Connective tissue is the medium which surrounds and supports the other tissues of the body.

Composed predominantly of intercellular material (extracellular matrix) which is secreted mainly by the connective tissue cells, cells and fibres.

The cells are usually widely separated by their matrix, which is composed of fibrous proteins and a relatively amorphous ground substance.

There are two types of cells

Resident cells (fixed cells)

Wondering cells

Resident Cells are cells are always found residing in the connective tissue

These include

Fibroblasts

Macrophages (histiocytes)

Mast cells

Adipocytes

Pigment cells (melanocytes)

Undifferentiated mesenchymal cells

Fibroblasts

Are flat, spindle shaped cells with branching of cytoplasmic processes with elliptical and have elongated nucleus

The shape differs depending on activity

Fibroblasts are found in nearly all types of tissues

Active fibroblasts have abundant cytoplasm

Inactive fibroblasts (fibrocytes) are smaller and have diminished cytoplasm

Functions of Fibroblasts

Synthesis

Secretes connective tissue fibres, these include collagen, elastic and reticular fibres.

They also secrete substances such as glycosaminoglycans and glycoproteins found in the extracellular matrix (ground substance).

Differentiation into other cells: Fibroblasts are capable of changing into a variety of cell types depending on the body need.

Wound healing

Fibroblasts secrete connective tissue elements which help in remodeling the extracellular matrix, which is important in wound healing and tissue repair.

During wound healing the fibroblast acquires contractile proteins to become the myofibroblast which play an important role during wound healing. o Contraction causes the wound edges to unite, which enhances wound healing.

They are polymorphic cells and can be ovoid or irregular with short and blunt processes.

They are derived from blood monocytes, which migrate into the tissues and transform to become histiocytes.

Main role in tissue is phagocytosis and antigen presentation.

Are large cells, round or oval in shape

Play an important role in allergic reactions

Mast cells are filled with secretory granules that are filled with substances of inflammation such as histamine and heparin

Heparin, is an anticoagulant and Histamine, is released by the cells under allergic conditions give rise to oedema, bronchospasm and other forms of allergic reactions to the surrounding tissues

These are the cells which are temporarily found within the connective tissues depending on the needs of the body, example during infection.

They include

monocytes

lymphocytes

granulocytes

There three types of connective tissue fibres

Collagen fibres

Elastic fibres

Reticular fibres

Most abundant fibres formed by the union of many collagen fibrils that are made up of collagen proteins

They are tough, inextensible and posses a high tensile strength

They appear white in fresh sections

Form major part of tendons, ligaments, cartilage, teeth (Dentin and cementum) and bones

There different types of collagen fibres

There are many types of collagen fibres but the most important are type I, II, III, and IV

Stretching is restricted

Each fiber is composed of a number of fibrils

The fibres are strong and flexible but not elastic

Found in all types of connective tissue proper and in cartilages and bones, but are especially abundant in tendons and aponeurosis

Chemically, fibres are composed of an albuminous protein (tropocollagen) which forms the fundamental building units of collagen and contains gelatin

They are acidophilic in their staining reaction and therefore take up eosin stain

These are fine fibres made up of elastin protein (tropoelastin)

They allow some degree of distention and stretching

When stretched they usually recover the original form and dimension when the force is eliminated and the elastic limit is not exceeded

Elastic fibres changes as the age advances where it looses its resilience

Appear yellow in fresh sections

Stained by special stain such as Silver stain

Elastic fibres exist as accompanying structure of collagen fibres in the capsule of many organs, vascular walls and the elastic cartilage

Also in ligamentum nuchae and ligamentum flava of the spinal cord

Smaller fibres that branch and anastomose to form a netlike supporting framework known as reticulum

They are closely related to the collagen fibres because they contain collagen fibrils and they show cross-banding pattern, and are sometimes continuous with collagen fibres

Reticular fibres form the supporting framework in the hemopoietic (Bone marrow) and lymphoid organs such as the thymus, lymph node, spleen

In these organs the reticular fibres are produced by reticular cells

Also found in endocrine glands, small blood vessels, veins, muscle cells, fat tissue, and in spaces between the epithelium with connective tissue

In these locations the reticular fibres are produced by fibroblasts, smooth muscle cells, and the Schwann cells produce reticular fibres that surround the nerve fibres

In wound healing the reticular fibres are the first to be formed and as the wound improves they gradually change to become collagenous

Classification of Connective Tissue

Connective tissue proper, which consists of

Loose connective tissue

Dense connective tissue

Specialized connective tissue, which consists of

Adipose tissue

Blood tissue

Cartilage

Bone tissue

Lymphoid tissue

Loose Connective Tissue (Areolar Tissue)

Contains more cells than fibres and the fibres are thinner, delicate, sparse and loosely arranged.

Found around vessels, between muscle fibres, lamina propria of the intestine and in fascial spaces

It forms the essential medium for the nutrients and waste materials exchange between tissues and blood, and also maintains osmotic pressure.

