what is tissue .....explain

Introduction

The human body consists of around 50 trillion to 106 trillion individual structural working
units called cells (Marieb and Hoehn, 2008). Cells work together to ensure that homeostasis
is maintained.
Cells come in many different shapes, sizes and life spans; however, they can
be categorised
depending on their structure and functions. A group of cells that have a
similar structure and function are called tissue, and within the human body there are four
distinct types of tissue. Cells that provide a covering for organs and structures, for example,
are referred to as epithelial tissue, whereas cells that provide support for structures are
called connective tissue. Cells that govern body movement are muscle tissue, and cells that
help control homeostasis are nervous tissue. Most organs of the body contain a selection of
all four tissue types. The heart, for example, contains muscle tissue, is controlled by nervous
tissue, lined by epithelial tissue and supported by connective tissue. Tissue also has the
capacity to repair itself. This chapter examines
all four types of tissue and the process of
tissue
repair.

Epithelial tissue

Epithelial tissue is essentially a sheet of cells that covers an area of the body. Epithelial tissue
covers or lines body surfaces (i.e. skin), or it lines the walls and the organs within body cavities.
The major role of epithelial tissue is as an interface; indeed, nearly all the substances absorbed
or secreted by the body must pass through epithelial tissue. Broadly speaking, epithelial tissue
has six main functions:
•• absorption
•• protection
•• excretion
•• secretion
•• filtration
•• sensory reception.
Not all epithelial tissue carries out all six functions. In many areas of the body epithelial tissue
specialises in just one or two functions. Epithelial tissue in the digestive system, for example,
specialises in absorption of nutrients, whereas epithelial tissue within skin provides a protective
layer.
Epithelial tissue cells are closely bonded together in continuous sheets, which have an
apical
and a basal surface. The apical surface faces outwards, towards the exterior of the
organ it covers. Apical surfaces can be smooth, but most have hair‐like extensions called
microvilli. Microvilli dramatically increase the surface area of the epithelial tissue and therefore
increase its ability for absorption and secretion. Some areas, within the respiratory tract for
example, possess larger hair‐like extensions called cilia, which are also capable of propelling
substances. Lying close to the basal surface is a thin sheet of glycoproteins that acts as a
selective filter, governing which substances can enter epithelial tissue. Epithelial tissue is
innervated by neurones, but it has no blood supply as such. Rather than being served by a
network of capillaries, epithelial tissue receives a supply of nutrients from nearby blood
vessels.
Owing to its protective role, epithelial tissue needs to endure a great deal of abrasion
and environmental damage, and epithelial cells need to be very hardy and tough. This hardiness
is generated by their ability to divide and regenerate rapidly, resulting in the swift
replacement of damaged epithelial cells. However, this regenerative capacity is reliant upon
a plentiful supply of nutrients.
Epithelial tissue can be categorised into the following three distinct types:
•• simple
•• stratified
•• glandular.
Simple epithelium consists of a single layer of cells bound into a continuous sheet. Stratified
epithelium is also arranged into a continuous sheet but is thicker with numerous layers of cells.
Glandular epithelium forms the glands of the body.
All epithelial cells have six sides; indeed, under a microscope a cross‐section of epithelial
tissue
looks like a honeycomb. Epithelial cells can be subdivided further into the following three
different six‐sided shapes:
•• cuboidal
•• columnar
•• squamous.
As their names suggest, cuboidal and columnar epithelial cells are square and tall respectively,
whereas squamous epithelial cells are rather flat and scaly (see Figure 4.1). When examining the
many different types of epithelial cell it is easy to work out its size and shape by its name.
For instance, simple squamous epithelium is thin, flat and scale‐like.

Simple epithelium

Because simple epithelia consist of a single cellular layer it specialises in absorption, secretion
and filtration rather than protection.
Simple squamous epithelium is quite often very permeable and is found where the diffusion
of nutrients is essential. Capillary walls, the alveoli of the lungs and the glomeruli in the kidneys
are all lined with simple squamous epithelium, which facilitates the rapid diffusion of nutrients.
Simple squamous epithelium is also found within the heart and blood and lymph vessels. Simple
squamous epithelium found within the heart and blood and lymph vessels is called endothelium
Simple cuboidal epithelium specialises in secretion as well as absorption. Simple cuboidal
epithelium is found in the lining of the ovaries, the kidney tubules and the ducts of smaller
glands. It also forms part of the secretory portions of glands such as the thyroid and pancreas
(see Figure 4.3).
Simple columnar epithelium can be ciliated or non‐ciliated. As its name suggests, ciliated
simple columnar epithelium has cilia on its apical surface. It is found in areas of the body where
movement of fluids, mucus or other substances is required. Ciliated simple columnar epithelial
tissue, for example, lines the passageways of the central nervous system and helps propel cerebrospinal
fluid. It also lines the Fallopian tubes and helps move oocytes recently expelled from
the ovaries (see Figure 4.4). A common location for non‐ciliated simple columnar epithelium is
the lining of the digestive tract from the stomach to the rectum (see Figure 4.5). Non‐ciliated
simple columnar epithelium performs two broad functions. Some possess microvilli, greatly
increasing their surface area for absorption; others specialise in the secretion of mucus. Such
cells are referred to as goblet cells owing to their cup‐like shape. Simple columnar epithelial cells
are generally of equal size. However, in some instances simple columnar epithelial cells vary in
height, with only the tallest reaching the apical surface. This gives the illusion that the tissue has

