It would be a basic biologic protection system. The "core" would need to produce a sort of totipotent stem cells and, probably, keep the blood saturated in them. The stimulus to produce such cells would then be the depletion of such cells in the blood.

At the same time the organ has to "retire" the cells grown too old and weak, and implement some kind of regenerative check on all cell lines to prevent random mutations from devolving into cancers (see "integrity check", below). And/Or maybe the Hayflick limit for "normal" or fully grown cells is much lower, so they don't have the time to degenerate but rather go into apoptosis - they live fast, die early and leave a beautiful corpse.

TL;DR the greatest risk to a long-lived organism is cancer, when cells start to reproduce incorrectly. To prevent this, not only you need a method to supply fresh stem cells to all organs but also a "check" (almost certainly an enzyme of some kind) to kill off runaway cells, to be replaced by core-produced stem cells. This mechanism does not protect the core itself: if it's overused, to the contrary, the core either withers away, or kills off the rest of the organism.

It would be a basic biologic protection system. The "core" would need to produce a sort of totipotent stem cells and keep the blood saturated in them. The stimulus to produce such cells would then be the depletion of such cells in the blood.

At the same time the organ has to "retire" the cells grown too old and weak, and implement some kind of regenerative check on all cell lines to prevent random mutations from devolving into cancers (see "integrity check", below). And/Or maybe the Hayflick limit for "normal" or fully grown cells is much lower, so they don't have the time to degenerate but rather go into apoptosis - they live fast, die early and leave a beautiful corpse.

At the same time the organ has to "retire" the cells grown too old and weak, and implement some kind of regenerative check on all cell lines to prevent random mutations from devolving into cancers. Or maybe the Hayflick limit for "normal" or fully grown cells is much lower, so they don't have the time to degenerate but go into apoptosis - they live fast, die early and leave a beautiful corpse.

For this to work, at least two very hard bounds are required.

One: reproduction becomes incredibly complicated, unless the core allows for a "grace period" before going active (maybe just after puberty?) and pregnancy shuts down the core enough to let a foetus grow and mature to term undisturbed.

(It might be possible to have an Evangelion regenerate by pumping him full of some metabolism-slowing drug, and keeping him hypothermic and in a coma. Or transplanting a large enough compatible core chunk).

• at birth: the core begins to grow (e.g. somewhere under the heart).
• age three to fifteen: the core grows but is otherwise nonfunctional.
• age fifteen (more or less): the core activates and takes some roles of a human's Major Histocompatibility Complex, plus some really wicked, DNA-based integrity check system. Some biological effects and possibly a shock occurs. Also, any damaged or mutated tissues are swiftly killed and replaced, possibly triggering some form of septic shock or rhabdomyolisis. Bottom line: not all Evas survive to see their sixteenth birthday.
• age eighteen onwards: the core has the blood saturated in repair workers. Most large scale cellular damage can be repaired. Actually, large damage to the brain will be repaired but leave the victim with severe mental effects, amnesia being the least.
• during pregnancy, pregnancy hormones shut down the core and floating cells so that they don't "think" it's a good idea to do something for that poor swollen uterus full of fluids and who-knows-what. Downside: a pregnant Eva is much more vulnerable than a human, since it has next to no regenerative powers (they'd be superfluous with a working core). As a stopgap measure, a wounded pregnant Eva would immediately abort the pregnancy and return to full defense mode.
• normally, the core awakens periodically and tops up the reservoirs at leisure, and it always filters out weakened cells from the bloodstream, destroying them and using the raw materials to build new cells.
• a large systemic shock sends the Eva in a regenerative coma, with metabolism reduced to the bare minimum and just enough circulation to keep repairs going. The core goes into low-level, long-term activation and wears out a bit, eating into its own regenerative reservoirs. It's important to keep the Eva well fed, otherwise his chances on recovery depend on body fat and muscle reserves. In ancient times, fat was synonym of health; for Evas it is still so.
• in case of multiple, repeated activations, while the rest of the body gets "iguana-like" regeneration, the core itself has "lizard-like" regeneration, and the mechanism goes awry. As a result, the Evangelion dies, not too horribly since the heart is the first to go.
• lower genetic diversity between Evas allow easier core transplants between them.

At the same time the organ has to "retire" the cells grown too old and weak, and implement some kind of regenerative check on all cell lines to prevent random mutations from devolving into cancers (see "integrity check", below). And/Or maybe the Hayflick limit for "normal" or fully grown cells is much lower, so they don't have the time to degenerate but rather go into apoptosis - they live fast, die early and leave a beautiful corpse.

If we make this work this way, at least two awkward limitations follow.

One: reproduction becomes incredibly complicated, unless the core allows for a "grace period" before going active (maybe just after puberty?) and pregnancy shuts down the core enough to let a foetus grow and mature to term undisturbed (a newborn will have a DNA which isn't the same as the parent's, and the DNA check routine would kill it at a very early stage).

