ANIMAL TISSUES
BY
AUGUSTINE
BY
AUGUSTINE
This is a collection of closely packed single and multilayered compound sheets of cells covering the external and internal surface of the body of an organism.
At the bottom epithelial cells rest on a basement membrane.
Epithelial tissue protects the underlying structures from injury.
In some situations, the free surface of epithelium is highly differentiated may be absorptive, secretory or excretory.
CHARACTERISTICS OF EPITHELIAL TISSUE
1. Epithelial tissue consists of tightly packed cells that are firmly attached to each other with little intercellular material between them. Epithelial cells are held firmly together by small amounts of carbohydrate cementing substances and by special intercellular junctions between the cells.
2. The bottom of epithelial cells rest on a basement membrane composed of a network of fibres which include collagen.
The portion of epithelial cells attached to the basement membrane is called the basal surface, the opposite end facing the external environment or the lumen of the body cavity is called the apical surface/free surface.
CONTI….
3. There are no blood vessels in the epithelial tissues hence the tissue lacks vascularity. As the epithelial cells are not supplied with blood vessels, they rely on diffusion of nutrients and oxygen from lymph vessels which run through nearby intercellular spaces. However, nerve endings may occur in the epithelium.
4. Regeneration- Epithelial cells have a high regeneration capacity due to rapid cell division. This gives the epithelial tissue quick recovery after any injury or abrasions.
CLASSIFICATION OF EPITHELIAL TISSUES.
Epithelial tissues are classified according to the number of cells/layers and the shape of the individual cells.
1. Squamous epithelium (pavement epithelial):
Cells form a single layer attached to a basement membrane. In surface view, the cell outlines are irregular and closely packed.
The cells are thin, shallow and flattened.
Adjacent cells may be joined by strands of cytoplasm.
Location:
• Alveolar lining of the lungs.
• Inner lining of blood vessels and lining of blood capillaries.
• Bowman’s capsule of kidney.
Functions:
Diffusion of materials and gaseous exchange across the membrane.
Provides a friction free lining for blood flow in blood vessels
Adaptations:
Made up of thin flattened cells to reduce diffusion distance across tissues.
Their smooth surface provides a friction free lining for blood flow in blood vessels.
2. Cuboidal epithelial tissue:
Its cells are cuboidal and form a single layer attached to a basement membrane.
The nuclei are spherical and centrally located.
The cells appear pentagonal or hexagonal in surface view.
Structure of cuboidal epithelium
Location:
Lining of salivary, collecting and pancreatic ducts (kidneys).
Salivary, sweat and thyroid glands.
Functions:
Secretion
The ciliated cuboidal epithelium are for flow of nephric filtrate.
3. Simple columnar epithelium
Tall, column like narrow cells with nucleus at the basal end. It is often interspersed by goblet cells.
Location:
• Lining of stomach, intestine and gall bladder.
• Intestinal and gastric glands.
Functions:
Secretion and/or absorption.
Mucus protects the lining from the acidic content in the stomach and from digestion by enzymes and also lubricates the passage.
Side view of simple columnar
4. Brush bordered columnar epithelium
These are tall and narrow cells with a nucleus near their base.
The surface area is increased by micro villi at the free end.
Secretory goblet cells are found within the columnar cells.
Location:
Intestinal mucosa
Function:
Increased surface area for absorption of nutrients.
Adaptations:
Microvilli at free surface increase surface area for absorption of nutrients.
Secretory goblet cells for secretion of mucus.
5. Ciliated columnar epithelium.
It comprises of columnar cells with cilia at their free edges.
They have many mucus secreting goblet cells in between the cells.
Location:
Oviducts, respiratory passage (bronchioles) and spinal canal.
In flat worms, lines underside of the body where they aid locomotion.
Functions:
Locomotion in flatworms.
Movement of materials in a particular direction. The cilia sets up currents to move materials in a particular direction.
The mucus protects the lining and lubricates the passage.
6. Pseudostratified epithelium
Consist of one layer of columnar cells that appear to be in two layers due to:
• Nuclei at different levels and All cells do not reach the surface.
Has two types of cells: Longer cells have cilia and Shorter cells lack cilia and secrete mucus.
All cells rest on the basement membrane.
Location:
• Lining of trachea and primary bronchi.
• Part of the nasal epithelium.
