· Definition: A group of specialized cells similar in embryonic origin, form and function is called tissue.
· But the term histology was given by a German histologist Mayer 1819.
· Study of tissue is called histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Types of Animal Tissues:
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
B. MUSCULAR TISSUE
b. Simple cuboidal epithelium
i. Structure: They are cube-like in shape and have a centrally placed spherical nucleus (cube-shaped).
ii. Location: They are found in the salivary gland, pancreatic duct and collecting tubules of kidney thyroid vesicles ovaries seminiferous tubules of testes choroid an iris of eyes, the inner surface of the lens, pigmented layer of the retina.
iii. Function: They have secretary, absorptive, gametes formation and excretory in functions.
· This epithelium is further divided into 2 types:
I. Brush bordered cuboidal epithelium
· In this epithelium, microvilli are present on the free surface of the cell to increase the absorptive surface area
i. Location: Proximal convoluted tubule (PCT) of the kidney is an example of this epithelium.
ii. Function: They enhance the absorption of the required materials.
· It has many layers of horny scale-like remains of dead squamous cells due to deposition of keratin protein which ultimately flakes away and several deeper layers have many polygonal cells.
· Functions: impervious to water and highly resistant to mechanical abrasions.
2. Non keratinised squamous epithelium
· This epithelium does not have keratin protein. So they are unable to check the water loss and provide moderate protection against abrasion.
· Location: They are located in the lining of the buccal cavity, pharynx, tongue, oesophagus, anal canal, vocal cord, the inner surface of eyelids conjunctiva and cornea of the eye.
II. Stratified cuboidal epithelium
· It has outer layers of cuboidal cells and basal layer of columnar cells.
· Location: epidermis of fishes and amphibians, duct of sweat gland, salivary gland and pancreas.
· Function: This epithelium protects the other tissues from chemical and mechanical injuries.
III. Stratified columnar epithelium
· It has many columnar cells on the superficial layers and layer of cuboidal cells.
· Location: the lining of ducts of mammary glands and parts of the urethra. It also covers the epiglottis.
· Function: It is protective, secretary in function.
2. Apocrine glands
· In these glands, the secretory products first accumulate in the apical part of the cells and later this apical part break off to discharge the secretion with some amount of the cytoplasm.
b. Compound glands
· They have many ducts and tubular or alveolar secretory parts.
C. CONNECTIVE TISSUE
D. NERVOUS TISSUE
Epithelial tissue (Gr. Epi-upon, thelia-grows)
· The term “epithelium” was given by a Dutch anatomist Ruysch which means the tissues that grow upon the other tissues.1. General Structure
· They have no intercellular spaces because the cells are held together by the small amount of cementing materials called desmosomes and intercellular bridges.
· The tissue is arranged in a single or multilayered sheet.
· They have no intercellular spaces because the cells are held together by the small amount of cementing materials called desmosomes and intercellular bridges.
· The tissue is arranged in a single or multilayered sheet.
· The cells of the lowest layer always rest upon the non-cellular basement membrane.
· Free surface of the cell may have cilia, stereocilia and microvilli.
· Blood vessels are absent in epithelial tissue and nutrients enter into it through a simple diffusion process from underlying connective tissue.
· Free surface of the cell may have cilia, stereocilia and microvilli.
· Blood vessels are absent in epithelial tissue and nutrients enter into it through a simple diffusion process from underlying connective tissue.
· But nerve endings may penetrate in the epithelium.
·They are derived from all embryonic germ layers: ectoderm (mouth, skin, anus), mesoderm (endothelium of blood vessels) or endoderm (inner lining of respiratory system, digestive tract, pancreas, liver).
· They have good power of regeneration after injury.
·They are derived from all embryonic germ layers: ectoderm (mouth, skin, anus), mesoderm (endothelium of blood vessels) or endoderm (inner lining of respiratory system, digestive tract, pancreas, liver).
· They have good power of regeneration after injury.
2. Location
· They cover the external surface of the body, internal surface of visceral organs, body cavity, blood vessels and various ducts.
