From Embryology

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Fig. 495. Isolated villi from chorion frondosum of a human embryo of eight weeks

Kollmann's Atlas.

THE CHORION FRONDOSUM or foetal portion of the placenta consists of two layers which are not, however, sharply separated.

The compact layer

This lies next to the amnion and consists of connective tissue. " At first the latter is of the more cellular embryonal type. Later it resembles adult fibrous tissue.

The villous layer

The chorionic villi, when they first appear, are short simple projections from the epithelial layer of the chorion and consist wholly of epithelium. Very soon, however, two changes take place in these projections. They branch dichotomously giving rise to secondary and tertiary villi, forming tree-like structures (Fig. 495). At the same time mesoderm grows into each villus so that the central part of the originally solid epithelial villus is replaced by connective tissue, which thus forms a core or axis. This connective tissue core is at first free from blood vessels, but toward the end of the third week terminals of the umbilical (allantoic) vessels grow out into the connective tissue and the villus becomes vascular. Each villus now consists of a core of vascular mesodermic tissue (embryonal connective tissue) covered over by trophoderm (epithelium). At first the epithelium of the villus consists of distinctly outlined cells. Very soon, however, the epithelium shows a differentiation into two layers. The inner layer lying next to the mesoderm is called the layer of Langhans or cyto-trophoderm. Its cell boundaries are distinct and its nuclei frequently show mitosis. The outer covering layer consists of cells the bodies of which have fused to form a syncytium the syncytial layer or plasmoditrophoderm. This is a layer of densely stained protoplasm of uneven thickness (Figs. 496 and 497). It contains small nuclei which take a dark stain. As this layer is constantly growing, and as these nuclei do not show mitosis, it has been suggested that they probably multiply by direct division.

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Bailey FR. and Miller AM. Text-Book of Embryology (1921) New York: William Wood and Co.

Cite this page: Hill, M.A. (2024, May 25) Embryology Bailey495.jpg. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G

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current23:45, 1 February 2011Thumbnail for version as of 23:45, 1 February 2011802 × 730 (142 KB)S8600021 (talk | contribs){{Template:Bailey 1921 Figures}} Category:Coelom Category:Placenta