Marrow Circulation. Schematic of Marrow Circulation. The arterial flow enters the marrow through the nutrient artery. The foramen through which it flows is angulated about 45 degrees; the vessels do not enter and leave perpendicular to the bone surface. The nutrient artery bifurcates into ascending and descending arterial branches from which originate the radial arteries that either anastomose with the very small vessels in the bony cortex or terminate to form the primary venous sinuses. Note that the blood in systemic arteries supplying the periosteum can comingle with the marrow arterial circulation through the vessels in the bony canals. The venous sinuses anastomose with one another forming secondary and tertiary sinuses, ultimately becoming larger collecting sinuses that drain into the very large diameter central sinus. Thereafter, blood flows out emissary veins through the cortical foramen to enter the systemic venous circulation. Hematopoiesis occurs in the intersinusoidal spaces, sometimes referred to as the hematopoietic cords. Newly formed mature blood cells penetrate the sinus wall to enter the blood. Images XII.A.002-004 show electron micrographs of the venous circulation.
Marrow Venous Circulation. The bony cortex has been stripped from a rat femur. The very large diameter central sinus is evident. The orifices of collecting sinuses can be seen opening into the central sinus. At the top of the central sinus one can see the transected outline of two sinuses fusing into a collecting sinus entering the central sinus. The marrow surrounding the central sinus shows the hematopoietic cords full of developing hematopoietic cells. Sinuses in longitudinal or cross section among the hematopoietic cords can be discerned, but will be shown in higher magnification in the subsequent images. (Scanning electron micrograph).
Marrow Venous Circulation. Enlargement of area of a collecting sinus entering central sinus shown in XII.A.002. Sinus orifices opening into the collecting sinus are evident. The collecting sinuses drain into the very large central sinus. It is in this marrow venous circulation that blood carrying newly formed blood cells enters the systemic circulation after leaving the marrow. At this magnification one can appreciate the individual marrow cells in the hematopoietic cords and some adventitial reticular cell fibers can be seen making up the stromal network. Empty spaces in the hematopoietic cords are artifacts resulting from cutting the surface open for microscopy. (Scanning electron micrograph)
Marrow Venous Circulation. Collecting sinus orifices opening into the central sinus are evident. The collecting sinuses drain into the very large ...