Connective tissue

Question 1

What is meant by connective tissue

Answer 1

Connective tissue is the general name given to a family of tissues including cartilage, bone, tendons and ligaments as well as those commonly referred to as loose or dense connective tissue

Question 2

List some features of connective tissues

Answer 2

The support cells which maintain the tissue are not necessarily attached to one another (i.e. unlike epithelial cells).
The majority of the volume in most (though not all) connective tissues is taken up by extracellular matrix.
Other cells may be present, for example those with immunological functions.
The tissues are derived from mesenchyme, an embryonic tissue principally associated with the mesoderm layer of the early three layer embryo. (You will earn more about embryonic development in other parts of the medical course).

Question 3

What are the different types of ECM

Answer 3

Fibrillar structures which provide strength against tensile forces applied to the tissue. Their main protein components are collagen, elastin and fibrillin.
A viscous liquid component which provides resistance to compressive forces applied to the tissue. The main molecular components are complex negatively charged polysaccharides which bind water and Na+ ions and are mainly glycosaminoglycans or proteoglycans. The liquid component also provides a solvent in which oxygen, nutrients, waste products and small signalling molecules can diffuse.
Other molecules whose main function is in binding to cells via specific cell surface receptors and mediating their attachment to the ECM. The best known example is fibronectin.

Question 4

Describe the connective tissue of the skin

Answer 4

The outside surface of the skin can be recognised because it is covered with an epithelium (called the epidermis) whose cells stain a pale blue colour. Below this is a layer of dense connective tissue called the dermis. Individual cells (recognised by their nuclei) are separated by pink staining ECM. The pink colour is actually due to a high density of interwoven collagen fibres, though they are so closely packed you may not be able to distinguish individual fibres.

The fibrous components are more clearly seen in the loose connective tissue found deeper down in the skin – near the opposite side of the section. The fibres are less closely packed and you will see that both thick and thin fibres are present. Without special stains it is not possible to distinguish elastic fibres from collagen fibres though both are present.

Question 5

Describe the cellular components of loose connective tissue

Answer 5

The support cells for both dense and loose connective tissue are known as fibroblasts, and they have a spindly shape with correspondingly pointed looking nuclei. Other cells are mainly migratory white blood cells such as lymphocytes (which are especially abundant where the tissue is likely to be exposed to foreign substances, e.g. the loose connective tissue underlying the intestinal epithelium) and neutrophils (which become abundant during an acute inflammatory response). Two types of cell which are normally resident in loose connective tissue but are related to white blood cells are the macrophages (corresponding to blood monocytes) and mast cells (corresponding to basophils).

Question 6

What is also found in dense connective tissue

Answer 6

Dense connective tissue also has fibroblasts as its support cell type, responsible for synthesising the ECM. The dense connective tissue of skin is known as dense irregular connective tissue because the fibres are randomly oriented to give the tissue equal strength in all directions. In some locations – such as tendons and ligaments – strength is required in a single direction, and in this case the collagen and other fibres are aligned in parallel and the tissue is known as dense regular connective tissue.

Question 7

What may be found in loose connective tissue

Answer 7

Part of the loose connective tissue in the skin contains a large number of fat cells or adipocytes. These appear white and large – about 10 times the diameter of most other cells. In fact the majority of the cell is taken up by a single large lipid droplet with all the rest of the cell (nucleus, cytoplasm, organelles) squeezed in a thin rim around the outside. The ECM components of the tissue lie between the fat cells

Question 8

How does cartilage appear cut in cross section

Answer 8

They are cut in cross section on this slide, and appear as large purple staining regions easily identified at low magnification.

Question 9

how does cartilage appear at higher magnifications

Answer 9

At higher magnification look for the support cells, recognising them by their blue staining nuclei. The support cells of cartilage are called chondrocytes, and they are well separated from each other. They each lie in their individual pale staining regions, called lacunae. The remaining space, staining purple, is the ECM of the cartilage.

Question 10

Why may it be difficult to see the structure of ECm in cartilage

Answer 10

You will find it difficult to see evidence of the structure of the ECM since the dense fine network of collagen fibres and the glycosaminoglycans rich in sulphated sugars produce a fairly homogenous appearance in this type of cartilage which is called hyaline cartilage. Other types of cartilage have modified ECMs.

