Review: How eggs and sperm get started

germ plasm figure

Above: my hand-drawn illustration of mouse, frog and fish embryos growing.  Green: starting materials for eggs and sperm.

Earlier this year I spent a few months writing up the background of my thesis work for a chapter in The Immortal Germline.  While it’s directed toward is mainly scientists who are already in my field (or are graduate students interested in entering it), I want to share the final product!  It’s available online here, free for the rest of this month: Chapter One – The vertebrate Balbiani body, germ plasm, and oocyte polarity.

(Thanks to Ruth Lehmann for editing this volume and giving me the chance to write for it!)

I’ve spent uncountable hours, now, fascinated by eggs (oocytes, in scientific parlance).  They’re life’s beginning.  Sperm swims up and says, “hey, here’s some genetic variation for the good of the species” and doesn’t contribute much else.

The egg has all the good life-goop.  It splits in half, then half again, and it doesn’t grow until that’s gone on for many doublings.  The egg made all of that; all it’s doing is getting divvied up.

Image result for human embryo development 5 days

Above: First five days of human embryonic development.  (Embryos destined for in vitro fertilization.)

Not only that, but the life-goop is organized.  It’s not like chopping up a potato where you get eight largely identical beige chunks.  More like red cabbage: Some parts have more stem, some parts more purple leaves.  The growing critter takes that initial distinction between cabbage-wedge 1 and cabbage-wedge 2 and spins it out, fractal-like, into the information needed to make a whole animal.

My thesis is about the way that organization gets going.  I want to know how eggs go from a ziplock bag full of nutritious life-soup to a ziplock bag full of life-soup with all the carrots on the left side and all the dumplings on the top right.  It’s a tricky problem, but life is incredibly good at solving it.  Someday, we’ll know how.

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