Structural and Functional Organization

Animal Tissues:

Simple Epithelial Tissues:

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 a ^^little intercellular matrix.^^
There are two types of epithelial tissues namely * %%Simple epithelium%% * %%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 a protective function as it does in our skin.%%
On the basis of the structural modification of the cells, the simple epithelium is further divided into three types.

Squamous Epithelium:

It 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.%%

Cuboidal Epithelium:

It 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 the ^^proximal convoluted tubule (PCT) of the nephron in the kidney has microvilli.^^

Columnar Epithelium:

It is composed of a single layer of tall and slender cells.
Their nuclei are located at the base.
The %%free surface may have microvilli.%%
They are found in the ^^lining of the stomach and intestine and help in secretion and absorption.^^
If the %%columnar or cuboidal cells bear cilia on their free surface they are called the ciliated epithelium.%% * ^^Their function is to move particles or mucus in a specific direction over the epithelium.^^ * They are mainly present on the inner surface of ^^hollow organs like bronchioles and fallopian tubes.^^

Glandular Epithelium:

%%Some of the columnar or cuboidal cells%% %%get%% %%specialized for secretion%% and are called the glandular epithelium.
They are mainly of two types: * ^^Unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal)^^ * ^^Multicellular, consisting of a cluster of cells (salivary gland).^^
On the basis of the mode of pouring of their secretions, glands are divided into two categories namely 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:

It is made of more than one layer (multi-layered) of cells and thus has a limited role in ^^secretion and absorption.^^
Their main function is to %%provide protection against chemical and mechanical stresses.%%
They cover the %%dry surface of the skin, the moist surface of the buccal cavity, the pharynx, the inner lining of ducts of salivary glands, and pancreatic ducts.%%

Junctions:

All cells in the epithelium are held together with little intercellular material.
In nearly all animal tissues, ^^specialized junctions provide both structural and functional links between their individual cells.^^
Three types of cell junctions are found in the epithelium and other tissues.
These are called %%tight, adhering, and gap junctions.%% * Tight junctions help to ^^stop substances from leaking across a tissue.^^ * Adhering junctions perform %%cementing to keep neighboring 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.^^

Connective Tissue:

They 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 specialized types, which include ==cartilage, bone, adipose, and blood.== * In all connective tissues except blood, the cells secrete fibres of $structural proteins called collagen or elastin.$
The fibres provide ==strength, elasticity and flexibility== to the tissue. * These cells also secrete modified polysaccharides, which accumulate between cells and $fibres and act as a matrix (ground substance).$
Connective tissues are classified into three types: * ==Loose connective tissue.== * Loose connective tissue has cells and fibres loosely arranged in a semi-fluid ground substance, for example, $areolar tissue present beneath the skin.$ * Often it serves as a ==support framework for epithelium.== * It contains $fibroblasts$ $(cells that produce and secrete fibres),$ $macrophages and mast cells.$ * ==Adipose tissue== is another type of loose connective tissue located mainly ==beneath the skin.== * $The cells of this tissue are specialised to store fats.$ * The excess nutrients which are not used immediately are converted into fats and are stored in this tissue. * $Dense connective tissue.$ * ==Fibres and fibroblasts== are compactly packed in dense connective tissues. * The orientation of fibres shows a regular or irregular pattern and is called $dense regular and dense irregular tissues$ $.$ * In the dense regular connective tissues, the collagen fibres are present in rows between many parallel bundles of fibres. * Tendons, which attach skeletal muscles to bones and ligaments which attach one bone to another are examples of this tissue. * Dense irregular connective tissue has fibroblasts and many fibres (mostly collagen) that are oriented differently. * $This tissue is present in the skin$ $.$ * ==Specialised connective tissue.== * $Cartilage is solid and pliable and resists compression.$ * Cells of this tissue ==(====chondrocytes====)== are enclosed in small cavities within the matrix secreted by them. * $Most of the cartilage in vertebrate embryos is replaced by bones in adults.$ * Cartilage is present in the ==tip of the nose, outer ear joints, and between adjacent bones of the vertebral column, limbs and hands in adults.== * $Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give bone its strength.$ * It is the ==main tissue that provides a structural frame== to the body. * Bones support and protect softer tissues and organs. * The bone cells (osteocytes) are present in the $spaces called lacunae.$ * ==Limb bones, such as the long bones of the legs, serve weight-bearing functions.== * They also interact with skeletal muscles attached to them to bring about movements. * $The bone marrow in some bones is the site of the production of blood cells.$ * Blood is a fluid connective tissue containing plasma, ==red blood cells (RBC), white blood cells (WBC) and platelets.== * It is the $main circulating fluid$ that helps in the $transport$ of various substances.

Muscle Tissue:

Each muscle is made of many %%long, cylindrical fibres%% arranged in parallel arrays.
These fibres are composed of numerous fine fibrils, called myofibrils.
Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion.
Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body.
In general, ^^muscles play an active role in all the movements of the body.^^
Muscles are of three types, %%skeletal, smooth, and cardiac.%%

Skeletal Muscle:

It is closely attached to skeletal bones.
In a typical muscle such as the biceps, striated (striped) skeletal muscle fibres are bundled together in a parallel fashion.
A sheath of tough connective tissue encloses several bundles of muscle fibres

Smooth Muscle:

They taper at both ends (fusiform) and do not show striations.
Cell junctions hold them together and they are bundled together in a connective tissue sheath.
The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue.
Smooth muscles are ‘involuntary’ as their functioning cannot be directly controlled.

Cardiac Muscle:

It is a contractile tissue present only in the heart.
Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together.
Communication junctions %%(intercalated discs)%% %%at some fusion points allow the cells to%% %%contract as a unit, i.e., when one cell receives a signal to contract, its neighbours are also stimulated to contract.%%

Neural Tissue:

Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions.
Neurons, the unit of the neural system are excitable cells.
The ==neuroglial cell== which constitutes the rest of the neural system protects and supports neurons.
Neuroglia makes up more than one-half the volume of neural tissue in our body.
When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma