudal end: The opposite end, toward the future tail.

Gastrulation and the three germ layers

Gastrulation is presented as the major event of the third week. It begins when the primitive streak forms on the epiblast. Initially indistinct, it becomes a recognizable groove around days 15–16. At its cephalic end, the primitive node surrounds the primitive pit.

The process proceeds as follows:

1. Epiblast cells migrate toward the primitive streak.
2. They detach from the epiblast and move beneath it, or invaginate.
3. Some migrating cells displace the hypoblast, forming the endoderm.
4. Other cells settle between the endoderm and the epiblast, forming the mesoderm.
5. The epiblast cells that remain at the surface become the ectoderm.

These layers form the trilaminar germ disc: ectoderm on the outside, mesoderm in the middle, and endoderm on the inside. Gastrulation also extends toward the cephalic end, where the buccopharyngeal membrane is located. This membrane will become the opening of the oral cavity.

The video emphasizes that gastrulation is delicate and may be affected by toxic substances—including alcohol, nicotine, and drugs—as well as infectious agents.

Formation of the notochord

The video describes notochord formation as a dynamic process:

1. Prechordal cells migrate from the primitive pit toward the cephalic end and the prechordal plate.
2. These cells intercalate into the hypoblast, forming the notochordal plate.
3. As the hypoblast is replaced by endoderm, cells in the notochordal plate proliferate and separate from the endoderm.
4. They form a solid cord called the definitive notochord, which the video identifies as a basis for the embryo’s axial skeleton.

The notochord extends toward the prechordal plate at the cephalic end and toward the primitive pit at the caudal end. A temporary neuroenteric canal forms at the primitive pit, connecting the amniotic cavity with the definitive yolk sac.

Other structures noted

• The cloacal membrane forms at the caudal end. Like the buccopharyngeal membrane, it consists of endoderm and ectoderm in close contact.
• Around day 16, the allantois appears as a diverticulum extending toward the connecting stalk.

Speakers and sources featured

• Professor Ángela López — identified in the introduction as the guide for the lesson.
• Instructional narrator/voiceover — presents the explanations; no additional speakers or named external sources are identified.