Summary: The median nerve passes through the humeral and ulnar heads of the pronator teres muscle (PT), although variations such as absence of the ulnar head may exist. We observed histological sections of the upper extremity from 24 embryos and fetuses. In the early stage, the PT extended between the radius and the medial epicondyle of the humerus, but no candidate for the ulnar head was found. In mid-term fetuses, the ulnar margin of the PT was attached to the elbow joint capsule. Moreover, in late-stage fetuses, a small deep part of the PT arose from the thick joint capsule of the humero-ulnar joint near the coronoid process of the ulna. This joint capsule also provided the most proximal origin of the flexor digitorum profundus muscle. Therefore, we considered fetal PT origin from the capsule as a likely candidate for the ulnar head. Consequently, the PT seemed to develop from a single anlage through which the median nerve passed, but laterpossibly after birth -a small PT origin from the joint capsule appeared to obtain an aponeurosis connecting the muscle fiber to the ulna. This secondary change in PT morphology might explain the muscle variation seen in adults.

Summary: In comparative anatomy, the musculocutaneous nerve is hypothesized to pass between the superficial and deep muscle bellies of the coracobrachialis muscle. The superficial belly is supplied by nerve branches of the lateral cord of the brachial plexus, while the deep belly by the musculocutaneous nerve. Observations of longitudinal sections of ten human embryonic arms (7 weeks; crown-rump length 26-32 mm) demonstrated that the coracobrachialis muscle was always continuous with the short head of the biceps muscle. If the aforementioned hypothesis was applied, the deep belly behind the musculocutaneous nerve course was continuous with the biceps. However, such a close relation between the coracobrachialis and biceps was not known in supplying nerves in adults. A further study using embryos of some apes without the deep belly of the coracobrachialis would be necessary for the comparison between a pattern of the embryonic muscle division and the muscle classification in comparative anatomy.bryos for suitable materials of the present observation. Materials and MethodsThe study was performed in accordance with the provisions of the Declaration of Helsinki 1995 (as revised in 2013). We observed serial paraffin sections of the body with upper extremities of 10 human embryos at 7 weeks (crown-rump length [CRL], 26-32 mm). All sections were part of the large collection kept at the Institute of Embryology, Universidad Complutense Madrid, and were products of miscarriages and ectopic pregnancies managed at the Department of Obstetrics at the university. These sections in Madrid were stained with hematoxylin and eosin (HE) or azan staining. The sectional planes were sagittal to the body and longitudinal (almost frontal) to the arm. However, because of the curved course, the humerus was not always cut longitudinally.Okajimas Folia Anat. Jpn., 93(1): 15

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.