Ahh! I have blood!

June 7, 2006 7:05 PM

BloodFilter: How do new blood cells make their way out of bones? Help this anatomy-challenged Mefite understand!

This may seem like a silly question. But I've Googled like crazy, and I've still been unable to find anything that will explain in minute detail how exactly the blood cells created in humans' bone marrow make their way out of the bones.

I've found the Wikipedia stub about hematopoiesis, but that only explains how blood cells are formed, not the path they take to get out of the bones.

So how does it work? Do blood cells make their way out of "pores" or "channels" in the spongy part of the bone? That was the best guess my friends and I could come up with—but we don't know if it's correct. And even if it is correct, it still doesn't explain how the cells get into the bloodstream. Are there veins that directly abut the bone? If so, how do the blood cells move into the veins—do they somehow go through the blood vessel wall?

I never took an anatomy class, and the only biology I had was eight years ago—I took physics courses in college. Please forgive my ignorance!

posted by limeonaire to health & fitness (14 comments total)

White blood cells follow chemical "scents" -- hormones of the immune system, if you will, that are stronger in the circulation than they are in the bone marrow. These "scents" go by various names -- chemokines, interleukins, growth factors, colony stimulating factors, and so on, and are responsible for white blood cells getting "directions" and knowing both how to exit the bone marrow, and how to exit from circulation into tissues, into lymphoid organs, etc.
posted by NucleophilicAttack at 7:16 PM on June 7, 2006

Do red blood cells also do that?
posted by limeonaire at 7:18 PM on June 7, 2006

There are pores in all bones and blood vessels go through those pores into the bones.

Bone cells are alive, just like all other cells are alive, and they have to have access to the things that the blood carries, like oxygen, and minerals from the intestines, and have to have metabolic wastes carried away. So all the cells in the bones, including the marrow, have access to blood capillaries, just as do almost all other cells in the body.

The marrow has to have a particularly good blood supply, because it needs proteins and nucleic acids and lipids in order to make new blood cells. The cells that get made then leave the marrow through the same vessels/veins which brought in the nutrients that were used to make them.
posted by Steven C. Den Beste at 7:20 PM on June 7, 2006

The word that you're looking for is hematopoiesis (or haematopoiesis). (erythropoiesis for red blood cell generation - erythrocytes toss their DNA upon maturation thus causing the rolled-up condom appearance of red blood cells)

Someone's PPT slides html-ized.

As to squeezing out of pores in the bone part - it's really difficult to see first hand, but yes, bone is quite porous so it's most likely that cells will transmigrate out of the marrow and into circulation..
posted by porpoise at 8:05 PM on June 7, 2006

Mm. Thanks for the answers so far. I understand it all a lot better now, especially after getting that PowerPoint as a reference.
posted by limeonaire at 8:15 PM on June 7, 2006

Making the calcium in bone micro-porous is good anyway, because it slightly reduces the total strength but substantially reduces mass, for a net gain in actual strength (since the bone also supports itself). Metabolically it's also a win because it reduces the amount of calcium the critter has to come up with for bone generation.
posted by Steven C. Den Beste at 8:25 PM on June 7, 2006

If you're interested in the blood, here's some things to think about:

Given that a cell can only divide a certain number of times before death, due to telomere shortening, how does the body continue to make new blood cells at such a rate throughout our life?

Are there cells that have the ability to continually divide and divide over and over, or is there a quiescent reservoir of blood cells, each one of which is capable of a fixed number of doublings, which slowly gets used up over our lifespan?

If the cells have the ability to continually divide over and over, what distinguishes them from cancer cells? And how does a recently-divided "hematopoetic stem cell" decide to remain a stem cell or become a blood cell and divide no further?
posted by Mr. Gunn at 8:43 PM on June 7, 2006

Oh, the other part of your question; since histology is really confusing, this is an artist's rendition of a capillary snaking into the bone through the pores.

The blood vessel isn't like a rubber hose, it's like a bicycle tire made entirely out of patches (each patch is a single cell). The contact points between each cell can increase and decrease. Cells can squeeze into the bloodstream through these contact points (and are 'directed' to enter the bloodstream by moving in the direction of increasing concentration of various small soluble signalling proteins present in the bloodstream but not in marrow [the small signalling proteins permeat through tissue - the further away from the source, the less concentrated it becomes - cells can sense these gradients and move towards/away from them.).
posted by porpoise at 8:45 PM on June 7, 2006

Mr. Gunn - immortal cell vs. cancerous cell; a cancerous cell is an senile immortal cell =)

Word of the day: Telomerase
posted by porpoise at 8:48 PM on June 7, 2006

Mr. Gunn, I believe it's similar to sperm production. There's a fixed supply at the beginning of cells which can eventually be transformed into sperm or red blood cells respectively, but that initial supply is so huge that normal people won't run out in their lifetimes.
posted by Steven C. Den Beste at 10:31 PM on June 7, 2006

A bit more: each initial cell in the testes can be converted ultimately into four sperm cells. I believe each individual beginning cell in the marrow can produce a lot more red blood cells than that -- and also, the initial supply is much bigger.
posted by Steven C. Den Beste at 10:32 PM on June 7, 2006

No one's mentioned the Haversian system, which is really the answer to this question on an ultrastructural level. The Haversian system, with a longitudinal Haversian canal at its center, is the structural unit of bones. The transverse canals are known as Volkmann's canals, and they perforate the rim of compact bone surrounding the marrow.

Each canal contains a pair of blood vessels - artery and vein - and these are in communication with the bone marrow, which is itself really, in a way, part of the intravascular space. (Injecting into a bone marrow is pretty much equivalent to an IV injection; anyone who's ever participated in a bone marrow biopsy knows that it's quite bloody in there.)

The reasons that bones form this way has to do with the way they develop in the embryo and thereafter; as you might imagine there are scientists who devote their lives to understanding bone development. It is an elegant and complex process.

Here's a larger picture, which seems a little clearer to me than the one in the Wikipedia article.
posted by ikkyu2 at 1:04 AM on June 8, 2006

Ikkyu2, your casual description of a bone biopsy made me grit my teeth-- hard.

Because red blood cells have no DNA, I have been assuming they were being produced by a process not subject to cell division limits; I thought that was probably the 'point' of no DNA, in fact. But if they initially have DNA which is later discarded, I don't see how that can be so.

Thank you, porpoise.
posted by jamjam at 11:12 AM on June 8, 2006

Ikkyu2, your casual description of a bone biopsy made me grit my teeth-- hard.

Sorry about that! The first time I saw one, I actually felt a little faint - the only time that's ever happened to me in eleven years of medical training and practice.

Mature erythrocytes contain no DNA. But erythroblasts, which sit around dividing in the marrow, do indeed contain a complete nucleus, which shrinks, condenses and is eventually spit out the side of the cell and degraded.

The question as to how do CD34+ stem cells know when to divide and reproduce themselves in their pluripotent state, and when to divide and mature and differentiate into lineages that have a determined fate instead, is another of the fascinating questions that people are working very hard to unravel.
posted by ikkyu2 at 11:46 AM on June 8, 2006