High power TEM of the nuclear organization of a blastomere in a stage 2, 4-cell embryo in vitro 44 hours post-insemination (original magnification x33,380). The outer and inner membranes of the nuclear envelope converge at the pores which are traversed by diaphragms. Each pore complex contains dense material and small clumps of heterochromatin. Pores are points through which nuclear-cytoplasmic exchanges occur. The dense oval body in the perinuclear space appears to originate from the inner membrane apposite a pore complex (red arrowhead). It contains granular material that resembles heterochromatin. The oval body may be small packets of nuclear material being extruded into the cytoplasm. Organelles resembling oval body are present in the cytoplasm in the vicinity of the nucleus (blue arrowhead).
H = heterochromatin
N = nucleus
S = smooth endoplasmic reticulum
From: Trounson and Sathananthan, 1984. Reproduced with permission of Plenum Publishing Corporation.
Source: The Virtual Human Embryo.