Anat Cell Biol.  2012 Sep;45(3):185-192. 10.5115/acb.2012.45.3.185.

Initial stage of fetal development of the pharyngotympanic tube cartilage with special reference to muscle attachments to the tube

• Katori Y
• Rodriguez-Vazquez JF
• Verdugo-Lopez S
• Murakami G
• Kawase T
• Kobayashi

• KMID: 2046746
• DOI: http://doi.org/10.5115/acb.2012.45.3.185

Abstract

Fetal development of the cartilage of the pharyngotympanic tube (PTT) is characterized by its late start. We examined semiserial histological sections of 20 human fetuses at 14-18 weeks of gestation. As controls, we also observed sections of 5 large fetuses at around 30 weeks. At and around 14 weeks, the tubal cartilage first appeared in the posterior side of the pharyngeal opening of the PTT. The levator veli palatini muscle used a mucosal fold containing the initial cartilage for its downward path to the palate. Moreover, the cartilage is a limited hard attachment for the muscle. Therefore, the PTT and its cartilage seemed to play a critical role in early development of levator veli muscle. In contrast, the cartilage developed so that it extended laterally, along a fascia-like structure that connected with the tensor tympani muscle. This muscle appeared to exert mechanical stress on the initial cartilage. The internal carotid artery was exposed to a loose tissue facing the tubal cartilage. In large fetuses, this loose tissue was occupied by an inferior extension of the temporal bone to cover the artery. This later-developing anterior wall of the carotid canal provided the final bony origin of the levator veli palatini muscle. The tubal cartilage seemed to determine the anterior and inferior margins of the canal. Consequently, the tubal cartilage development seemed to be accelerated by a surrounding muscle, and conversely, the cartilage was likely to determine the other muscular and bony structures.

Keyword

Pharyngotympanic tube cartilage; Levator veli palatini muscle; Internal carotid artery; Tensor tympani muscle; Human fetuses

MeSH Terms

Arteries
Carotid Artery, Internal
Cartilage
Eustachian Tube
Fetal Development
Fetus
Humans
Muscles
Palate
Pregnancy
Stress, Mechanical
Temporal Bone
Tensor Tympani

Figure

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  Fig. 1 Initial stage of development of the pharyngotympanic tube cartilage: a fetus with 102 mm crown-rump length. Hematoxlyin and eosin staining. Horizontal sections. (A [E]) is the most superior (or inferior) side of the figure: the distance is 3 mm. The posterior or lateral orientation is shown by arrows in (D). (F-H) are higher-magnigfication views of the pharyngotympanic tube (PTT) shown in (B-D), respectively. (A [C]) displays the superior (or inferior) end of the tubal cartilage (C): thus, the initial cartilage, almost 1 mm in diameter, is restricted to the posterior side of the pharyngeal opening of the PTT. The superior end of the levator veli palatini muscle (LVP) is seen 1 mm below the cartilage (D). A thick, band-like mesenchymal condensation or an "adventitia" in the text (triangles) extends between the cartilage and the tensor tympani muscle (TT). A facia (arrows) connects between the tensor veli palatini muscle (TVP) and Meckel's cartilage (MC). The internal cartotid artery (ICA) does not run through the bony carotid canal but is exposed to a large loose space (stars) on the posterior side of the PTT. ATN, auriculotemporal nerve; LP, lateral pterygoid muscle; MN, mandibular nerver; MP, medial pterygoid muscle; OG, otic ganglion; PT, pterygoid process; TMJ, temporomandibular joint. Scale bars in (A)=1 mm (A-E); in (F)=0.5 mm (F-H).

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  Fig. 2 (A-C) Laterally extending cartilage and the tensor tympani muscle in a crown-rump length 150-mm fetus. Hematoxylin and eosin staining. Horizontal sections. (A [C]) is the most inferior (or superior) side of the figure: the distance is 2 mm. The posterior or lateral orientation is shown by the arrows in (C). The cartilage (C) of the pharyngotympanic tube (PTT) extends along a fascial structure or an adventitia (triangles) that connects with the tensor tympani muscle (TT). Arrows indicate another fascia connecting between the tensor veli palatini muscle (TVP) and Meckel's cartilage (MC). The internal cartotid artery (ICA) is exposed to a loose space (stars) on the posterior side of the PTT. ATN, auriculotemporal nerve; LC, longus capitis muscle; LP, lateral pterygoid muscle; MP, medial pterygoid muscle; PT, pterygoid process. Scale bar=1 mm (A-C).