(It might be possible to have an Evangelion regenerate by pumping him full of some metabolism-slowing drug, and keeping him hypothermic and in a coma, so the core doesn't exceed its design specs. Or transplanting a large enough compatible core chunk).

• at birth: the core begins to grow (e.g. somewhere under the heart).
• age three to fifteen: the core grows but is otherwise nonfunctional. Normal body growth can take place, bones are digested and remade longer, new organs can develop and mature.
• age fifteen to eighteen (more or less): the core activates and takes some roles of a human's Major Histocompatibility Complex, plus some really wicked, DNA-based integrity check system. Some biological effects and possibly a shock occurs. Also, any damaged or mutated tissues are swiftly killed and replaced, possibly triggering some form of septic shock or rhabdomyolisis. Bottom line: not all Evas survive their coming of age.
• age eighteen onwards: the core has the blood saturated in "repair workers". Most large scale cellular damage can be repaired. Actually, large damage to the brain will be repaired but leave the victim with severe mental effects, amnesia being the least.
• during pregnancy, pregnancy hormones shut down the core and floating cells so that they don't "think" it's a good idea to do something for that poor swollen uterus full of fluids and who-knows-what. Downside: a pregnant Eva is much more vulnerable than a human, since it has next to no regenerative powers (they'd be superfluous with a working core). As a stopgap measure, a wounded pregnant Eva might immediately abort the pregnancy and return to "full defense mode".
• normally, the core awakens periodically and tops up the reservoirs at leisure, and it always filters out weakened stem cells from the bloodstream, destroying them and using the raw materials to build new cells.
• a large systemic shock (e.g. loss of a limb) sends the Eva in a regenerative coma, with metabolism reduced to the bare minimum and just enough circulation to keep repairs going. The core goes into low-level, long-term activation and wears out a bit, eating into its own regenerative reservoirs. It's important to keep the Eva well fed, otherwise his chances on recovery depend on body fat and muscle reserves. In ancient times, fat was synonym of health; for Evas it is still so.
• in case of multiple, repeated activations, while the rest of the body gets "iguana-like" regeneration, the core itself has "lizard-like" regeneration, and the mechanism goes awry. As a result, the Evangelion dies, not too horribly since the heart is the first to go.
• lower genetic diversity between Evas allow easier core transplants between them.

It would be a basic biologic protection system. The core would need to produce a sort of totipotent stem cells and, probably, keep the blood saturated in them. The stimulus to produce such cells would then be the depletion of such cells in the blood.

It would be a basic biologic protection system. The "core" would need to produce a sort of totipotent stem cells and, probably, keep the blood saturated in them. The stimulus to produce such cells would then be the depletion of such cells in the blood.

Hypothetical core development

• at birth: the core begins to grow (e.g. somewhere under the heart).
• age three to fifteen: the core grows but is otherwise nonfunctional.
• age fifteen (more or less): the core activates and takes some roles of a human's Major Histocompatibility Complex, plus some really wicked, DNA-based integrity check system. Some biological effects and possibly a shock occurs. Also, any damaged or mutated tissues are swiftly killed and replaced, possibly triggering some form of septic shock or rhabdomyolisis. Bottom line: not all Evas survive to see their sixteenth birthday.
• age eighteen onwards: the core has the blood saturated in repair workers. Most large scale cellular damage can be repaired. Actually, large damage to the brain will be repaired but leave the victim with severe mental effects, amnesia being the least.
• during pregnancy, pregnancy hormones shut down the core and floating cells so that they don't "think" it's a good idea to do something for that poor swollen uterus full of fluids and who-knows-what. Downside: a pregnant Eva is much more vulnerable than a human, since it has next to no regenerative powers (they'd be superfluous with a working core). As a stopgap measure, a wounded pregnant Eva would immediately abort the pregnancy and return to full defense mode.
• normally, the core awakens periodically and tops up the reservoirs at leisure, and it always filters out weakened cells from the bloodstream, destroying them and using the raw materials to build new cells.
• a large systemic shock sends the Eva in a regenerative coma, with metabolism reduced to the bare minimum and just enough circulation to keep repairs going. The core goes into low-level, long-term activation and wears out a bit, eating into its own regenerative reservoirs. It's important to keep the Eva well fed, otherwise his chances on recovery depend on body fat and muscle reserves. In ancient times, fat was synonym of health; for Evas it is still so.
• in case of multiple, repeated activations, while the rest of the body gets "iguana-like" regeneration, the core itself has "lizard-like" regeneration, and the mechanism goes awry. As a result, the Evangelion dies, not too horribly since the heart is the first to go.
• lower genetic diversity between Evas allow easier core transplants between them.