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An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities.
It serves for protection, secretion and excretion.
The word ‘epithelium’ was introduced by Ruysch.
Epithelial tissue evolved first in animal kingdom.
It originates from all the three primary germ layers. e.g. Epidermis arises from ectoderm, Coelomic epithelium from the mesoderm and epithelial lining of alimentary canal from the endoderm.
Types of Epithelium
Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their ducts system as distinguishing characteristics.
Shape include tubular and alveolar (Sac like).
Simple exocrine glands e.g. intestinal glands, mammalian sweat glands, cutaneous glands of frog etc. have only one duct leading to surface.
Compound exocrine glands have two or more ducts e.g. liver, salivary glands etc.
Structural classification of exocrine glands:
Type
Example
Simple tubular
Intestinal glands, crypts of Lieberkuhn in ileum.
Simple coiled tubular
Sweat glands in man
Simple branched tubular
Gastric (stomach) gland, and Uterine gland.
Simple alveolar
Mucous gland in skin of frog, Poison gland of toad and seminal vesicle.
Simple branched alveolar
Sebaceous glands
Compound tubular
Brunner’s gland, bulbourethral gland and liver.
Compound alveolar
Sublingual and submandibular parotid salivary gland
Compound tubulo alveolar
Parotid salivary glands, Mammary gland and Pancreas.
Study of tissue outside the body in a glass tube is known as in vitro, while study of living tissues in situ is known as in vivo.
Among epithelia, simple epithelia were first to evolve.
Transitional epithelium also called plastic epithelium or urothelium. It lacks basement membrane
False epithelium derived from mesenchyma a diffuse network of tissue derived from embryonic mesoderm) and lining the synovial cavities.
Mammary glands without teats are present in prototheria.
A malignant tumour arising from an epithelium is called a carcinoma. If it arises from a squamous epithelium it is a squamous cell carcinoma and if it arises from glandular epithelium it is called an adenoma.
The epithelial lining of brain ventricles and central canal of spinal cord is known as ependyma.
Stereocilia are elongated membrane outgrowths found in certain parts of male reproductive tract.
Muscle cells are highly contractile (contracting to 1/3 or 1/2 the resting length).
Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology.
About 40% to 50% of our body mass is of muscles.
The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres.
These possess large numbers of myofibrils formed of actin and myosin.
(f) Difference between three types muscle fibres
S.No.
Feature
Striated or Striped or Skeletal or Voluntary muscle fibres
Non-striated or Unstriped or Smooth or Visceral or Involuntary muscle fibres
Cardiac muscle fibres
1.
Shape
Long cylindrical
Fusiform (thick in middle tapering at ends) (0.02 nm to 0.2 nm long)
Network of fibres
2.
Stripes
Dark A bands and light I bands present
Absent
Present
3.
Nucleus
Many (syncytial) at periphery
Single at the centre of each cell
Many nuclei between successive end plates central position
4.
Unit
Sarcomeres, cylindrical long myofibrils placed end to end forming cylindrical myofibrils
Fusiform cells with inconspicuous borders
Oblique cross-connecting fibres make this muscle an interconnected bundle of myofibrils
5.
Attachment
To bones
To soft organs or viscera
Not attached to other organs except major blood vessels which are isolated and covered by pericardium
6.
Sarcolemma
Distinct
7.
Sarcoplasmic Reticulum
Well developed
Less extensive
Poorly formed
8.
Blood supply
Rich
Poor
9.
Contraction
Quick, fatigue fast
Slow, sustained contraction
Rhythmic, contractions originate in heart (pace maker immune to fatigue)
10.
Location
Generally peripheral, tongue, proximal part of oesophagus
Central, in hollow visceral organs, iris of the eye, dermis of the skin
Only in heart
11.
Intercalated discs
12.
T-tubule system
Lacking
13.
Innervated nerves
Motor nerves from central nervous system (neurogenic)
Nerves from autonomic nervous system (neurogenic)
Nerves from central and autonomic nervous system (myogenic)
14.
Fibres
Unbranched
Fibres join by short oblique bridges
15.
Action
Voluntary
Involuntary
It connects and supports all the other tissues, the intercellular element predominating.
The cellular element is usually scanty. In function this tissue may be mechanical, nutritive and defensive.
It is a tissue made up of matrix (abundant intercellular substance or ground substance) and living cells that connects and support different tissues.
Connective tissue was called mesenchyme by Hertwig (1893).
Types of connective tissues
(1) On the basis of their texture:
The bones are divided into two categories spongy or cancellous or tubecular bones and compact or periosteal bones
Bone
Cartilage
1. Matrix is composed of a tough, inflexible material, the ossein.
1. Matrix is composed of a firm, but flexible material, the chondrin.
2. Matrix is always impregnated with calcium salts.
2. Matrix may be free or impregenated with calcium salts.
3. Bone cells lie in lucunae singly.
3. Cartilage cells lie in lacunae singly or in groups of two or four.
4. Osteocytes are irregular and give off branching processes in the developing bone.
4. Chondroblasts are oval and devoid of processes.
5. Lacunae give off canaliculi.
5. Lacunae lack canaliculi.
6. There are outer and inner layers of special bone forming cells, the osteoblasts, that produce new osteocytes, which secrete new lamellae of matrix.
