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