· They cover the external surface of the body, internal surface of visceral organs, body cavity, blood vessels and various ducts.
3. General functions of Epithelial tissue
· They serve a variety of functions which are as follows:
· They have protective functions. They protect from mechanical injuries, infection, harmful chemicals and drying up.
· They produce many exoskeleton structures such as hair, nail, hoof, feather and teeth etc.
· They produce sperms and ova from the germinal epithelium of testes and ovaries.
· Sensory epithelia help to receive and conduct the stimuli.
· Epithelium of the lung helps in gaseous exchange, absorption, excretion filtration and conduction of materials.
· Pigmented epithelia of retina darken the cavity of the eyeball.
· They serve a variety of functions which are as follows:
· They have protective functions. They protect from mechanical injuries, infection, harmful chemicals and drying up.
· They produce many exoskeleton structures such as hair, nail, hoof, feather and teeth etc.
· They produce sperms and ova from the germinal epithelium of testes and ovaries.
· Sensory epithelia help to receive and conduct the stimuli.
· Epithelium of the lung helps in gaseous exchange, absorption, excretion filtration and conduction of materials.
· Pigmented epithelia of retina darken the cavity of the eyeball.
4. Various Types of Epithelial Tissues
1. Simple epithelium (Types: Squamous, Cuboidal, Columnar, Pseudostarified)
2. Compound epithelium (Types: Stratified squamous, Stratified cuboidal, Stratified columnar, Transitional)
3. Specialized epithelium (Types: Ciliated, Sensory, Germinal, Glandular)
1. Simple Epithelium
· It consists of a single layer of cells so all the cells rest upon the non-cellular basement membrane.· They occur mainly in secretary and absorptive areas but are never present in the area that is exposed to mechanical and chemical abrasions.
· Simple epithelium also further divisible into the following types:
a. Simple squamous or Pavement epithelium
a. Simple squamous or Pavement epithelium
b. Simple cuboidal epithelium (Types: Brush Bordered & Ciliated cuboidal)
c. Simple columnar epithelium (Types: Brush Bordered & Ciliated columnar)
d. Pseudostratified epithelium
a. Simple squamous or Pavement epithelium
· The cells of this epithelium are flattened thin with centrally placed large nuclei with tessellated or regular margin fit closely together like the tiles on the floor (polygonal-shaped).
· So they are also known as pavement epithelium.
· So they are also known as pavement epithelium.
i. Location: They are present in the inner lining of bronchioles, alveoli, Bowman’s capsules of kidney, in the endothelium of blood vessels, lymph vessels, coelomic cavities and heart and a membranous labyrinth of the internal ear.
ii. Function: They help to protect, excrete, secrete and exchange the gases.
b. Simple cuboidal epithelium
i. Structure: They are cube-like in shape and have a centrally placed spherical nucleus (cube-shaped).
ii. Location: They are found in the salivary gland, pancreatic duct and collecting tubules of kidney thyroid vesicles ovaries seminiferous tubules of testes choroid an iris of eyes, the inner surface of the lens, pigmented layer of the retina.
iii. Function: They have secretary, absorptive, gametes formation and excretory in functions.
· This epithelium is further divided into 2 types:
· In this epithelium, microvilli are present on the free surface of the cell to increase the absorptive surface area
i. Location: Proximal convoluted tubule (PCT) of the kidney is an example of this epithelium.
ii. Function: They enhance the absorption of the required materials.
II. Ciliated cuboidal epithelium
· In this epithelium, cilia are present on the free surface of the cell.
i. Location: In certain parts of nephrons. It maintains the flow of nephric filtrate and other mucus or liquid substances.
ii. Function: The height of the cell exceeds the width and the nucleus is usually situated in the basal region of the cell.
· In this epithelium, cilia are present on the free surface of the cell.
i. Location: In certain parts of nephrons. It maintains the flow of nephric filtrate and other mucus or liquid substances.
ii. Function: The height of the cell exceeds the width and the nucleus is usually situated in the basal region of the cell.
c. Simple columnar epithelium
iii. Location: They are located in the lining of the stomach, intestine, gall bladder, bile duct, pancreatic lobules.
iv. Function: They are protective, secretory and absorptive in function.
