Second stage: Gastrulation
The essence of gastrulation lies in the profound reorganization of the embryo. At this stage, blastula cells actively migrate and organize into a multilayered structure. The main outcome of this process is the formation of three distinct sheets of cells called germ layers:
- The outer layer is called the ectoderm.
- The middle layer forms the mesoderm.
- The inner layer becomes the endoderm.
The embryo at this stage of development is no longer a blastula—it is now called a gastrula.
Third stage: Formation of axial primordia
Upon completion of gastrulation, the differentiation of the germ layers begins. From their material, the complex of main axial structures is laid down, determining body symmetry and the general plan of body structure. These include the neural tube, notochord, and primitive gut.
Special attention is paid to the derivatives of the mesoderm, which actively divides into specific regions:
- Somites are segmented structures that subsequently divide into dermatomes, myotomes, and sclerotomes.
- Nephrotomes are regions associated with the development of the excretory and reproductive systems.
- Splanchnotomes are the lateral plates of the 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 a number of other tissues.
Morphological changes and extraembryonic organs
Concurrently with internal differentiation, the external shape of the embryo also changes. Initially, the flat embryo begins to actively fold. As a result of this process, so-called body folds are formed. Their main function is to clearly demarcate the body of the embryo itself from the extraembryonic parts.
From the extraembryonic parts of the ectoderm, mesoderm, and endoderm, provisional (temporary) organs develop, which are necessary to support the vital activity of the embryo in the early stages:
- Yolk sac.
- Amnion.
- Allantois.
- Chorion (characteristic of mammals) or serosa (in birds).
Briefly: answers to topical questions
What tissues and organs develop from the ectoderm?
Structures of the nervous system, skin epidermis, and epithelium of some internal organs develop from the ectoderm. The ectoderm differentiates in the following directions:
- Neural tube — forms the spinal cord and brain.
- Neural crest cells (ganglion plates) — form peripheral nerve ganglia and skin melanocytes.
- Surface ectoderm — gives rise to the epidermis of the skin and its derivatives (sebaceous and sweat glands, hair, nails), as well as the epithelium of the oral cavity, vagina, and anal region 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. Endoderm derivatives include:
- Epithelium of the stomach and intestines.
- Epithelium of the digestive glands: liver and pancreas; the epithelium of the pancreas gives rise to exocrine acini and endocrine islets.
- Derivatives of the hepatic diverticulum: hepatocytes and cholangiocytes develop from the cranial division, while the epithelium of the common bile duct and gallbladder develops from the caudal division.
- Inner layer of the allantois — extraembryonic endoderm, a derivative of the intestinal endoderm.
What organs and tissues develop from splanchnotomes?
The myocardium, epicardium, and mesothelium of the serous membranes develop from splanchnotomes (lateral mesoderm). The splanchnotome divides into two layers, between which the coelomic cavity forms:
- Visceral layer (splanchnopleure) forms the myoepicardial plate — the primordium of the myocardium and epicardium of the heart.
- Parietal layer (somatopleure) is the source of the mesothelium of the serous membranes.
- Coelomic cavity subsequently divides into the pericardial, pleural, and peritoneal (abdominal) cavities.
To easily remember the three derivatives of the somites (mesoderm), use the abbreviation DSM: Dermatome, Myotome, Sclerotome.
Class and exam questions
What is the essence of gastrulation?
The essence of the process is the transformation of the blastula into the gastrula with the formation of three germ layers: the outer (ectoderm), middle (mesoderm), and inner (endoderm).
What is mesenchyme and where is it located?
This is a population of loosely arranged cells located between the developing axial primordia, serving as a source for the development of a number of future tissues.
What are body folds for?
They are formed during the folding of the flat embryo and are necessary for the spatial separation of 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.
What else to study on this topic
- Mechanisms of ectoderm, mesoderm, and endoderm formation from blastula cells
- Histogenesis and derivatives of dermatomes, myotomes, and sclerotomes
- Role and development of nephrotomes and splanchnotomes
- Structure and functions of the amnion, allantois, and yolk sac
- Structural features of the mammalian chorion and avian serosa
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