The development and growth of the human body is an extremely complex and fascinating process in which the different structures work with millimetric precision to give birth to the different organs and body systems.

One of these structures is the endoderm , a layer or stratum of tissue that we will talk about throughout this article. This layer is one of the oldest biological parts at the developmental level and gives rise to important vital organs such as those found in the digestive system.

What is the endoderm?

The endoderm refers to the innermost layer of tissue of the three layers that develop during the embryonic growth of animals. These layers known as germ layers are the ectoderm, which is the outermost layer, and the mesoderm or middle layer.

However, it is necessary to specify that not all species possess these three germination layers. Depending on each animal group, embryonic cells can derive into two or three layers, forming diblastic and triblastic beings respectively. Even so, all possess the endoderm layer, which is found below the rest.

In the case of the endoderm, it appears around the third week of gestation, being considered one of the oldest layers within the process of embryonic differentiation. Moreover, it is from this layer of cells that many vital organs are born for the survival of the adult living being.

From this germinating layer, the majority of the most important internal organs will be formed. Some of them are the alveoli that are housed in the lungs, the entire digestive system as well as its secretory glands, the epithelia of some glands such as the thyroid or thymus, and finally some parts of the kidneys, the bladder and the urethra.

How does it work?

During the early stages of embryonic development, the embryo consists of a single layer of cells. It then folds back on itself in a process called gastrulation, thanks to which the first cell layers are born. The first of these layers to appear is the endoderm.

Around the second week of gestation, a group of migratory cell organisms slide into the cells of the hypoblast , an inner mass formed by cubic cells, and becomes the final endodermal layer.

The next phase in the evolution of the embryo is called organogenesis. This is responsible for producing the corresponding changes in the embryonic layers and for giving rise to the formation of the appropriate organs and tissues.

As indicated above, in the case of the endoderm, this will give rise to different organs of the digestive and respiratory system , as well as the epithelial envelope of some parts of the body. However, it is necessary to specify that these organs are not the definitive structures, but primitive limbs yet to be fully developed.

Types of endoderm

As a result of the differentiation of the embryonic body, the endoderm is sectioned into two parts that have their own characteristics. These parts are the embryonic endoderm and the extra-embryonic endoderm. These two divisions are connected by a wide orifice which will later become the umbilical cord .

1. Embryonic endoderm

The embryonic endoderm is the section of the endodermal layer that will give rise to the internal structures of the embryo, forming the primary intestine. In addition, this embryonic layer works together with the mesodermic layer to form the notochord . When this structure is fully developed, it is the main one in charge of emitting the necessary signals to enable cell migration and differentiation; an extremely important process to enable the formation of organic structures such as the brain.

From this point on, the notochord and the endoderm develop in parallel, with the former generating a series of folds that form the cranial, caudal and lateral axes of the embryo, while the folds of the endoderm remain inside the body, forming the intestinal tube.

2. Extraembryonic endoderm

The second division of the endoderm is that which remains outside the embryo f ormating the well-known yolk sac . This membranous adnexa is connected to the embryo, providing sufficient nutrients and oxygen, as well as disposing of metabolic waste.

However, this division of the embryonic endoderm does not remain until the end of the embryonic development but usually disappears around the tenth week of gestation.

Sections of the intestinal tract

In the previous section it was mentioned that the embryonic endoderm gives rise to a structure called the intestinal tube. This structure can in turn be differentiated into different sections that can correspond to both the embryonic and extra-embryonic endoderm. These sections are:

1. Cranial bowel

Known as the cranial or internal intestine , this structure is located inside the skull of the embryo. During the early stages of development it forms the oropharyngeal membrane, which gradually changes into the pharynx. The lower boundary then forms a structure known as the respiratory tract.

Finally, the intestinal tube dilates to become what will eventually correspond to the stomach .

2. Flowable intestine

Located within the caudal fold is the precursor of the allantoic membrane . An extra-embryonic web that appears through the formation of folds located next to the yolk sac.

3. Midgut

Finally, the midgut is located between the cranial and caudal structures. It expands until it reaches the yolk sac through the umbilical cord. Thanks to it, the embryo is supplied with sufficient nutrients that come from the maternal organism and allow its correct development.

What organs does it transform into?

It has already been mentioned many times that the endoderm is the primitive structure thanks to which a large part of the organs and body structures emerge, a process known as organogenesis , that is to say the birth of the organs.

This process of developing the final organs usually takes place between the calves and the eighth week of management. However, it has been found that identifiable organs can be observed in individuals from the fifth week onwards.

To be more specific, from the endodermal layer derive these structures :

  • Yolk sac.
  • Allantoids.
  • Respiratory tract , specifically the bronchi and pulmonary alveoli.
  • Epithelium and connective tissue of tonsils, pharynx, larynx and trachea. As well as the epithelium of lungs and certain parts of the gastrointestinal tract.
  • Urinary bladder.
  • Epithelium of the Eustachian tubes, ear cavities, thyroid and parathyroid glands, thymus, vagina and urethra
  • Glands of the digestive system , specifically the gastrointestinal tract; as well as the liver, gallbladder and pancreas.