An ultrastructural and cytological study of preimplantation development of the mouse

Abstract

Abstract Each stage of preimplantation development in the mouse from the fertilized egg to the blastocyst stage (including the unfertilized egg) was studied cytologically and ultrastructurally. Observations were made on the appearance and elaboration of several cellular organelles, inclusions and cell surface specilizations. The fertilized egg exhibits many intranuclear annulate lamellae, an increase in cytoplasmic vesicle number when compared to the unfertilized egg, and small amounts of crystalloids; mitochondria are vacuolated and small. The 2‐cell stage is very similar to the fertilized egg but shows an increase in the number of cytoplasmic vesicles. The 4‐cell stage is characterized by many changes: functional nucleoli appear, vacuolated mitochondria enlarge, cytoplasmic vesicles continue to increase in number, rough endoplasmic reticulum appears (as mitochondria‐associated sacs), and some ribosomes are localized near the plasma membrane. At the 8‐cell stage, large numbers of free ribosomes are observed in the cytoplasm, but clusters (polysomes) predominate at the 16‐cell stage (morula). Morulae develop junctional complexes and exhibit differences in cytoplasmic basophilia between cells, which may be a prelude to differentiation. At the blastocyst stage, nucleoli change to an elongate form and differences in cytoplasmic background density can be observed ultrastructurally. Observations suggest that the contents of the blastocoel may be derived from the cytoplasmic vesicles, which increase in number and size subsequent to fertilization and discharge their contents into the intercellular spaces; the blastocoel arises as these fluid‐filled spaces become confluent and enlarge.