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J Embryol Exp Morphol 4, 254-264 (1956)
Published by The Company of Biologists 1956
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The Ectodermal Thickening and Ridge in the Limbs of Staged Human Embryos1

Ronan O'Rahilly, Ernest Gardner and D. J. Gray2

Departments of Anatomy, Wayne University College of Medicine, Detroit, Michigan, and Stanford University School of Medicine, Stanford, California

2 address: Department of Anatomy, Wayne University College of Medicine, 1401 Rivard, Detroit 7, Michigan, U.S.A.

Received for publication 10 November 1955.

SUMMARY

The limbs of amniote embryos are characterized by a thickening of the epiblast at their tips (Balfour, 1885). Kölliker (1879) illustrated such a thickening in a section of a chick embryo and, according to Braus (1906), was responsible for the term ‘ectodermal cap’ (Ektodermkappe). Saunders (1948) showed in the chick by experimental means that ‘the thickened ectoderm at the apex of the bud’ is ‘essential to the formation of the limb’.

A similar thickening has frequently been noted at the apex of developing mammalian limbs. It can be seen in figures 3, 4, and 5 of Lewis's (1902) paper onthe development of the upper limb in man. Bardeen & Lewis (1901, fig. 19) called attention to the thickened epithelium at the free edge of the human lower limb bud. Steiner (1929, fig. 5), in a study of the development of human skin, illustrated the ‘ectodermal cap’ in the upper limb of a 9-mm. human embryo.

Footnotes

1 This investigation was supported by research grants A-550(C2) and A-532(C7) from the National Institute of Arthritis and Metabolic Diseases, of the National Institutes of Health, Public Health Service, Bethesda, Maryland.


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© The Company of Biologists Ltd 1956