From Embryology

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Fig. 494. Semi-diagramatic sagittal section of human uterus containing an embryo of about five weeks

Allen Thompson. (note historic drawing and information)


  • a Ventral (anterior) surface
  • c cervix uteri
  • ch chorian
  • g outer limit of decidua
  • m muscularis
  • p dorsal (posterior) surface

The Decidua Basalis

As the decidua basalis is that part of the mucosa to which the chorion frondosum is attached, it is convenient to consider the two structures together.

The Decidua Capsularis

Early in its development this has essentially the same structure as the decidua parietalis. Its older or more common name, decidua reflexa, indicates the earlier idea that this portion of the decidua represents a growing around or reflection of the uterine mucosa upon the attached ovum. Peters, after examining the very early ovum which bears his name, came to the apparently warranted conclusion that instead of the uterine mucosa growing out around the ovum, the ovum buries itself in the mucosa, and that by the time the ovum had reached the size of the one he examined (1 mm.), it was almost entirely covered over by the mucosa (Fig. 74). See also Fig. 493. In Peters' ovum a coagulum consisting of blood cells, other cast off cells and fibrin marked the point at which the ovum probably entered the stroma. Later this is replaced by connective tissue and for a considerable time the point is marked by an area of scar tissue.

By about the fifth month the rapidly growing embryo with its membranes has filled the uterine cavity, and the decidua capsularis, now a very thin transparent membrane, is everywhere pressed against the decidua parietalis. It ultimately either disappears (Minot) or blends with the decidua parietalis (Leopold, Bonnet).

The Decidua Parietalis

The changes in the uterine mucosa which result in the formation of the decidua parietalis are similar to, though more extensive than, the changes which take place during the earlier stages of menstruation. There is congestion of the stroma with proliferation of the connective tissue elements and increase in the length, breadth and tortuosity of the glands. These changes result as in menstruation in thickening of the mucosa so that at the height of its development the decidua parietalis has a thickness of about 1 cm. It extends to the internal os where it ends abruptly, there being no decidua formed in the cervix.

In the superficial part of the mucosa the glands wholly or almost wholly disappear and their place is taken by the proliferating connective tissue of the stroma. The result is a layer of comparatively dense connective tissue the compact layer. Beneath this layer are found remains of the uterine glands in the shape of widely open, somewhat tortuous spaces which extend for the most part parallel to the muscularis. Some of these glandular remains retain part of their epithelium. Lying in the proliferating stroma, these spaces give to this layer the structure which has led to its being designated the spongy layer.

During the latter half of pregnancy the decidua parietalis becomes greatly thinned, due apparently to pressure from the growing embryo with its membranes. With this thinning, the few remaining glands of the compact layer disappear. The character of the spongy layer changes, the glands collapsing or being reduced to elongated, narrow spaces parallel to the muscularis. The entire tissue also becomes much less vascular than in early pregnancy.

If the foetal membranes are in situ the compact layer is in contact with the ectodermic (epithelial) layer of the chorion. Next to this lies the mesodermic (connective tissue) layer of the chorion. Delicate adhesions connect the mesodermic tissue of the chorion with the mesodermic layer of the amnion. Covering the latter is the amniotic ectoderm (epithelium).

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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, June 19) Embryology Bailey494.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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