Striated muscle fiber crossings at almost right angle are known to exist in the face, soft palate, pharyngeal wall and tongue. We aimed to identify a specific interface tissue at the crossing. We observed histological sections from 22 halfheads of 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). For comparison, we also observed tongue frontal sections from 5 elderly cadavers (75–85 years old). At the angle of mouth as well as in the soft palate and pharyngeal wall, a solitary striated muscle fiber (e.g., levator) consistently crossed a fiber bundle of the antagonist muscle (e.g., depressor), but a solitary-to-solitary fiber interdigitation was unlikely with the antagonist muscle. Near the external nasal orifice as well as in the tongue intrinsic muscle layer, at every section, there was a crossing with an endomysium-to-endomysium contact:the nasalis and platysma muscles and; the vertical and transverse (or inferior longitudinal) tongue muscles. Therein, the functional vectors crossed at almost right angle. Also in adult tongue, the vertical and transverse muscle fibers sometimes (0–2 sites per section) crossed with an endomysium-to-endomysium contact. At the muscle crossing with an endomysium contact, the endomysium and basement membrane seemed to receive a friction stress between two muscles. Although some crossings might disappear due to high muscle activity after birth, not a few of them were likely to maintain. To minimize the mechanical stress, a minute nervous control of the timing, duration and strength of muscle contraction seemed to be necessary.

Striated muscle fiber crossings at almost right angle are known to exist in the face, soft palate, pharyngeal wall and tongue. We aimed to identify a specific interface tissue at the crossing. We observed histological sections from 22 halfheads of 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). For comparison, we also observed tongue frontal sections from 5 elderly cadavers (75–85 years old). At the angle of mouth as well as in the soft palate and pharyngeal wall, a solitary striated muscle fiber (e.g., levator) consistently crossed a fiber bundle of the antagonist muscle (e.g., depressor), but a solitary-to-solitary fiber interdigitation was unlikely with the antagonist muscle. Near the external nasal orifice as well as in the tongue intrinsic muscle layer, at every section, there was a crossing with an endomysium-to-endomysium contact:the nasalis and platysma muscles and; the vertical and transverse (or inferior longitudinal) tongue muscles. Therein, the functional vectors crossed at almost right angle. Also in adult tongue, the vertical and transverse muscle fibers sometimes (0–2 sites per section) crossed with an endomysium-to-endomysium contact. At the muscle crossing with an endomysium contact, the endomysium and basement membrane seemed to receive a friction stress between two muscles. Although some crossings might disappear due to high muscle activity after birth, not a few of them were likely to maintain. To minimize the mechanical stress, a minute nervous control of the timing, duration and strength of muscle contraction seemed to be necessary.

Striated muscle fiber crossings at almost right angle are known to exist in the face, soft palate, pharyngeal wall and tongue. We aimed to identify a specific interface tissue at the crossing. We observed histological sections from 22 halfheads of 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). For comparison, we also observed tongue frontal sections from 5 elderly cadavers (75–85 years old). At the angle of mouth as well as in the soft palate and pharyngeal wall, a solitary striated muscle fiber (e.g., levator) consistently crossed a fiber bundle of the antagonist muscle (e.g., depressor), but a solitary-to-solitary fiber interdigitation was unlikely with the antagonist muscle. Near the external nasal orifice as well as in the tongue intrinsic muscle layer, at every section, there was a crossing with an endomysium-to-endomysium contact:the nasalis and platysma muscles and; the vertical and transverse (or inferior longitudinal) tongue muscles. Therein, the functional vectors crossed at almost right angle. Also in adult tongue, the vertical and transverse muscle fibers sometimes (0–2 sites per section) crossed with an endomysium-to-endomysium contact. At the muscle crossing with an endomysium contact, the endomysium and basement membrane seemed to receive a friction stress between two muscles. Although some crossings might disappear due to high muscle activity after birth, not a few of them were likely to maintain. To minimize the mechanical stress, a minute nervous control of the timing, duration and strength of muscle contraction seemed to be necessary.