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  Fig. 3 Mucosal fold of the lateral pharyngeal wall and pharyngotympanic tube cartilage: three fetuses. Azan staining. Horizontal sections. All panels exhibit the inferior part or wall of the pharyngotympanic tube (PTT). The cartilage has not yet appeared in the specimen shown in (A) (crown-rump length [CRL], 101 mm), but the levator veli palatini muscle (LVP) has begun to extend into the putative mucosal fold at the pharyngeal opening of the PTT. In (B) (CRL, 100 mm) and (C) (CRL, 116 mm), the tubal cartilage (C) is almost 2 mm in length and these panels show the medial end. The LVP employs the mucosal fold containing the cartilage to guide its downward path to the palate. A fascia (circles), along the LVP, extends posteromedially to cover the longus capitis muscle (LC, panel C). Arrows indicate another fascia along the tensor veli palatini muscle (TVP). The internal cartotid artery (ICA) is exposed to a loose space (stars) on the posterior side of the PTT. LP, lateral pterygoid muscle; MP, medial pterygoid muscle; PT, pterygoid process;TB, temporal bone. Scale bar=1 mm (A-C).

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  Fig. 4 Pharyngotympanic tube and the levator veli palatini muscle in a crown-rump length 125-mm fetus. Hematoxylin and eosin staining. Frontal sections. (A [E]) is the most posterior (or anterior) side of the figure: the distance is 3.3 mm. The superior or lateral orientation is shown by arrows in (A). The internal carotid artery (ICA with arrow) enters the brain cavity immediately posterior to (A). (A) and (B) display the most superior part or the origin (arrowheads) of the levator veli palatini muscle (LVP): because of the lack of a bony structure (star) near the pharyngotympanic tube (PTT), the origin has not yet become attached to the temporal bone, but extends along the inferior aspect of the PTT. The tubal cartilage (C) is divided into two fragments in (B). The levator muscle extends anteriorly along the inferior aspect of the PTT and its cartilage (C-E). ATN, auriculotemporal nerve; CPS, constrictor pharyngis superior muscle; ECA, external carotid artery; ICN, internal carotid nerve; MC, Meckel's cartilage; MH, mylohyoideus muscle; MP, medial pterygoid muscle; OG, otic ganglion; PG, parotid gland; SB, sphenoid bone (body); SG, styloglossus muscle; SMG, submandibular gland; SP, salpingopharyngeus muscle; TG, trigeminal nerve ganglion; TT, tensor tympani muscle; TVP, tensor veli palatini muscle; VN, Vidian's nerve (nerve of the pterygoid canal). Scale bar=1 mm (A-E).

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  Fig. 5 Pharyngeal recess and the pharyngotympanic tube cartilage: two fetuses. Azan staining. Sagittal sections. (A and B [or C and D]) display fetuses with a crown-rump length of 103 mm (or 100 mm). (A) and (C) are located medially to (B) and (D), respectively. The superior or posterior orientation is shown by arrows in (A). The pharyngotympanic tube (PTT) and the tubal cartilage (C) are located between the pharyngeal recess (PR) and the PTT (A, B). Instead of the bony carotid canal, a loose space (stars in B and D) is evident between the PTT cartilage and the cochlea. Thus, the internal carotid artery (ICA) is exposed to the loose space. An inset (lower angle of D) displays a control specimen (30 wk) in which the bony carotid canal is established: the levator veli palatini muscle (LVP) attaches to the bony part that develops later (asterisks). CPM, constrictor pharyngis medius muscle; CPS, constrictor pharyngis superior muscle; FA, facial artery; HB, hyoid bone; LC, longus capitis muscle; LP, lateral pterygoid muscle; MC, Meckel's cartilage; MP, medial pterygoid muscle; PP, palatopharyngeal muscle; PT, pterygoid process; SG, styloglossus muscle; SH, stylohyoid muscle; SMG, submandibular gland; TVP, tensor veli palatini muscle. Scale bars=1 mm (A-D, inset in D).

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