Zoology is studied as a ‘pure science’ (knowledge gaining) and it has application in other branches such as euphenics, eugenics, biotechnology, bioenergetics, bioinformatics, etc. As applied science, it has tremendous scope in agriculture, aquaculture, animal husbandry, human health, diseases, ...
Basophils
They are the least common type of WBC and constitute about 0.4% of the total
leucocytes. Nucleus is partially divided into 2 or 3 lobes. Cytoplasmic granules are
‘fewer’ and ‘irregular’ in shape. They take basic stains. They produce heparin,
histamine, etc. They supplement the function of mast cells when needed.
Eosinophils (acidophils)
They constitute about 2.3% of the total leucocytes. Nucleus is distinctly bilobed.
Cytoplasm has large granules which stain with acidic dyes such as ‘eosin’. They play
a role in allergic reactions. Their number increases during ‘allergic reactions’ and
‘helminth infections’. They remove ‘antigen - antibody complexes’, from the blood.
They are capable of phagocytosis and are not as effective as neutrophils in killing bacteria.
Neutrophils
They are the most common type of WBC and constitute about 62% of the total
leucocytes. Nucleus is many lobed (2-5). Specific cytoplasmic granules are small and
abundant. They stain with ‘neutral dyes’. These are active phagocytic cells commonly
described as ‘microscopic policemen’. Certain neutrophils of female mammals have
sex chromatin body or Drumstick body (an extra ’X’ chromosome) attached to the
nucleus .
Agranulocytes
Cytoplasmic granules are absent in agranulocytes. Nucleus of these cells is not
divided into lobes. These are of two types:
Lymphocytes: They constitute about 30% of the total leucocytes. They are
small, spherical cells with large spherical nucleus and scanty peripheral
cytoplasm. There are functionally two types of lymphocytes -‘B’ lymphocytes,
which produce ‘antibodies’ and ‘T’ lymphocytes which also have an important
role in the immunological reactions of the body.
Lymphocyte Some lymphocytes live for only a few days while others survive many years.
,Structural organisation in Animals 53
Monocytes
They constitute about 5.3% of the leucocytes . The nucleus is kidney
shaped (reniform). These are the largest, motile phagocytes. They engulf
bacteria and cellular debris (wastes). They differentiate into macrophages,
when they enter the connective tissues.
iii. Blood platelets (Thrombocytes)
Monocyte
These are colourless non-nucleated, round or oval biconvex discs.
Number of platelets per cubic mm of blood is about 2,50,000 -
4,50,000. They are formed from giant megakaryocytes produced
in the red bone marrow, by fragmentation. The average life-span of
blood platelets is about 5 to 9 days. They secrete thromboplastin
and play an important role in blood clotting. They adhere to the
damaged endothelial lining of capillaries and ‘seal’ minor vascular
openings.
2. Lymph Blood platelets
Lymph is a colourless fluid. It lacks RBC, platelets and large
plasma proteins, but has more number of leucocytes. It is chiefly composed of plasma
and lymphocytes. When compared to the tissue fluid, it contains very small amounts of
nutrients (except fats) and oxygen, but has abundant CO2 and other metabolites. The
most important site of formation of lymph is interstitial space. As blood passes through
the blood capillaries, some portion of blood that includes water, solutes and proteins of
low molecular weight passes through the walls of capillaries, into the interstitial spaces
due to hydrostatic pressure at the arteriolar ends. This fluid forms the interstitial
fluid (tissue fluid). Most of the interstitial fluid is returned directly to the capillaries due
to osmotic pressure at the venular ends. Little amount of this tissue fluid passes through
a system of lymphatic capillaries (lymph capillaries of the intestinal villi are called
‘lacteals’), vessels, lymph ducts and finally reach the blood through the subclavian veins.
The extracellular ‘tissue fluid’ after passing into the lymph capillaries and lymph vessels
is called ‘lymph’. Lymphatic system represents an ‘accessory route’ by which interstitial
fluid flows from tissue spaces into blood.
2.4.3 Muscular Tissue
Muscular tissue is mesodermal in origin, except muscles of the iris and ciliary
body, which are ectodermal in origin. Muscles show three essential properties such as
excitability, conductivity and contractility. Muscle fibers contract (shorten) in
response to stimulation, then relax (lengthen or return to their un-contracted state), in
a coordinated fashion. Muscles play an active role in the movements of the body to
adjust to changes in the surrounding environment and to maintain the posture of the
body. The study of muscular tissues is known as myology. Muscular tissue has
,54 Zoology
elongated cells called ‘muscle fibers’ (myocytes) which are surrounded by a connective
tissue sheath. Extracellular matrix is absent. The plasma membrane of a muscle fibre is
called sarcolemma. The cytoplasm of a muscle fibre is called sarcoplasm, the
endoplasmic reticulum, the sarcoplasmic
Bone
reticulum, and the mitochondria, the
Perimysium
Blood vessel
sarcosomes. The cytoplasm of a muscle fibre has
several myofibrils. Each myofibril has thick
(myosin) and thin (actin) myofilaments. The
regular arrangement of myosin and actin
filaments is responsible for the alternate dark and
Muscle
Fibre light bands of a ‘striated muscle’. Sarcoplasm also
Epimysium contains ATP, phosphocreatine, glycogen and
Tendon Endomysium Fascicle myoglobin. Muscles are of three types - skeletal,
Fig. 2.20 (a) Skeletal muscle
smooth and cardiac.