Striated muscle fiber crossings at almost right angle are known to exist in the face, soft palate, pharyngeal wall and tongue. We aimed to identify a specific interface tissue at the crossing. We observed histological sections from 22 halfheads of 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). For comparison, we also observed tongue frontal sections from 5 elderly cadavers (75–85 years old). At the angle of mouth as well as in the soft palate and pharyngeal wall, a solitary striated muscle fiber (e.g., levator) consistently crossed a fiber bundle of the antagonist muscle (e.g., depressor), but a solitary-to-solitary fiber interdigitation was unlikely with the antagonist muscle. Near the external nasal orifice as well as in the tongue intrinsic muscle layer, at every section, there was a crossing with an endomysium-to-endomysium contact:the nasalis and platysma muscles and; the vertical and transverse (or inferior longitudinal) tongue muscles. Therein, the functional vectors crossed at almost right angle. Also in adult tongue, the vertical and transverse muscle fibers sometimes (0–2 sites per section) crossed with an endomysium-to-endomysium contact. At the muscle crossing with an endomysium contact, the endomysium and basement membrane seemed to receive a friction stress between two muscles. Although some crossings might disappear due to high muscle activity after birth, not a few of them were likely to maintain. To minimize the mechanical stress, a minute nervous control of the timing, duration and strength of muscle contraction seemed to be necessary.

Striated muscle fiber crossings at almost right angle are known to exist in the face, soft palate, pharyngeal wall and tongue. We aimed to identify a specific interface tissue at the crossing. We observed histological sections from 22 halfheads of 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). For comparison, we also observed tongue frontal sections from 5 elderly cadavers (75–85 years old). At the angle of mouth as well as in the soft palate and pharyngeal wall, a solitary striated muscle fiber (e.g., levator) consistently crossed a fiber bundle of the antagonist muscle (e.g., depressor), but a solitary-to-solitary fiber interdigitation was unlikely with the antagonist muscle. Near the external nasal orifice as well as in the tongue intrinsic muscle layer, at every section, there was a crossing with an endomysium-to-endomysium contact:the nasalis and platysma muscles and; the vertical and transverse (or inferior longitudinal) tongue muscles. Therein, the functional vectors crossed at almost right angle. Also in adult tongue, the vertical and transverse muscle fibers sometimes (0–2 sites per section) crossed with an endomysium-to-endomysium contact. At the muscle crossing with an endomysium contact, the endomysium and basement membrane seemed to receive a friction stress between two muscles. Although some crossings might disappear due to high muscle activity after birth, not a few of them were likely to maintain. To minimize the mechanical stress, a minute nervous control of the timing, duration and strength of muscle contraction seemed to be necessary.

The temporal fascia is a double lamina sandwiching a thick fat layer above the zygomatic bony arch. To characterize each lamina, their developmental processes were examined in fetuses. We observed histological sections from 22 half-heads of 10 mid-term fetuses at 14–18 weeks (crown-rump length, 95–150 mm) and 12 near-term fetuses at 26–40 weeks (crown-rump length, 215–334 mm). The superficial lamina of the temporal fascia was not evident at mid-term. Instead, a loose subcutaneous tissue was attached to the thin, deep lamina of the temporal fascia covering the temporalis muscle. At near-term, the deep lamina became thick, while the superficial lamina appeared and exhibited several variations: i) a monolayered thick membrane (5 specimens); ii) a multi-layered membranous structure (6) and; iii) a cluster of independent thick fasciae each of which were separated by fatty tissues (1). In the second and third patterns, fatty tissue between the two laminae was likely to contain longitudinal fibrous bands in parallel with the deep lamina. Varying proportions of the multi-layered superficial lamina were not attached to the zygomatic arch, but extended below the bony arch. Whether or not lobulation or septation of fatty tissues was evident was not dependent on age. The deep lamina seemed to develop from the temporalis muscle depending on the muscle contraction. In contrast, the superficial lamina developed from subcutaneous collagenous bundles continuous to the cheek. Therein, a difference in development was clearly seen between two categories of the fasciae.

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.

