I am currently studying the textbook Hoffbrand's Essential Haematology, eighth edition, by A. Victor Hoffbrand and David P. Steensma. Chapter 1 Haemopoiesis says the following:

During normal childhood and adult life, the marrow is the only source of new blood cells. The developing cells are situated outside the bone marrow sinuses; mature cells are released into the sinus spaces, the marrow microcirculation and so into the general circulation.

I don't understand this description. The authors state that the developing cells are situated outside the bone marrow sinuses. Presumably, the bone marrow sinuses are deeper within the bone, right? If so, then this description would be implying that the developing cells are located on the outer regions of the bone, away from the sinuses, and then are released from the outer region into the sinus space, and then into marrow microcirculation and general circulation. But isn't this path of circulation leading towards the exterior of the bone? So it sounds like the authors are saying that the developing cells start on the outer regions of the bone, then are released deeper into the bone (into the sinuses), and then are taken via marrow microcirculation and general circulation back towards the outer parts of the bone, which is where they came from in the first place. Am I misunderstanding something here?

I would greatly appreciate it if people would please take the time to clarify this.


I found a slide that seems to agree with the textbook description:

enter image description here

  • A comment for potential answerers: The central point of interest is the authors' claim that "The developing cells are situated outside the bone marrow sinuses; mature cells are released into the sinus spaces, the marrow microcirculation and so into the general circulation."; any answer to the question must necessarily specifically address this (otherwise, it isn't actually clear how it is addressing the question). – The Pointer Jul 15 at 19:23
  • "... developing cells start on the outer region of bone..." Can you tell what do you mean by "outer region of the bone." What do you understand by it? – Sikander Jul 16 at 2:12

It's really simple, your misunderstanding just comes from mixing macro- and microanatomy.
Macroscopically, all of bone marrow is located inside the bone, in the medullar cavity.
Microscopically, bone marrow tissue consists of different cells, some of which are endothelial cells that comprise sinuses, and alongside them (or outside of them, if you will; in the bone marrow's stroma) you find all other cell types.

I'll attach this wiki page as reference just in case, your textbook is a much better option though I suppose, this was just a clarification question.

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  • So how specifically does this clarify the point of the textbook explanation? – The Pointer Jul 15 at 1:52
  • @ThePointer your question was where the developing cells are located in respect to the bone, right? – practiZ Jul 15 at 10:41
  • Yes, but you're providing a general answer to that question, rather than an answer that is within the context of my question and specifically addresses it. – The Pointer Jul 15 at 19:18
  • 1
    @ThePointer addressing your comment to the question: the developing cells are outside the sinuses, then once they mature they are released into the sinuses, meaning in the marrow microcirculation. This all happens in the marrow - inside the bone. From there, via veins that perforate the outer bone layer (compact bone), they find connection to bigger veins and systemic circulation. – practiZ Jul 16 at 0:40
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    @ThePointer sinuses (sinusoids), capillaries, then veins - this is what bone marrow's microcirculation consists of. So once released into the bloodstream, mature cells follow this path to get out of bone marrow into central circulation. britannica.com/science/sinusoid – practiZ Jul 20 at 1:16

enter image description here

Image source: cancer .gov.

Bone marrow is a semi-solid tissue found within the spongy or cancellous (denoting bone tissue with a meshlike structure containing many pores, typical of the interior of mature bones) portions of bones. As you can see in the image above the hematopoiesis occur at the ends of long bones where red bone marrow is present. In red bone marrow the ratio of stem cells to fat cells are more in comparison to yellow bone marrow which is present in shaft of long bones of adults.

Look at the image below:.

bone marrow sinuses

Image source: flipper.diff.org

This is a microstructure of bone marrow sinuses with the hematopoietic stem cells near by. These hematopoietic stems cells traverse the walls of sinuses once they are differentiated and mature and then comes into circulation. These sinuses are just lined by endothelial cells, they ultimately forms connection between arterioles and venules. For megakaryocyte, it extends cytoplasmic process inside the sinus which ultimately breaks down to form platelets.

the HSC in bone marrow

Image source:frontiersin.org

Here is another image showing how HSC clinging to endosteum differentiate and matures as it moves toward the sinuses which then ultimately traverse through the sinus wall.

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  • So the hematopoietic stem cells are outside of the area shown in the image and are therefore not shown? – The Pointer Jul 15 at 12:55
  • @ThePointer I added an image for more clarification – Sikander Jul 15 at 13:50
  • The problem is that this does not directly address my question; rather, it is just stating a set of related facts. – The Pointer Jul 15 at 19:16
  • @ThePointer: I really like the second image: you can see the sinusoids (dark red), and surrounding them are hematopoetic islands (orange and purple). This entire snippet is from within the bone. Blood cells differentiate in these islands (outside of the sinusoids) and then enter the bloodstream through the endothelium of the sinusoids (shown with a neutrophil at the top). – Narusan Jul 18 at 6:50
  • These hematopoetic islands are outside of the bone marrow sinusoids but within the bone marrow. The sinusoids are microscopically small structures (~10-50 micrometers) and run through basically the whole bone. – Narusan Jul 18 at 6:52

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