What is compact bone and cancellous bone?

The bones are made up of woven bonea type of specialized connective tissue characteristic of vertebrate animals.

In each bone we can find two types of bone tissue, the compact or dense bone tissuehighly calcified and poorly vascularized, and the cancellous or areolar bone tissueless calcified and much more vascularized.

Next we will see the structure, characteristics and functions of compact bone and cancellous bone, and then make a summary with the main differences between them.

general bone structure

Structure of compact and spongy bone tissue
Structure of compact bone and cancellous bone

The woven bone is made up of a set of specialized cells (osteoblasts, osteocytes and osteoclasts) dispersed in a calcified extracellular matrix called bone matrix.

The osteoblasts are the cells that synthesize the bone matrix, osteocytes they are mature osteoblasts with the capacity to synthesize (formative osteocytes) and reabsorb matrix (resorptive osteocytes), and the osteoclasts are the cells responsible for destroying bone matrix (bone resorption).

In addition to these cells of the bone tissue, immune cells also appear in the tissue.

The bony structure of mature bone is called lamellar bone. Histologically, lamellar bone consists of groups of concentric lamellae of calcified bone matrix.

Each group of lamellae form cylindrical structures called osteons or the Haversian system, considered the functional unit of bone.

In the center of the lamellae appears a duct, known as haver's duct, through which blood vessels, nerves and lymphatic vessels run longitudinally. Between the lamellae appear gaps that host the osteocytesall of them connected through the bony canaliculi.

The lamella system is similar in compact bone and in the cancellous bonealthough in the spongy bone tissue, as we will see below, true osteons do not appear and the calcification of the collagen fibers is partial, forming trabeculayes

Inside the long bones, vertebrae, ribs, sternum, skull bones, shoulder girdle and pelvis, another type of tissue appears, the bone marrowwhere takes place hematopoiesis.

Unlike bone tissue, bone marrow is a flexible tissue. From compact bone tissue to bone marrow, the lamellae system gradually transforms into a much more porous and vascularized trabecular system, with intermingled bone marrow and calcified tissue.

The transition between compact bone, cancellous bone, and bone marrow is diffuse and gradual. There is no clear boundary between them.

compact bone tissue

The compact bone tissuealso called dense bone tissueas we have seen, is made up of true osteons and highly calcified bone tissue. It appears in all types of bones.

In long bones it appears mainly in the external zone forming the diaphysis. In flat bones it is located inside and outside. In short bones, the location of compact bone tissue is more variable.

The osteons appear highly packagedintimately united with each other, which, together with the high calcification, makes the compact bone tissue a very hard tissue.

The lamellae and Haversian canal are easily distinguished microscopically. Canals connecting the Haversian canals of adjoining osteons, called Volkmann's canals, as well as the bony canaliculi connecting the osteocyte lacunae are also discernible.

Compared with cancellous bone tissue, it is much harder and heavier. At the macroscopic level, it has the appearance of a solid mass with eminently structural function. Compact bone forms around the 80% of the human skeleton.

cancellous bone tissue

Bone Anatomy: Compact Bone, Spongy Bone, and Bone Marrow
Bone anatomy: location of compact bone, cancellous bone, and bone marrow in a long bone.

The cancellous bone tissuealso called areolar or trabecular bone tissuehas a similar structure, but the collagen fibers and the extracellular matrix are calcified in an interrupted way, forming trabeculaewhich gives rise to a very porous and vascularized.

The pores of cancellous bone tissue, called trabecular cavities, contain bone marrow (popularly called marrow). Here takes place the hematopoiesis.

From hemocytoblasta multipotential stem cell, two major cell lines arise, the red serieswhich gives rise to the formed elements of the blood (erythrocytes, platelets), and the white series or leukocytes, which gives rise to all the cells of the immune system.

The greatest amount of spongy bone tissue appears in the epiphyses of long bones, such as the femur, and in short, flat, irregular bones, such as the pelvis, ribs, or sternum. In the adult, these areas of spongy bone tissue constitute the largest reservoir of bone marrow.

The trabeculae, although they may appear to be irregularly arranged, follow the stress lines of the bone, contributing to the strength of the tissue.

Main differences between compact and cancellous bone

Bone is a calcified organ that forms the skeletonessential in the musculoskeletal system of the human being.

But in the bone also takes place the hematopoiesisthe process by which blood cells and cells of the immune system are formed.

The greater mechanical strength of bone is due to the structure of the compact bone tissuemade up of osteons. A system of highly packed calcified lamellae.

The hematopoiesis happens in the bone marrowwhich in the adult appears mainly in the trabecular spaces of the cancellous bone tissue.

From a histological point of view, there is no clear separation between both types of bone tissue, but rather a gradual transition between compact bone and bone marrow, with areolar or spongy bone appearing in between.

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