100 BIOLOGY
C HAPTER 7
S TRUCTURAL ORGANISATION IN
ANIMALS
7.1 Animal Tissues In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
7.2 Organ and Organ
organisms, all functions like digestion, respiration and reproduction
System
are performed by a single cell. In the complex body of multicellular
7.3 Earthworm animals the same basic functions are carried out by different groups of
7.4 Cockroach cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
7.5 Frogs
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
2020-21
, STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium. Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
Flattened cell
(a)
Cube-like cell
(b)
Tall cell
(d)
(c)
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
2020-21
, 102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
unicellular are mainly of two types: unicellular,
gland
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
Multicelluar
gland (salivary gland). On the basis of the mode of
(a) (b) pouring of their secretions, glands are
divided into two categories namely
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
Multi-
layered than one layer (multi-layered) of cells and thus
cells has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
Figure 7.3 Compound epithelium
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
2020-21
C HAPTER 7
S TRUCTURAL ORGANISATION IN
ANIMALS
7.1 Animal Tissues In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
7.2 Organ and Organ
organisms, all functions like digestion, respiration and reproduction
System
are performed by a single cell. In the complex body of multicellular
7.3 Earthworm animals the same basic functions are carried out by different groups of
7.4 Cockroach cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
7.5 Frogs
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
2020-21
, STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium. Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
Flattened cell
(a)
Cube-like cell
(b)
Tall cell
(d)
(c)
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
2020-21
, 102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
unicellular are mainly of two types: unicellular,
gland
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
Multicelluar
gland (salivary gland). On the basis of the mode of
(a) (b) pouring of their secretions, glands are
divided into two categories namely
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
Multi-
layered than one layer (multi-layered) of cells and thus
cells has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
Figure 7.3 Compound epithelium
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
2020-21
