∞ generated and posted on 2018.09.01 ∞

The skeleton consists of bones which interact at articulations, where they are held together by ligaments, and which are manipulated spatially through the action of skeletal muscles via tendons.

The Skeleton of the adult human body possesses over 200 bones which individually consist of connective tissue that is associated with highly mineralized ground substance.

This page contains the following terms: Ground substance, Fibroblasts, Bone, Periosteum, Osseous tissue, Diaphysis, Compact bone, Trabeculae, Spongy bone, Bone marrow, Osteocytes, Haversian canal, Osteoblasts, Osteoclasts, Mesenchymal stem cells, Ossification

The above video provides a nice introduction to bones, their physiology, structure, and, to a somewhat lesser extent, your skeleton.

Ground substance

Extracellular matrix other than fibers that is associated with connective tissue.
Ground substance typically is gel like, consisting especially of carbohydrate or carbohydrate-associated polymers (polysaccharides) that draw water to themselves particularly by displaying electrical charges (that is, they are ionized and as a consequence are well hydrated). These polymers go by the names such as glycoproteins, glycosaminoglycans, and proteoglycans (where the "glyc-" refers to the carbohydrate component).

This drawing of water to themselves results in these substances taking on the relative incompressibility of water. That is, squeeze ground substance and the water both resists being compressed and resists squirting out to the side because it does not want to leave the charged carbohydrates making up the ground substance. This property is seen especially with cartilage.

Additional, less gel-like ground substances exist. Bone, for example, represents a solid, that is, crystalline ground substance. Blood, on the other end of the spectrum, possesses a liquid ground substance (i.e., plasma). Note that the definition I use here for "Ground substance" may be more easily read as "Extracellular matrix, other than fibers, that is associated with connective tissue."

Links to terms of possible interest: Connective tissue, Ground substance, Protein fibers


Cell types that secrete the extracellular matrix of connective tissue.
Fibroblasts are the dominant cells found within most connective tissue. As connective tissue in many cases is defined by its extracellular matrix, the fibroblasts are involved basically with the formation, maintenance, and repair of connective tissue.

As part of this role, fibroblasts also play important roles in wound healing, which involves in many cases repair not just of connective tissue but also replacement of other tissues instead with connective tissue (what we call scar tissue). When fibroblasts are less active, that is, when they are not actively creating new connective tissue, then they are described instead as fibrocytes.

Links to terms of possible interest: Chemokines, Connective tissue, Cytokines, ECM, Fibroblast, Ground substance, Growth factors, Myofibroblast

The above video provides an outline of what fibroblasts are all about.


Rigid, ossified connective tissue found within animals possessing endoskeletons.
Bone in most vertebrates is ossified, that is, has been hardened via the deposition of minerals. These minerals consist primarily of what is known as calcium hydroxyapatite, which can be considered to be bone ground substance (versus collagen, which is present as well but which is fiber rather than ground substance).

Bone often is associated with bone marrow, from which blood cells originate, and bone also is associated with less rigid surrounding connective tissue (periosteum) that serves to further strengthen bone as well as afford some flexibility.

The ossification of bone occurs upon softer connective tissue "forms", as does body development generally. Certain fish, described as cartilaginous, e.g., sharks and rays, mostly lack this ossification and thereby technically also lack bones, though they don't lack endoskeletons nor vertebral columns. Our own bodies possess various skeleton-like parts that also are lacking in ossification (e.g., the cartilage supporting our nose). The mineralization of bones occurs upon a cartilage-like organic matrix which serves a similar function in bone as steel reinforcement can serve in concrete.

Bones are connected together via ligaments, which also represent connective tissue, and muscles are attached to bone via yet another form of connective tissue, the tendons. Bones come together at joints, most of which can move relative to each other. Above all, bone is living tissue that is able to grow, repair itself, and remodel itself in response to changes in how the bone is used, for example, such as during weight training.

Links to terms of possible interest: Carpel, Flat bone, Femur, Irregular bone, Long bone, Parietal bone, Short bone, Skull, Sphenoid bone

The above video provides a nice introduction to the variation in what types of bones are present in our bodies.

