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The upper and lower limb initially develops as a small bulge called a limb bud, which appears on the lateral side of the early embryo. The upper limb bud appears near the end of the fourth week of development, with the lower limb bud appearing shortly after.
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Joints form during embryonic development in conjunction with the formation and growth of the associated bones. The embryonic tissue that gives rise to all bones, cartilage, and connective tissues of the body is called mesenchyme.
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In anatomy, several standard anatomical positions are used as references for describing the position and orientation of different body parts. These positions help provide a common frame of reference when discussing anatomical structures. The anatomical position is the standard reference point for describing the body's position and orientation. In this position:
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The upper limb consists of the arm, forearm, wrist, and hand bones. The humerus is the single bone of the upper arm region. Proximally, it has a large, spherical, smooth head that articulates with the glenoid cavity of the scapula to form the glenohumeral or shoulder joint. The margin of the head is the anatomical neck, a residual epiphyseal plate. Laterally it extends to form bony projections called the greater tubercle and the lesser tubercle. Next to the tubercles is the surgical neck, a...
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Intramembranous ossification is one of the two processes involved in the development of bones within an embryo. The flat bones of the face, most of the cranial bones, and the clavicles are formed via this process. During intramembranous ossification, the bones develop directly from sheets of undifferentiated mesenchymal connective tissue.
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Human shoulder development is adapted to obstetrical constraints.

Mikaze Kawada1, Masato Nakatsukasa1, Takeshi Nishimura2

  • 1Laboratory of Physical Anthropology, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan.

Proceedings of the National Academy of Sciences of the United States of America
|April 12, 2022
PubMed
Summary

Human shoulder development differs from primates, with slower fetal growth and faster postnatal growth. This perinatal shoulder development may have evolved to reduce obstetrical difficulties like shoulder dystocia.

Keywords:
childbirthobstetrical dilemmaontogenetic allometryshoulder dystocia

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Area of Science:

  • Human anatomy
  • Evolutionary biology
  • Developmental biology

Background:

  • Human birth canal size presents obstetrical challenges due to neonate head and shoulder dimensions.
  • Cranial development in humans shows adaptations to birth constraints, but shoulder development remains unexamined.

Purpose of the Study:

  • To investigate if human shoulder development exhibits adaptations for obstetrical constraints.
  • To compare shoulder width development trajectories across human and nonhuman primate species.

Main Methods:

  • Tracking shoulder width development from fetal to adult stages.
  • Comparative analysis of developmental trajectories in humans, chimpanzees, and Japanese macaques.

Main Results:

  • Human shoulder development follows a distinct trajectory compared to nonhuman primates.
  • Humans exhibit reduced shoulder growth relative to trunk length prenatally.
  • Postnatal growth shows enhanced shoulder development in humans.

Conclusions:

  • Human perinatal shoulder development is characterized by a unique growth pattern.
  • This developmental pattern likely evolved to mitigate obstetrical difficulties, including shoulder dystocia.