Biology Today - February 2018

Table: Types of dense regular connective tissue

White fibrous connective tissue Yellow elastic connective tissue

Structure

It consists of mainly white (collagen) fibres which are

arranged in bundles. The fibroblasts are present in rows

between the bundles.

This tissue is mainly made up of much thicker branched

loose network of yellow elastic fibres.

The fibroblasts are irregularly scattered.

Types

Cords

It forms cords called tendons, which connect the skeletal

muscles with the bones.

This tissue forms cords called ligaments which join

bones to bones.

Sheets

It forms flat plates or sheets which occur in the dermis of

the skin, connective tissue sheaths of muscles and nerves

and tunica adventitia of large blood vessel, periosteum,

perichondrium, pericardium, duramater and renal capsule, etc.

The sheets formed by this tissue occur in the walls

of blood vessels, lungs, bronchioles, true vocal cords,

cartilage of larynx and trachea.

Function

It has great strength, however its flexibility is limited.

The presence of white fibrous tissue at the joints between

skull bones makes them immovable.

This tissue has considerable strength and remarkable

elasticity. Thus, it allows stretching of various organs.

Specialised Connective Tissue

This tissue includes cartilage, bone and blood; each having unique cells and extracellular matrices that allow special functions.

Cartilage

It is a soft skeletal tissue, not rigid like bone. It is found more abundantly in vertebrate embryos because most of the bones forming
skeleton of the adult are cartilaginous in the early stage. A typical cartilage consists of cartilage cells and ground substance (matrix).

(i) Cartilage cells or chondrocytes : They are present in a fluid-filled space, called lacunae. Young cartilage cells chondroblasts
are small and flattened whereas mature cartilage cells chrondrocytes are large and rounded. Chondroblasts are metabolically
more active cells than chondrocytes.

(ii) Ground substance (matrix) : It essentially consists of water, proteoglycans, some lipid, collagen, non-collagenous protein and
collagen fibres. The core protein is known as aggrecan. Carbohydrates are glucosaminoglycans (GAG) including chondroitin
sulphate, keratin sulphate and hyaluronic acid.

The cartilage is divided into three types as shown in the table below.

Table : Types of cartilage

Hyaline cartilage Fibrous cartilage Calcified cartilage

Structure

It contains clear, large amount of transluscent, slightly

elastic matrix with less fibres. It is the most prevalent

cartilage.

It is of two types: White fibrous

cartilage (strongest cartilage) and

yellow elastic cartilage (makes

cartilage flexible).

The matrix contains

granules of calcium

carbonate, which makes

cartilage hard and elastic.

Location

Forms the articular surfaces at the joints of long bones

such as knees, where it is called articular cartilage.

It also forms part of larynx and sternum (breast bone),

rings of trachea and bronchi, sternal parts of ribs (=

costal cartilages), hyoid apparatus and nasal cartilages.

White fibrocartilage occurs in the

intervertebral disc and in the pubic

symphysis. Yellow elastic cartilage

occurs in pinna, external auditory

canal, epiglottis, etc.

It is found in supra scapula

of pectoral girdle of frog,

and vertebrae of shark.

Bone

Bone is a solid, rigid and strong connective tissue which gives support to various organs. A bone which has been exposed to
drying conditions for a long time so that all living cells die, decay and disappear is called dried bone. This bone then contains
only inorganic matrix.

When a bone is kept in dilute acid (HCl or HNO 3 ) for long hours due to which calcium, magnesium, potassium and sodium salts
of inorganic part of matrix get dissolved whereas organic part of matrix remains intact is known as decalcified bone.