Tight Junctions between Cells in the Early Chick Embryo as Visualized with the Electron Microscope

Once embryonic cells have begun a course of somatic differentiation, they are characterized by a remarkable degree of adhesiveness which can be disrupted by trypsin in calcium-poor solutions (1-3) and which seems attributable in part to desmosomelike specializations, the zonula ad-haerens and macula adhaerens (4-5). Another cell contact specialization, one which has received little attention from embryologists, is the so-called tight junction which has been reported to take the form of a zonula, macula, or fascia occludens in mature tissues (4, 10). Potter, Furshpan, and Len-nox (6) have recently discovered that electrical coupling between embryonic cells appears prior to overt cell differentiation in the squid embryo. A preliminary report by Sheridan (7) demonstrating low electrical resistance between cells in chick embryos shows that a similar phenomenon exists in developing vertebrates. In view of the possibility that tight junctions are involved in electrical con-ductance between cells (6, 8) and because of the relation to Sheridan's work, we are publishing a preliminary account of observations made over the past few years on the fine structure of cell contact specializations in the primitive germ layers and developing tissues of the chick embryo. MATERIAL AND METItODS Chick embryos from the four somite stage through the period of formation of the definitive organ anlage (Hamburger-Hamilton stages 4-18) were dissected from the yolk and fixed with the vitelline membrane intact for 15 min at 25°C in 2.5% glutaraldehyde with 4.0% paraformaldehyde and 0.075% CaC12 at pH 7.5 in 0.1 M cacodylate (9). The tissues were washed briefly with two or three changes in 0.1 M Na cacodylate and postfixed in 1.2% osmium tetrox-ide in collidine buffer at 4°C for 1 hr. After a brief wash in dilute HC1 (pH 5.0), the whole embryos were immersed in 0.5% uranyl acetate solution in collidine buffer for 1~/,~ hr at 25°C. The uranyl acetate solution was prepared by adding an appropriate amount of uranyl acetate to a solution of collidine at pH 6.1 (10). Following treatment with the uranyl acetate, the tissues were dehydrated in alcohol, flat-embedded in Araldite (R. P. Cargille Laboratories, Inc., Cedar Grove, Illinois), and sectioned in transverse and longitudinal planes for study in the light microscope and in the electron microscope (Siemens Ehniskop I, RCA EMU 3 F). RESULTS At the beginning of incubation the chick embryo already has two germ layers, the epiblast (pre-sumptive ectoderm) and the hypoblast (presump-tive endoderm). By 20 hr of …