· They are also 2 types:
I. Brush bordered columnar epithelium
· Here, microvilli are present at the free ends of the columnar epithelium to increase absorption.
iv. Function: They are protective, secretory and absorptive in function.
· They are also 2 types:
I. Brush bordered columnar epithelium
· Here, microvilli are present at the free ends of the columnar epithelium to increase absorption.
· For example columnar epithelium of intestinal mucosa.
II. Ciliated columnar epithelium
· In this type of epithelium, cilia are present at their free surface for conduction and filtration.
Location: They are found in bronchioles, oviducts, spinal canal, ventricular of the brain and Eustachian tube etc.
II. Ciliated columnar epithelium
· In this type of epithelium, cilia are present at their free surface for conduction and filtration.
Location: They are found in bronchioles, oviducts, spinal canal, ventricular of the brain and Eustachian tube etc.
d. Pseudostratified epithelium
· This epithelium is made up of a single layer of columnar cells but appears as the double-layered structure because of tall and short cells and their nuclei appear to lie in various layers in the cells
· Location: They are located at the large duct of parotid glands, urethra of male, olfactory mucosa, trachea and bronchi etc.
· Functions: protection, secretion and conduction.
· This epithelium is made up of a single layer of columnar cells but appears as the double-layered structure because of tall and short cells and their nuclei appear to lie in various layers in the cells
· Location: They are located at the large duct of parotid glands, urethra of male, olfactory mucosa, trachea and bronchi etc.
· Functions: protection, secretion and conduction.
2. Stratified or Compound epithelium
· This epithelium is made up of more than one layer of the cell.· So only the lowermost layer (germinative layer) of this epithelium rests on the basement membrane.
· The cells of the upper layers are different in shape from the germinal layer.
· Function: protective, secretory or absorptive layer.
· It may be of the following types.
I. Stratified squamous epithelium
· The cells in the germinal layer are cuboidal or columnar with oval nuclei.
I. Stratified squamous epithelium
· The cells in the germinal layer are cuboidal or columnar with oval nuclei.
· The cells of the germinal layer divide by mitosis and new cells shift gradually outward. So the cells of the superficial layer are flat with the elongated nucleus.
· These layers are called squamous layers and are called squames.
· In the intermediate layers have many polyhedral cells with rounded nuclei.
· This type of epithelium is further divided into 2 types:
1. Keratinised squamous epithelium· It has many layers of horny scale-like remains of dead squamous cells due to deposition of keratin protein which ultimately flakes away and several deeper layers have many polygonal cells.
· Functions: impervious to water and highly resistant to mechanical abrasions.
· Eg skin (epidermis), nail, claw, hairs
2. Non keratinised squamous epithelium
· This epithelium does not have keratin protein. So they are unable to check the water loss and provide moderate protection against abrasion.
· Location: They are located in the lining of the buccal cavity, pharynx, tongue, oesophagus, anal canal, vocal cord, the inner surface of eyelids conjunctiva and cornea of the eye.
II. Stratified cuboidal epithelium
· It has outer layers of cuboidal cells and basal layer of columnar cells.
· Location: epidermis of fishes and amphibians, duct of sweat gland, salivary gland and pancreas.
· Function: This epithelium protects the other tissues from chemical and mechanical injuries.
III. Stratified columnar epithelium
· It has many columnar cells on the superficial layers and layer of cuboidal cells.
· Location: the lining of ducts of mammary glands and parts of the urethra. It also covers the epiglottis.
· Function: It is protective, secretary in function.
IV. Transitional epithelium
· This epithelium consists of 4-6 layers of cells.
· This epithelium consists of 4-6 layers of cells.
· It has a single layer of cuboidal or columnar cells at the base, 2-3 layers of polygonal or pear-shaped cells at the middle and superficial layers have large globular or umbrella-shaped cells.
· There is no basement membrane so the innermost layer always rests on connective tissue.
· Cells of this epithelium are stretchable and they can modify the thickness of tissue.
· Cells of this epithelium are stretchable and they can modify the thickness of tissue.
