NAME: EFE-ODENEMA KOBIRUO
MATRIC NO: 14/MHS02/022
DEPT: NURSING SCIENCE
COURSE CODE: ANA 203
HISTOLOGY OF HAVERSIAN SYSTEM
The osteon or Haversian system is the fundamental functional unit of much compact bone.
Osteons are roughly cylindrical structures that are typically several millimeters long and around
0.2mm in diameter. They are present in many bones of most mammals and some bird, reptile,
and amphibian species.
Histology of compact bone showing osteon
Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a
central canal, the Haversian canal. The Haversian canal contains the bone's blood supplies. The
sssboundary of an osteon is the cement line.
Between adjoining osteons, there are angular intervals that are occupied by interstitial lamellae.
These lamellae are remnants of osteons that have been mostly destroyed. Near the surface of the
compact bone, the lamellae are arranged parallel to the surface; these are called circumferential
lamellae. Some of the osteoblasts develop into osteocytes, each living within its own small
space, or lacuna. Osteocytes make contact with the cytoplasmic processes of their counterparts
via a network of small transverse canals, or canaliculi. This network facilitates the exchange of
nutrients and metabolic waste. Collagen fibers in a particular lamella run parallel to each other,
but the orientation of collagen fibers within other lamellae is oblique. The collagen fiber density
is lowest at the seams between lamellae, accounting for the distinctive microscopic appearance
of a transverse section of osteons. The space between osteons is occupied by interstitial lamellae,
which are the remnants of osteons that were partially resorbed during the process of bone
remodeling.
Osteons are connected to each other and the periosteum by oblique channels called Volkmann's
canals or perforating canals.
COMPONENTS OF HAVERSIAN SYSTEM
Each osteon has a cylindrical structure that consists of the following components:
✦ Haversian canals are located at the center. Small blood vessels that are present in the central
canal perform the function of supplying blood to the osteocytes. The central canal also contains
nerve filaments. These canals are connected to each other by Volkmann's canals, which are
transverse vessels that run perpendicular to the long axis. Volkmann's canals also connect the
blood vessels that are present in the central Haversian canals to the periosteum (dense tissue
covering the outer surface of the bone). Volkmann's canals are not surrounded by concentric
lamellae.
The Haversian canal is surrounded by the following structures:
✦ Lamellae are concentric rings of a strong bone matrix. Mineral salts and collagen fibers lend
strength to the matrix. The collagen fibers in a lamella in an osteon run parallel to one another,
but the collagen fibers run in different directions in the lamella that lies next to it. This
arrangement also strengthens the osteon. It is the presence of mineral salts such as calcium and
phosphates in the matrix that makes the bone hard, whereas collagen fibers strengthen the matrix.
✦ Between the lamellae are small
✦ Tiny channels called canaliculi connect the lacunae. Nutrients are supplied to the osteocytes
via these channels. Waste products also leave the osteocytes through these channels.
✦ The remnants of previous concentric lamellae can be found between the osteons. These are
referred to as interstitial lamellae.
Investigative applications
In bio archaeological research and in forensic investigations, osteons in a bone fragment can be
used to determine the sex of an individual and age, as well as aspects of taxonomy, diet, health
and motor history.
Osteons and their arrangement vary according to taxon, so that genus and sometimes species can
be differentiated using a bone fragment not otherwise identifiable. However, there is
considerable variability among the different bones of a skeleton, and features of some faunal
osteons overlap with those of human osteons; therefore, examination of osteons is not of primary
use in the analyses of osteological remains. More research is needed, but osteohistology has the
potential to positively affect the studies in bio archaeology, paleontology and forensic
investigations.
NOTE
It must be noted that osteocytes are living cells, which is why they need nutrition. The osteon
comprises several channels which facilitate waste disposal, as well as the supply of nutrients.
These canaliculi also help the cells stay connected, as they can make contact with the
cytoplasmic processes of other osteocytes through the canaliculi. Thus, the main function of this
system is to facilitate the supply of oxygen, as well as nutrients to the bone cells. It is the central
canal that is responsible for the nerve supply, as well as the vascular supply to the bone.
On a concluding note, the Haversian system or the osteon plays a vital role, as it facilitates the
supply of oxygenated blood and nutrients to the bones. Moreover, the network of canaliculi also
facilitates the passage of substances between the blood vessels and the mature bone cells.