Functions:
• Mucus traps bacteria and dust particles and prevent them from reaching the lungs.
• Cilia move mucus with trapped foreign particles up to the throat for swallowing.
COMPOUND EPITHELIAL TISSUES
Characteristics:
Consist of many layers of cells.
Only the lower most layer of cells rest on the basement membrane.
Compound epithelial may be stratified or transitional.
Types of compound epithelial tissues.
1. Stratified epithelium
It is made of many layers of cells and therefore thicker than the simple epithelium.
The cells are formed by mitotic division of the germinal layer which rests on the basement membrane.
As new cells form, older ones are pushed near to the surface changing shape and flattening to form squamous.
In some areas the squamous cells may remain un keratinized as in the oesophagus or may be heavily thickened with keratin (cornified) e.g. the skin where there is a dead layer of cells.
Location:
• Areas or ducts that are delicate or have large flow of fluids.
• External skin surface, lining of buccal cavity and vagina.
• Lining of pharynx and oesophagus.
Function:
Protection from abrasion to areas exposed to wear and tear.
Protection of the tissue from mechanical damage by the food that is swallowed.
Protection of the underlying tissue from mechanical damage.
Adaptations of stratified epithelium
It is composed of several layers of cells which are tough impervious and some cells keratinized/cornified for protection against mechanical abrasions.
Cells of germinal layer divide repeatedly by mitosis to replace the cells that are breaking off wearing off at the surface.
Some cells can change their shape when subjected to tension to allow stretching where they are located e.g. in urinary bladder
2. Transitional epithelium.
It comprises of 3 or 4 layers of cells which may be flattened towards the surface which are not shed but can change their shape thus allowing stretching. Transitional epithelium lacks a basement membrane.
Location:
It is found in structures which must stretch e.g. the urinary bladder, ureter and urethra.
Function:
• By changing the shape, the transitional epithelium allows the expansion of the organ.
• It prevents the loss of water from blood to urine.
• Due to its thickness, it prevents the urine from escaping into the surrounding tissue.
GLANDULAR EPITHELIUM
1. Based on the kind of secretion and the duct present, glands are of two types;
i) Exocrine glands: these pour their secretions through the ducts to their respective sites of action e.g. salivary, tear intestinal and gastric glands. Their secretions are called enzymes.
ii) Endocrine glands: these do not possess ducts and pour their secretions directly into the blood stream. Their secretions are known as hormones.
2. Based on number of cells, > the glands are of 2 types;
i) Unicellular: an individual epithelial cell is modified into a glandular cell as in goblet cells.
ii) Multicellular: number of glandular cells aggregate to form a multicellular gland. Multicellular glands can further be divided into simple or compound glands e.g. sweat glands.
3. Based on the shape and complexity, the exocrine glands are of 2 main types;
simple and compound glands which may further be modified.
Simple glands:
these have a single unbranched duct.
The secretory part could be in the form of tube (called tubules) or sacs (alveolar/saccular).
These could be coiled or uncoiled, branched or unbranched.
4. Based on the mode of secretions, the exocrine glands are of 3 types;
i) Merocrine glands: the secretions produced within the cell are discharged on its cell surface without losing any of its cytoplasm. E.g. goblet cells, pancreatic glands and sweat glands.
ii) Apocrine glands: in these glands, the cell loses a part of its cytoplasm while releasing its secretions. The secretions are stored in the apical part of the cell which bursts open to release the contents e.g. mammary glands.
iii) Holocrine glands: the entire cell breaks down in order to release its secretions which extrude from the epithelial surface e.g. sebaceous glands.
5. Based on the form of secretion, glands are of 3 types.
Mucous glands: secretion is in form of viscous mucous fluid. They are called mucocytes.
ii) Serous glands: secretion is clear, watery fluid containing enzymes. They are called serocytes.
iii) Mixed glands: secret both.
CONNECTIVE TISSUE
Connective tissue is a composite tissue and has three basic components.
i) Cells: they are the living components that are widely separated from each other. They originate from the embryonic mesoderm.
ii) Fibres: there are several types of fibres scattered in between the cells. They form the extracellular material. Blood is devoid of any fibres.
iii) Matrix: tit forms the basic ground tissue in which both the cells and the fibres are suspended. It is nonliving, transparent, fluid or semi fluid in nature. It contains various organic and inorganic substances, the most important being hyaluronic acid. The kind of matrix varies in different tissues.
