Anat Cell Biol.  2024 Jun;57(2):278-287. 10.5115/acb.24.048.

Insertions of the striated muscles in the skin and mucosa: a histological study of fetuses and cadavers

• Kim JH
• Murakami G
• Rodríguez-Vázquez JF
• Sekiya R
• Yang T
• Abe Si

• KMID: 2556572
• DOI: http://doi.org/10.5115/acb.24.048

Abstract

Striated muscle insertions into the skin and mucosa are present in the head, neck, and pelvic floor. We reexamined the histology of these tissues to elucidate their role in transmission of the force. We examined histological sections of 25 human fetuses (gestational ages of ~11–19 weeks and ~26–40 weeks) and 6 cadavers of elderly individuals. Facial muscle insertion or terminal almost always formed as an interdigitation with another muscle or as a circular arrangement in which muscle fiber insertions were sandwiched and mechanically supported by other muscle fibers (like an in-series muscle). Our examination of the face revealed some limited exceptions in which muscle fibers that approached the dermis were always in the nasalis and mentalis muscles, and often in the levator labii superioris alaeque nasi muscle. The buccinator muscle was consistently inserted into the basement membrane of the oral mucosa. Parts of the uvulae muscle in the soft palate and of the intrinsic vertical muscle of the tongue were likely to direct toward the mucosa. In contrast, the pelvic floor did not contain striated muscle fibers that were directed toward the skin or mucosa. Although ‘cutaneous muscle’ is a common term, the actual insertion of a muscle into the skin or mucosa seemed to be very rare. Instead, superficial muscle insertion often consisted of interdigitated muscle bundles that had different functional vectors. In this case, the terminal of one muscle bundle was sandwiched and fixed mechanically by other bundles.

Keyword

Striated muscle; Facial muscles; Soft palate; Intrinsic tongue muscle; Pelvic floor

Figure

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  Fig. 1 Striated muscles in the eyelid skin and mucosa of fetuses. Two horizontal sections of a fetus with a crown-rump length of 262 mm. Panel (A) is 2.6 mm superior to panel (C); panels (B, E) are magnified views of rectangles in panel (A); and panels (D, F) are magnified views of rectangles in panel (C). In the upper eyelid, muscle fibers of the orbicularis oculi muscle (OOM) ran parallel to the skin and conjunctiva (B, D), but the levator palpebrae superioris muscle (LPSM) consisted of muscle bundles that cut transversely (E). (F) Shows the orbicularis oculi muscle bundle cut transversely in the lower eyelid. Insert at the bottom of the figure shows topographical relation of the orbicularis oculi and levator palpebrae superioris muscles with the skin (S) or mucosa (M). (A–D, F) Scale bars=1 mm, (E) scale bars=0.1 mm; H&E staining: all panels. FB, frontal bone.

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  Fig. 2 Circularly-arrayed muscle in the eyelid skin and mucosa and another muscle near the eye of fetuses. Sagittal section of a fetus with a crown-rump length of 328 mm. Panels (B, C) are magnified views of the rectangles in panel (A). Eyelids contain many muscle bundles of the orbicularis oculi muscle (OOM in A). Small muscle bundles surround the lacrimal canaliculus (stars in B). Some fibers (arrows in C) of the levator labii superioris alaeque nasi muscle (LLSANM) are superficial to (but do not reach) the dermis. Insert at the bottom of the figure shows topographical relation of the levator labii superioris alaeque nasi muscle with the skin (S). Scale bars=1 mm. H&E staining: all panels. LB, lacrimal bone; LLSM, levator labii superioris muscle; MX, maxilla; NM, nasalis muscle.

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  Fig. 3 Nasalis muscle in the nose skin and the mentalis muscle in the lip skin in near-term fetus with 215 mm crown-rump length (CRL). Frontal sections of a fetus with a CRL of 215 mm (A, B, E, F) and a fetus with a CRL of 170 mm (C, D); panels (B, D, E) are magnified views of the rectangles in panels (A, C). Some fibers of the nasalis muscle (NM) ran toward the skin and inserted into the dermis (arrows in D), but did not reach the basement membrane (stars in B, D, E). Fibers of the mentalis muscle (MM) were orthogonal to the skin and they approach the hair follicle (hair, F). Insert in panel (E) shows the topographical relation of the mentalis and nasalis muscles with the skin (S). (A, C, D, F) Scale bars=1 mm, (B, E) scale bars=0.1 mm. (A, B, E, F) H&E staining, (C, D) Azan staining. MA, mandible; NS, nasal septum cartilage; OORM, orbicularis oris muscle.

