During observations of mid-term and late-stage fetuses, we found a joint disk-like structure at the anterior component of the median atlanto-axial joint. At mid-term, the disk-like structure was thick (0.1-0.15 mm) relative to the sizes of bones surrounding the joint. However, it did not completely separate the joint cavity, and was absent in the inferior and/or central part of the cavity. This morphology was similar to the so-called fibroadipose meniscoid of the lumbar zygapophysial joint that is usually seen in adults. In mid-term fetuses, there was evidence suggesting that a mesenchymal tissue plate was separated from a roof of the joint cavity. In late-stage fetuses, the thickness (less than 0.15 mm) was usually the same as, or less than that at mid-term, and the disk-like structure was often flexed, folded and fragmented. Therefore, in contrast to the zygapophysial meniscoid as a result of aging, the present disk-like structure was most likely a temporary product during the cavitation process. It seemed to be degenerated in late-stage fetuses and possibly also in newborns. Anomalies at the craniocervical junction such as Chiari malformations might accompany this disk-like structure at the median atlanto-axial joint even in childhood.

stage, characterized by lamellae or an outer bulb of perineural origin and observed in fetuses of CRL ≥120 mm; and (3) a vascular stage, characterized by the gradual incorporation of surrounding blood vessels into the lamellae and observed in fetuses of CRL ≥200 mm [1]. To verify these observations, we studied serial transverse sections of the distal and middle segments of the thumb and all fingers obtained from 12 hands of 12 late-stage fetuses of gestational age 20-34 weeks (CRL,. We found that the corpuscles in late-stage fetuses can be classified into two types: (1) thin, small, immature corpuscles with tightly packed lamellae, of thickness 30-70 µm and length 0.3-0.7 mm; and (2) thick, long, mature corpuscles with loosely packed lamellae, of thickness 80-150 µm and length 0.5-2.0 mm and morphology similar to that in IntroductionThree stages in the development of digital Pacinian corpuscles in the human fetus have been described: (1) a primordial stage, consisting of one type of cell distributed along "stag-horn-like" nerve twigs and observed in fetuses of crown-rump length (CRL) 70-90 mm; (2) Abstract: Pacinian corpuscle-like structures were identified in the digital tendon sheaths and nail beds of hands obtained from eight of 12 human fetuses of gestational age 20-34 weeks (crown-rump length, 150-290 mm). The aberrant corpuscles were present in tight fibrous tissue connecting the flexor tendon sheath to the dorsal aponeurosis (138 corpuscles in the thumbs and all fingers of eight fetuses); loose fibrous tissue inside the sheath on the dorsal side of the tendon (37 corpuscles in the thumbs and all fingers of four fetuses); and the nail bed (10 clusters in the thumbs and second fingers of four smaller fetuses). The aberrant corpuscles in the tendon sheath were classified into two types: thin and short, with tightly packed lamellae, of diameter 20-40 µm and length 20-200 µm; and thick and long, with loosely packed lamellae, of diameter 70-150 µm and length 0.5-1.5 mm. The small corpuscles tended to form clusters, each containing 5-10 structures. Their similarity indicated that the tight and loose lamellae in these two types of corpuscles corresponded to typical immature and mature corpuscles, respectively, usually distributed along the palmar digital nerve. However, mature, large corpuscles were absent from the nail bed, and most aberrant corpuscles were smaller than typical corpuscles along the nerve. The aberrant corpuscles were apparently incorporated into the tendon sheath or nail bed during fetal vascular development, but they appeared to degenerate after birth due to mechanical stress from the tendon or nail.

Summary:The teres major and latissimus dorsi muscles (TM, LD) are considered to be the so-called brother muscles. Actually, being similar to the TM, an uppermost part of the LD usually arises from the scapular plate. In embryos of 11 mm CRL, anlagen of the TM and LD appeared to be fused to provide a single mass at an angle between the axillary and radial nerves. However, splitting had already finished in not only the TM and LD but also the other muscles at and around the shoulder in specimens of 14 mm CRL. Thus, muscle splitting at the region appeared to occur simultaneously at a short stage of 12-13 mm CRL.The TM and LD carried a common tendon still at 6 weeks (14-16 mm CRL), but their muscle bellies were separated clearly. A concept of brother muscles might be applied to the TM and LD according to a bias from the gross and comparative anatomy, not from the embryological view.Shin-ichi Abe and Koichiro Sakanaka contributed equally to this work.

No abstract