· The form of these cells changes according to the degree of distension of the organs.
· Location: They are found mainly in the urinary tracts such as the urinary bladder, ureters, upper part of the urethra and renal pelvis. So, the transitional epithelium is also called the urothelium.
· Functions: The thickness of this tissue prevents the loss of water in the outer tissues.
· It allows the expansion of organs due to their capacity to stretch and modify.
· Location: They are found mainly in the urinary tracts such as the urinary bladder, ureters, upper part of the urethra and renal pelvis. So, the transitional epithelium is also called the urothelium.
· Functions: The thickness of this tissue prevents the loss of water in the outer tissues.
· It allows the expansion of organs due to their capacity to stretch and modify.
3. Specialized Epithelium
· These are modified cuboidal or columnar cells to perform a special function.· They are of following types:
a. Ciliated epithelium
· It has cilia on the free surface of the cuboidal or columnar cells.
a. Ciliated epithelium
· It has cilia on the free surface of the cuboidal or columnar cells.
· They are found in vasa differentia, oviduct.
· Their functions are conduction of the material towards their destination.
b. Sensory epithelium
· Some columnar cells are modified into the sensory cells to receive and conduct the stimuli.
· They are located in the retina of the eye, ampulla of the semicircular canal of the internal ear, nasal chamber and tongue.
· Some columnar cells are modified into the sensory cells to receive and conduct the stimuli.
· They are located in the retina of the eye, ampulla of the semicircular canal of the internal ear, nasal chamber and tongue.
c. Germinal epithelium
· It is present in the testes and ovaries.
· It is present in the testes and ovaries.
· Cells of this epithelium divide by meiotic division and produce the gametes.
d. Glandular epithelium
· It is present in the glands such as liver, pancreas, gastric glands and secretes fluids.
· It is present in the glands such as liver, pancreas, gastric glands and secretes fluids.
· Various Types of Glands:
A. Based on the number of cells
1. Unicellular glands – a single cell represents the gland.
A. Based on the number of cells
1. Unicellular glands – a single cell represents the gland.
· Eg goblet cells produce mucus.
2. Multicellular glands- a gland is formed by many cells.
2. Multicellular glands- a gland is formed by many cells.
· Eg gastric and sweat gland.
B. Based on the kinds of secretion and the duct present
1. Exocrine glands- these glands have ducts and pour their secretions through the ducts to their respective site of action.
1. Exocrine glands- these glands have ducts and pour their secretions through the ducts to their respective site of action.
· They secrete enzymes or bio- catalysts or hydrolases.
· They are located near their site of action.
· Eg sweat gland, gastric gland, salivary glands.
2. Endocrine glands- these glands have no ducts and so their secretions are directly diffused into the blood and reach the targeted organ.
2. Endocrine glands- these glands have no ducts and so their secretions are directly diffused into the blood and reach the targeted organ.
· They secrete hormones or chemical messengers or autocoids.
· They are always located far from their site of action.
· Eg pituitary gland, adrenal glands etc.
3. Mixed glands – These glands have exocrine as well as endocrine portions to secrete both hormones and enzymes.
3. Mixed glands – These glands have exocrine as well as endocrine portions to secrete both hormones and enzymes.
· So they are also known as dual gland or heterocrine glands.
· Eg Pancreas.
C. Based on their mode of secretion
1. Merocrine glands
· They secrete their secretions by the simple diffusion process through their cell membranes without losing any amount of cytoplasm.
1. Merocrine glands
· They secrete their secretions by the simple diffusion process through their cell membranes without losing any amount of cytoplasm.
· So there is no loss of cells and their parts.
· Examples- mucous glands, sweat glands and salivary glands.
2. Apocrine glands
· In these glands, the secretory products first accumulate in the apical part of the cells and later this apical part break off to discharge the secretion with some amount of the cytoplasm.
· Example- Mammary glands.
3. Holocrine glands
· The entire cell breaks down in order to release its secretions when the cells fill with their secretory products.
· The entire cell breaks down in order to release its secretions when the cells fill with their secretory products.