Stratified epithelium

Unlike simple epithelia, stratified epithelia have many layers. The cells regenerate from below,
with new cells dividing on the basal layer pushing the older cells towards the surface. As stratified
epithelium is thicker, its principal function is protection.
Stratified squamous epithelium is the most common stratified epithelium and forms the
external part of skin (see Chapter 17). Stratified squamous epithelial tissue is keratinised,
toughened
by the presence of keratin, a special tough fibrous protein. Non‐keratinised stratified
squamous epithelial tissue lines wet areas of the body – the mouth, the tongue and the vagina
for example (see Figure 4.7). Only the outer layers of stratified squamous epithelium are actually
squamous in shape; the basal layers may be cuboidal or columnar.
Stratified cuboidal epithelium is found in the oesophagus, sweat glands and in the male
urethra (see Figure 4.8). Stratified columnar epithelium, however, is quite rare. Small amounts
can be found in the male urethra and in the ducts of some glands. Another common example of
stratified epithelium is transitional epithelium, which may have both squamous and cuboidal
cells in its apical surface. The basal surface may contain both cuboidal and columnar cells.
Transitional epithelium can withstand a great deal of stretch and is found in organs such as the
bladder, which is subject to considerable distension (see Figure 4.9)

Glandular epithelia

The glands of the body are formed by glandular epithelia. All glands are classified as endocrine
or exocrine. Glands that secrete their products internally are called endocrine glands. Endocrine
glands release hormones, regulatory chemicals for use elsewhere in the body (see Chapter 15).
Exocrine glands release their products onto the surface of epithelial tissue. Exocrine glands are
either unicellular or multicellular. Unicellular exocrine glands consist of a single cell type and the
main example is the goblet cell, which releases a glycoprotein called mucin. Once dissolved in
water mucin forms mucus, which lubricates and protects surfaces. Multicellular exocrine glands
are far more complex, coming in several shapes and sizes. Some exocrine glands are simple and
consist of a single branched duct, whereas others are more complex with multibranched ducts
(see Figure 4.10). However, they all contain two distinct areas: an epithelial duct and secretory
cells (acinus). Exocrine glands that are tubular in shape can be found within the digestive system
and stomach. Other exocrine glands are spherical and referred to as alveolar or acinar. The oil
glands within skin and mammary glands are two examples of spherical‐ or acinar‐shaped
exocrine
glands. Glands that are both tubular and acinar are referred to as tubulacinar.
The salivary
glands, for example, are tubulacinar.

Connective tissue

Connective tissue is the most abundant tissue in the human body. Its main functions are to bind
tissues together, reinforcement, insulation, protection and support. All epithelial tissue is
reinforced
by the connective tissue base it rests upon (see Figure 4.11). There are four types of
connective tissue:
•• connective tissue proper
•• cartilage
•• bone
•• liquid connective tissue.
Connective tissue is not present on body surfaces and, unlike epithelial tissue, is highly
vascular
and receives a rich blood supply.

The following types of cell are present in connective tissue

•• adipocytes
•• primary blast cells
•• macrophages
•• plasma cells
•• mast cells
•• leucocytes (white blood cells).
Adipocytes are fat cells. Within connective tissue adipocytes store triglycerides (fats). Primary blast
cells continually secrete ground substance and produce mature connective tissue cells. Each type of
connective tissue contains its own unique primary blast cells (see Table 4.1). Macrophages, plasma
cells and white blood cells form part of the body’s immune system. Their functions are as follows:
•• Macrophages engulf invading substances and plasma cells produce antibodies.
•• White blood cells are not normally found in significant numbers within connective tissue;
however, they do migrate into connective tissue during inflammation.
•• Mast cells produce histamine, which promotes vasodilatation during the body’s inflammatory
response.