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PRACTICAL HISTOLOGY
LAB.5
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Bone
Bone is a strong, flexible and semi-rigid supporting tissue. It can
withstand compression forces, and yet it can bend. Like
cartilage, and other types of connective tissue, bone is made up
of Cells and Extracellular matrix:
Cells - which in bone are called osteoblasts and osteocytes,
(osteo - bone). There are also two other cell types:
osteoprogenitor cells and osteoclasts.
What is bone for?
Support - bones make up a structural framework for the .1
body, and provide attachment sites for muscles.
Protection - protection of internal organs - i.e. brain, heart .2
and lungs,
Assisting movement. .3
Mineral homeostasis - the bone is a store for calcium and .4
phosphorus
Blood cell production - takes place in the bone marrow. .5
Types of bone:
There are two types of mature bone:
1. Compact - which is found in the shafts of long bones (in the
diaphyses). This makes up 80% of all bone.
2. Spongy (cancellous) bone - which is found at the ends of
long bones (in the epiphysis). This makes up 20% of all bone.
This type of bone contains red bone marrow and a network of
bony trabeculae.
A 'periosteum' is found on the outside of bone. This is a dense
fibrous layer, where muscles insert. It contains bone forming
cells. It is not found in the regions of bone covered by articular
cartilage.
The endosteum is the name given to the tissue that lines the
inner surfaces of bones.
Lymphoid Tissue
Lymphoid tissue covers all of the various tissues that are
important in mounting an immune response. This includes
discrete organs such as the spleen, thymus and lymph nodes, as
well as more diffuse aggregations of lymphocytes.
A fluid called lymph, (lymph = clear fluid) flows in lymphatic
vessels, lymphatic tissue and red bone marrow.
Fluid filters out of capillaries and drains into lymphatic vessels
to become lymph.
The content of lymph is the same as interstitial fluid, the fluid
around tissue cells.
Lymph eventually drains into venous blood.
Lymph drains interstitial fluid, transports dietary lipids and
facilitates immune responses.
Primary and Secondary Lymphoid Tissues
Primary lymphatic organs are where lymphocytes are formed
and mature. They provide an environment for stem cells to
divide and mature into B- and T- cells:
There are two primary lymphatic organs: the red bone marrow
and the thymus gland.
Both T-cell and B-cells are 'born' in the bone marrow.
However, whereas B cells also mature in the bone marrow, Tcells have to migrate to the thymus, which is where they mature
in the thymus.
Secondary lymphoid tissues are arranged as a series of filters
monitoring the contents of the extracellular fluids, i.e. lymph,
tissue fluid and blood. The lymphoid tissue filtering each of
these fluids is arranged in different ways. Secondary lymphoid
tissues are also where lymphocytes are activated.
These include: lymph nodes, tonsils, spleen, Peyer's patches
and mucosa associated lymphoid tissue (MALT).
Tonsils are large partially-encapsulated masses of lymphoid
tissue, found in the walls of the pharynx and nasopharynx, and
at the base of the tongue. They form an incomplete ring around
the gastrointestinal and respiratory tracts, where they cross over.
Peyer's patches are large masses of confluent lymphoid
follicles, found in the walls of the ileum, part of the small
intestine.
The spleen is a large encapsulated organ in left upper part of
abdomen, the outer capsule is fibro-elastic.