Connective tissues

Dr. Hersh Abdul Ham-KarimBVM&S, PG Dip, MSc and PhD

Connective tissue is a term applied to a basic type of tissue of mesodermal origin,

which is sparsely populated by cells and contains an extensive extracellular matrix

consisting of protein fibers, glycoproteins, and proteoglycans. CT provides structural

and mechanical support for other tissues, and to mediate the exchange of nutrients and

waste between the circulation and other tissues.

▪ All connective tissues are composed of:

1. Cells

2. Extracellular matrix (ECM)

a. Fibers

b. Ground substance

Connective tissue (CT)

Functions of CT

Support: Structural support is the major function of connective tissue, which forms

the framework upon which all other body tissues are assembled.

Defense physical: The viscosity of the extracellular matrix, slows the progress of

many bacteria and foreign particles.

Defense immunologic: Foreign bodies that successfully penetrate epithelia are

intercepted by immuno-responsive cells that inhabit the underlying connective

tissue.

Repair: Rapidly closing any breaches in the body's protective barriers.

Storage: Reserves of water, electrolytes, and reserve energy in the form of lipids

are stored in adipocytes.

Transport: CT is thus a crossroads for transporting substances to and from other

tissues.

There are two major groups of cell in connective tissue:

1. Fixed cells (resident cells): Are derived from mesenchyme and

are continuously present in the tissue (e.g., fibroblasts,

adipocytes, mast cell, etc).

2. Free cells (visitant cells): Enter and leave the blood stream to

migrate through and function in connective tissues (e.g.,

neutrophils, eosinophil, basophil, monocyte, lymphocytes, and

plasma cells)

Cells of CT

1. Fibroblasts:

• Fibroblasts are spindle-shaped flat cells

with minute processes spreading out of

the cell body.

• The cells have flattened nucleus

containing 1-2 nucleoli. When fibroblasts

are active, cellular organelles like Golgi

apparatus and rough endoplasmic

reticulum become more prominent.

• Secrete both fibers and ground substance

of the matrix

2. Mast cells.

Mediate immediate hypersensitivity reaction

by releasing immune modulators (Histamine)

from cytoplasmic granules, in response to

antigen binding with cell surface antibodies.

Structure:

• Round to oval-shaped cells.

• Round, usually centrally located nucleus.

• Well-defined cytoplasm filled with

secretory granules containing immune-

modulatory compounds (e.g., histamine).

3. Macrophages.

▪ Derived from blood monocytes; monocytes enter

connective tissue from the bloodstream and rapidly

transform into macrophages that function in

phagocytosis, antigen processing, and cytokine

secretion.

▪ Comprise the mononuclear phagocyte system of

the body; include Kupffer cells in the liver,

alveolar macrophages in the lung, microglia the

central nervous system, Langerhan’s cells in the

skin, and osteoclasts in bone marrow.

Extracellular matrix

• Collagen Fibers: Large fibers made of theprotein collagen and are typically the mostabundant fibers. Promote tissue flexibility.

• Elastic Fibers: Intermediate fibers made ofthe protein elastin. Branching fibers thatallow for stretch and recoil.

• Reticular Fibers: Small delicate, branchedfibers that have same chemical compositionof collagen. Forms structural framework fororgans such as spleen and lymph nodes.

1. Fiber

It is an amorphous gelatinous material. It is transparent,

colourless, and fills the spaces between fibres and cells

Functions:

▪ Forms a gel-like matrix (cells and fibers are

embedded).

▪ Provides a medium for passage of molecules

and cells migrating through the tissue.

2. Ground substance

Types of CT

The main criteria for classification are the amount and type of ECM, arrangement and

kinds of fibers, and abundance cell types;

Connective tissue

Embryonic

Mesenchyme Mucous

Adult

Proper

Loose Dense

Regular Irregular

supportive

Bone Cartilage

specialized

Adipose Reticular Elastic Blood

▪ Embryonic CT

• Mesenchyme: Unspecialized CT of early week of embryonic life, which further

differentiates into all CT.

• Mucous: Embryonic connective tissue with abundant ground substance and delicate

collagen fibers; present in the umbilical cord.

a. Loose (areolar) CT b. Dense CT

▪ Highly cellular, numerous cell types

present.

▪ Fewer cells, mostly fibroblasts.

▪ Fewer and smaller caliber collagen fibers

compared with dense.

▪ Highly fibrous with larger caliber collagen

fibers.

▪ Abundant ground substance. ▪ Minimal ground substance.

▪ Highly vascularized. ▪ Poorly vascularized.

▪ diffusion of nutrients, wastes and provides

padding between and around organs and

tissues.

▪ provides strength.

▪ Adult CT

1. True (proper) CT

Dense regular CT Dense irregular CT

▪ The parallel arrangement of bundles of

collagen fibers.

▪ Randomly-arranged collagen fibers

and a few fibroblasts

▪ Forms tendons, ligaments ▪ Forms the capsule of organs and the

dermis of the skin.

▪ Provide strong attachment between

various structures

▪ Provide strength

b. Dense connective tissue

fibroblasts

collagen

2. Supportive connective tissue

a. Cartilage

▪ It is a type of connective tissue which is tough, and flexible.

▪ Like other connective tissues, it consists of cells and ECM.

▪ Unlike other CT, does not contain vessels and nerves.

▪ The strength of cartilage is due to collagen fibers and the resilience is

due to the presence of chondroitin.

▪ Most cartilage is surrounded by the perichondrium. Fibrocartilage is

the exception.

Components of Cartilage

Perichondrium:

▪ Capsule-like sheath of dense connective tissue that

surrounds cartilage.

▪ Harbors the vascular supply for avascular cartilage.

▪ Connects cartilage with the surrounding tissues.

a) Fibrous layer; Outer portion, composed of

dense connective tissue, serves as a source of

reserve cells for the chondrogenic layer.

b) Chondrogenic layer; Inner more cellular

portion contains chondroblasts which create the

major component, the extracellular matrix, of

the cartilage.

❖ Cells:

▪ Chondroblasts

• Lie on the surface of cartilage in the

chondrogenic layer of perichondrium.

• Secrete extracellular matrix around them, thus

becoming.

▪ Chondrocytes

• Are chondroblasts that have surrounded

themselves with matrix.

• Lie within cartilage in potential spaces called

lacunae.

• Secrete and maintain extracellular matrix.

• Are frequently located in isogenous groups, a

cluster of chondrocytes, resulting from the

proliferation of a single chondrocyte.

❖ Extra cellular matrix: Fiber & ground substance

1. Hyaline Cartilage:

Composition :

▪ Presence of isogenous group.

▪ Ground substance appears homogenous & takes blue stain.

▪ Collagen fibers in ground substance have same refractive index as

that of ground substance.

▪ Perichondrium is present in all except in articular cartilage.

Types of Cartilage

Incidence: articular surfaces, wall of large respiratory passages – larynx,

trachea, bronchi, epiphyseal plate, ventral ends of ribs, embryonic temporary

skeleton

Functions: Reduces friction at joints, movement and support

2. Elastic Cartilage:

Composition

▪ Chondrocytes – similar to hyaline

cartilage, housed in lacuna singly or in

pairs.

▪ Extracellular matrix – fibrils of collagen

II and network of fine elastic fibers, less

amount of ground amorphous substance

▪ On the surface - perichondrium is

identifiable.

▪ Itprovides both strength and elasticity to

certain parts of the body.

Incidence: epiglottis, auricle of the ear

,Eustachian tube, cuneiform and corniculate

cartilages in larynx

auricle

Eustachian tube

Composition

▪ Chondrocytes – small, spindle-shaped, similar to fibroblasts

▪ arranged singly or in long rows

▪ Extracellular matrix – great number of collagen I fibers – acidophilic, amorphous matrix

less abundant

▪ Perichondrium is not identifiable.

Incidence: Intervertebral disc, articular disc,

pubic symphysis, labrum of ball and socket

joint.

Functions: Shock absorbers,

Provides sturdiness without

impeding movement

3. Fibrocartilage

b. Bone

▪ Bone is a hard and rigid tissue, consists of living cells with large

amounts of matrix.

Functions of bone:

▪ Mechanical structures for movement and protection of vesira

▪ Home for hematopoietic tissue

▪ Storage for elements and minerals – homeostatic regulation of

blood calcium level

▪ Storage of adipose tissue: yellow marrow

▪ Cells:

1. Osteo-progenitor cells: Flattened, elongated ovoid nuclei,

derived from mesenchymal cells, located in periosteum,

endosteum and stromal component of BM.

2. Osteoblasts:

▪ They are cuboidal to columnar cells have the epitheloid

appearance with intensely basophilic cytoplasm.

▪ The bone-forming cells derived from osteo-progenitor cells,

which produce and secrete matrix proteins and transport

mineral into the matrix.

3. Osteocytes:

▪ Osteocytes are spider-shaped cells.

▪ Transport materials between blood and bone and to

maintain surrounding matrix; they do not divide or

secrete matrix.

4. Osteoclasts:

▪ Osteoclasts are large, multinucleated cells, have small

projections (microvilli) that is the cell’s active region.

▪ They function in bone resorption by removing local

mineralized matrix.

1. Periosteum:

The outer surface of the bone and consists of two

layers.

1. Fibrous layer: consists mainly of the dense

irregular connective tissue, contains arteries,

veins, lymphatic vessels and sensory nerves.

2. Osteogenic layer: An inner (cambium) layer of

loose, richly vascularized CT contains progenitor

cells that develop to osteoblasts.

▪ Function:

• Attachment Sites for the Muscle Tendons

and Ligaments

• Nourishment of the Bones and Muscles

• Bone Growth and Repair

Surface coverings of bone

Endosteum:

▪ It is a thin layer of connective tissue that lines

the inner surfaces of all bones (the cavities

within the bone).

▪ Also contains both osteoblasts, osteoclasts,

and/or osteo-progenitor cells that line all

interior surfaces of bone except for lacunae and

canaliculi.

▪ Serves as a means of bone growth and/or

resorption.

Microscopic Appearance of Bone:

The orientation of collagen fibers in the bony matrix determines whether a bone is

either primary (woven) or mature (lamellar) bone.

1. Lamellar or mature bone or secondary.

▪ Composed of cylindrical units called osteon.

▪ Osteon consist of concentric lamellae of bone matrix surrounding a central canal

called Haversian canal, which contain the vascular and nerve supply of osteon.

▪ Within the bone matrix are spaces called Lacunae, each containing osteocyte.

▪ The osteocyte extends numerous processes into a small tunnels called canaliculi.

▪ Canaliculi serves for the passage of substances between the osteocytes and

blood vessels.

▪ Mature spongy bone is structurally similar to mature compact

bone except that the tissue is arranged as trabeculae.

▪ The matrix of bone is lamellated.

▪ Osteocytes get nutrients directly from circulating blood.

2. Woven or immature bone or primary bone

▪ Possibly will be either spongy or compact. Woven

bone is found on the growing ends of an immature

skeleton or, in adults, at the site of a healing fracture.

▪ Characterized by random deposition of fine collagen

and increased cellularity contains osteocytes that are

more numerous and spherical than those of lamellar

bone.

▪ Relatively low mineral contents.

▪ Seen in embryonic development, fracture, and repair.

▪ It is a biological red liquid circulating in the arteries and veins

under the impulse of the heart.

A. Blood

Functions of blood

▪ Transports gases, nutrients, waste, cells and hormones

throughout the body.

▪ Regulates temperature and water content of cells.

▪ Protects against blood loss through clotting.

▪ Protects against disease through phagocytic white blood cells

and antibodies.

2. Specialized CT:

▪ The main components of blood include is made up of plasma which is a highly viscous

liquid and 3 different types of blood cells that are floating around in it.

1. Plasma:

▪ A typical sample of plasma is composed of 90% water, 8% protein, 1%

inorganic salts, 0.5% lipids and 0.1% sugar, the rest being made of

lesser components.

▪ The three main groups of proteins in plasma are the blood coagulation

(fibrinogen) proteins, albumin, and the globulins.

• The globulins can be divided into;

• alpha globulins (proteases, anti-proteases and transport

proteins).

• beta globulins (transferrin, other transport proteins)

• gamma globulins (mainly immunoglobulins).

▪ The plasma proteins are nearly all derived from synthesis in the liver,

with the exception of the immunoglobulins which are synthesised by

plasma cells.

2. Cellular component

a. Erythrocyte (RBC)

▪ RBCs constitute the largest number of cells in the blood

▪ about 5.2 million mm3 (man), 4-5 million (woman)

▪ biconcave disc shape

▪ no nucleus

▪ contain hemoglobin

▪ Life span in blood 120 days

▪ main function is gases transport

b. Platelets (thrombocytes)

• Small, biconvex disks.

