Ultrastructural changes in the interhaemal membrane and junctional zone of the murine chorioallantoic placenta across gestation

Abstract

The mouse is an extremely useful experimental model for the study of human disease owing to the ease of genetic and physiological manipulation. A more detailed knowledge of murine placental development will, we hope, increase our understanding of the pathogenesis of placentally related complications of human pregnancy. The murine placenta consists of two main fetally derived compartments: the labyrinthine zone and the junctional zone. Exchange in the labyrinthine zone takes place across an interhaemal membrane comprising an outer layer of cytotrophoblast cells and two inner layers of syncytial trophoblast. The cytotrophoblast layer thins as gestation advances, and in addition becomes highly perforated after embryonic day (E)12.5. Furthermore, as gestation advances cytotrophoblast nuclear volume and DNA content increase, suggesting the formation of labyrinthine trophoblast giant cells. The syncytial layers become increasingly microvillous, enlarging the surface area for exchange. Separate basement membranes support the syncytium and the fetal capillary endothelium throughout gestation, although these appear to fuse where the capillaries are closely approximated to the trophoblast. The junctional zone consists of two principal trophoblast cell types, spongiotrophoblasts and invasive glycogen cells, yet the functions of each remain elusive. Spongiotrophoblasts vary in their appearance even when not fully differentiated, but a striking feature is the extensive endoplasmic reticulum of the more mature cells. Early glycogen cells are distinguished by the presence of electron-dense glycogen granules, and large amounts of surrounding extracellular matrix. Later the accumulations of glycogen granules occupy almost all the cytoplasm and there are few organelles. This is the first study to use both scanning and transmission electron microscopy in an ultrastructural description of murine placental development and is complementary to contemporary genetic investigations.

Fig. 1

Development of the cytotrophoblast with gestational age.(A)E12.5 cytotrophoblast (CT) attached to the underlying syncytial layer (TII) by desmosomes (indicated by arrows).(B)E14.5 binucleate cytotrophoblast spanning a maternal blood space (MBS).(C)E16.5 cytotrophoblast.(D)E19.5 cytotrophoblast intimately attached to the syncytial layer beneath (TII). FC, fetal capillary; FE, fetal endothelium; l, lipid droplet; m, mitochondrion; n, nucleus; T III, syncytial trophoblast Layer III. Arrows indicate desmosome adhesion complexes between cytotrophoblast and cytotrophoblast or syncytial trophoblast Layer II. Scale bar = 1 µm.

Fig. 2

Transmission and scanning electron micrographs of cytotrophoblastic bridges. (A) E14.5 cytotrophoblast (CT) with cytoplasmic bridges attached to several areas of labyrinthine zone interhaemal membrane (LIM) surrounding the maternal blood space (MBS). (B,C) E16.5 MBS adjacent to a fetal capillary (FC) with cytotrophoblast joining both sides of the LIM. (D–F) E16.5 labyrinthine zone with three-dimensional images of cytotrophoblast bridges. Arrows indicate cytotrophoblast bridges connecting separate sides of the lumens of blood spaces. Scale bar = 1 µm.

Fig. 3

Development of the labyrinthine interhaemal membrane with gestational age.(A) E12.5 interhaemal membrane showing maternal blood space (MBS) surrounded by cytotrophoblast (CT), directly apposed to the first syncytial trophoblast layer (TII), then the second syncytial trophoblast layer followed by basement membrane (BM), separating from the fetal endothelium (FE) that surrounds the fetal capillary (FC).(B)E14.5 interhaemal membrane.(C)E16.5 interhaemal membrane.(D) E18.5 interhaemal membrane. Scale bar = 1 µm.

Fig. 4

Interhaemal membrane of an E16.5 mouse placenta illustrating where a gap junction, indicated by the arrow, is observed at the interface between syncytial trophoblast layers (T II & T III). T I, cytotrophoblast layer. Scale bar=200nm.

Fig. 5

Areas of the interhaemal membrane where two separate basement membranes (BM) can be clearly identified associated with the fetal endothelium (FE) or syncytial trophoblast (TIII) surrounding the fetal capillary (FC). (A) E12.5, (B) E14.5, (C) E16.5, (D) E19.5. CT, cytotrophoblast; MBS, maternal blood space; TII, syncytial trophoblast Layer II. Scale bar = 1 µm.

Fig. 6

Perforations in the cytotrophoblast layer. (A) E14.5 cytotrophoblast (CT) layer with two distinct perforations indicated by arrows, where the syncytial layer beneath appears to be secreting some electron-lucent material into the maternal blood. (B) E16.5 interhaemal membrane where the syncytial trophoblast layer (TII) has direct contact with maternal blood through the cytotrophoblast perforation. (C) E18.5 perforation directly over a channel running into the syncytial trophoblast layer beneath. (D–F) Scanning electron micrographs of E16.5 extensively fenestrated cytotrophoblast layer. BM, basement membrane; FC, fetal capillary; FE, fetal endothelium; MBS, maternal blood space; TIII, syncytial trophoblast layer. Scale bar = 1 µm.

Fig. 7

Spongiotrophoblast development with gestational age. (A) E12.5, (B) E14.5, (C) E16.5, (D) E18.5. d, desmosome; er, endoplasmic reticulum; g, Golgi apparatus; m, mitochondrion; asterisks indicate fluid-filled spaces where spongiotrophoblast cytoplasmic projections are present. Scale bar = 1 µm.

Fig. 8

Glycogen cell differentiation from E12.5 in gestation. (A) E12.5. (B) Close up of A. (C–E) E14.5. ECM, extracellular matrix; er, endoplasmic reticulum; FC, fetal capillary; gly, glycogen granules; m, mitochondrion; MBS, maternal blood space; mvs, maternal venous sinous in the junctional zone; n, nucleus. Scale bar = 1 µm (A–D), 200 nm (E).