The Spongy Bone And Medullary Cavity

Organ 2

Organ 2

The interior of a long bone is displayed back in Figure 7.3 (B). Very prominent within the ''middle'' (medull) of the bone shaft (diaphysis) is the medullary (MED-you-lair-ee) or marrow cavity. The medullary (marrow) cavity is somewhat parallel to the central storage cavity found in the middle of a long Thermos bottle (Figure 7.3, C). Further, we can pretend that our Thermos bottle has its central storage cavity lined with an insulating membrane. In the case of the actual bone's medullary cavity, however, we have the endosteum

  • end-AHS-tee-um). This is defined as a thin membrane "present" (-um) deep ''within'' (endo-) the ''bone'' (oste), as the lining of the medullary (marrow) cavity.
  • Okay, so what's all this marrow cavity stuff, about?'' the involved reader may ask at this point. The medullary cavity is alternately called the marrow cavity, because it is filled with yellow marrow. Named for its yellowish color, the yellow marrow chiefly consists of thousands of adipocytes. Your memory may serve to remind you (Chapter 6) that adipocytes are ''fat cells'' that make up adipose connective tissue. Therefore, the yellow marrow has an energy-storing physiology for the long bone.

There are actually two types of marrow in a long bone. Remember the red marrow that was mentioned earlier, located within the spongy bone tissue of a flat bone ''sandwich'' in the skull (see Figure 7.2).

For long bones like the femur, most of the spongy bone tissue is located within the bone ends or epiphyses. Remember that spongy bone is also called cancellous bone. The word, cancellous, translates as ''pertaining to'' (-ous) a ''lattice'' (cancell). In spongy or cancellous bone tissue, then, there is a crisscrossing latticework of slender bone trabeculae (trah-BEK-yuh-lie) - ''little beams'' of hard bone tissue. The dark holes or gaps in between the latticework of these hard, white, beam-like trabeculae give this type of tissue the gross appearance of a sponge.

The reason that red bone marrow is colored red, of course, is that it largely consists of a soft, pulpy meshwork of red-colored blood vessels! These blood vessels of the red bone marrow snake in and out of the holes within the latticework of slender white trabeculae of spongy bone. The major function of red bone marrow is hematopoiesis (he-muh-toh-poy-EE-sis), which is the process of ''blood'' (hemat) ''formation'' (-poiesis). Specifically, hematopoiesis is the process of forming red blood cells, white blood cells, and blood cell fragments. The blood cells start off in the red marrow of spongy bone tissue, but they don't stay there. Instead, they circulate out of the long bone through its surface foramina (holes), within blood vessels that eventually join the general bloodstream.

Tissue 1

Tissue 1

Tissue 2

Tissue 2

Bone marrow summary

Let us make an overall summary of the two types of bone marrow:

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