Biology 133 lecture AY 2021- 2022

Good day, Ma'am Leonardo! Here are our answers:

1. Please elaborate on the Carnegie system of naming the stages of the human embryos.

 

Human prenatal development is comprised of an embryonic period and fetal period, where the former spans the first 8 weeks following fertilization (Tay et al., 2007; Rogers et al., 2020). With that, the Carnegie staging of human embryos was introduced by Franklin P. Mall of the Carnegie Institution of Washington in 1914 as a standardized system used to provide a unified developmental chronology of the human embryo (Wellner, 2009). It consists of 23 stages, each stage spanning a period of 2 to 3 days, that delineate the embryonic period on the basis of the development of structures, and not by size nor the number of days of development (Tay et al., 2007). The main rationale behind the creation of the Carnegie stages is that the sequence in which the various structures in the embryo development is always the same; however, embryos of the same age might be at different developmental conditions. Hence, it is practical to divide the embryonic development into stages that account for inner and outer morphological indicators. This allows for accurate claims to be made and relevant comparisons between various embryos. Shown in the attached PDF file is a summary of the developmental events and associated Carnegie stages.

PDF Link: https://drive.google.com/file/d/11mea0-vzoMsP4RjA1ISv01lUBm7jI5b8/view?usp=sharing   

References

• Rogers, K., Sapunar, D., & Arey, L. (2020). prenatal development. Encyclopedia Britannica. https://www.britannica.com/science/prenatal-development
• Tay, V. S., Kornberg, A., & Cook, M. (2007). CHAPTER 38 - SPINE AND SPINAL CORD: DEVELOPMENTAL DISORDERS (A. H. V Schapira, E. Byrne, S. DiMauro, R. S. J. Frackowiak, R. T. Johnson, Y. Mizuno, M. A. Samuels, S. D. Silberstein, & Z. K. B. T.-N. and C. N. Wszolek (eds.); pp. 488–506). Mosby. https://doi.org/https://doi.org/10.1016/B978-0-323-03354-1.50042-0
• Weller, K. (2009). Carnegie stages. https://embryo.asu.edu/pages/carnegie-stages

2. The lungs and the liver both develop as ventral outgrowths of the foregut. How are they eventually separated such that the lungs are enclosed in the pleural cavity and the liver contained in the peritoneal cavity?

In Carnegie stage 10, the cranial section of the intraembryonic coelom is composed of a middle part, the pericardial cavity and two lateral thin canals, the pericardioperitoneal canals. These canals connect the pericardial cavity with the portion of the intraembryonic coelom that is open towards the outside, the future peritoneal cavity. During this point, there is still no pleural cavity as the lungs have not yet developed. It is still in the form of a widened terminal portion of the laryngotracheal sulcus known as the respiratory primordium. In late Carnegie stage 11, the respiratory primordium is already characterized by a knoblike thickening of the endodermal wall of the foregut.

In early Carnegie stage 12, the lung bud appears and is growing outward in a ventral direction. The liver diverticulum develops in a similar manner, but in the region inferior to the lung bud. As the embryo progresses through late stage 12 and 13, the lung bud continues to grow ventrally and eventually bends downward into the mesenchyme ventral to the foregut, where its distance from the liver diverticulum is also progressively increased. Eventually, progressive development of the tracheoesophageal septum separates the respiratory and digestive tracts. The liver diverticulum can be found in the regions below the tracheoesophageal septum, along with the other structures of the digestive system.

The pericardioperitoneal canal widens as the lung anlage grows larger, forming the pleural cavity, which is separated from the pericardial cavity by the pleuropericardial membrane. Likewise, it is separated from the peritoneal cavity by the pleuroperitoneal membrane. The pleuroperitoneal membranes are a pair of membranes that form when the pleural cavities invade the body wall and expand. These membranes are attached to the body wall dorsolaterally, and their free edges project into the caudal end of the pericardioperitoneal canals. By the sixth week, they grow medially and ventrally; to which by the end of the week, their free edges fuse with the dorsal mesentery of the esophagus and septum transversum to separate the pleural and pericardial cavities. The closure of the canals’ openings is further supplemented by the growth of the liver and muscle extension into the said membranes. To this end, the lungs will be enclosed in the pleural cavity while the liver will be contained in the peritoneal cavity.

References

• Carlson, B. M., & Kantaputra, P. N. (2019). Human Embryology and Developmental Biology. Elsevier.
• Human Embryology. (n.d.). Somatic cavities. https://www.embryology.ch/anglais/rrespiratory/korperhohlen01.html
• Pansky, B. (1982). Body cavities: Pleuropericardial and pleuroperitoneal membranes. Review of medical embryology. https://discovery.lifemapsc.com/library/review-of-medical-embryology/chapter-45-body-cavities-pleuropericardial-and-pleuroperitoneal-membranes

Hi! Thank you for your inquiries.