1. Skeletal (striped and voluntary) muscle
It is usually attached to skeletal structures by ‘tendons’. In a typical
muscle such as the ‘biceps’ muscle, skeletal muscle fibre is
Dark band
surrounded by a thin connective tissue sheath, the endomysium.
A bundle of muscle fibres is called a fascicle. It is surrounded by a
Light band
connective tissue sheath called perimysium.A group of fascicles form
Nucleus a ‘muscle’ which is surrounded by an epimysium (outer most
Sarcolemma
connective tissue sheath).These connective tissue layers may extend
beyond the muscle to form a chord-like tendon or sheet-like
aponeurosis.
A skeletal muscle fibre is a long, cylindrical and unbranched
Fig. 2.20 (b) Striped cell. It is a multinucleated cell with many oval nuclei
muscle fibre
characteristically in the “peripheral” cytoplasm (a syncytium formed
by fusion of cells). Sarcoplasm has many myofibrils which show alternate dark and
light bands. So it is called striped or striated muscle. Skeletal muscle
usually works under the conscious control of an organism (a
voluntary muscle). Skeletal muscle contracts quickly and
undergoes fatigue quickly. They are innervated by the ‘somatic
nervous system’. Satellite cells are quiescent (quiet and inactive),
Nucleus
mononucleate and myogenic cells (cells that can generate muscle
cells) and help in regeneration, which is ‘limited’.
2. Smooth (unstriped and involuntary) muscle
It is located in the walls of the visceral organs such as blood
Fig. 2.20 (c) Smooth vessels, trachea, bronchi, stomach, intestine, excretory and genital
muscle fibre
, Structural organisation in Animals 55
ducts, and so this is also called ‘visceral muscle’. As cross striations are absent, it is
called ‘smooth muscle’. It is also found in iris and ciliary body of eye and in the skin
as ‘arrector pili muscles’ that are attached to hair follicles.
Usually smooth muscles are arranged in ‘layers’/‘sheets’. A smooth muscle fibre
is a spindle shaped (fusiform), uninucleate cell. Myofibrils do not show alternate dark
and light bands due to irregular arrangement of actin and myosin molecules. They do
not work under the conscious control, and so they are called involuntary muscles.
Smooth muscles exhibit ‘slow’ and ‘prolonged’
contractions. They may remain contracted for long Intercalated
Nucleus disc
periods without fatigue (show sustained
involuntary contractions called ‘spasms’). The
contraction of smooth muscles is under the control
of the autonomous nervous system.
3.Cardiac (striped and involuntary) muscle
The cardiac muscle is striated like the skeletal
muscle (shows sarcomeres). Cardiac muscle is
Fig. 2.20 (d) Cardiac muscle
found in the ‘myocardium’ of the heart of vertebrates.
The cardiac muscle cells or the ‘myocardial cells’ are short, cylindrical, mononucleate
or binucleate cells whose ends branch and form junctions with other cardiac muscle
cells. Each myocardial cell is joined to adjacent myocardial cells by ‘electrical synapses’
or ‘gap junctions’. They permit ‘electrical impulses’ to be conducted along the long
axis of the cardiac muscle fibre. The dark lines across cardiac muscle are called
intercalated discs (IDs). These discs are characteristic of the cardiac muscle and are
called communication junctions which allow the cells to contract as a unit. Intercalated
discs contain three different types of cell junctions and the ‘gap junctions’ which are
responsible for rapid conduction of action potentials.
The cardiac muscle of vertebrates does not require any nerve stimulus to contract
because it can produce impulses spontaneously from a specialised auto-rhythmic
structure, the ‘pace maker’ (myogenic heart). As such, the cardiac muscle is
involuntary in its function. However, the rate of heart beat is regulated by an
autonomic innervation and hormones such as epinephrine/adrenaline. The
excitation of one myocardial cell results in the excitation of all other myocardial
cells quickly to produce a ‘whole hearted’ contraction’, of the entire muscle as a
single unit. Thus the cardiac muscle is described as a ‘functional syncytium’. The
cardiac muscle is highly resistant to fatigue, because it has numerous sarcosomes,
many molecules of myoglobin (oxygen storing pigment) and rich supply of blood
which facilitate ‘continuous aerobic respiration’.
2.4.4 Nervous Tissue
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