Istologia - tessuto connettivo

It’s however very abundant in animals that undergo hibernation, since it’s specialised in

obtaining heat and not energy. The quantity of this tissue in our body can be modified depending

on climatic condition, since in cold areas humans have more brown tissue. Brown tissue can be

transformed in white tissue and viceversa.

The pink adipose tissue is suggested to derive from white or brown adipocytes and to

differentiate, during pregnancy and lactation, into milk-producing epithelial cells. It’s therefore

typical of the mammary gland.

Histology - Cartilage

Cartilage is a specific connective tissue specialised in supportive functions. It’s characterised by

the presence of big molecules in the ECM called proteoglycans. The cartilage contains both cells

and ECM, as all connective tissue. Cells are chondroblasts or chondrocytes, depending on

whether they’re active in positioning of cartilage or not. ECM is made of fibrillary components

(mainly collagen type II) and ground substances. There are no blood vessel nor nerves in the

cartilage. It’s always surrounded by another connective tissue, the perichondrium, containing

the vessels and the nerves. Only articular cartilage is not surrounded by this tissue. The absence

of blood vessel means that the tissue receives nutrients from other sources and as a

consequence, in adults, cartilage can’t repair itself.

In the embryo, the skeleton is made only by cartilage, which gives the structure for bone

production that replaces it. During growth, where long bones have to form, there is cartilage to

guide the bone development. This cartilage is called “metaphyseal cartilage”. It’s present up to

21 years-old. Cartilage is also present where it provides a smooth surface to allow sliding, which

is called articular cartilage. It’s also present in some areas such as the nose, the ears and so on.

Cells of the connective tissue derive from the mesenchymal cell.

Chondroblasts are cells that are very active in the production of the ECM. They’re evident where

they produce new cartilage in a layer of the perichondrium called chondrogenic layer. When

these cells remain in the ECM, they modify their phenotype and become chondrocytes, able to

maintain the ECM but not as able as the chondroblasts in producing new cartilage. These cells

are grouped in isogenous groups. In the perichondrium, fibroblasts can be found. Chondrocytes

group in isogenous groups because they live in a solid ECM and therefore when they divide they

remain one close to the other. All cells of an isogenous group derive from the same cell through

mitosis. Isogenous groups are characterised by 2-4 cells close one to another and surrounded by

a less dense cartilage ECM.

The production of cartilage, called chondrogenesis, follows two possible methods: the

appositional growth and the interstitial growth. The appositional growth means that cartilage is

added to the surface of pre-existing cartilage; fibroblasts of the perichondrium divide and then

differentiate in new chondroblasts. In the interstitial growth, cartilage is produced through the

division of chondrocytes already present in the cartilage and by ECM production.

The articular cartilage is particular since it lacks the perichondrium. It derives the nutrition from

the synovial fluid in the joint capsule. The main problem is that articular cartilage is

continuously consumed by movements and cannot be repaired due to the lack of perichondrium.

Only the interstitial growth is possible in this type of cartilage.

Cartilage is specialised in resisting both tensile and compressive forces. It can be therefore

compared to reinforced concrete, made by mortar and iron together. In cartilage, mortar is

equivalent to proteoglycans while iron is equivalent to collagen. When cartilage is compressed,

it works as a sponge. It releases the water inside the ECM and when the compression ends, the

cartilage absorbs water and goes back to the original shape and dimension. Cartilage can be

classified in three groups: hyaline cartilage, elastic cartilage and fibro cartilage.

Hyaline cartilage is a glassy tissue, containing scarce collagen fibres. This is the cartilage that

provides the embryo’s skeleton and in adults it’s located in the main joints. It also supports the

organs of the respiratory system, such as the trachea, and is surrounded by perichondrium

(except the articular cartilage).

Elastic cartilage has a yellowish colour and is characterised by abundant elastic fibres running in

all directions. It’s flexible and can be found in regions continuously stimulated and deformed by

external forces.

Fibrocartilage has smaller and less cells, arranged in short rows, squeezed between big collagen

fibres. Collagen is therefore very abundant and is mainly type I, with some type II. This cartilage

is very dense and due to the similarity to the dense connective tissue is very difficult to

recognise the perichondrium. Fibrocartilage can be found in some specific joints, such as

between the vertebrae, in the symphysis pubis and other joints. Fibrocartilage can be

considered as a transition between hyaline cartilage and dense connective tissue. In

intervertebral disks the fibrocartilage is found in the Anulus Fibrosus, the outer part of the disk

that surrounds the Nucleus Pulposus. In joints, pieces of fibrocartilage form the menisci, which

have the role to interact to the surfaces of the two different bones.