To address your question:
In Carnegie stage 10, the cranial section of the intraembryonic coelom is composed of a middle part, the pericardial cavity and two lateral thin canals, the pericardioperitoneal canals. These canals connect the pericardial cavity with the portion of the intraembryonic coelom that is open towards the outside, the future peritoneal cavity. During this point, there is still no pleural cavity as the lungs have not yet developed. It is still in the form of a widened terminal portion of the laryngotracheal sulcus known as the respiratory primordium. In late Carnegie stage 11, the respiratory primordium is already characterized by a knoblike thickening of the endodermal wall of the foregut.

In early Carnegie stage 12, the lung bud appears and is growing outward in a ventral direction. The liver diverticulum develops in a similar manner, but in the region inferior to the lung bud. As the embryo progresses through late stage 12 and 13, the lung bud continues to grow ventrally and eventually bends downward into the mesenchyme ventral to the foregut, where its distance from the liver diverticulum is also progressively increased. Eventually, progressive development of the tracheoesophageal septum separates the respiratory and digestive tracts. The liver diverticulum can be found in the regions below the tracheoesophageal septum, along with the other structures of the digestive system.

The pericardioperitoneal canal widens as the lung anlage grows larger, forming the pleural cavity, which is separated from the pericardial cavity by the pleuropericardial membrane. Likewise, it is separated from the peritoneal cavity by the pleuroperitoneal membrane. The pleuroperitoneal membranes are a pair of membranes that form when the pleural cavities invade the body wall and expand. These membranes are attached to the body wall dorsolaterally, and their free edges project into the caudal end of the pericardioperitoneal canals. By the sixth week, they grow medially and ventrally; to which by the end of the week, their free edges fuse with the dorsal mesentery of the esophagus and septum transversum to separate the pleural and pericardial cavities. The closure of the canals’ openings is further supplemented by the growth of the liver and muscle extension into the said membranes. To this end, the lungs will be enclosed in the pleural cavity while the liver will be contained in the peritoneal cavity.

Hello, Lexearl! Apologies for my earlier post, as I have mistakenly placed the answer to a previous question. Please disregard it instead. To answer your question:

During embryonic development, the trachea and esophagus start forming as a single tube and the failure of the development of the tracheoesophageal septum results in tracheoesophageal fistula and esophageal atresia. While both abnormalities are associated with each other, it is important to note that esophageal atresia is a result of the failure of the primitive foregut to recanalize while tracheoesophageal fistula is the failure of the lung bud to completely separate from the foregut (Madanick & Kaila, 2002). With this information, two major points can be noted: (1) the formation of the lung bud precedes the development of the esophageal atresia and tracheoesophageal fistula; and (2) the lung bud does not develop into an esophageal atresia, rather the failed recanalization of the primitive foregut.

Furthermore, the outgrowth was dubbed as the lung bud even if there is a development of the esophageal atresia (and formation of tracheoesophageal fistula) because there is a theory stating that it is the respiratory diverticulum that fails to elongate which results in the failure of the trachea to grow while the lung bud may still develop normally. With this, the foregut’s overall elongation and growth continue with the bronchial structures that originated from the foregut. After some time, the foregut rostral to the bronchi will assume tracheal histological characteristics while the foregut distal to the bronchial origin connects to the stomach and is known as the fistula. The upper atretic esophagus, on the other hand, is a result of the rearrangement of the anterior foregut (Ioannides & Copp, 2009).

To answer the second question, the tracheoesophageal sulcus must take the form of a saddle-shaped fold as it is crucial to the formation of the tracheoesophageal septum and thus, to the normal separation of the esophagus and trachea. Catenoidal configurations also possess growth-limiting properties that aid in the correct alignment of the different components of a developing body organ. Hence, since the normal development of the tracheoesophageal septum is dependent in the saddle-shaped fold, tracheoesophageal malformations (e.g., esophageal atresia and tracheoesophageal fistula) can be attributed to any configurational abnormalities arising during the development of the embryo (Sutliff & Hutchins, 1994).

References

• Ioannides, A. S., & Copp, A. J. (2009). Embryology of oesophageal atresia. Seminars in pediatric surgery, 18(1), 2–11. https://doi.org/10.1053/j.sempedsurg.2008.10.002
• Madanick, R. D., & Kaila, V. (2002). Anatomy, histology, embryology, and developmental anomalies of the esophagus. Sleisenger & Fordtran's gastrointestinal and liver disease : pathophysiology, diagnosis, management. Philadelphia :Saunders.
• Sutliff, K. S., & Hutchins, G. M. (1994). Septation of the respiratory and digestive tracts in human embryos: crucial role of the tracheoesophageal sulcus. The Anatomical record, 238(2), 237–247. https://doi.org/10.1002/ar.1092380210