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  Fig. 4 Muscle fiber configuration in the skin at the angle of mouth and in the mucosa of the soft palate of fetuses. Frontal sections of a fetus with a crown-rump length (CRL) of 334 mm (A, C) showing muscle convergence at the angle of the mouth; sagittal sections of a fetus with a CRL of 328 mm (D, G) showing interdigitation of the levator veli palatini (LVPM) and uvulae muscles (UM); panels (B, C) are magnified views of the rectangles in panel (A); panels (E, F) are magnified views of the rectangles in panel (D). In the subcutaneous tissue at the angle of mouth, the risorius muscle (RM) interdigitated with the depressor anguli oris (DAOM) and orbicularis oris muscles (OORM) (B). Likewise, the levator anguli oris muscles (LAOM) interdigitated with the orbicularis oris muscles (C). In the soft palate, the levator veli palatini muscle was divided into multiple bundles by the uvulae muscle (E). A solitary fiber or bundle of the uvulae muscle approached the oral mucosa (F), and ended in the dense submucosal tissue (G). Insert in panel (C) shows a facial muscle interdigitation providing the insertions at the angle of mouth. Another insert in panel (F) shows the topographical relation of the uvulae muscle with the mucosa (M). (A, D, E, F) Scale bars=1 mm, (B, C, G) scale bars=0.1 mm. H&E staining: all panels.

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  Fig. 5 Insertion of the buccinator muscle into the oral mucosa of fetuses. Sagittal sections of fetuses with a crown-rump length (CRL) of 62 mm CRL (A, B), a CRL of 150 mm (C, F), and a CRL of 215 mm (D, E); panels (B, E, F) are magnified views of the rectangles in panels (A, D, C). Muscle fibers of the buccinator muscle (BM) ran along the oral mucosa (B, E, F), and some of them (E) appeared to reach the basement membrane (stars). The muscle fibers ended at a dense fibrous layer (arrowheads in F) that was distant from the basement membrane. Insert at the lower right-hand angle of the figure shows the topographical relation of the buccinator muscle with the mucosa (M). (A–E) Scale bars=1 mm, (F) scale bars=0.1 mm. (A, B) Azan staining, (C–F) H&E staining. MA, mandible; MX, maxilla; PM, platysma muscle.

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  Fig. 6 Intrinsic vertical muscle approaching the lingual mucosa of fetuses and adults. Frontal sections of fetuses with a crown-rump length (CRL) of 70 mm (A) and a CRL of 258 mm (B), and of a 72-year-old female (C, D); panel (D) is a magnified view of the rectangle in panel (C). Intrinsic vertical muscle fibers of the tongue (VM) approached the lingual mucosa, but did not reach the basement membrane of the squamous epithelium (stars in A, B, C). A dense fibrous lamina was in the submucosal tissue of fetuses (arrowheads in A, B). The epimysium appeared to continue to a fibrous band in adults (triangles in D) and disperse into the submucosal tissue. Insert in panel (A) shows the topographical relation of the intrinsic tongue muscles with the mucosa (M). (A–C) Scale bars=1 mm, (D) scale bars=0.1 mm. (A) Verde Luz-orange G-acid fuchsin staining, (B) Masson’s trichome staining, (C, D) Azan staining. SLM, superior longitudinal muscle; TM, transverse muscle.

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  Fig. 7 Striated muscle sphincters beneath the anal and urethral mucosae of fetuses and adults. Frontal sections of the subcutaneous part of the external anal sphincter (EAS) in a 75-year-old male (A, B); anal sphincters of a fetus with a crown-rump length of 100 mm (C); and external urethral sphincter (URS) in a 69 year-old male (D, E); panel (E) is a magnified view of the rectangle in panel (D). The adult lateral anal wall (A) had subcutaneous striated muscle fibers (B) of the sphincter that were bundled by fibrous tissue septa (arrows in A), and the septa were dispersed into subcutaneous tissue. The fetal external sphincter became thick before the smooth muscle-made internal sphincter (IAS in C). The external urethral sphincter (D) and the external anal sphincter were ‘sandwiched’ or suspended by the bilateral levator ani muscles (LAM; C, D). Adult muscle fibers of these sphincters did not reach or connect to the dermis or mucosa (B, E). Adult septa (stars in E) divided the external urethral sphincter into multiple bundles or sheets, and they joined to insert into the urethral wall. Insert in panel (A) shows the topographical relation of the external sphincters of the anus and urethra with the mucosa (M). (D) Scale bars=10 mm, (A, C, E) scale bars=1 mm, (B) scale bars=0.1 mm. (A, B) H&E staining, (C) Azan staining, (D, E) Elastic-Masson’s staining.

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  Fig. 8 Schematic representation of muscle insertions beneath the skin. The perimysium (green), endomysium (orange), and basement membrane (black dots and lines). (A) Left shows striated muscle fibers that approached the skin with rare morphologies, in which muscles were almost orthogonal to the dermis (a) or were oblique to the dermis (b). (A) Right shows the more usual morphologies, in which muscle fibers were parallel to the skin and were solitary fibers enclosed by the endomysium (c) or were in a bundle surrounded by the perimysium (d). (B) Shows the usual insertion of cutaneous and submucosal striated muscles, in which the two muscles with different functional vectors (levator and lateral tractor muscles) were interdigitated and mechanically fixed due to bundle-to-bundle crossing with perimysial contact.

Reference

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