HISTOLOGY:

EPITHELIA AND GLANDS

CONNECTIVE TISSUE PROPER

CARTILAGE AND BONE

Olaleye O.O.

2B10

INTRODUCTION

The four basic types of tissues in the body are the:

•Epithelial tissue

•Connective tissue

•Muscular tissue

•Nervous tissue

These tissue exist and function in close association with one another.

Epithelia Epithelia are a diverse group of tissues which cover or line all body

surfaces, cavities and

Tubes. They function as interfaces between different biological

compartments. As such, the mediate a wide range of activities such

as;

•Selective diffusion

•Absorption and/or secretion

•Physical protection

•Containment

They are closely bound to one another by a variety of membrane

specialisations called cell junctions which provide physical strength

and exchange of info and metabolites.

All epithelia are supported by basement membrane which separates

epithelia from underlying supporting tissues and are never penetrated

by blood vessels. Thus, epithelia are dependent on the diffusion of

oxygen and metabolites from adjacent supporting tissues.

Arrangement and structure of Epithelial cells

Epithelium is named according to:

• Shape

• Structure

• Arrangement of cells

• Squamous- thin and flat shaped cells

• Cuboidal- cube shaped cells

• Columnar- column shaped cells

• Simple- single layer of cells

• Stratified- multilayered cells

• Pseudo-stratified- false stratified

• Transitional- stretchable

• Ciliated- cells possess cilia

Arrangement and structure of Epithelial cells

Basement Memebrane

The basement membrane is a

thin, noncellular region that

separates the epithelium from

the underlying conective tissue.

Can easily be seen with a light

microscope.

Classification of Epithelia tissues

Epithelium is traditionally classified according to three

morphological characteristics:

•The number of cell layer: a single layer of epithelial cells is

termed SIMPLE epithelium, whereas epithelia composed of

several layers are termed STRATIFIED epithelium

•The shape of the component cells: This is based on appearance of

sections taken at right angles to the epithelial surface.

NB: In stratified epithelia the shape of the outermost layer of cells

determines the descriptive classification. Cellular outline are often

difficult to distinguish, but the shape of epithelial cells is usually

reflected in the shape of their nuclei.

•The presence of surface specialisation such as cilia and keratin:

an example is the epithelia surface of skin whis is classified as

stratified squamous keratinized epithelium.

NB: Epithelial maybe derived from ectoderm, mesoderm or

endoderm origin.

Glands Epithelia

Epithelium which is primarily involved in secretion is often arranged into

structures called GLANDS

Glands are merely invaginations of epithelial surfaces which are formed

during embryonic development by proliferation of epithelium into the

underlying tissues

There different types of glands:

Glands which maintain their continuity with the epithelial surface,

discharging their secretions onto the free surface via a duct, are called

EXOCRINE glands.

In some cases, the duct degenerates during development leaving

isolated islands of epithelial secretory tissue deep within other tissues.

The secretory products of such glands, known as ENDOCRINE or

DUCTLESS glands, pass into the bloodstream; their secretions are

know as hormones.

Glands are cells or aggregations of cells whose function is secretion.

Exocrine glands release the secretory product via a system of ducts that

opens upon one of the surfaces of the body which are in contact with the

external world (skin, gastrointestinal tract etc.).

Endocrine glands release their secretory product (typically hormones)

into the spaces between the secretory cells (extracellular space) from

which it enters the bloodstream.

Both endocrine and exocrine glands are developmentally derived from

epithelia, which form a down-growth into the underlying connective tissue.

The cells forming this down-growth then develop the special

characteristics of the mature gland.

NB: Exocrine glands maintain the connection with the surface epithelium,

whereas the connection is lost by endocrine glands.

Exocrine glands

Exocrine glands may be classified according to cell number,

and/or the shape and branching pattern of their secretory portions

and ducts.

Unicellular Glands: consist of a single secretory cell. In mammals the

only example of unicellular exocrine glands are goblet cells, which occur

in the epithelium of many mucous membranes. Goblet cells secrete the

glycoprotein mucin, which by the uptake of water is converted into a slimy

substance, mucus.

Multicellular glands: The simplest form of a multicellular gland is a secretory

epithelial sheath - a surface epithelium consisting entirely of secretory cells (e.g.

the epithelium lining the inner surface of the stomach, where the mucous secretion

protects the stomach wall from the acidic contents of the stomach). Other

multicellular glands have their secretory portion embedded in the connective

tissue underlying the epithelium. The secretion is either discharged directly from

the secretory portion onto the epithelium or reaches the epithelium via a duct

system that consists of non-secretory cells.

The secretory portion may have a variety of shapes. Secretory cells

may form

tubes in tubular glands,

acini in acinar glands or

alveoli in alveolar glands

Combinations exist - the pancreas is a tubulo-acinar gland, in which each

section of the secretory system has a specialized function.

The precursors of digestive enzymes are produced by the acinar cells. Tubular

cells secrete the alkaline bicarbonate solution which eventually neutralizes the

acidic contents of the stomach that are released into the duodenum.

Multicellular glands with an unbranched excretory duct are

called simple.

When the excretory duct is branched, it is called a compound gland.

Finally, the part of the gland consisting of secretory cells is branched in a

branched gland.

Secretory Mechanisms

The secretory cells can release their secretory products by one of three

mechanisms:

• Merocrine secretion

• Apocrine secretion

• Holocrine secretion

Merocrine secretion: corresponds to the process of

exocytosis. Vesicles open onto the surface of the cell, and the

secretory product is discharged from the cell without any

further loss of cell substance.

Apocrine secretion: designates a mechanism in which part of the

apical cytoplasm of the cells is lost together with the secretory product.

The continuity of the plasma membrane is restored by the fusion of the

broken edges of the membrane, and the cell is able to accumulate the

secretory product anew. This mechanism is used by apocrine sweat

glands, the mammary glands and the prostate.

Holocrine secretion: designates the breakdown and

discharge of the entire secretory cell. It is only seen in

the sebaceous glands of the skin.

There are two additional mechanisms by which secretory cells can release their

products. Lipid soluble substances may diffuse out of the secretory cell (e.g.

steroid hormone-producing endocrine cells). Transporters (membrane proteins)

may actively move the secretory product across the plasma membrane (e.g. the

acid producing parietal cells of the gastric glands). These secretory mechanisms

may not involve any light microscopically visible specialisations of the cell.

Connetive Tissue Proper

This is the most widespread and abundant type of tissue in the human

body. Its function is primarily to support, anchor and connect various

parts of the body. Although connective tissue exists in a number of

forms, all types have three basic structural elements --

cells, fibres and intercellular substance (ground substance).

Connective tissue serves as:

• Packing

• Holds the cells of organs together

• Passes on nutrients to other tissues from the blood

• And active in fighting disease-causing organisms.

The cells in connective tissue are always well spaced in a thick, fluid base

substance, or matrix, in which there may also be long, thin threads called

fibers.

The most common cell types are fibroblasts, which produce fibres and other

intercellular materials. The two most common types of fibres are:

• collagen (collagenous)

• and elastic.

Collagen fibres are for strength while the elastic ones are for elasticity of the

tissue. Both the cells and the fibres are embedded in the intercellular

substance. The consistency of this substance is highly variable from gelatin-

like to a much more rigid material.

The proportions of the cells, fibres, and intercellular substance vary, depending

on a particular nature and function of the connective tissue. For example, a

strong connective tissue needs a greater proportion of the collagen fibres and

fewer cells. An example would be a dense regular connective tissue, which is

found in tendons and ligaments. On the other hand, a connective tissue

composed of mostly cells would not be very strong. An example would be an

adipose (fat) connective tissue.

