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Journal of the Anatomical Society of India

Development of the Human Trachea

Author(s): Harjeet, Sahni D, Jit I

Vol. 53, No. 1 (2004-01 - 2004-06)

Postgraduate Institute of Medical Education and Research, Chandigarh

Abstract:

The development of the human trachea was studied in serial sections of six embryos and fetuses of 4.2, 11.5, 12.5, 22, 30, and 85 mm CR stages. In addition, token transverse/sagittal sections at 10 µm were obtained from trachea of six fetuses between 38 and 100 mm CR stages. Sections were stained appropriately. Paries membranaceus (PM) differentiates at 12.5 mm CR stage. The mesenchymal 'anlagen' of the tracheali rings were evident at 22 mm CR stage, they became cartilagenous at 30 mm CR stage. The trachealis muscle was found to consist of an inner circular and an outer longitudinal layer. While the former was present both in PM and in intervals between the cartilagenous rings and was evident at 38 mm CR stage, the longitudinal layer which was represented by scattered bundles of nonstriated muscle fibres was seen only in the region of PM and developed at 62 mm CR stage.

Key words: Human trachea, tracheal rings, paries membranaceus, muscle, trachealis muscle.

Introduction:

Very little information is available on the time of development of the tracheal cartilagenous rings and 'paries membranaceus' (PM). According to Grosser (1912) the anlagen of the tracheal cartilages appear as condensation of "tissues" in embryos of 17 mm stage and cartilage becomes evident at 20 mm stage. It is not described as to when the constituents of the PM come into existence and nonstriated muscle fibres of the trachealis develop.

The present study was undertaken to find out whether the observation made by Grosser about 90 years back could be verified. At that time the technique of taking a photomicrograph had not fully developed and therefore he was not able to illustrate his observations by actual photomicrographs. It is also to be found out whether differentiation of the cartilagenous rings occurs craniocaudally or vice versa. The development of trachealis muscle (TM) will also be studied.

Material and Methods:

Material for the present study included complete serial sections of 4.2, 11.5, 12.5, 22, 30 and 85 mm CR length embryos and fetuses. In addition, transverse or sagittal sections of the trachea of 38, 42, 50, 62, 80 and 100 mm CR length embryos/fetuses were cut at 10µm and stained. Staining methods included hematoxylin and eosin, Masson's trichrome, Heidenhain's iron hematoxylin, Verhoeff and de Castro. The specific muscle stain of Masson's trichrome was found to be exceedingly useful in identifying early myoblasts; coarse myofibrils in the muscle fibres became better defined with Heidenhain's iron hematoxylin.

Results

At 4.2 mm CR stage the floor of the pharynx of the embryo shows five branchial arches and endodermal pouches in between. The hypobronchial eminence is present as a median elevation between the ventral ends of the third and fourth arches. Behind this eminence there appears a vertical groove called the laryngotracheal groove. From this groove a diverticulum (TBD in Fig.3) arises which extends caudally anterior to the esophagus. By development of a groove on either side, the pouch separates from the pharynx and esophagus and constitutes a tube, the upper part of which becomes the larynx and the caudal part develops into trachea. However, the cranial connection with the esophagus persists. The tube is named as laryngotracheal tube. The caudal portion of the tube bifurcates to form the principal bronchi.

At 11.5 mm CR stage the tracheal tube is lined by pseudostratified columnar epithelium. The basement membrane can be identified. Outside this, there are whorls of several layers of mesenchymal cells with a few fine delicate collagen fibres (Fig. 1). No cartilage is visible. The two main bronchi are also seen which are lined by pseudostratified columnar epithelium. In the neighbouring esophageal wall, differentiation is more advanced.

At 12.5 mm CR stage the trachea shows a well marked layer of pseudostratified ciliated columnar epithelium which is better indicated cranially than caudally. Now the basement membrane is better defined; it is surrounded by several layers of mesenchymal cells in which delicate collagen fibres are also present. Lamina propria has differentiated. The lumen of the trachea is small as compared to that of the esophagus (Fig. 2).

At 22 mm CR stage the epiglottis and thyroid cartilage are well indicated (Fig.3). The laryngobracheal tube is lined by a layer of pseudostratified columnar epithelium in which some cells bear cilia. The epithelial layer is surrounded by several layers of mesenchymal cells in which the 'anlagen' of the tracheal rings are seen as condensation of mesenchymal cells, some of which are in precartilagenous stage (Fig. 4). The basement membrane and lamina propria are well indicated. Differentiation is more marked cranially than caudally.

At 30 mm CR stage the trachea (TR) shows incomplete rings in the cartilagenous stage in which chondrocytes are well defined. The cartilages are typically hyaline. The PM shows folds (Fig. 5) in which the primordium of the circular layer of the trachealis muscle (TM) is indicated by the presence of myoblasts which stain pink selectively by Masson's trichrome. Myofibrils have not developed. The submucosa is defined but is devoid of glands.

At 38 mm CR stage, the epiglottis and thyroid cartilage have developed. All the layers of the tracheal wall including the cartilagenous rings are well defined. The PM is protruding into the tracheal lumen (Fig. 5). The circular coat of the TM containing spindle shaped myoblasts with elongated nuclei and some myofibrils is seen in PM and in the intervals between the tracheal cartilages. Tracheal glands are not seen in the submucosa.

At 42 and 50 mm CR stages, the circular muscle coat of the trachealis is well defined. It is present both in PM and in the intervals between the cartilages. Some muscle fibres show coarse myofibrils. No glands are visible.

