It is one of the most important parts of the human embryo, since vital organs come out of it.
The development and growth of the human body is an extremely complex and fascinating process in which the different structures work with millimeter precision to give rise to the birth of 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.
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What is endoderm?
The endoderm refers to the innermost tissue layer of the three layers that develop during embryonic growth of animals. These strata 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 have these three germ layers. According to each animal group, embryonic cells can derive in two or three layers, forming diblastic and triblastic beings respectively. Still, they all have the endoderm layer, which is underneath the rest.
In the case of endoderm, it appears around the third week of gestation, being considered one of the oldest layers within the embryonic differentiation process. Furthermore, it is from this layer of cells that many vital organs are born for the survival of the adult living being.
From this stratum germinates, most 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 develop?
During early embryonic development, the embryo is made up of a single layer of cells. This 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 that of the endoderm.
Around the second week of gestation, a group of migrating cellular organisms slip into the cells of the hypoblast, an inner mass made up of cuboidal cells, and become 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 giving way to the formation of suitable organs and tissues.
As indicated above, in the case of endoderm, it will give rise to different organs of the digestive and respiratory systems, 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 rather primitive members that are yet to be fully developed.
Types of endoderm
Following the differentiation of the embryonic body, the endoderm is divided 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 that, later, will 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. Furthermore, this embryonic stratum works together with the mesodermal 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 migration and cell differentiation; an extremely important process to enable the formation of organic structures such as the brain.
From here, the notochord and the endoderm carry out a parallel development in which the first generates a series of folds that will form the cranial, caudal and lateral axes of the embryo; while the endoderm folds remain inside the body forming the intestinal tube.
2. Extra-embryonic endoderm
The second division of the endoderm is the one that remains outside the embryo, forming the well-known yolk sac. This membranous annex is connected to the embryo, supplying sufficient nutrients and oxygen, as well as discarding metabolic waste.
However, this division of embryonic endoderm does not remain until the end of embryonic development, but usually disappears around the tenth week of gestation.
Intestinal tube sections
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 intestine
Known as the cranial or inner intestine, this structure is located inside the embryo’s skull. During the early stages of development this forms the oropharyngeal membrane, which gradually transforms into the pharynx. The lower limit then forms a structure known as the respiratory tract.
Finally, the intestinal tube dilates until it becomes what will finally correspond to the stomach.
2. Caudal intestine
Located within the caudal fold it is the precursor of the allantoic membrane. An extra-embryonic web that appears through the formation of localized folds next to the yolk sac.
3. Middle intestine
Finally, the midgut is located between the cranial and caudal structures. Its extension is dilated 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 on multiple occasions that the endoderm is the primitive structure thanks to which a large part of the organs and body structures arise, a process known as organogenesis, that is, the birth of organs.
This process of development of the final organs usually takes place between the calf and the eighth week of management. However, it has been shown that identifiable organs can be seen in people after the fifth week.
To be more specific, these structures derive from the endodermal layer :
- Yolk sac.
- Respiratory tract, specifically the bronchi and pulmonary alveoli.
- Epithelium and connective tissue of tonsils, pharynx, larynx and trachea. As well as the epithelium of the 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.