A WELL LABELLED DIAGRAM OF HAVERSIAN SYSTEM
HISTOLOGY OF MUSCLE AS A TISSUE
MUSCLE
Contractility is a fundamental property of cells and the majority of them contain essentially the
same contractile machinery as that found in muscle cells. In muscle cells, however, a larger
proportion of the cells' resources are given over to this function than in other cell types.
Muscle function:
1. contraction for locomotion and skeletal movement
2. contraction for propulsion
3. contraction for pressure regulation
Muscle classification: muscle tissue may be classified according to a morphological
classification or a functional classification.
Morphological classification (based on structure)
There are two types of muscle based on the morphological classification system
1. Striated
2. Non striated or smooth.
Functional classification
There are two types of muscle based on a functional classification system
1. Voluntary
2. Involuntary.
Types of muscle: there are generally considered to be three types of muscle in the human body
Skeletal muscle: which is striated and voluntary
Cardiac muscle: which is striated and involuntary
Smooth muscle: which is non striated and involuntary
These are the three types of muscle:
Skeletal Muscle
Contractions move part of the skeleton. Also called 'voluntary' because usually its contractions
are under your control. Skeletal muscle cells are elongated or tubular. They have multiple nuclei
and these nuclei are located on the periphery of the cell. Skeletal muscle is striated. That is, it has
an alternating pattern of light and darks. Skeletal muscle: moves joints by strong and rapid
contractions
It has a stripy appearance, because of the repeating structure of the muscle: there are many
myofibrils (fibers), each one of which is made up of repeating units called muscle sarcomeres.
Each sarcomere is 2.5 m long. Can you work out how many sarcomeres are there (placed end to
end) in your biceps muscle, which approximately 25cm long? Skeletal muscle is designed as a
bundle within a bundle arrangement
Cardiac Muscle
Cardiac muscle cells are not as long as skeletal muscles cells and often are branched cells.
Cardiac muscle cells may be mononucleotide or binucleated. In either case the nuclei are located
centrally in the cell. Cardiac muscle is also striated. In addition, cardiac muscle contains
intercalated discs.
Cardiac muscle makes up the muscular walls of the heart (myocardium). It is 'involuntary'
because its contractions are not under your control. However, it has a similar ultrastructural
organisation to skeletal muscle. So, it too has a stripy appearance because of the repeating units
called muscle sarcomeres.
(click here to find out more)
Smooth Muscle
Smooth muscle cell is described as spindle shaped. That is, they are wide in the middle and
narrow to almost a point at both ends. Smooth muscle cells have a single centrally located
nucleus. Smooth muscle cells do not have visible striations although they do contain the same
contractile proteins as skeletal and cardiac muscle, these proteins are just laid out in a different
pattern.
Found in the walls of most blood vessels and tubular organs such as the intestine. It is also
'involuntary'. However, it does NOT have a stripy appearance, because it does not have repeating
sarcomeres. The contractile proteins, myosin and actin are much more randomly arranged than in
skeletal or cardiac muscle.
NOTE
Skeletal Muscle and Cardiac Muscle are also called 'striated muscle', because they have dark and
light bands running across the muscle width when they are looked at under the microscope.
Confusingly the prefixes myo- and sarco- (respectively from the Latin and Greek, both meaning
muscle) are often used when naming structures and organelles associated with muscle.
Thus the plasma membrane of muscle cells is sometimes called the sarcolemma and their
cytoplasm sarcoplasm.
Their endoplasmic reticulum is called sarcoplasmic reticulum and their mitochondria are
sometimes called sarcosomes.
The contractile fibres that lie in the sarcoplasm are known as myofibrils and the embryonic
precursors of skeletal muscle cells are called myoblasts.
The sarcolemma is the name given to the plasma membrane of the muscle cell. There are
specialized invaginations of the sarcolemma that run transversely across the cell. These
invaginations are known as T tubules (short for transverse tubules). The T tubules are essential
for carrying the depolarization brought to the cell by a motor nerve impulse down into the
muscle cell where it can have an affect on the terminal cisternae. We will cover more about this
in the unit on the physiology of muscle contraction.
The cytosol is the cytoplasm of the muscle cell.
The sarcoplasmic reticulum is the endoplasmic reticulum of the muscle cell. There are sac-like
regions of the sarcoplasmic reticulum known as terminal cisternae. The terminal cisternae act as
calcium storage sites. The calcium ions stored in the terminal cisternae are essential in muscle
contraction. We will cover more about this in the unit on the physiology of muscle contraction.
NOTE: this is not calcium storage for use in general body physiology as we would see with bone
tissue, but rather is calcium storage for muscle contraction.
