Bones: Structure and Types

Professor Dave here, let’s check out some
bones. We were first introduced to bones when we
learned about connective tissue, as bone falls into this category, but now it’s time to
get much more specific about the structure of bone, and take a look at all the types
of bones found in the body. Before diving into the bones that we are familiar
with, let’s start with the bits of skeletal cartilage that can be found in various regions
of the skeleton. We already learned about cartilage when we
discussed connective tissue, but let’s recall a few points. First, it contains a lot of water, which gives
it the ability to withstand tension and compression. Cartilage contains cells called chondrocytes
which sit in cavities called lacunae, inside of an extracellular matrix filled with ground
substance and fibers. There are three types, hyaline, elastic, and
fibrocartilage, and skeletal cartilage is comprised of all three. Hyaline cartilage makes up the most skeletal
cartilage, found in the nose, the ribs, the larynx, and the ends of most bones, providing
support. Elastic cartilage is more stretchy, found
in the ears, and the epiglottis, the flap that covers the opening of the larynx when
we swallow. Then, highly compressible fibrocartilage can
be found in areas that must withstand lots of pressure, like between the vertebrae of the spine. Now that we have the cartilage out of the
way, let’s take a look at bones. Bones can be placed into two categories, axial
and appendicular. Axial bones are found in the head and torso,
making up the spine, rib cage, and skull. Appendicular bones make up our appendages,
or limbs, those being the arms and legs, as well as the pelvis and shoulders. Bones can also be classified by shape, being
either long, short, flat, or irregular. Long bones are longer than they are wide,
like the ones in our limbs. Short bones are cubelike, found in the ankles
and wrists among other places. Flat bones are thin and often curved, like
the sternum and shoulderblades. And irregular bones are the ones that have
complicated shapes that don’t fit into the other three categories, like vertebrae and hip bones. What do bones do? Clearly the main function is support. The rest of the body essentially hangs on
the skeleton, supported by the bones as we stand and walk around. Also, organs are protected by bones, like
the heart within the rib cage. Bones act as levers that allow us to perform
physical tasks. Bones also provide mineral storage, like calcium
and phosphate, which can be released into the bloodstream as necessary, as well as fat
storage, hormone production, and blood cell formation. So what exactly are bones made of? How can we connect this image with what we
know about molecules and cells? First let’s make the distinction between
bone, a type of tissue, and an actual complete bone, which is an organ, because it is made
of several types of tissue. Most of a bone is made of bone, but there
is also nervous tissue, connective tissue, cartilage, and blood vessels. We will have to examine bones at a few levels
of complexity, starting with gross anatomy, meaning the part that is visible to the naked eye. The outer layer of any bone is made of compact
bone, which is very dense and smooth. Inside there is lots of spongy bone, which
is like a honeycomb of little needles. Typically the open spaces will be filled with
bone marrow, which we will discuss in a moment. The precise arrangement of compact and spongy
bone depends on the bone type. Short, irregular, and flat bones all consist
of thin plates of spongy bone covered by compact bone. There is no well-defined cavity for the marrow
to sit in, and hyaline cartilage covers portions of the surface that are involved with joints. Long bones are a little different. These contain a tubular shaft, called a diaphysis. This is made of a thick collar of compact
bone surrounding a medullary cavity, or marrow cavity. In adults, this cavity contains yellow bone
marrow which is high in fat. The ends of a long bone are called epiphyses. These parts do contain spongy bone inside
the compact bone, and again, cartilage covers the joint surface for cushion and stress absorption. Beyond the yellow marrow we mentioned, there
is also red marrow, which can be found inside the cavities of spongy bone, and this type
of marrow produces blood cells. We can also see an epiphyseal line, which
is a remnant of the epiphyseal plate, a disc of cartilage that grows during childhood,
which is how these bones get longer as a child gets taller. A white membrane called the periosteum covers
the exterior of the bone, consisting of an outer fibrous layer made of dense irregular
connective tissue, and an inner osteogenic layer, containing primitive stem cells. This membrane is attached to a network of
nerve fibers and blood vessels, which then pass through the shaft to the marrow cavity,
and perforating fibers connect the periosteum to the bone. Endosteum covers the internal spongy bone
layer, as well as canals that pass through the compact bone. In addition, the outside of a bone will display
markings, which can be projections that bulge out, or depressions and openings like fossae,
foramina, and grooves. Now that we have this view covered, let’s
zoom in a little more and check out the microscopic anatomy of a bone. We can find a few different types of cells
in here, so let’s go through each one. First, osteogenic cells. These are a type of stem cell that actively
divide, and they are found in the periosteum and endosteum that we mentioned. If the bone is growing, these are flattened
or squamous cells, and they can differentiate into other types at certain times. Next are osteoblasts. These are the ones that secrete the bone matrix
that consists of collagen and other proteins, meaning they are responsible for bone growth. These are also actively mitotic, and cube-shaped
while active. Once surrounded by matrix, they become our
next type of cell, osteocytes. These are mature bone cells that monitor and
maintain the bone matrix, communicating this information to other cells. Next are bone lining cells, which are flat
cells found on the surface of the bone. These also help maintain the matrix. And lastly, osteoclasts. These are large cells with multiple nuclei
that use enzymes to break down bone, which is a normal process called resorption that
releases minerals to be transferred to the blood. Now that we are familiar with the five types
of cells, let’s zoom in on a long bone and see what’s what. First let’s check out the compact bone in
the shaft. This is actually not solid all the way through,
there are many cylindrical units with open canals at the center, and each of these units
is called an osteon. If we were to pull one of these out of the
bone, we would see that it is made of a series of lamellae, which are hollow tubes, and these
are arranged like the rings of a tree trunk. Within each lamella are collagen fibers running
in a specific direction, with crystals of bone salts in between, and as we proceed inward,
the next lamella will have its fibers running in another direction, continuing in this fashion
all the way to the center. This alternating pattern is what gives compact
bone the ability to withstand torsion, or twisting force. The open region at the center is called the
central canal, and it contains blood vessels and nerve fibers that serve the cells in that osteon. There are also shorter canals running perpendicular,
allowing for connections to run all the way from the periosteum to the central canals
to the medullary cavity. Where the lamellae meet we can find tiny gaps
called lacunae, and these are filled with osteocytes. The lacunae are connected by extra-tiny canals
called canaliculi. Beyond the lamellae found within osteons,
there are others called interstitial lamellae, that fill in gaps between osteons, as well
as circumferential lamellae, which make up the circumference of the diaphysis, surrounding
all the osteons. Lastly, we can discuss the chemical composition of bone. There are organic components, which are all
the cells we discussed, as well as osteoid, which is the organic part of the matrix. This is made of ground substance and collagen
fibers, which are secreted by osteoblasts. These materials contribute to the structure,
flexibility, and tensile strength of the bone. There are also inorganic components inside
any bone, such as hydroxyapatites. These are needle-like crystals of calcium
phosphate surrounding the collagen fibers, which largely accounts for the hardness of bone. The organic and inorganic components work
together to ensure that bones are durable and strong without being brittle, and bones
are almost as good as steel at resisting tension and compression, which is pretty miraculous
when you think about it. Now that we know more about the structure
of bones, let’s zoom back out and take a look at how they come together to produce
an entire human skeleton.


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