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Nuclear organization of a blastomere in a 4-cell embryo

High power TEM showing nuclear organization of a blastomere in a stage 2, 4-cell embryo in vitro 44 hours post-insemination (original magnification x33,380). The nucleus contains a dense, compact nucleolus and scattered clumps of heterochromatin but mainly euchromatin. The nuclear envelope is a two-layered membrane and is perforated by well-defined pores (small arrowheads). A dense, oval body is present in the perinuclear space and arises from the inner membrane where the outer membrane forms a bleb (large blue arrowhead).

M = mitochondrion
S = smooth endoplasmic reticulum

From: Trounson and Sathananthan, 1984. Reproduced with permission of Plenum Publishing Corporation.

Keywords: blastomere, bleb(s), euchromatin, four-cell embryo, heterochromatin, nuclear membrane, nucleolus, nucleus, perinuclear space, smooth endoplasmic reticulum, stage 2 embryo

Source: The Virtual Human Embryo.