CLASSIFICATION OF CONNECTIVE TISSUE

• Connective tissue proper

• Specialized connective tissues

Connective tissue proper: encompasses all organs and body cavities

connecting one part with another and, equally important, separating one

group of cells from another. This is a very large and diverse group of

tissues and includes adipose tissue (fat), areolar (loose) tissue, and

dense regular tissue, among others.

Specialized connective tissues: this group includes cartilage, bone,

and blood. Cartilage and bone form the skeletal framework of the body

while blood is the vascular (transport) tissue of animals.

Connective tissue proper

Areolar (Loose) Connective Tissue

• Is the most widespread connective tissue of the body.

• It is used to attach the skin to the underlying tissue.

• It also fills the spaces between various organs and thus holds them in

place as well as cushions and protects them.

• It also surrounds and supports the blood vessels.

• The fibres of areolar connective tissue are arranged in no particular

pattern but run in all directions and form a loose network in the

intercellular material. Collagen (collagenous) fibres are predominant.

• They usually appear as broad pink bands.

• Some elastic fibres, which appear as thin, dark fibres are also

present.

• The cellular elements, such as fibroblasts, are difficult to distinguish in the

areolar connective tissue. But, one type of cells - the mast cells are

usually visible.

• They have course, dark-staining granules in their cytoplasm.

• Since the cell membrane is very delicate it frequently ruptures in slide

preparation, resulting in a number of granules free in the tissue

surrounding the mast cells.

• The nucleus in these cells is small, oval and light-staining, and may be

obscured by the dark granules.

Adipose Connective Tissue

The cells of adipose (fat) tissue are characterized by a large internal fat droplet,

which distends the cell so that the cytoplasm is reduced to a thin layer and the

nucleus is displaced to the edge of the cell.

These cells may appear singly but are more often present in groups.

When they accumulate in large numbers, they become the predominant cell

type and form adipose (fat) tissue.

Adipose tissue, in addition to serving as a storage site for fats (lipids), also pads

and protects certain organs and regions of the body. As well, it forms an

insulating layer under the skin which helps regulate body temperature.

Dense (Fibrous) Regular Connective Tissue

• Dense connective tissue is characterized by:

• an abundance of fibres with fewer cells, as compared to the loose

connective tissue.

• It is also called fibrous or collagenous connective tissue because of the

abundance of collagen (collagenous) fibres.

• Little intercellular substance is present.

• Furthermore, in this tissue type, the fibres are organized in a regular, parallel

pattern. Hence, the name – dense regular (fibrous or collagenous)

connective tissue.

• In addition to the tendons, this type of tissue is also found in ligaments.

Hence, the function of this tissue is to anchor skeletal muscle to bone, to

attach bone to bone as well as to stabilize the bones within a joint.

Fibroblasts are the only cells visible, and are arranged in rows between the

fibres. These fibroblasts function to lay down or create the fibres of the tissue

Cartilage: Specialized Connective Tissues

Cartilage is a somewhat elastic, pliable and compact type of connective

tissue.

It is characterized by three traits:

• lacunae,

• chondrocytes,

• a rigid matrix.

The matrix is a firm gel material that contains fibres and other

substances.

There are three basic types of cartilage in the human body:

• hyaline cartilage,

• elastic cartilage and

• fibrocartilage.

The most common type of cartilage is the hyaline cartilage.

Most of the skeleton of the mammalian fetus is composed of hyaline cartilage.

As the fetus ages, the cartilage is gradually replaced by more supportive bone.

In the mammalian adult, hyaline cartilage is mainly restricted to:

• the nose,

• trachea,

• Bronchi,

• ends of the ribs,

• and the articulating surfaces of most joints

The function of the hyaline cartilage is to provide

slightly flexible support and reduce friction within

joints. It also provides structural reinforcement.

The matrix appears as a smooth, solid, blue or pink-coloured

substance. Fine protein fibres, are embedded in the matrix, but

they are not visible with the light microscope since they do not

stain well.