Question 11

Describe fibrocartillage

Answer 11

Fibrocartilage contains thicker collagen fibres oriented to provide tensile strength in a particular direction, for example in the pubic symphysis (which is the joint connecting the two halves of the pelvis at the front) or the intervertebral discs (connecting adjacent vertebrae of the spinal column.

Question 12

Describe elastic cartilage

Answer 12

Elastic cartilage contains additional elastic fibres to provide pliability and resilience (for example in the external ear).

Question 13

What are the key features of cartilage

Answer 13

Cartilage is unusual in that it has no blood vessels and substances reach the chondrocytes by simple diffusion through the ECM – despite the solid appearance of cartilage, it consists of about 70% water.

Question 14

What is meant by bone

Answer 14

Bone (slide C3) is a specialised connective tissue where the ECM (termed osteoid) has become mineralised with calcium phosphate crystals to provide great compressive strength.

Question 15

Describe the solid region around the edge of the bone

Answer 15

The relatively solid region around the edge of the bone shaft is known as compact bone or dense bone. Within it there are the support cells of bone, called osteocytes, within lacunae. There are also other cavities containing blood vessels. The mineral prevents free diffusion, and small channels called canaliculi provide a pathway for diffusion of O2 and nutrients from the blood vessels to the osteocytes.

Question 16

What can be seen in transverse section of bone

Answer 16

In transverse section on that slide it can be seen that bone is arranged in structural units called osteons or Haversian systems which have a central canal containing blood vessels, with the ECM of the bone arranged in concentric layers around this central canal. The collagen fibres are oriented differently in each layer to provide additional strength.

Question 17

Describe the central parts of. bone

Answer 17

The central part of a bone has spaces between the bone tissue, and is known as spongy bone or cancellous bone. The bony structures here are called trabeculae, and they are usually arranged in the way that best resists the mechanical stresses on the bone. Most of the space between the trabeculae is filled with bone marrow, the site of developing blood cells. There may also be fat cells.

Question 18

Describe the formation of new bone

Answer 18

New bone is laid down by cells called osteoblasts and bone is resorbed by cells called osteoclasts (these cell types are not easily identified in histological sections). Normally the two processes are in balance but they allow deposition of new bone to be adjusted to strengthen bone in regions of mechanical stress or to heal a wound. There can be net bone production during growth, or net resorption resulting from disuse in immobile patients or from osteoporosis. Because the mineral component of bone can act as a store for Ca2+ and phosphate, these activities are under hormonal control from calcitonin and parathyroid hormone to maintain the normal blood concentrations of these ions.

Question 19

Describe how bones develop

Answer 19

Most (though not all) bones start as cartilage precursors in the foetus, which then get converted to bone – a process known as endochondrial ossification. In long bones, a cartilaginous region persists at the growth plates between the shaft and heads until the bone has reached its adult length. Cartilage also lines the bone surface where it forms part of a joint. Elsewhere the bone is lined with a layer of dense connective tissue – the periosteum

Question 20

Describe myofibrils

Answer 20

These are made up of a large number of precursor cells which have fused together to make a syncytium in which the contractile apparatus in the cytoplasm becomes continuous. Attached round the edge of each myofibril is an ECM containing collagen fibres that are ultimately continuous with those of the tendons or bones on which the muscle acts.

Question 21

Describe how cardiac muscle differs

Answer 21

Cardiac muscle (which you will study in more detail next term) is also a form of striated muscle, but in this case the muscle cells remain separate and are connected end to end at structures called intercalated discs. Some cardiac muscle cells have become specialised to form the conducting system of the heart.

Question 22

Describe smooth muscle

Answer 22

Smooth muscle is the form associated with organs and vessels, and it is controlled by the autonomic nervous system. Examine the wall of the bladder (slide U4) to see its structure. It is made up from individual cells which can be hundreds of micrometres in length, and which have a central nucleus with a characteristic cigar shape, appearing circular in transverse section. The contractile apparatus is differently organised from striated muscle, with no repeating units being visible. Smooth muscle is not capable of the rapid powerful contraction of skeletal muscle, but can contract to a smaller fraction of its relaxed length and can maintain contraction for very much longer times.