Location of connective tissue
It is present in between different tissues and organs.
It is present inside and around the body organs.
The skeletal tissue is present in the form of bone and cartilage.
Fluid connective tissue is present throughout the body.
Functions of connective tissue
Connective tissue is basically a binding and packaging tissue but has many other important functions as well.
It binds various tissues together like skin with the muscles and muscles with the bones.
It forms sheaths around the body organs and makes a kind of packaging tissue.
The areolar tissue protects the body against wounds and infection.
The adipose tissue stores fat and insulates the body against heat loss.
The supportive tissue forms shape and the frame work of the body.
The haemopoietic tissue produces blood.
The lymphatic tissue helps the body to build immunity by producing antibodies.
Types of connective tissue
The type depends on the kind of matrix present.
They include connective tissues proper, skeletal tissue and vascular (fluid/haemopoietic) tissue.
This is found around all organs in the body. It consists of a semi-fluid matrix containing a variety of cells and fibres.
The cells are:
A. Fibroblasts: These are spindle shaped flattened cells with an oval nucleus. They produce fibres and so are generally seen close to them.
B. Mast cells: These are large oval shaped cells and contain granular cytoplasm. They secrete the matrix and chemicals heparin and histamine. Heparin is an anticoagulant while histamine is anti-inflammatory and is released from tissue when they are injured.
C. Macrophage or histocytes: These are large amoeboid cells with a kidney shaped nucleus. They engulf bacteria or other foreign particles. They are capable of amoeboid movement and can ingest damaged cell tissues.
D. Plasma cells: these are small round or irregular cells. They produce antibodies that help in self-defense.
The collagen/white fibres: these are long, wavy and unbranched fibres present in bundles. They are flexible but inelastic.
The elastic/yellow fibres: these are long, straight and branched fibres arranged singly. They are flexible and elastic as they contain protein elastin.
2) White fibrous tissue:
This consists of glycoprotein matrix containing densely packed collagen fibres.
The collagen fibres are strong, flexible yet inelastic and have a high tensile strength.
They are abundant in tendons and ligaments.
3) Yellow elastic fibrous tissue:
This consists of a glycoprotein matrix containing loose network of fibres. It is strong and elastic.
Such tissue is found in ligaments where it binds bones to other bones.
It is also found around the walls of arteries and it is also found as a component of the lungs and associated air passages as well as in the great cords of the neck.
4) Adipose tissue (fatty tissue):
This is areolar tissue containing many fat cells which act as an energy reserve, for insulation and also act as a shock absorber.
There are two types of adipose tissue;
i) White adipose tissue:
The white adipose tissue is called so because the cells appear white due to accumulation of fats. It is distributed throughout the body particularly the deep layers of the skin.
ii) Brown/yellow adipose tissue:
This is commonly in young mammals and some hibernating mammals. It is important in temperature regulation.
5) Haemopoietic tissue:
This forms the red and white blood cells and is located in the red bone marrow and lymphoid tissue of mammals
Haemopoietic (or hematopoietic) tissue is a specialized connective tissue responsible for the production of all cellular components of the blood, including red blood cells, white blood cells, and platelets. In adults, it is primarily found in the red bone marrow, though the spleen and lymph nodes also play supportive roles
i) Cartilage:
Cartilage is a tough, hard but flexible connective tissue.
It can resist strain and can absorb the mechanical shock.
It consists of solid or semi-solid matrix in which are embedded the cartilage cells called chondrocytes and the fibres.
Structure of cartilage:
The cartilage is enclosed in a sheath of white fibrous tissue called perichondrin.
Next to perichondrin is a layer of chondroblasts which eventually form the chondrocytes.
The chondrocytes are dispersed in the matrix and occur in the fluid filled spaces called lacunae.
Each lacuna contains two to three chondrocytes.
Each chondrocyte is a large, angular cartilage cell with a distinct nucleus.
The matrix has a protein chondrin and lacks blood vessels.
Types of cartilage
a) Hyaline cartilage
The simplest form of cartilage is known as hyaline cartilage which consists of only chondrin matrix and chondroblasts which secret it.
It is glassy and semi-transparent in appearance and has very few or no fibres. It is slightly elastic and compressible.
It is found at the ends of the bones, larynx (voice box) and trachea.
It forms the skeleton of cartilage fish.
b) White fibrous cartilage
The matrix has bundles of densely packed white collagen fibres.
It provides great strength and a little degree of flexibility.
It acts as a shock absorber by giving a cushioning effect.
It is found between the adjacent vertebrae.
c) Elastic cartilage
This has a semi-opaque matrix with many yellow elastic fibres.
It is highly flexible and elastic.
The tissue recovers the shape quickly.
It is found in external ear, eustachian tube, nose and the epiglottis.
ii) Bone:
Bone is a supportive and protective tissue.
The matrix is solid and calcified. Most bone mass consist of salts of calcium and phosphate.
Small amounts of sodium, magnesium, potassium, chloride and fluoride are also present.
The phosphates and carbonates of calcium and magnesium give hardness and strength to the bone.
The matrix contains protein ostein. In the matrix are embedded the bone cells osteocytes and mainly the collagen fibres.
Each bone is enclosed in a layer of white fibrous connective tissue called the Periosteum. It is through the Periosteum, the blood vessels and nerves pierce in.
In a bone, the matrix is arranged in concentric circles called lamellae.
In between the lamellae are present, a number of living bone cells called osteoblasts or osteocytes, in the fluid-filled cavities called lacunae. Osteoblasts are active bone cells while osteocytes are inactive osteoblasts.
Each lacuna has fine cytoplasmic extensions called canaliculi which pass through lamellae and make connections with other lacunae.
A compact bone consists of concentric cylinders of bony lamellae surrounding a central Haversian canal with an artery and vein.
Osteoblasts interspersed between the lamellae aid bone deposition.
A layer of dense connective tissue, the periosteum covers the surface of the bone.
In the centre of a compact bone is present a bone marrow cavity lined by endosteum.
Throughout the bone, the lamellae are present in concentric circles except at the periphery where they are arranged circumferentially.
THE DEVELOPMENT OF BONE (OSSIFICATION)
Ossification is the process of formation and development of bone.
A bone originates in two ways; intramembranous ossification, Osteonal and endochondral ossification
Added concepts
a) Intramembranous ossification
The thin bony plates of the skull and parts of some other bones e.g. clavicles are formed directly by clusters of ossification which
b) Endochondral/Intra-cartilaginous ossification
Endochondral ossification is the process of replacement of cartilage by bone.
c) Osteonal ossification. Bone is laid down on walls or spaces of existing bone leading to formation of harversian bone.
Occurs in case of repair of broken bone structure.
MUSCULAR TISSUE
This is composed of specialized thin and elongated cells called muscle fibres.
These muscle fibres have the capability to contract and relax.
This property of contractility is due to presence of protein filaments myosin and actin present in their cytoplasm.
Basic structure of muscular tissue
All muscles are made up of elongated and thin cells called muscle fibres.
The muscle fibres contain specialized cytoplasm called sarcoplasm that contains a network of membranes called sarcoplasmic reticulum.
The muscle fibre may be bound by a cell membrane called sarcolemma.
Each muscle fibre may contain numerous thin myofibrils.
Types of muscles.
1. Skeletal (striated/stripped/voluntary) muscles.
2. Smooth (involuntary/unstriated/unstripped) muscles.
3. Cardiac muscles.
1) Skeletal/striated muscles.
These are voluntary in action i.e. they work under one’s own will.
They bring about the movement of the organs and the locomotion of the body.
They undergo powerful and rapid contractions with short rest periods and hence get fatigued easily.
Location: attached to the skeleton in the head, trunk and limb region
Structure:
A skeletal muscle possesses distinct cross-striations in the form of light and dark bands or I and A bands respectively.
I means isotropic i.e. that allows the light to pass through and so appears lighter while A means Anisotropic i.e. that does not allow light to pass through and so appears darker.
A muscle is composed of a number of multi-nucleate cylindrical muscle fibres.
Each muscle fibre further consists of numerous thin myofibrils.
2) Smooth or unstriated/involuntary muscles.
Smooth muscles are involuntary in action and cannot be moved by one’s own will.
The cells undergo prolonged and slow contractions and relaxations.
Location: walls of visceral organs like stomach, intestine, ureters, kidneys, blood vessels, etc.