Links to terms of possible interest: Blood vessels, Bone marrow, Compact bone, Long bone, Red marrow, Spongy bone, Yellow marrow

The above video provides a nice overview to the very beginnings of learning the names of bones as well as features associated with individual bones.


Connective tissue surrounding bones.
Bones are encased in two-layer irregular dense connective tissue called periosteum. The outer layer contains fibroblasts, the cells that give rise to connective tissue, while the inner layer contains osteoblasts, which are the cells that give rise to bone.

Osseous tissue

Living material making up the compact and spongy, rigid aspects of bones.
Osseous tissue consists of a combination of connective tissue cells (osteoblasts), collagen (specifically what is known as type-I collagen), and deposited mineral (i.e., calcium hydroxyapatite). Osseous tissue is rigid but not excessively brittle. Osseous tissue also is capable of both growing and repairing itself.


Links to terms of possible interest: Articular cartilage, Bone marrow, Cancellous bone, Compact bone, Diaphysis, Endosteum, Epiphyseal plate, Epiphysis, Hematopoiesis, Long bone, Marrow cavity, Perforating fibers Periosteum, Sharpey's fibers, Spongy bone, Trabeculae, Woven bone

The above video provides a nice introduction to long-bone anatomy.

This video titles itself, "Gross anatomy of bone" and focusses on that of long bones.


The shaft, non-end portion of long bones.
The diaphysis is tube-shaped mid-section of long bones, consisting of compact bone in its exterior but relatively little spongy bone in its interior. Instead, the interior consists of bone marrow. This is predominantly red bone marrow at younger ages and predominantly yellow bone marrow at older ages. Basically it is the presence of the diaphysis that makes long bones long.

Compact bone

Rigid and somewhat densely ossified connective tissue found within animals possessing endoskeletons.
Compact bone, though dense, is not solid but instead is filled with microscopic channels which allow movement of interstitial fluid. Compact bone in addition possesses larger channels known as Haversian canals through which blood vessels and nerves are routed. Nutrients, oxygen, and wastes consequently can penetrate relatively easily into compact bone despite its macroscopically solid appearance.

Compact bone makes up the portions of bones that contribute substantially to their rigidity, durability, and weight-bearing ability, but nonetheless compact bone represents only a portion of bones. In addition, compact bone is heavy, since ossified tissue is heavier than other body materials, and therefore compact bone is employed only sparingly in bones, that is, its production is avoided to the extent that local or total body mass is an issue.

Compact bone also does not possess bone marrow directly within its substance, so represents volume within bodies that has limited function other than serving in a body-supporting capacity.

Links to terms of possible interest: Blood vessel, Bone marrow, Canaliculus, Cancellous bone, Compact bone, Lamellae, Osteoblast, Osteoclast, Osteocyte, Spongy bone, Trabeculae


Strut- or beam-like connective tissue elements.
Trabeculae are found in many locations within the body, taking up space and/or providing rod-like rigidity. They serve, for example, as the osseous tissue of spongy bone.

Links to terms of possible interest: Canaliculus, Compact bone, Endosteum, Lacuna, Osteocyte, Osteon, Osteonic canal, Perforating canal, Periosteum, Spongy bone

Spongy bone

Rigid and somewhat sparsely ossified connective tissue found within animals possessing endoskeletons.
Spongy bone is superficially sponge-like in its structure, that is, possessing a structure that is full of holes and passages. Here, though, that structure is ossified.

Unlike an actual sponge (which is a kind of animal), the volume found within this sponge-like structure is filled with other tissues rather than with water. This other tissue predominantly is bone marrow, which is another form of connective tissue, though bone marrow is not found exclusively within the confines of spongy bone but instead also in non-boney volumes within bones called marrow cavities (i.e., medullary cavities).

Spongy bone adds to the strength and rigidity of bone, though less so than compact bone adds to that strength and rigidity. Spongy bone, however, is not as heavy as compact bone. With spongy bone an animal thus can possess dual use bone material that also is lighter, allowing the formation of larger bones without incurring as great a penalty, in terms of weight and versatility, as is the case when and where compact bone is used more extensively.

