Foundations Practical - Week 3 and 4

Germ Layers and Dynamic Processes

From 1 layer of cells to 3 layers that define all tissues of the entire embryo.

The conceptus is now fully implanted in the uterine wall. This time is now when a small cluster of cells (the inner cell mass) will differentiate into two simple cell layers (epiblast and hypoblast). From the single epiblast layer cells will migrate through a narrow region (primitive streak) to form firstly two layers epiblast and endoderm, then later the three layers ectoderm, mesoderm and endoderm. These final three layers will form all the tissues of the body.

During implantation cells on the outside (trophoblast layer) of the conceptus are forming part of the early placenta. The cells on the inside (inner cell mass) now begin to form the three germ layers that will form the entire embryo. Note that as the conceptus also forms the fetal component of the placenta, not all cells from our blastocyst will form part of the developing embryo.

Stage 7 Human Embryo (15 to 17 days)

Gastrulation

The first migration of cells forms the endoderm germ layer. An epithelial layer of cells which contributes all the epithelial lining (and glands) of the gut as well as the respiratory tract.

Somitogenesis

The later migration of cells forms the mesoderm germ layer. An embryonic connective tissue (mesenchyme) which forms nearly all the connective tissues of the body (the head is different).

Neurogenesis

The remaining cells that do not migrate from the epiblast layer remain and form the ectoderm. An epithelial layer of cells which contributes all neural and external epithelium of the embryo.

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers

Cite this page: Hill, M.A. (2024, May 1) Embryology Foundations Practical - Week 3 and 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Foundations_Practical_-_Week_3_and_4

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