The large cartilage cells called chondrocytes, are trapped within

the matrix in spaces called lacunae (singular, lacuna).

Cartilage is a non-vascular tissue. As such, the chondrocytes rely

on blood vessels in the tissue surrounding the cartilage for nutrient

supply and waste removal.

BONE

Bone is the main component of the skeleton in the adult human.

Like cartilage, bone is a specialised form of dense connective tissue.

Bone gives the skeleton the necessary rigidity to function as attachment

and lever for muscles and supports the body against gravity.

Two types of bone can be distinguished macroscopically:

• Trabecular or cancellous or spongy bone

• Compact bone

Trabecular or cancellous or spongy bone consists of delicate bars

and sheets of bone, trabeculae, which branch and intersect to

form a sponge like network. The ends of long bones

(or epiphyses) consist mainly of trabecular bone.

Compact bone does not have any spaces or hollows in the bone matrix

that are visible to the eye. Compact bone forms the thick-walled tube of

the shaft (or diaphysis) of long bones, which surrounds the marrow cavity

(or medullary cavity). A thin layer of compact bone also covers the

epiphyses of long bones.

Bone is, again like cartilage, surrounded by a layer of dense connective

tissue, the periosteum.

A thin layer of cell-rich connective tissue, the endosteum, lines the surface of

the bone facing the marrow cavity.

Both the periosteum and the endosteum possess osteogenic potency.

Following injury, cells in these layers may differentiate into osteoblasts (bone

forming cells) which become involved in the repair of damage to the bone.

Compact Bone

Compact bone consists almost entirely of extracellular substance, the matrix.

Osteoblasts deposit the matrix in the form of thin sheets which are

called lamellae.

Lamellae are microscopical structures.

Collagen fibres within each lamella run parallel to each other.

Collagen fibres which belong to adjacent lamellae run at oblique angles to each

other.

Fibre density seems lower at the border between adjacent lamellae, which

gives rise to the lamellar appearance of the tissue. Bone which is composed by

lamellae when viewed under the microscope is also called lamellar bone.

In the process of the deposition of the matrix, osteoblasts become encased in

small hollows within the matrix, the lacunae.

Unlike chondrocytes, osteocytes have several thin processes, which extend

from the lacunae into small channels within the bone matrix , the canaliculi.

Canaliculi arising from one lacuna may anastomose with those of other lacunae

and, eventually, with larger, vessel-containing canals within the bone.

Canaliculi provide the means for the osteocytes to communicate with each other

and to exchange substances by diffusion.

In mature compact bone most of the individual lamellae form concentric rings

around larger longitudinal canals (approx. 50 µm in diameter) within the bone

tissue.

These canals are called Haversian canals. Haversian canals typically run

parallel to the surface and along the long axis of the bone. The canals and the

surrounding lamellae (8-15) are called a Haversian system or an osteon. A

Haversian canal generally contains one or two capillaries and nerve fibres.

Irregular areas of interstitial lamellae, which apparently do not belong to any

Haversian system, are found in between the Haversian systems.

Immediately beneath the periosteum and endosteum a few lamella are found

which run parallel to the inner and outer surfaces of the bone. They are

the circumferential lamellae and endosteal lamellae.

A second system of canals, called Volkmann's canals, penetrates the

bone more or less perpendicular to its surface.

These canals establish connections of the Haversian canals with the inner

and outer surfaces of the bone.

Vessels in Volkmann's canals communicate with vessels in the Haversian

canals on the one hand and vessels in the endosteum on the other.

A few communications also exist with vessels in the periosteum.

Trabecular Bone

The matrix of trabecular bone is also deposited in the form of lamellae.

In mature bones, trabecular bone will also be lamellar bone.

However, lamellae in trabecular bone do not form Haversian systems.

Lamellae of trabecular bone are deposited on pre-existing trabeculae

depending on the local demands on bone rigidity.

Osteocytes, lacunae and canaliculi in trabecular bone resemble those in

compact bone.