• Non-nucleated cell formed in the bone marrow

from the cytoplasm of cells called

megakaryocytes.

• Platelets have a life span of about 10 days.

• Their numbers in circulating blood range from

150 000 to 400 000/mL.

• Function: initiate blood clots

c. Leucocyte WBC

▪ 5000-9000 mm3 (healthy adult)

▪ act mainly outside blood vessels in the tissues

▪ main function is defence.

▪ classification of leukocytes

• classification system is based on appearance of the granule

o Granulocytes

✓ Neutrophil, Basophile and Eosinophil

o Agranulocytes

✓ Lymphocytes and Monocytes

1. Granulocyte

▪ Neutrophil

• the most common type of leucocyte in blood

• Compose 60-70% of the leukocytes

• Life Span: < 1 week

• About 9-12 µm in diameter (thus larger than

RBC).

• Nucleus is polymorphonuclear - has 3-5 lobes

linked together by fine threads of chromatin

• Cytoplasm contains granules, which are

lysosomes that contain enzymes for digestion

of phagocytosed particles, e.g. bacteria.

▪ Eosinophil

• Compose 1-6% of the leukocytes

• Nucleus is bilobed

• Life span < 2 weeks

• Cytoplasm contains ovoid, acidophilic

granules (bright pink), which are larger than

those of neutrophils

• Granules are lysosomes that contain enzymes

that can degrade phagocytosed particles.

• Important in allergic reactions, parasitic

infections, and phagocytosis of Ab-Ag

complexes.

▪ Basophil

• constitute less than 1% of leucocytes.

• Lobulated nucleus often obscured by

granules

• Life Span: 1-2 years

• basophilic cytoplasmic granules (dark

blue), containing heparin (anti-coagulant)

and, histamine (vasodilator).

• Function in allergic reactions and

inflammatory response.

2. Agranulocyte

a. Lymphocytes

• are the smallest cells in the white cell series,

being only slightly larger than erythrocytes.

• constitute 20-30% of leucocytes.

• Life Span: variable (few days to several

years)

• Round dense nucleus.

• Small lymphocytes (inactivate) have very

little cytoplasm.

• Two populations, one that can become T-

lymphocytes and the other B- lymphocytes.

• When activated by encounter with foreign

antigen presented by a macrophage - become

large lymphocytes that are capable of mitosis.

Small lymphocytes

Large lymphocytes

Round nucleus

Ovoid nucleus

Structure:

▪ Oval-shaped cells.

▪ Round, eccentrically located nucleus.

▪ Basophilic cytoplasm due to large amounts of rough

endoplasmic reticulum.

▪ Well-developed Golgi complex appears as a distinct,

unstained region in the cytoplasm near the nucleus

and, for that reason, is often referred to as a

“negative Golgi.”

* Plasma cells.

▪ Secrete antibodies to provide humeral immunity.

▪ Derived from B-lymphocytes.

b. Monocytes

• They are the largest cells in the white cell series,

comprise 3-10 per cent of the leukocytes

• Large, eccentric nucleus either oval, kidney-

shaped or horseshoe-shaped with delicate

chromatin that is less dense than that of

lymphocytes.

• Life Span: few days in blood, several months in

connective tissue

• Migrate through blood to the tissues; once in

tissues they differentiate into phagocytes

(macrophages)

• Macrophages, by releasing cytokines after

activation, coordinate inflammatory and

defensive reactions. nucleuschromatin

The vacuoles contain chemicals

that digest the foreign particles

B. Adipose cells (adipocytes, fat cells)

Their functions are to Store lipids inside the body.

Types:

a. Yellow fat (unilocular).

▪ Each cell contains a single droplet of neutral fat

(triglycerides) for energy storage and insulation.

▪ Minimal cytoplasm, present as a rim around the lipid droplet.

▪ Flattened, crescent-shaped nucleus that conforms to the

contour of the lipid droplet.

b. Brown fat (multilocular).

▪ Cells contain numerous, small lipid droplets.

▪ Large numbers of mitochondria.

▪ Present mostly during early postnatal life in humans,

abundant in hibernating animals for heat production.