Structure of smooth muscle:
A smooth muscle consists of sheets of densely packed elongated fibres running parallel to each other, bound together by connective tissue.
Each muscle fibre is spindle shaped, tapering at both ends and uninucleate. It lacks sarcolemma.
Each muscle fibre contains numerous fine contractile myofibrils arranged longitudinally.
The nucleus is centrally placed and is surrounded by little sarcoplasm.
The actin and myosin filaments are randomly distributed and hence there are no striations or light and dark bands.
It is shorter than skeletal muscle
It has less mitochondria and other organelles and much less extensive sarcoplasmic reticulum.
3) Cardiac muscle.
The cardiac muscles are myogenic meaning the contractions are generated within the muscle itself. They do not have to be initiated by the nervous system.
The rate of contraction can be influenced by the autonomic nervous system.
The cardiac muscles have rhythmical contractions and relaxations and do not get fatigued. They need a constant supply of a large amount of energy.
Location: Found only in the heart.
Structure of cardiac muscle
The muscle cells are short, cylindrical and branched cells joined end to end to form rows.
They show faint, but regular cross striations (light and dark bands) indicating a regular arrangement of myosin and actin filaments.
Each muscle fibre has numerous mitochondria, myofibrils with sarcomeres and many nuclei. It has abundant cytoplasm and glycogen granules.
The cells are connected to each other by special zigzag junctions called intercalated discs.
In between such discs generally one nucleus is present.
Nervous tissue.
The nerve tissue is made up of millions of nerve cells called neurons. The neurons are highly specialized cells and form the nervous system of the body.
They provide the quickest mean of communication within the body and help the body give response to the external stimuli.
The nervous tissue does not regenerate when damaged.
Each neuron posses a cell body and cytoplasmic extensions (nerve fibers’).
Each cell body contains a nucleus and abundant granular cytoplasm.
The cytoplasm also contains prominent conical granules called Nissl’s granules which are groups of ribosomes and rough endoplasmic reticulum rich in RNA and associated with protein synthesis.
From the cell body extends out two types of cytoplasmic extensions; a Dendron and axon.
TYPES OF NEURONS/NERVE CELLS
Sensory/afferent neurons:- These conduct impulses from the receptors to the central nervous system.
Motor /efferent neurons:- These conduct impulses from the central nervous system to the effectors.
Relay/intermediate neurons:- These transmit impulses from the sensory neuron to the motor neuron. They are only found in the central nervous system.
NB:
The process which brings impulses towards the cell body is called a Dendron and the one which conducts
impulses from the cell body is called the axon.
ADDITIONAL CONCEPTS
Roles played by dead cells in multicellular organisms.
i) Support and protection:
• Nails, horns, hooves, down feathers and dead skin cells in animals.
• Sclerenchyma has thick walls for mechanical support.
• Heart wood gives a strong internal support.
ii) Protection from loss of fluids and invasion of harmful microbes in:
• The outer layer of skin in animals.
• The cork and bark of the tree.
iii) Create channels for conducting without leakage. Xylem vessels have dead walls which help conduct water and minerals to great heights without leakage.
iv) Development of other organs:
• During metamorphosis many tissues and organs like gills and tail in tadpole stage die and help it to become a frog.
• During maturation of xylem in cells of flowering plants, the cross walls dissolve giving rise to long columns to help in the conduction of water and minerals.
Biological life fact on death:
Death is a biological phenomenon which is a part of an organism’s lifecycle. Each organism has a certain length of time for which he will survive. This is called its lifespan. It is a period from birth to death. It is different for all species.
Even if an organism does not meet an accident, does not suffer from a disease, does not fall prey to predators, death comes as a last event of aging.
Aging is a prerequisite for natural death. The deterioration in the structure of body cells starts after adulthood where there is a gradual degeneration of organs. This is the final stopping of the vital organs like heartbeat and respiration of the body.
It is an irreversible process. It is necessary for the continuity of life on earth.
Some of the advantages of death are:
Some of the advantages of death are:
• The cells that die over a period of time play very important roles.
• Helps to regulate the number of individuals in a population. It prevents overcrowding and so maintains an ecological balance.
• The decomposers (microbes) act on dead and decaying organisms. They convert the complex organic molecules into simple forms like C, H, O, N, S, etc. these are returned to soil, air and water. The recycling of minerals helps in maintaining a balance in nature.