Links to terms of possible interest: Bone marrow, Cancellous bone, Compact bone, Spongy bone, Trabeculae, Trabecular bone

Bone marrow

Soft connective tissue found within bone and used for generation of blood cells as well as for energy storage.
Bone marrow can be differentiated into two types, red bone marrow and yellow bone marrow. It is from red bone marrow that blood cells are formed via mitosis followed by maturation and differentiation. This type of bone marrow is found predominately within spongy bone.

Yellow bone marrow serves as fat storage. Fat, as found within adipose cells is both a compact and lightweight energy storage molecule. Fat possesses both of these properties basically because fat is not hydrated – that is, does not draw water to itself – and also fat itself is less dense than water (oil as a kind of fat, for example, tends to float upon water rather than water upon it).

This means that in the limited space found within bones a relatively large amount of energy can be stored, as fat. Also, by storing fat there rather than denser or heavier materials, the bones themselves, as a whole, are lighter than they otherwise would be and therefore can more rapidly change direction, allowing for more effective as well as efficient movement of the body.

Links to terms of possible interest: Bone marrow, Cartilage, Medullary cavity, Red marrow, Spongy bone, Yellow marrow

The above video has poor sound quality but nonetheless does a good job of describing what bone marrow is all about.


Mature cells responsible for maintaining the mineralized ground substance making up bone.
Osteocytes mature from osteoblasts. Both are relatively metabolically inactive and are found trapped within mineralized bone. They are responsible for "turning over" bone, that is, breaking down as well as building up bone as needed.

Due to this ongoing remodeling, bone can be strongest or heaviest where such strength is most needed (i.e., where bone is most stressed in the course of normal activity) and lighter where such strength is less needed. Bone, that is, is dynamic in terms of its structure and this dynamism occurs due to the action of osteocytes.

Since osteocytes are trapped within bone, are required for normal bone functioning, and are not readily replaced, they are long-lived cells, basically surviving as long as the bone surrounding them survives.

Links to terms of possible interest: Compact bone, Haversian canal, Osteoblast, Osteocyte, Osteon

The above video illustrates the functioning of osteoclasts and osteoblasts along with the formation of osteocytes.

Haversian canal

Capillary- and nerve cell- containing structure found within normal mineral deposits within bone.
The Haversian canal or associated Haversian system is a basic functional unit of compact bone, representing the product of layered depositing of calcium hydroxyapatite, the mineral making up bone. This layering occurs around existing nerves and capillaries.

Links to terms of possible interest: Cancellous bone, Compact bone, Haversian canal, Inner circumferential lamellae, Interstitial lamellae, Osteon, Outer circumferential lamellae, Trabeculae, Volkmann's canals

The above video goes through a lot more than just Haversian canals, or the more general Haversian systems as the video describes them, but nonetheless sets things up well for giving a good sense of what Haversian canals are all about.


Metabolically active cells responsible especially for building up new bone in the course of ossification.
Chondroblasts are responsible first for synthesizing a form of cartilage described as organic matrix that serves as bone precursor. Osteoblasts are responsible for the mineralization of that cartilage into bone.

Osteoblasts as well as osteoblast derivatives (i.e., osteocytes) serve as the cellular aspect of bone versus the extracellular bone matrix consisting of protein and mineral. One can distinguish bone-building cells in terms of their locations, with osteoblasts located on surfaces and not trapped within mineral whereas osteocytes are found within the interior of mineral and are trapped. Osteocytes, however, remain in contact via cytoplasmic extensions with osteoblasts.

Osteoblasts themselves do not divide but instead arise from cells (mesenchymal stem cells) that are associated with periosteum, which is the non-mineralized connective tissue that surrounds bone, as well as from associated bone marrow.

Links to terms of possible interest: Bone homeostasis, Bone marrow, Cellular differentiation, Osteoblast, Osteoclast,

This video does a good job a cataloguing the numerous roles played by osteoblasts in bone health.


