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Gastrulation and Axial Primordia

Gastrulatio

For medical students2 min readUpdated 2026-10-10

Gastrulation is the critical second stage of embryonic development during which the blastula cells reorganize into three germ layers. It is immediately followed by the third stage—the formation of the axial primordia complex—when the flat embryo begins to fold and the fundamental structures of the future organism are established.

Germ layersEctoderm (outer), mesoderm (middle), and endoderm (inner).
Axial structuresInclude the neural tube, notochord, and primitive gut.
MesenchymeLoosely arranged cells between the primordia that serve as a source for tissue development.
Provisional organsYolk sac, amnion, allantois, and chorion (serosa).

Stage Two: Gastrulation

The essence of gastrulation is a profound reorganization of the embryo. At this stage, blastula cells actively migrate and organize into a multilayered structure. The primary outcome of this process is the formation of three distinct cell sheets called germ layers:

At this stage of development, the embryo is no longer a blastula—it is now called a gastrula.

Stage Three: Formation of Axial Primordia

Following gastrulation, the differentiation of the germ layers begins. From their material, the complex of main axial structures is established, determining body symmetry and the overall structural plan. These include the neural tube, notochord, and primitive gut.

Particular attention is paid to mesodermal derivatives, which actively divide into specific regions:

  1. Somites—segmented structures that subsequently differentiate into dermatomes, myotomes, and sclerotomes.
  2. Nephrogonotomes (intermediate mesoderm)—regions associated with the development of the urinary and reproductive systems.
  3. Splanchnotomes (lateral plate mesoderm).

The space between these developing organs does not remain empty. It is filled with mesenchyme—an aggregation of loosely arranged cells that act as a universal source for the formation of various other tissues.

Morphological Changes and Extraembryonic Organs

In parallel with internal differentiation, the external shape of the embryo also changes. Initially flat, the embryo begins to actively fold. As a result of this process, body folds (lateral and head/tail folds) form. Their main function is to clearly demarcate the body of the embryo itself from the extraembryonic parts.

From the extraembryonic portions of the ectoderm, mesoderm, and endoderm, provisional (temporary) organs develop, which are necessary to support embryonic viability in the early stages:

Mnemonic

To easily remember the three somite derivatives (of the mesoderm), use the acronym DSM: Dermatome, Sclerotome (or Myotome), Myotome (Dermatome, Myotome, Sclerotome).

Frequently asked questions

Which tissues and organs develop from the ectoderm?

The nervous system structures, cutaneous epidermis, and epithelium of certain internal organs develop from the ectoderm. The ectoderm differentiates into the following pathways:

  • Neural tube—forms the brain and spinal cord.
  • Ganglionic plates (neural crest)—form peripheral nerve ganglia and skin melanocytes.
  • Surface ectoderm—gives rise to the skin epidermis and its derivatives (sebaceous and sweat glands, hair, nails), as well as the epithelium of the oral cavity, vagina, and anal portion of the rectum.
What structures are formed from the endoderm?

Epithelial derivatives of the primitive gut and the inner layer of the allantois are formed from the endoderm. Endodermal derivatives include:

  • Epithelium of the stomach and intestines.
  • Epithelium of digestive glands: liver and pancreas; the pancreatic epithelium gives rise to exocrine acini and endocrine islets.
  • Derivatives of the hepatic diverticulum: hepatocytes and cholangiocytes develop from the cranial portion; the epithelium of the common bile duct and gallbladder develops from the caudal portion.
  • Inner layer of the allantois—extraembryonic endoderm, a derivative of gut endoderm.
Which organs and tissues develop from splanchnotomes?

The myocardium, epicardium, and mesothelium of serous cavities develop from splanchnotomes (lateral plate mesoderm). The splanchnotome divides into two layers, between which the coelomic cavity forms:

  • Visceral layer (splanchnopleure) forms the myoepicardial mantle—the primordium of the heart myocardium and epicardium.
  • Parietal layer (somatopleure) is the source of the mesothelium of serous membranes.
  • Coelomic cavity subsequently divides into the pericardial, pleural, and peritoneal (abdominal) cavities.
What is the core essence of gastrulation?

The essence of the process is the conversion of the blastula into a gastrula with the formation of three germ layers: the outer (ectoderm), middle (mesoderm), and inner (endoderm).

What is mesenchyme and where is it located?

It is a population of loosely arranged cells located between the developing axial primordia that serves as a source for the development of several future tissues.

What is the purpose of body folds?

They form during the folding of the flat embryo and are necessary to spatially demarcate the embryo's body from its extraembryonic parts.

What do provisional organs develop from?

Extraembryonic organs develop from the cellular material of the extraembryonic parts of the ectoderm, mesoderm, and endoderm.

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