Alteration of gametophyte and sporophyte generations

All living species in their life cycle go through 2 phases or generations- the gametophytic and sporophytic-for cytogenetics we call it the N and 2N phases where N refers to the complement or set of chromosomes.

After browsing through one of my old books on botany I found graphs throughout this book showing alterations and dominance of these 2 cycles or phases. Basically there was a shift from dominance of the gametophytic or N generation found in the lower forms of life to the sporophytic or 2N degeneration in the higer forms of life like the gymnosperms and angiosperms. I cant imagine an animal kingdom sporophyte being a "parasite" first before shifting or evolving to the dominant phase in the higher forms of life but it would appear that this evolutionary shift also occurred in the animal kingdom.

Unfortunately, there is no way to include these cycles (diagrams) in a more descriptive blog because the book was published over a century ago. Still, this is a most fascinating piece of evolution and survival.

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Neural crest embryology

My recent internet search reconfirms the comments made in my earlier blog that the islet cells in the pancreas were of neural origin and therefore the treatment and/or management of any pathology of these cells including cancer should be approached from the category of "Diseases of the Nervous System ". Also surgeons doing islet transplantation should be aware that they are doing transplants of brain tissue - except in a location where lymphocytes are readily available to trigger the homograft rejection reaction.

Some of other adult tissues which have their origin in the neural crest of the developing embryo include melanocytes situated in the basal layer of the ectoderm and the medullary or chromaffin cells of the adrenal gland which produces adrenalin. Embryologists consider the medullary cells to be modified neurons which have lost both their axons and dendritic components! For example hypersecretion of epinephrine and norepinephrine by the medullary cells (chromaffin cells) of the adrenal glands are called pheochromacytoma tumors . These tumors may also be located in other tissues or glands besides the adrenal.

By searching the internet the reader will note that there are many other cells and tissues which are derived from the neural crest including post gangiolic cells of the parasympathetic nervous system, craniofascial cartilage, enteric neurones and glia cells.