Metabolically active cells responsible for breaking down bone in the course of bone resorption.
Bone is a dynamic tissue that is both ongoingly built up (ossification) and ongoingly broken down (bone resorption). Osteoblasts are responsible for ossification on bone surfaces and osteoclasts are responsible for bone resorption in similar locations (the trapped osteocytes, by contrast, effect both processes).

Unlike osteoblasts, osteoclasts possess multiple nuclei rather than having only a single nucleus. They exist within pits in bones where bone resorption takes place and possess substantially folded membranes, which facilitate the uptake of the bony matrix they are actively eroding. The erosion itself is effected by reducing the pH immediately proximate to the bone, which results in a literal dissolving of the calcium hydroxyapatite (i.e., just as the acidity of vinegar can be used to dissolve calcium deposits associated with hard water).

Also unlike osteoblasts, osteoclasts form via the fusion of immune system cells called macrophages, which themselves are formed from precursors that form within bone marrow. This osteoclast origin is fitting as osteoclasts need to seek out areas of bone that require resorption, i.e., microfractures, just as macrophages also are involved in seeking out specific locations within bodies within which they function.

Links to terms of possible interest: Bone resorption, Bone remodeling, Cell-cell fusion, Multinucleated cells, Osteoclast

The above video arguably goes into far too much detail (unless you are interested in that detail), but does start out with a nice, more general overview of what osteoclasts are all about.

Cleaning calcium-salt hard water deposits using vinegar. This is a demonstration of the ability of acids to dissolve calcium-based minerals.

Links to terms of possible interest: Endosteal sinus, Monocyte, Osteoblast, Osteoclast, Osteocyte, Osteoid, Stem cells

Mesenchymal stem cells

Precursor cells to adipocytes, chondrocytes, and osteoblasts/osteocytes.
Mesenchymal stem cells were originally discovered in association with primitive, poorly differentiated connective tissue known as mesenchyme (tissue also from which cancers known as sarcomas are derived). As stem cells they are multipotent but not pluripotent, that is, they are limited in what sorts of cells they are able to give rise to. Notably, they are not hematopoietic, that is, they do not give rise to blood cells (where blood is yet another form of connective tissue).


The building up of new bone material during bone growth, repair, or remodeling.
Bone formation during bone growth is a two-step process that involves first the laying down of a cartilage-like organic matrix and then the mineralization of that matrix with calcium hydroxyapatite, with the second of these two processes described as ossification. The first process is effected by connective tissue cells called chondrocytes (trapped cartilage-producing cells) and the second by both osteoblasts and their derivative osteocytes.

The actual mineralization process involves a partial dissolving of the already laid down cartilage, as well as the cellular suicide (apoptosis) of associated chondrocytes, though with sufficient collagen remaining to serve as the organic matrix of the resulting bone.

The growth of long bones occurs in length near their ends, beneath sites called epiphyses, specifically within epiphyseal growth plates, though bone growth in terms of breadth can occur as well.

Links to terms of possible interest: Bone formation, Bone resorption, Cartilage, Chondrification, Chondrocytes, Collagenase, Macrophage, Mesenchymal stem cells, Ossification, Osteoblasts, Osteoclasts, Osteogenesis,

Links to terms of possible interest: Articular cartilage, Bone collar, Chondrocytes, Compact bone, Endochondral ossification, Epiphyseal plate, Hyaline cartilage, Intramembranous ossification, Medullary cavity, Mesenchymal stem cells, Ossification, Ossification center, Osteoblasts, Osteoid, Periosteum, Primary ossification center, Secondary ossification center, Spongy bone

The above video provides an overview of endochondral ossification, i.e., bone growth as this mostly occurs within the body.

The above video is not flashy, and can be a little hard to follow, but is short while still relatively comprehensive.

Intramembranous ossification: Wow, what an awesome video on the basics of ossification!

More on Intramembranous ossification, the means of formation, in utero, of cranial bones and the clavicle, that is, flat bones.