Initial stages of respiratory system formation
The respiratory system is laid down in the early stages of embryogenesis—at the 3rd week. It is important to emphasize that it does not arise in isolation, but is a derivative of the ventral wall of its foregut (its endodermal layer).
The process begins with the appearance of a longitudinal groove on the primitive digestive tube. This groove gradually evaginates outward, forming the respiratory diverticulum, also known as the laryngotracheal outgrowth.
As it develops, this diverticulum begins to grow actively in a caudal (downward) direction. To ensure that the respiratory tract and digestive tract can function separately, the process of their separation occurs: the tracheoesophageal septum forms between the respiratory diverticulum and the future esophagus, completely separating these two structures.
Differentiation of the respiratory tube regions
After successful separation from the digestive tube, the respiratory bud undergoes strict spatial differentiation, forming the main regions of the respiratory tract:
- Cranial end: transforms into the larynx (larynx).
- Tubular part: forms the trachea (trachea).
- Caudal end: divides into two sac-like evaginations called lung buds. These represent the primordia of the main bronchi.
Further development of the bronchial tree proceeds in an asymmetric manner, which reflects the future structure of the lungs. The right lung bud divides into three branches, and the left into two (these are the primordia of the future lobar bronchi). Subsequently, these branches undergo dichotomous division, sequentially forming segmental and progressively smaller bronchi.
Derivatives of the dorsal and ventral gastric mesenteries
Since the respiratory system originates from the foregut, its development is conceptually linked to the transformations of the digestive tube. The logic of abdominal structure development is inextricably linked to the process of gastric rotation, which determines the final position of ligaments, omenta, and mesenteries.
Derivatives of the dorsal mesentery (mesogastrium dorsale):
- The spleen develops within the thickness of this mesentery as it grows extensively to the left and downward.
- The omental bursa (*bursa omentalis)* forms as a pocket behind the stomach as a result of its rotation.
- The greater omentum (*omentum majus)* forms when the dorsal mesentery elongates downward as a fold (duplication), anteriorly covering the transverse colon and loops of the small intestine. The layers of this fold fuse to form a single four-layered plate. The upper part of the greater omentum fuses with the mesentery of the transverse colon.
Derivatives of the ventral mesentery (mesogastrium ventrale): It connects the lesser curvature of the stomach and the initial part of the duodenum with the liver.
- Forms the lesser omentum (*omentum minus), which consists of the hepatogastric (*lig. hepatogastricum) and hepatoduodenal (lig. hepatoduodenale) ligaments.
- Part of the ventral mesentery extends from the liver to the anterior abdominal wall and diaphragm, forming the falciform ligament of the liver (*lig. falciforme hepatis)*.
Fate of the midgut and hindgut mesenteries
The relationship of various intestinal segments to the peritoneum changes during embryogenesis. The fate of the dorsal mesentery of the midgut and hindgut depends on the specific site:
- Intraperitoneal position (persistence of the mesentery). It forms the mesenteries of the small intestine, appendix (mesoappendix), transverse colon (mesocolon transversum), and sigmoid colon (mesocolon sigmoideum).
- Mesoperitoneal position (reduction of the mesentery). The ascending and descending colons are displaced toward the posterior abdominal wall. Their mesenteries fuse with the parietal peritoneum, so these segments end up covered by peritoneum on three sides only.
- Peritoneal relations of the rectum. Formed extremely unevenly: the upper third retains its mesentery (intraperitoneal), the middle third is covered on three sides (mesoperitoneal), and the lower third completely lacks a peritoneal covering (extraperitoneal).
Remembering the division of the lung buds is simple: the right bud gives 3 branches (like the 3 lobes in the right lung), and the left gives 2 branches (like the 2 lobes in the left).
Class and exam questions
From what source does the respiratory system develop?
The respiratory system is a derivative of the digestive tube. It arises from the endoderm of the ventral wall of the foregut during the 3rd week of embryogenesis.
How does the separation of the respiratory tract from the digestive tract occur?
The respiratory diverticulum grows in a caudal direction and separates from the future esophagus due to the formation of a specialized tracheoesophageal septum.
How is the greater omentum formed?
The greater omentum forms from the dorsal gastric mesentery, which extends downward as a fold. Its layers then fuse to form a single four-layered plate.
What else to study on this topic
- Mechanisms of gastric and intestinal rotation in embryogenesis
- Anomalies of development of the tracheoesophageal septum
- Topography and recesses of the omental bursa
- Histogenesis and maturation of lung tissue in later stages
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