Observing the cartilage at the light microscope it’s possible to understand if there is a

pathological conditions influencing the mechanical functioning of the tissue. One of the main

pathological conditions are related to ageing, since it reduces the concentration of

proteoglycans and increase the presence of non-collagen fibres, less stable and resistant.

Histology - Bone

Bone tissue is, together with the cartilage, supportive connective tissue.

The ECM is mineralised, which provides resistance, rigidity and hardness, properties required in

the skeleton. However, the bone is hard and rigid but also resistant and light. It’s not static but

continuously remodelled and pieces of bone are removed and replace by new pieces of bone.

This is controlled by different mechanisms, such as hormones. Bone tissue is present also in

dentin and cementum.

The bone tissue is the tissue that we can find in bones. It has many functions: the skeletal

function, since it provides the scaffold for the organism by creating different areas for specific

organs; the mechanical function, since the bones are connected to ligaments and tendons that

allow them to move; the protection function, since bones’ hardness allows them to protect the

different organs; the trophic function, because calcium is stored in bones but can be released in

bloodstream when needed; the hemotopoietic function, since in specific bones we can find bone

marrow made by stem cells leading to the formation of different blood cells.

The mineralization of the ECM is provided by the accumulation of hydroxyapatite crystals that

provide rigidity and hardness to the bone tissue matrix, while the organic part of the tissue

(collagen mainly) provide resistance to tensile forces and pressing. Bone components are

destroyed by specific chemical reagents and provoke different reactions: the destruction of

inorganic components leads to a loss of hardness and rigidity, making the bone more flexible but

still resistant; the destruction of the organic components leads to the bone becoming much more

fragile, but not losing its shape.

There are three main types of bones: the long, the flat and the short bones. Long bones are

made by two parts: two epiphyses at the ends and a diaphysis in the middle. The diaphysis is

rich in blood vessels and has a thick wall, while the epiphyses are spongy bones.

Bone tissue can be classified according to different characteristics. The main classification is

between lamellar (mature or secondary) bone and non-lamellar (woven or primary) bone.

Lamellar bone can be further classified in compact or spongy bone. Non lamellar bone, in

humans, is not further classified. Non lamellar bone can be find in fetal bones or during fracture

repair. This bone is then modified and replaced by lamellar bone. The compact (or cortical) bone

can be found in the outer shell of flat bones, on the surface of short bones and epiphysis and in

the diaphysis in long bones. The spongy (or trabecular or cancellous) bone can be found in the

internal part of flat and short bones and in the epiphysis of long bones.

Non lamellar bone is formed during embryogenesis or during fracture repair, but is then modified

in lamellar bone. It’s characterised by a lower mineralization and therefore a more irregular and

loose structure.

Compact lamellar bone tissue is characterised by lamellae of bone tissue arranged in particular

ways: concentric to a canal with blood vessels, which leads to osteons; parallel one to another

on the outer parts of the bone to form circumferential lamellae; randomly distributed to form

the matrix between osteons, called interstitial bone lamellae. In each osteon we find different

layers or concentric lamellae with a central (Haversian) canal containing a blood vessel. This

canal is connected to nearby osteons’ canals thanks to perforating (Volkmann’s) canals. This

network of canals with blood vessels is important for the bones nourishment. Lamellae are made

by different layers of osteocytes, the main bone cell, placed in some holes (lacunae) in the

mineralised ECM. Each osteocyte has some protrusions in the ECM which allows those in the

inner layers to reach the blood vessel, collect nourishment and move it to outer layers’ cells

through canaliculi. Collagen fibres are arranged in a very specific and ordered way. In each

lamella they are parallel and their orientation is different from a lamella to the adjacent one.

This specific arrangements of fibres is very useful to resist to different tensile forces from any

direction. When new bones are produced, collagen fibres give the structure and the mineralised

ECM is produced between them. The strength of the bone depends both on the mineralised ECM

and on the collagen fibres.

Bones can be analysed at the microscope with two methods: decalcified bone, obtained by the

destruction of inorganic components (decalcification); ground bone, obtained with the

elimination of blood vessels and cells and the preservation of collagen and mineralised ECM. In

the first case, it can be stained as usual. In the second case, it’s black and white.

Spongy bone (spongy lamellar bone) is made by lamellae not organised in osteons, but arranged

to form a network of trabeculae with many cavities, called medullary cavities, filled with bone

marrow. Since bone marrow is highly vascularised, there is no need for canals in lamellae and

blood vessels reach each osteocyte directly. These osteocytes are in lacunae of the trabecular

lamellae. All the space inside the bone tissue is lined by the endosteum.

Bone tissue is surrounded by periosteum, which is dense connective tissue. However it has three

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