It is unclear whether forearm and crural muscle fibers extend distally across the wrist and ankle joints, respectively.We hypothesized, in late-term fetuses, an over-production of muscle bellies extending over the joint. Muscle fibers in histological sections from unilateral wrists and ankles of 16 late-term fetuses (30–40 weeks) were examined and compared with 15 adult cadavers. Muscle fibers of the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) in fetuses, especially muscle bellies to the third and fourth fingers, were found to extend far distally beyond the radiocarpal joint.The extensor digitorum and extensor pollicis longus on the extensor side of the wrist were found to carry distally-extending muscle fibers, but these fibers did not extend beyond the distal end of the radius. In the ankle, most muscle bundles in the flexor hallucis longus (FHL), fibularis brevis (FB) and extensor digitorum longus extended distally beyond the talocrural joint, with most FB muscle fibers reaching the level of the talocalcaneal joint. In adult cadavers, muscle fibers of the FDP and FHL did not reach the levels of the radiocarpal and talocrural joints, respectively, whereas the FB muscle belly always reached the talocalcaneal joint. Similarly, some of the FDS reached the level of the radiocarpal joint. Generally, infants’ movements at the wrist and ankle could result in friction injury to over-extended muscle. However, the calcaneal and FDP tendons might protect the FB and FDS tendons, respectively, from friction stress.

The human fetal sacroiliac joint (SIJ) is characterized by unequal development of the paired bones and delayed cavitation. Thus, during the long in utero period, the bony ilium becomes adjacent to the cartilaginous sacrum. This mor phology may be analogous to that of the temporomandibular joint (TMJ). We examined horizontal histological sections of 24 fetuses at 10–30 weeks and compared the timing and sequences of joint cartilage development, cavitation, and ossification of the ilium. We also examined histological sections of the TMJ and humeroradial joint, because these also contain a disk or disk-like structure. In the ilium, endochondral ossification started in the anterior side of the SIJ, extended posteriorly and reached the joint at 12 weeks GA, and then extended over the joint at 15 weeks GA. Likewise, the joint cartilage appeared at the anterior end of the future SIJ at 12 weeks GA, and extended along the bony ilium posteriorly to cover the entire SIJ at 26 weeks GA. The cavitation started at 15 weeks GA. Therefore, joint cartilage development seemed to follow the ossification of the ilium by extending along the SIJ, and cavitation then occurred. This sequence “ossification, followed by joint cartilage formation, and then cavitation” did not occur in the TMJ or humeroradial joint. The TMJ had a periosteum-like membrane that covered the joint surface, but the humeroradial joint did not. After muscle contraction starts, it is likely that the mechanical stress from the bony ilium induces development of joint cartilage.

Solitary distal vaginal atresia is generally caused by a transverse septum or an imperforate hymen. We found a novel type of distal vaginal atresia in a late-term fetus (gestational age approximately 28 weeks) in our histology collection. This fetus had a vaginal vestibule that was closed and covered by a thick subcutaneous tissue beneath the perineal skin in the immediately inferior or superficial side of the imperforate hymen. The uterus, uterine tube, anus, and anal canal had normal development. The urethral rhabdosphincters were well-developed and had a normal topographical relationship with the vagina, but the urethrovaginal sphincter was absent. Thus, vaginal descent seemed to occur normally and form the vestibule. However, the external orifice of the urethra consisted of a highly folded duct with hypertrophied squamous epithelium. Notably, the corpus cavernosum and crus of the clitoris had poor development and were embedded in the subcutaneous tissue, distant from the vestibule. Normally, the cloacal membrane shifts from the bottom of the urogenital sinus to the inferior aspect of the thick and elongated genital tubercle after establishment of the urorectal septum. Therefore, we speculate there was a failure in the transposition of the cloacal membrane caused by decreased elongation of the genital tubercle. The histology of this anomaly strongly suggested that the hymen does not represent a part of the cloacal membrane, but is instead a product that appears during the late recanalization of the distal vagina after vaginal descent. The transverse septum was also likely to form during this recanalization.