6. There are no special cartilage-forming cells. Cartilage grows by division of all chondroblasts.
7. Matrix occurs largely in concentric lamellae.
7. Matrix occurs in a homogenous mass.
8. Bone is highly vascular.
8. Cartilage in nonvascular.
9. Bone may have bone marrow at the centre.
9. No such tissue is present.
(2) On the basis of origin of bone:
Ossification or osteogenesis is the process of bone formation. A bone is classified into four categories.
Characters
Spongy bone
Compact bone
Arrangement of lamellae
There is no regular Haversian system so have spongy texture.
Have regular Haversian system
Occurrence
In skull bones, ribs, centrum of vertebrae and epiphyses of long bones
In the shaft (diaphysis) of long bones
Marrow cavity
Broad
Narrow
Type of bone marrow
Red marrow in the spaces between lamellae
Yellow marrow in marrow cavity
Function
Marrow forms RBCs and Granular WBCs
Marrow stores fats
(3) On the basis of treatment:
These are of two types :-
Dried bone
Decalcified bone
Type of treatment
Subjected to high temperature.
Subjected to dilute solution of HCl.
Nature of matter left
With only mineral matter.
With only organic matter.
Empty.
With bone-marrow.
Fate of cells
Periosteum, endosteum, osteoblasts and osteocytes are absent being killed by high temperature.
Periosteum, endosteum, osteoblasts and osteocytes all are present.
Lacunae
Lacunae present.
Lacunae absent.
(6) Number of RBC: The number of RBCs is counted by instrument haemocytometer. The total number of RBC per cubic mm of blood is called RBC count. RBC count is slightly lower in women than a man and number of RBC is more in people who live on mountains because there is less oxygen. RBC are absent in cockroach.
Organism
Number of RBCs
Male
5 – 5.4 million / cubic mm of blood
Female
4.5 – 5 million / cubic mm of blood
Infants
65 – 70 lacs/ cubic mm of blood
Embryo
85 lacs/ cubic mm of blood
Rabbit
70 lacs / cubic mm of blood
Frog
4 lacs / cubic mm of blood
(7) Life span of RBC: The life span of red blood corpuscles circulating in the blood stream varies in different animals. RBCs have longest life span in blood. The mammalians RBC have short life span due to absence of nucleus, which is disappeared during development.
Life span of RBCs
Mammals and Human
120 days or 4 months
80 days
100 days
New born
(8) Function of RBCs: The major function of erythrocytes is to receive O2 of respiratory surfaces and then transport and readily deliver it to all cells of body. This important function is performed by haemoglobin which has a great ability to combine loosely and reversibly with O2 and is, hence, called “respiratory pigment”. Haemoglobin, in annelids, is dissolved in the plasma because of absence of red blood corpuscles. In mollusc and some arthropods, etc., a different respiratory pigment, haemocyanin is found dissolved in the plasma. This pigment is bluish due to presence of copper in place of iron.
(9) Comparison Between Blood and Lymph
Blood
Lymph
1. Red corpuscles present.
1. These are absent.
2. White corpuscles fewer, neutrophils most numerous.
2. White corpuscles more; lymphocytes most numerous.
3. Soluble proteins more than insoluble proteins.
3. Insoluble proteins more than soluble proteins.
4. Amount of nutrients and O2 comparatively more.
4. Amount of nutrients and O2 comparatively less.
5. Amount of CO2 and metabolic wastes normal.
5. Amount of these much more.
Argentaffin cells which produce a precursor of serotonin, a potent vasoconstrictor hormone, occurs in intestinal cells.
The brown adipose tissue in human is restricted till third month of post natal life.
White fibres yield gelatin on boiling and are digestible with enzyme pepsin but yellow (elastic) fibres are not digestible by enzyme trypsin.
The fat in the globules is stored in the form of triglycerides.
The Cytoplasmic granules basophils contain histamine.
Sprain – Excessive pulling of ligaments.
Plasma cells are also called as “Cart wheel cells”.
Collagen constitutes about 33% of total body protein.
A most complex tissue in the body, composed of densely packed interconnected nerve cells called neurons (as many as 1010 in the human brain).
It specialized in communication between the various parts of the body and in integration of their activities.
Nervous tissue is ectodermal (from neural plate) in origin.
Axon
Dendron
1. Number
Always single
May be one or more in number
2. Structure
Formed of neuroplasm with only neurofibrils but no Nissl’s bodies.
Formed of neuroplasm with both neurofibrils and Nissl’s bodies
3. Size
Long sized processes
Small sized processes
4. Direction of new impulses
Always away from the cell body
Always towards the cell body
5. Nature
Efferent
Afferent
6. Branching
Generally absent
Generally present
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