Muscle: Cell Junctions
There are several kinds of cell-cell junctions. A description of these can be found here.
Cardiac cells are special, amongst the muscle types, because they are connected to each other by
intercalated discs - structures that are only found in cardiac muscle cells. These can be seen in
this diagram, as darkly staining irregular lines, at 90 degrees to the striped sarcomeric pattern.
Intercalated discs contain three different types of cell-cell junctions:
1. Fascia adherens junctions (anchoring junctions)
o where actin filaments attach thin filaments in the muscle sarcomeres to the cell
membrane.
2. Expanded desmosomes
o
sites of strong adhesion, that help to keep the muscle cells connected when they
contract.
3. Gap junctions
o large and small, which provide direct contact between the cardiac cells, facilitating
electrical communication, so that waves of depolarization spread rapidly over the entire
heart, by passing from cell to cell.
Skeletal muscle does not have any cell-cell junctions.
Smooth muscle contains gap junctions, to allow a rapid spread of depolarization, as in cardiac
muscle.
Muscle: Muscle regeneration
Skeletal muscle contains numerous 'satellite cells' underneath the basal lamina, as shown in the
photograph opposite. These are mononucleated quiescent cells. When the muscle is damaged,
these cells are stimulated to divide. After dividing, the cells fuse with existing muscle fibres, to
regenerate and repair the damaged fibres.
The skeletal muscle fibres themselves, cannot divide. However, muscle fibres can lay down new
protein and enlarge (hypertrophy).
Cardiac muscle can also hypertrophy. However, there are no equivalent to cells to the satellite
cells found in skeletal muscle. Thus when cardiac muscle cells die, they are not replaced.
Smooth cells have the greatest capacity to regenerate of all the muscle cell types. The smooth
muscle cells themselves retain the ability to divide, and can increase in number this way.
As well as this, new cells can be produced by the division of cells called pericytes that lie along
some small blood vessels.
Smooth muscle can also hypertrophy.
Muscle: Stimulation
This picture shows nerve fibres approaching skeletal muscle fibres, and forming a neuromuscular
junction. The structure of the neuromuscular junction is described in more detail here.
Skeletal muscle is stimulated via a nerve impulse, which depolarizes the muscle. However, not
all the muscle fibres in the muscle fibre will necessarily be activated at once. Sometimes, a
subset of muscle fibres is activated, depending on how much force is needed.
When the muscle is stimulated, calcium ions are released from its store inside the sarcoplasmic
reticulum, into the sarcoplasm (muscle).
Then the calcium ions bind to a protein called troponin, on the thin filaments, which in turn
allows myosin to bind to actin. This interaction makes the thick and thin filaments slide past each
other, to make the muscle shorten.
Invaginations of the plasma membrane (sarcolemma) of the muscle fibres are called T (or
transverse) tubules. The T-tubules lie over the junction between the A- and I-bands (see
diagram).
The two terminal cistemae of the SR together with their associated T tubule are known as a triad.
Inside the muscle fibre, the T-tubules lie next to the terminal cisternae of an internal membrane
system derived from the endoplasmic reticulum, called the sarcoplasmic reticulum (SR), which
is a store of calcium ions. Stimulation of the muscle fibre, causes a wave of depolarisation to
pass down the t-tubule, and the SR to release calcium ions into the sarcoplasm. Calcium is
pumped back up into the SR to lower calcium ion concentration in the sarcoplasm, to relax the
muscle (turn off contraction).
Cardiac Muscle also has T-tubules, and SR. However the T-tubules lie over the Z-line in
cardiac muscle, are less numerous and wider. The SR is smaller and less elaborate, and stores
less calcium ions. Cardiac muscle cells also depend on extracellular calcium ions, that enter
through the T-tubules and triggers release of calcium ions from the SR.
Cardiac muscle cells are electrically connected through gap junctions, so that waves of electrical
stimuli pass around the heart from cell to cell, and all the cells are stimulated to contract.
Smooth Muscle. The thick and thin filaments are attached to alpha-actinin in dense bodies
(equivalent to Z-lines in skeletal muscle), which are attached to the plasma membrane by
intermediate filaments. The thin filaments do not have troponin. This type of muscle responds to
a increase in calcium, following nerve stimulation through a protein called calmodulin. Binding
of calcium to calmodulin, results in the activation of an enzyme (myosin light chain kinase) that
phosphorylates myosin, which activates it, enabling it to interact with actin.