Contains more fibres than the cellular component

Fibres are densely packed with little space for ground substance

Based on the fiber arrangement and direction dense Connective tissue is divided into

Irregular dense connective tissue

Regular dense connective tissue

Fibres are irregularly arranged

Fibres are mainly collagen but in some made up of elastic fibres e.g. walls of elastic arteries, and elastic ligaments e.g. ligamentum flavum and ligamentum nuchae

It forms the dermis of the skin, superficial fascia, fibrous capsule of organs, tunica albuginea of the testis, periosteum, perichondrium, epimysium, dura matter, and septae and trabeculae in various glands

Also forms sheaths and fasciae e.g. the axillary sheath and fascia lata of the thigh

Dense regular connective tissue

Predominant fibres are the collagen fibres

The fibres are densely packed and regularly arranged parallel to each other

Its arrangement gives rise to a strong structure that withstands tension exerted in one direction

Comprises tendons, ligaments and aponeuroses

Adipose Tissue

Formed by aggregation of fat cells (adipocytes) with few other cells such as macrophages, fibroblasts, and leukocytes

Basically it is a storage tissue that stores nutritive material in the form of natural fat that can be used to produce energy when the need arises

Other functions includes; protection and insulation

There are two types

White adipose tissue

Brown adipose tissue

Is made up of unilocular adipocytes i.e. each cell contains one large lipid vacuole, which fills the entire cells

The nucleus and cytoplasm are pushed to the periphery of the cell

They are organized into lobes and lobules separated by septae that are predominantly made up of collagen fibres

When fat cells aggregate together they appear yellow

This is due to the presence of lipofuchsin pigment in fat cells

Made up of aggregation of multilocular adipocytes, i.e. one cell contains numerous vacuoles (fat droplets)

Richly supplied with blood vessels

This makes it appear light brown in colour when viewed in fresh conditions

Present in the infants and newborn, and decreases with age and may be replaced by white adipose tissue

Found in the posterior cervical part, axilla, suprailiac and peritoneal regions

Main function is to protect newborn from cold

It is a tough specialized connective tissue made up of cells, fibres and ground substances.

These elements make cartilage firm and compact

Cartilage is avascular

Cartilage cells

Chondroblasts

Chondrocytes

It is homogenous and stains with basic dyes due to presence of chondromucoprotein.

It surrounds the lacunae in which the cartilage cells lie.

Fibres: Are either collagen or elastic fibres.

Perichondrium is a specialized membrane that covers the cartilage

It is made up of dense regular connective tissue with many blood vessels and nerve fibres

There are two layers

Outer fibrous layer containing fibroblasts

Inner chondrogenic layer that contains undifferentiated cells which can become chondroblasts or chondrocytes

There are three types of cartilage based on the types of fibres it contains and the composition of the ground substance

Hyaline cartilage

Elastic cartilage

Fibro cartilage

Most abundant type of cartilage in human body

It is solid but flexible and can be cut with a knife

Consists of collagen fibres type II, cells and ground substance appear shiny as glass

Microscopically, chondrocytes are ovoid or spherical in shape with large spherical centrally placed nucleus

Hyaline cartilage is found in the following structures

Respiratory tract including nose, larynx, trachea and bronchi

On the sternal ends of ribs, (costal cartilages)

Covers the articular surfaces of joints

During embryonic life it forms the cartilage skeleton, from which the long bones develop

Functions of Hyaline Cartilage

Facilitation of joint movement as it lines the articular surfaces of all the synovial joints, making them to be smooth.

Support to the airways because of its firmness and does not collapse hence it assists in keeping the tubes patent.

Growth; it forms nearly all bones of the foetal skeleton; these are replaced by bone tissue except at the distal ends of long bones where they form the epiphyseal cartilage.

The epiphyseal cartilage is responsible for the longitudinal growth of long bones in the body.

Contains elastic fibres as major component but few collagen type II fibres.

Chondrocytes occurs singly and in groups.

Ground substance contains a network of branching and anastomosing elastic fibres.

It is more flexible and elastic.

Elastic cartilage found in epiglottis, auditory tube, pinna of the ear (external ear), and the coniculate, cuneiform and arytenoids cartilages of the larynx.

Functions of the elastic cartilage are to provide support and also to maintain the shape and flexibility of the organs.

Has an opaque appearance and fibrous texture and does not have perichondrium.

Has numerous visible type I collagen fibres and sparse ground substance.

Intercellular substance contains thick bundles of collagen fibres which run parallel to one another, and are separated by narrow areas of non-fibrous matrix in which cartilage cells are lodged.

Found in areas where firm support and tensile strength are required e.g. the intervertebral discs, pubic symphysis, articular discs in joints, the cartilaginous lining of bony grooves in which tendons are lodged and rims of certain articular cartilages.

Fibrocartilage functions as shock absorber and joint stability.