The fine structure of the rat umbilical cord at various times of gestation

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Abstract
The morphology of the umbilical cord of the rat has been investigated in fetuses of 17,19,20 and 21 days gestation. The development and differentiation of three major features of the cord, the covering amniotic epithelium, the mesenchyme of Wharton's jelly, and the major umbilical blood vessels, were studied. At 17 days gestation, the epithelium is composed of a simple layer of flattened cells without obvious specialization and later becomes bilayered and shows considerable irregularity at the surface and at cell‐interfaces, where desmosomes are a prominent feature in the stages just prior to birth. Within the underlying mesenchyme, there is a decrease in cellularity throughout the period under study. Mesenchymal cells are characterized by the presence of a well developed endoplasmic reticulum containing an electron dense homogeneous material. Ribosomes in association with the endoplasmic reticulum are in the form of polysomal aggregates by 17 and 19 days gestation which later become less marked. Unit fibrils of collagen increase in number in the days prior to term and in later stages appear in definite bundles. In the umbilical blood vessels, there is differentiation of a muscle coat from an aggregation of mesenchymal cells. No nerves in relation to the blood vessels and no vascular supply to the umbilical cord itself could be identified in any stage of the present study.