At 62 mm CR stage, the fibres of circular layer of the trachealis are found to be inserted on the inner surfaces of the ends of the cartilages for a short distance. In the region of PM some isolated bundles of muscle fibres are seen running longitudinally posterior to the circular muscle coat. The upper extension of the muscle can be traced to the lower part of the posterior surface of the lamina of the cricoid cartilage. Caudally the fibres get attached to the posterior surface of the carina.

The longitudinal muscle fibres are not seen in the region of the cartilages.

At 80 and 85 mm. CR stages, the development of the various layers of the tracheal wall has not progressed any further. Tracheal glands are not seen.

At 100 mm CR stage, both muscular layers are well indicated in the region of PM. Tracheal glands, elastic fibres and lymphocytes are not seen either in the lamina propria or submucosa.

Discussion

Grosser (1912) observed that the anlagan of the tracheal cartilagenous rings could be recognised as condensation of 'tissues' in embryos of 17 mm stage and the cartilage develops at 20 mm CR stage embryos. According to Hamilton et al (1957) the cartilagenous rings in the trachea appear at about 20 mm CR stage and by 50 mm CR stage they are well defined. Tucker and O'Rahilly (1972) studied the development of the human larynx in closely graded emrbyos of one to 23 stages (developmental horizons of Streeter). He described the presence of cartilagenous tracheal rings in 30 mm stage embryos (stage 23). The earlier stages of the trahceal rings are not described. Merkel (1902) found the presence of muscle fibres in the dorsal wall of the trachea before the appearance of the cartilagenous rings. In the present material the mesenchymal anlagen of the tracheal rings are seen at 22 mm CR stage. The rings became cartilagenous by 30 mm stage. Differentiation occurs craniocaudally. In PM circular muscle coat of the trachealis is seen at 38 mm stage and longitudinal muscle coat at 62 mm CR stage. Tracheal glands are not seen even at 100 mm CR stage.

According to Stirling (1982-83) the TM has been described in Quain's Anatomy as a muscle which lies between the ends of the cartilages and also in the intervals between them. A study by Stirling (1982-83) revealed that there are two layers of the muscle: an inner circular and an outer longitudinal; the fibres of the inner layer run transversely and get attached to the inner surface of the adjacent ends of a cartilage. He also found that the longitudinal muscle coat was represented by a few strands. Fisher (1962) described three layers of the muscle; transverse, oblique and longitudinal. Further, according to him the longitudinal layer of smooth muscle fibres exists between the circular layer and submucosa. Present observations indicate that there are only two layers in the muscle: an inner circular layer and an outer longitudinal layer. The former is present not only in PM but is also seen in the intervals between the cartilages. In the PM the fibres extend for some distance on the inner surfaces of the adjacent ends of the cartilages; the outer layer is not a compact layer but is represented by isolated bundles of longitudinal fibres in PM. They extend proximally to the inferior border of the lamina of the cricoid cartilage and caudally get inserted into the posterior aspect of the carina, some fibres extend on to the posterior surfaces of the main bronchi.

 

Fig. 1

Missing Image

A transverse section of the upper part of the trachea of a 11.5 mm CR stage embryo. The trachea is lined by a layer of pseudostratified columnar epithelium which is surrounded by whorls of mesenchymal cells in which a few delicate collagen fibres are also seen. X 140

Fig. 2

Missing Image

A transverse section of the trachea (TR) and esophagus (ES) of a 12.5 mm CR stage embryo showing the presence of the mesenchymal whorls in the wall of the the trachea. In the esophagus, the rumen and various coats are better defined. X 55

Fig. 3

Missing Image

A sagittal section of a 22 mm CR stage embryo showing tracheobronchial diverticulum (TBD) the upper part of which is the developing larynx. Epiglottis (EP) is also seen in the mesenchymal stage. Anlage of thyroid cartilage (TH) is clearly indicated. Tracheo-bronchial diverticulum (TBD) arises from the esophagus (ES). X 140

Fig. 4

Missing Image

A sagittal section of the the trachea of a 22 mm CR stage embryo showing mesenchymal primordia of tracheal rings (TRR) cut vertically. On the opposite wall the paries membranaceus (PM) is indicated which shows the anlagen of the circular muscle coat, the submucosa and lamina propria; x 140.

Fig. 5

Missing Image

A sagittal section of the trachea of a 30 mm CR stage embryo showing well defined rings of hyaline cartilage cut vertically. The PM is indicated on the opposite wall, it is wavy and protrude into the tracheal lumen. The outer layer is the layer of circular muscle coat which has myoblasts. X 275

Fig. 6

Missing Image

A transverse section of the trachea of a 38 mm CR stage embryo. In the posterior wall the PM is protruding into the lumen of the trachea. The circular muscle coat of the trachealis is well indicated. Other layers of the tracheal wall are also evident. X 550

References

  1. Fisher AWF. The trachealis muscle. Journal of Anatomy 1962 : 92: 139-140.
  2. Grosser O : Manual of Human Embryology. Vol. 2. Lippincott. Philadelphia; 1912, p 475 .
  3. Hamilton WJ, Boyd JD, Mossman HW: Human Embryology. Haffer. Cambridge;1957, p. 199 .
  4. Merkel F, Atmungsorgane: Bardeleben's Handbuch der Anatomie des Menschen. 6th Edn.; 1902.
  5. Stirling W. The trachealis muscle of man and animals. Journal of Anatomy 1982-83 ; 17: 204-206.
  6. Tucker JA, O'Rahilly R. Observations on the embryology of human larynx. Annals of Otology 1972; 81: 520-523.
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