Connective Tissue (CT)

Connective Tissue

• A really diverse group of tissues:

• Connective tissue (CT) is the most diverse of the four tissue types with a wide variety of functions. CT forms an extensive compartment in the body and can be considered as the "glue" that holds the body together.

Composition of Connective Tissue

• Cells

• Extracellular matrix– Ground substance– fibres

Cells of CT

• Fibroblasts: • the principal cells of connective tissue. • Matrix secreting cells• responsible for the secretion of all types of

fibres (collagen, reticular, elastin) and the complex carbohydrates of ground substance.

Cells of CT

Chondroblasts & chondrocytes: • the matrix-secreting cells of cartilage.

Osteoblasts & osteocytes: • the matrix-secreting cells of bone.

Macrophages: • are phagocytic cells derived from monocytes.

Cells of CTAdipose cells:• also called adipocytes, these cells are specialized to store lipids.

Mast cells: • have granules containing histamine, heparin and anaphylactic factors. • when released in response to an antigen, they cause hypersensitivity reactions,

allergy and anaphylaxis.

Lymphocytes: • cells responsible for immune responses that circulate in the blood. • normally present in small numbers but their number increases dramatically at

certain sites of tissue inflammation.

Plasma cells: • are derived from B-lymphocytes and produce antibodies against a specific

antigen.

Cells of CTNeutrophils: • are white blood cells that act as phagocytes in the early stages of

acute inflammation.

Eosinophils: • are white blood cells that are found at sites of allergic reaction and

parasitic infection.

Basophils: • are white blood cells that are similar to mast cells in having

vasoactive agents released in response to an allergen.

Monocytes: • are white blood cells that will give rise to all the phagocytes namely

macrophages

Ground substance:

• occupies the space between the cells and fibres of connective tissues.

• high water content

• consists largely of proteoglycans and hyaluronic acid.

Fibres:

• There are three types of fibres secreted by connective tissue cells: collagen fibres, reticular fibres, and elastic fibres. The abundance and preponderance of different types of fibres varies in different CTs.

Collagen fibres:

• the most common fibre type • flexible fibres with a high tensile strength.• Type I collagen is the most prevalent type

of collagen (90%) - found in the dermis of the skin, bone, tendon, organ capsules and many other areas.

• Type II collagen found in cartilage are finer• Type IV collagen is found in the basement

membrane of epithelia.

Reticular fibres:

• closely related to collagen fibres.

• They are made of type III collagen fibers

• given their name because they are arranged in a mesh-like pattern.

• provide a supporting framework for the cellular constituents of various tissues and organs

• most are produced by fibroblasts.

Elastic fibres:

• thinner than collagen fibres • they give tissue the ability to cope with

stretch and distension. • they are interwoven with collagen fibres in

order to limit excessive stretching and tearing.

• found in certain ligaments (elastic ligaments), some cartilage (elastic cartilage) and in large arteries (elastic arteries).

• most are produced by fibroblasts.

Classification of connective tissues:

• Connective tissues are classified on the basis of types and relative abundance of cells, fibres and ground substance, and on the organization of fibres. The images below will introduce you to the diversity of connective tissues.

Loose (or areolar) connective tissue

• abundant ground substance and thin and relatively sparse fibres.

• located beneath epithelia that line the internal surfaces of the body, glands and small vessels.

Loose (or areolar) connective tissue

Dense irregular connective tissue

• collagenous fibres make up the bulk of the tissue (give it strength)

• fibroblasts are scarce and usually the only cell type present.

• little ground substance is present. • the fibres are typically arranged in bundles in

various directions (hence irregular), which enables the tissue to withstand various stresses.

• found on the outside of many organs and in the dermis of the skin, meninges surrounding the spinal cord

Dense irregular connective tissue

Dense regular connective tissue

• collagenous fibres are packed in dense regular arrays, between which lie rows of cells (fibroblasts).

• found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones,

Dense regular connective tissue

Adipose tissue

• adipocytes, which are specialized to store fat, are found throughout loose connective tissue.

• Fat cells appear as empty circles in which a peripheral nucleus can often be identified

Adipose tissue

Cartilage

• tissue whose cells, called chondrocytes, secrete a very specialized matrix. specifically hyaluronic acid, chondroitin sulfate and keratan sulfate.

• Collagen (type II) fibrils are also present in the matrix

• the chondrocytes sit in spaces called lacunae, which they fill during life.

Hyaline Cartilage

Mostly collagen fibres found at the ends of ribs; in the larynx, trachea, and bronchi; and on the articular surface of bones.

Elastic Cartilage

•found in the external ear, epiglottis, eustachean tube and larynx.

Fibrocartilage

•consists of chondrocytes in combination with dense connective tissue – found are the intervertebral disks, the symphysis pubis, and certain places where tendons attach to bone.

Bone

• characterized by a mineralized extracellular matrix.

• the matrix is secreted by cells called osteocytes.

• consists of mostly of mineralized collagen fibres arranged in lamellae.

Compact Bone

• Also known as dense or cortical bone• consists of numerous units called osteons or

Haversian systems which consists of a central Haversian canal which contains its vascular and nerve supply around which lamellae of collagen fibres are concentrically arrayed. Osteocytes lie in lacunae between the lamellae and make contact with other osteocytes, and ultimately the Haversian canal, via cell processes which they extend in little channels or canaliculi.

• Found in the long axis and diaphysis of the long bones

Anatomy of a Bone

Compact Bone

Compact Bone

Spongy Bone

• Also known as trabecular and cancellous bone • the collagen is contained within the irregularly

shaped trabeculae. The spaces between the trabeculae are filled with bone marrow which is actively involved in red blood cell formation.

• Found in the margins of the bone marrow cavity, ends of bones (epiphyses)

Spongy Bone

Hemopoietic tissue

• The only hemopoietic tissue in the adult is red bone marrow. Red bone marrow gives rise to red blood cells, granulocytes (neutrophils, eosinophils and basophils), monocytes and platelets. Lymphocytes are formed both in the red bone marrow and in lymphatic tissue.

Blood

• fluid connective tissue that circulates throught the body.

• functions in bringing nutrients and oxygen to tissues, removing waste products, transporting hormones and with the immune response.

Figure 12 illustrates a blood smear (RBC, neutrophil and basophil)

Figure 13 illustrates a blood smear (eosinophil, RBC, platelets)

Figure 14 illustrates a blood smear (lymphocyte, eosinophil, RBC)

Figure 15 illustrates a blood smear (monocyte, neutrophil and lymphocyte,

platelets)