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Bone marrow structure/function


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What is the function of the bone marrow?
When does medullary hematopoeisis begin in the fetus?
at 10-11 weeks
when does the bone marrow become the primary site of hematopoeisis?
at 24 weeks
what is medullary hematopoeisis?
blood cell synthesis in the bone marrow
what is extramedullary hematopoeisis?

hematopoeisis at a site other than the marrow; like the liver, spleen, lymph nodes, and kidney.
What are the 2 main elements of the bone marrow structure?
Blood supply

Hematopoietic Tissue
How does blood initially get to the bone marrow?
Via nutrient arteries. They enter and then branch into arterioles.
what are sinusoids? what is their function?
spaces which arterioles give rise to in bones.

Function to pass blood cells from the marrow to the arteries through a seive-like barrier llining the spaces.
where exactly does hematopoeisis take place in bones?
in stroma within the bony matrix. NOT in the sinusoids.
what are adventitial cells, what is their function?
cells that form reticulin.

the function of this reticulin is to form a net that creates a hematopoeisis inducing microenvironment (HIM)
what kind of stain shows reticulin?
a silver stain
what type of cells lines the bone marrow cavity?
endothelial cells
what holds the vascular tissue in the bone marrow in place?
the basement membrane
what is the function of central longitudinal veins?
to carry blood cells/wastes out of the bone marrow cavity back to circulation
3 things that make up Hematopoietic tissue:
-Marrow stroma/bone matrix

-Hematopoietic cells

-Fat cells
Where is stroma in the bone marrow?

what does the stroma consist of?
It lies outside the vascular sinusoidal system, within the central cavity.

stroma is composed of endothelial cells and fibroblasts.
why are fibroblasts stromal cells?
they can stretch their cytoplasm considerably to form a meshy network of support.
what is another name for hematopoietic cells?
red marrow
what are lipocytes?
fat cells
what is the function of fat cells in marrow?
they convert to yellow marrow to act as space-fillers.

when called upon, have ability to revert to red marrow and produce blood cells.
Why is the bone marrow the site of erythropoeisis?
because it is central and insulated from external changes in the environment
What are the three possible types of bone marrow?
what is red marrow?
actively producing, hematopoeitic marrow.
what is yellow marrow?
a mix of fat and mesenchymal cells that can revert to red marrow if needed in a crisis.
what is white marrow?
marrow devoid of cellular material - it cannot revert to hematopoietic red marrow.
what happens to the marrow within our
-long bones
-flat bones
long: becomes yellow marrow, doesn't retain hematopoeitic ability.

flat: remain the sites of medullary hematopoeisis - stays red
as humans age, what happens to the composition of their bone marrow?
fat increases, cells decrease.
what is the proportion of fat to cells in:
child: 30% fat, 70% cells

adult: 50 yr old has 50:50

elderly: 70% fat, 30% cells
what is normocellular marrow?
bone marrow with a normal fat:cell ratio
what is hypocellular marrow?
bone marrow with less than 35% of ceullarity that it should have.
4 causes of hypocellular bone marrow:
aplastic anemia
3 consequences of hypocellular marrow
what may be triggered by hypocellularity in bone marrow?
extramedullary hematopoeisis.
What are examples of a Slight, Moderate, and Marked hypercellular bone marrow?
Slight = 50-59%

Moderate = 60-89%

Marked = 90-100%
What are 2 causes of hypercellular bone marrow?
hemolytic anemias
6 consequences of long-term hypercellularity:
skeletal aches and pains
bone tenderness
diffuse demineralization
osteolytic lesions
what is marrow egress?
movement of cells from the marrow into the sinusoids for circulation
What are the 4 regulators of marrow egress?
-cell surface
-releasing substances
-marrow structure
how does cell surface affect marrow egress?
young cells have sticky surfaces, and can't pass through wall from marrow to sinusoids
what are releasing substances?
enzymes that aid in cells passing through barrier to sinusoids
what is propulsion?

how does marrow egress depend on it?
the utilization of energy to propel through the cytoplasm of adventitial cells that line the sinusoids.

only older cells have this energy to get into circulation
how does marrow structure affect egress?
the blood is moving slowly through the sinusoids, so there is TIME for cells to cross into it
what happens to marrow egress in pathological architectural damage to the sinus wall?

what is the result?
immature cell are released early into circulation.

Clogs the system, so hematopoeisis stops.

The spleen and extramed. kicks in. This lacks the regulation of the marrow, so young cells are seen in the periph. blood too early.
What are the 3 periods of hematopoeisis in the Fetus?
-Mesoblastic period
-Hepatic period
-Bone marrow period
Where is the site of hematopoeisis during the mesoblastic period?
Yolk sac
What is the fetal age during the mesoblastic period?
10 days
what are the 2 components of hematopoeisis during the mesoblastic period - where, from what tissue?
In the yolk sac, in vessels derived from mesothelium.

Hemocytoblasts are derived from these vessels and produce fetal hemoglobin.
What are the 3 major Fetal Hemoglobins?
What are the constituents of the 3 fetal Hbs?
Gower 1: zeta 2 epsilon2

Gower 2: alpha 2 epsilon 2

Portland: delta 2 zeta 2
At what age does the hepatic period begin in the fetus? What happens during this period?
At 6 weeks.
Erythropoeisis in the liver
When is the liver the main erythropoetic site in the fetus?
9-24 weeks
What types of hemoglobin are produced during the hepatic period?
Hb F
Hb A (small amounts)
Why is there more Hb F in fetus, how does it affect the cell morphology?
O2 tension is low in the fetal environment. Hb F has higher O2 affinity, but it's also harder to release it in tissues. So, cells are bigger and there are more of them - megaloblastic
What are the 3 processes going on during the hepatic period?
-Erythropoiesis - in the liver

-Myelopoeisis and megakaryopoeisis

-extramedullary hematopoeisis in the spleen, kidney, conn. tissue, lymph nodes, and thymus.
When does the bone marrow peroid begin, and when does it become the major site?
10-11 weeks, major site of hematopo. at 24 weeks.
What is the typical cell morphology during the bone marrow period?
Megaloblastic - cells are big, w/ nuclear/cytoplasmic asynchrony
How does hemoglobin type shift during the bone marrow period?
Type shifts from HbF to HbA.

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