The role of the nucleus in organogenesis: Part 1.

Organogenesis is the result of cell allocation followed by cell differentiation, and the processes involved are still to be elucidated. Present-day teaching suggests that the cytoplasmic membrane, the extracellular matrix and the cytoskeleton, in combination, are responsible, but the literature also suggests that other factors must be working as well. Because organogenesis is under strict genetic control, the controlling mechanism must reside in the nucleus and when the biology of the embryonic stem cell's nucleus was looked into, some interesting features emerged. One noteworthy feature was that the cytoplasm formed an incomplete ring around the nucleus so that part of the nuclear envelope was bare of cytoplasm, and in the aggregates of stem cells which made up an organ rudiment, the bare margins were in intimate contact. Another noteworthy feature was that the cytoplasm of the embryonic cells then underwent dissolution prior to differentiation, and the specialized cytoplasm arose progressively from the bare nuclei. The argument is put that, when embryonic stem cells join to form an organ rudiment, the bare margins of the nuclei of adjoining cells make intimate contact, allowing the correct spatial orientation of the cells to be achieved by means of positional cues in the nuclei. The cytoplasm of the embryonic cells then undergoes dissolution and the specialized cytoplasm of the differentiated cells is constructed by the denuded nuclei.