system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What exactly is geneseed?

Is it like guts? So glowy goo that turns men into marines? And how exactly is it made? Why is recovering it so important?
gpt
OOC: c'mon guys.  It's somewhat obvious but it's fun to answer regardless.  Don't downvote him!

++++COMMUNICATION RECIEVED: AUTHORITY VERIFICATION, ALPHA PRIORIS++++

Thought for the day: The Emperor Protects.  What have you given to Him in turn?

Greetings, valued Imperial subject.  If you're reading this, you are of rank high enough to know of one of the Imperium's most important secrets.  How Astartes -Space Marines- are made.  This is a basic primer meant to familiarize you with Geneseed, and its importance to the many Chapters of Space Marines loyally serving the Imperium.

What Astartes Brothers refer to as "Gene-Seed" mostly refers to the special gene-forged organs - one in particular - that can be implanted into a human boy and turn him into an Astartes.  

Space Marines specially culture organs for grafting into neophyte's bodies from a particular gland called "The Progenoid Gland."  Each Astartes has two of these glands, one in the chest and one in the neck.  These glands mature after 5, and then 10 years respectively, and from them the rest of the array of organs can be cultured and grown.  They can otherwise be preserved if not needed for immediate implantation, and can also be harvested from dead Astartes if they are mature and intact.

The only way for the Marines to create more geneseed is through this process.  This is why preserving the organs, particularly the Progenoids, is so important to the Chapter in question, and indeed, the imperium itself.  Chapters of Marines have perished or been dissolved in the past in situations where they were unable to continue their genetic lineage through this.  

The actual process of implantation of each organ and the sequence of which is contained in a more classified document, should your clearance be high enough, feel free to access it.  If not, know that not all answers are meant for mortal eyes.

Geneseed, like all things good and holy, comes from The Emperor.  Over 10 millennia ago, he applied his infinite knowledge and skill towards the creation of Astartes, and therefore Geneseed.  To further illustrate his magnificence, the Astartes project is merely the leftovers of what he used to create his 9 sons, who now rest along side him until they are needed.  Indeed, all loyalist marines can trace the genetic lineage of their geneseed to one of the Emperor's own sons, which he created in his image.


As an aside, each Chapter loyal to the Imperium is required to send 5% of their geneseed stocks to Mars for examination by the techpriests, ensuring minimal mutation.