· Example- Sebaceous glands.
D. Based on their nature of secretions
1. Mucous glands
· They secrete their secretion in the form of viscous mucus fluid.
1. Mucous glands
· They secrete their secretion in the form of viscous mucus fluid.
· Example- goblet cells.
· The cells of these glands are called mucocytes.
2. Serous glands
· The secretion is the form of a clear watery solution containing enzymes as in sweat glands.
· The secretion is the form of a clear watery solution containing enzymes as in sweat glands.
· The cells are termed serocytes.
3. Mixed glands
· They have both mucocytes and serocytes so they produce mucus and enzymes.
· They have both mucocytes and serocytes so they produce mucus and enzymes.
· Eg Gastric glands and Intestinal glands.
E. Based on the shape and complexity, exocrine glands are divided into the following types
a. Simple glands
· This gland has a single duct with tubular or alveolar secretory portions.
a. Simple glands
· This gland has a single duct with tubular or alveolar secretory portions.
· They are of the following types:
1. Simple tubular glands: It has a single duct with a tubular secretory portion. Example- crypts of lieberkuhns
2. Simple coiled tubular glands- It has a single duct with a coiled tubular portion. Example- sweat or sudoriferous glands, glands present in the nuptial pads of frogs.
3. Simple branched tubular glands: It has a single duct with many branches of tubules. Example- gastric glands.
4. Simple alveolar or saccular glands: It has a single duct with flask or sac-like secretory part. Example-mucous glands.
5. Simple branched alveolar glands: It has a single duct with a number of sacs. Example- sebaceous glands.
1. Simple tubular glands: It has a single duct with a tubular secretory portion. Example- crypts of lieberkuhns
2. Simple coiled tubular glands- It has a single duct with a coiled tubular portion. Example- sweat or sudoriferous glands, glands present in the nuptial pads of frogs.
3. Simple branched tubular glands: It has a single duct with many branches of tubules. Example- gastric glands.
4. Simple alveolar or saccular glands: It has a single duct with flask or sac-like secretory part. Example-mucous glands.
5. Simple branched alveolar glands: It has a single duct with a number of sacs. Example- sebaceous glands.
· They have many ducts and tubular or alveolar secretory parts.
· They are divided into the following types:
1. Compound tubular glands- It has many ducts and tubular secretory portions. Example- Salivary glands.
2. Compound alveolar glands- It has many ducts and sac-like secretory portions. Examples- pancreatic acini, sublingual and submandibular glands.
3. Compound tubulo-alveolar glands- It has many ducts and tubular as well as alveolar secretory portions.
1. Compound tubular glands- It has many ducts and tubular secretory portions. Example- Salivary glands.
2. Compound alveolar glands- It has many ducts and sac-like secretory portions. Examples- pancreatic acini, sublingual and submandibular glands.
3. Compound tubulo-alveolar glands- It has many ducts and tubular as well as alveolar secretory portions.
· Examples- mammary glands, Cowper's glands in males and Bartholin glands in females.
Note: Specialization of the cell surface of epithelia
· The free surface of the epithelial cell is modified into various types to perform various functions. They are:
· The free surface of the epithelial cell is modified into various types to perform various functions. They are:
Differences between Kinocilia, Stereocilia and Microvilli
| Kinocilia | Stereocilia | Microvilli |
|---|---|---|
| Cilia has basal granules | Has no basal granule | |
| They have mobility | They have less mobility | They increase surface area for absorption |
| Present in the lining of respiratory and reproductive tract as well as neurocoel | Present in the inner lining of epididymis | Present in the inner wall of the small intestine or ileum |
Differences between Cilia and Flagella
| Cilia | Flagella |
|---|---|
| They are hair-like structures ranging from 5 to 10 millimicron in length | They are long whip-like which are about 150 millimicrons in length |
| They are usually present on one end of the cell. | They are usually present all over the body. |
| They are less in number (1 to 4) | They are numerous in number |
| They move independently. | They move in coordinating movements. |
| They exhibit undulating movement | They exhibit sweeping rhythm. |
Also, Read our Other Notes Related to Animal Tissues: