Connective (or Supporting) Tissues
- provide structural, metabolic and defensive support within other tissues and organs
- generally have few cells compared to other tissues
- occur in many different forms with diverse physical properties
- in their supporting role for epithelia are referred to as the stroma
- can be classified into the following categories
- Fibrocollagenous tissues
- Adipose tissue
- Bone (covered in the Locomotor Module)
- Blood (covered in the Cardiorespiratory Module)
Two major components of connective tissues:
- Extracellular matrix
Differentiated cells of connective tissue include:
- are embryologically derived from mesoderm.Mesenchymal cells are stem cells in the adult which retain the capability to differentiate into any cell type derived from mesoderm.
- fibroblasts- common in many types of connective tissue
- chondroblasts / chondrocytes (mature form) in cartilage
- osteoblasts / osteocytes in bone
- adipocytes- fat storing and metabolising cells
- blood cells.
2. Extracellular matrix of supporting tissue
- is produced by connective tissue cells
- defines the structural properties of the tissue
- is comprised of ground substance and fibres
- forms a semi-fluid gel through which metabolites can diffuse
- contains carbohydrates in chains (glycosaminoglycans- GAGs) and complexes
formed by linkages to proteins (proteoglycans) as well as fluid components
with dissolved substances including nutrients, electrolytes, gases and hormones
- adhesion of cells to extracellular matrix (laminin and fibronectin)
- control of fibre organisation (fibrillin)
- regulation of calcium binding and calcification in bone
Three main types:
- is the most common protein in the body
- is secreted by fibroblasts as the precursor tropocollagen which polymerizes to form collagen
- occurs in many variant types (>18)
Types I, II and III polymerize to form rope-like fibrils
Type I - provides tensile strength in dense connective tissues of the skin dermis, tendons and ligaments, also loose supportive tissue and bone.
Type II - predominant form in hyaline and elastic cartilage.
Type III - makes up reticular fibres (see below)
Type IV collagen forms a meshwork structure rather than distinct fibrils and
is an important constituent of basement membranes.
- Reticular fibres
- are delicate fibres made of type III collagen
- are found in most fibrocollagenous tissues. They form a mesh or framework
that supports the basement membrane and are the major fibre type in the
walls of small blood vessels. In reticular tissue, reticular fibres act
as scaffolding for highly cellular organs such as the liver, bone marrow
and lymphoid tissues.
- Elastic fibres
- are formed from elastin (secreted by fibroblasts and chondroblasts)
and the glycoprotein fibrillin
- allow stretch and give resilience to supporting tissues
- are found in arterial walls, dermis of the skin, elastic cartilage, lung
Types of connective tissues
Fibrocollagenous tissues are relatively unspecialised tissues (compared to
bone and cartilage) and are characterised by significant quantities of collagen
fibres (type I usually but also type III) made by fibroblasts.
Fibrocollagenous tissues are classified by:
Several classes of supporting tissue come under this heading:
- the number of collagen fibres relative to the amount of ground substance
- the organisation of collagen fibres
- the type of collagen fibres
- loose or areolar connective tissue
- dense connective tissues
- reticular connective tissue
Fibrocollagenous tissues provide a framework in many tissues and glands, permit
transport of materials and act as a form of biological packing material or connection
between cells and tissues with more specific functions.
A. Loose or areolar connective tissue
- has haphazardly arranged collagen bundles separated by open (areolar) spaces
which are filled with ground substance
- is found supporting epithelia in most mucous or serous membranes as well
as vessels and ducts in organs
- is an extremely important type of connective tissue acting as a 'glue'
between many tissues and supporting them metabolically by containing their
blood supply. Diffusion of nutrients and metabolites is facilitated by the
characteristics of its abundant ground substance.
- possesses fibroblasts that are responsible for synthesis of extracellular
components. It may also possess adipocytes for storage of fat.
- contains defence and immune cells. Intrinsic defence cells of areolar tissue
- Macrophages (Gr. makros- large + phagein- to eat) whose functions include
the ingestion of inert material and microorganisms
- Mast cells which are important in allergic reactions.
- Other types of white blood cells, especially lymphocytes and plasma
cells. These are particularly enriched in structures that are constantly
under threat of pathologic invasion (eg. gut, respiratory tract). Lymphocytes
are characterised by dense round nuclei with little surrounding cytoplasm.
Plasma cells are derived from B lymphocytes and synthesise and secrete
antibodies. They have eccentric nuclei and heavily stained cytoplasm.
B. Dense connective tissues
- is found in locations where mechanical support and tensile strength is required
- is characterised by a dense arrangement of collagen fibres and reduced ground substance compared to areolar connective tissue
- has collagen bundles whose alignment is dependent on the direction from which the structure must withstand tension.
- dense regular connective tissues such as tendons and ligaments
have collagen bundles in ordered, parallel orientations that provide maximum
- dense irregular connective tissues have interwoven collagen
bundles to resist tension from many directions and provide tough physical
support and protection. The dermis of the skin, organ capsules and sheaths
surrounding tendons and nerves are examples.
C. Reticular tissue
- forms a delicate supporting framework of reticular fibres (type III collagen).
Reticular tissue is sometimes classified as a type of 'loose' connective tissue
due to the sparsity of its fibres.
- supports some cellular tissues and organs
2. Adipose tissue
There are two main types of adipose tissue:
- is characterized by abundant adipocytes which store and metabolise fat
- is supported by collagen type I fibres synthesised by fibroblasts and is
- is usually surrounded by fibrocollagenous connective tissue which supports
and may divide the adipose tissue into pads which absorb shock (soles of the
feet, around kidneys, in the orbit around the eye)
- White adipose tissue
- is the main fat storing tissue in the adult where it acts as an energy
- acts as insulation (under the skin) and as a shock absorber (eg. around
- is made up of unilocular adipocytes (lipid fuses into 1 large droplet)
- has a widespread distribution
- Brown adipose tissue
- contributes to thermoregulation of newborns by producing heat
- is made up of multilocular adipocytes (fat is stored in multiple, small droplets)
- is organized into lobules separated by fibrocollagenous septa
- has a restricted distribution in the newborn and is mostly lost by adulthood
Varying proportions of collagen and elastic fibres define three main types:
- is a semi-rigid form of supporting tissue
- is characterised by an abundant extracellular matrix with a predominance
of proteoglycan-containing ground substance complexed to a fine collagen (mainly
Type II) fibre latticework. This arrangement forms a hydrated gel that resists
compressive forces while permitting free diffusion of small molecules.
- acts in a structural fashion in limited sites (eg. ear, trachea) and is
a precursor in bone formation
1. Hyaline cartilage (Gk hyalos- glass)
- is found in articular surfaces of synovial joints, in the respiratory
system and as a precursor to bone in the developing skeleton
- has an amorphous matrix of ground substance reinforced by collagen
(usually type II)
- except in articular cartilage, is surrounded by perichondrium consisting
of condensed fibrocollagenous supporting tissue containing chondroblasts
with cartilage forming potential
2. Elastic cartilage
- has a architecture similar to hyaline cartilage
- has a collagen-containing matrix with an abundant network of elastic
- is found in the external ear, epiglottis and auditory tube
- is a hybrid tissue between dense fibrocollagenous tissue and hyaline
- is found in places including intervertebral disks and in association
with dense fibrous tissue in tendons and ligaments
- Connective tissues have varied structures but have common origins, general
composition and functional characteristics. The connective tissue type is
dependant on its particular composition of cells and extracellular matrix
(fibres and ground substance).
- Fibrocollagenous tissue may be loose or dense indicating the ratio of collagen
fibres to ground substance.
- Loose connective tissue is found in most organs as its areolar form
and acts as a structural support as well as carrying the organ's blood
supply. Another type of 'loose' fibrocollagenous tissue is reticular tissue.
- Dense connective tissues may be regular or irregular depending on the
nature of the stresses the tissue must bear.
- Adipose tissue is characterised by adipocytes and is of two distinct forms-
white and brown- with different cellular structures and functions.
- Cartilage has unique properties because of its particular arrangement of
extracellular matrix and abundant proteoglycan-containing ground substance.
The fibres in the cartilage reinforce it and give it additional qualities
in different types of cartilage (ie. elasticity, tensile strength).
- Remember that bone and blood are also classed as types of connective or
supporting tissue. We will discuss them in the future.
A typical exam question
to WebCT for formative assessment.