Tissue can be defined as organized layers or masses of structurally similar cells of common embryonic origin and the same functions.
Types of Tissue
All complex animals consist of four basic types of tissues:
- Epithelial Tissue
- Connective Tissue
- Muscular Tissue
- Neural Tissue
Epithelial tissues protect the surface of the organs and form glands. They lack blood vessels and receive their nutrition from the underlying connective tissues by diffusion through the basement membrane.
Types of Epithelial Tissue
Epithelial tissues can be divided into the following forms:
- Simple Epithelium
- Compound Epithelium
1. Simple Epithelium
Simple epithelium is uni-layered or uni-laminar. It can be of the following types:
- (a) Squamous epithelium (pavement epithelium): It is made of a single thin layer of flattened cells. They are found in the walls of blood vessels and air sacs of the lungs.
- It functions as protection, excretion, gas exchange and secretion of coelomic fluid.
- (b) Cuboidal epithelium: It is composed of cube-like cells. It is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption.
- Cells often have microvilli which increase absorptive surface area.
- (c) Columnar epithelium: It is made of tall and slender cells with their nuclei at the base. In the intestine, this layer has microvilli and is called brush bordered columnar epithelium.
- It lines the stomach, intestine, gallbladder and bile duct.
- (d) Ciliated epithelium: When the columnar or cuboidal epithelia bear cilia on the free surface of the cells, they are called the ciliated epithelium.
- It helps in the movement of mucus, urine, eggs and cerebrospinal fluid in a particular direction.
- (e) Pseudostratified epithelium: It is one cell thick, yet it appears to be multi-layered because nuclei lie at different levels in different cells. It is of two types:
- Pseudostratified columnar epithelium (in the large ducts of parotid salivary glands and the urethra of the human male) and
- Pseudostratified columnar ciliated epithelium (in the trachea and large bronchi). The main functions of pseudostratified epithelium are protection, secretion, and movement of secretions.
2. Compound Epithelium
The compound epithelium is multilayered. It can be of the following types:
- (a) Transitional epithelium: It consists of fewer (3 to 4) layers of less flattened surface cells with remarkable flexibility.
- It is found in the ureters, urinary bladder, and urethra and is adapted for the distension of these organs.
- (b) Stratified epithelium: It consists of many layers of cells that vary in nature. It may be keratinized (in the skin epidermis) or non-keratinized (in the pharynx, vagina, conjunctiva, etc).
Specialized junctions provide both structural and functional links between individual cells. These are the following types:
- (a) Tight junctions (check leakage of substances): Tight junctions, also known as occluding junctions or zonulae occludentes (singular, zonula occludens) are multiprotein junctional complexes whose general function is to prevent leakage of transported solutes and water and seals the paracellular pathway.
- (b) Adhering junctions (cement neighbouring cells): Adhering junctions provides strong mechanical attachments between adjacent cells through the linkage of the cytoplasmic face with the cytoskeleton.
- (c) Gap junctions (facilitate communication between cells): Gap junctions are a type of cell junction in which adjacent cells are connected through protein channels.
- These channels connect the cytoplasm of each cell and allow molecules, ions, and electrical signals to pass between them.
Connective tissues link and support other tissues and organs of the body. The basic components of connective tissues are matrix connective tissue cells and fibres.
Types of Connective Tissue
The different types of connective tissues are as follows:
- Loose connective tissue
- Dense connective tissue (white fibrous and yellow elastic)
- Skeletal tissue (cartilage and bone)
- Vascular tissue (blood and lymph)
1. Loose Connective Tissue
Loose connective tissue consists of (a) Areolar and (b) Adipose tissues.
- (a) Areolar: Areolar tissue is present under the skin as subcutaneous tissue, in between and around muscles, nerves and blood vessels and between the lobes and lobules of compound glands.
- It binds parts together and also provides strength, elasticity, and support to the parts.
- (b) Adipose Tissues: Adipose tissue, located mainly beneath the skin, is specialized to store fats. Adipose tissue is a poor conductor of heat. It reduces heat loss through the skin.
- There are two types of adipose tissue: white (or yellow) fat and brown fat. Brown fact in infants plays role in non-shivering thermogenesis.
2. Dense Connective Tissue
Dense connective tissue consists of (a) white fibrous connective tissue and (b) Yellow elastic connective tissue.
- (a) White fibrous connective tissue: In white fibrous connective tissue, collagen is dominant and cells are mainly fibroblasts.
- A tendon is a modification of white fibrous tissue which connects skeletal muscles to bones. It is tough and inelastic.
- (b) Yellow elastic connective tissue: Yellow elastic connective tissue has strength and elasticity, thus, it allows stretching of various organs. It forms cords called ligaments which attach one bone to another.
3. Skeletal Tissue
Skeletal tissue consists of (a) cartilage and (b) bones.
- (a) Cartilage: It is soft skeletal tissue, consisting of cartilage cells (chondroblasts and chondrocytes) and matrix. Cartilage has the following forms:
- Hyaline cartilage: Hyaline cartilage is the most prevalent cartilage. It forms articular surfaces at the joints of long bones, rings of trachea and bronchi, sternal parts of ribs, hyoid apparatus, nasal septum and also part of the larynx.
- White fibrous: White fibrous cartilage is the strongest cartilage, it occurs in the intervertebral discs and in the pubic symphysis.
- Yellow elastic fibrocartilage: – Yellow elastic fibrocartilage is found in the pinna and external auditory canal of the ear, Eustachian tubes, epiglottis and tip of the nose.
- (b) Bones: Bones have a hard ground substance rich in calcium salts and collagen fibres which gives bone its strength.
- Bone cells are of three kinds: Osteoblasts (bone-forming cells), Osteocytes (bone maintaining cells) and Osteoclasts (bone cleaning cells). The matrix of the bone occurs as layers called lamellae.
- A Haversian canal with its surrounding lamellae and osteocytes constitute a cylindrical unit of bone called a Haversian system or osteon. Haversian systems are absent in the spongy bones of mammals.
- In the long bones such as limb bones, a cavity called bone marrow cavity is present. The bone marrow cavity is filled with neurovascular tissue termed bone marrow.
- Redbone marrow is present in the spongy parts of the bones. It produces RBCs, WBCs, and platelets.
- Yellow bone marrow is present in the shafts of long bones. It produces blood corpuscles in an emergency.
4. Vascular Tissue
Vascular tissue consists of (a) blood and (b) lymph
- (a) Blood: Blood is a highly specialized mobile fluid connective tissue that helps in the transport of various substances. It is composed of fluid, plasma and, blood corpuscles (RBCs, WBCs, platelets).
- (b) Lymph: Lymph is the fluid that flows through vessels and nodes of the lymphatic system. It is formed when the interstitial fluid (the fluid which lies in the interstices of all body tissues) is collected through lymph capillaries.
- It is then transported through larger lymphatic vessels to lymph nodes, where it is cleaned by lymphocytes, before emptying ultimately into the right or the left subclavian vein, where it mixes back with the blood.
Muscular tissue is present in muscles. It consists of cells in the form of contractile fibres, each containing fine fibrils called myofibrils, present in the cytoplasm known as sarcoplasm.
Types of Muscular Tissue
Muscular tissue is of the following three types:
1. Skeletal muscles or striated muscles:
They are voluntary in nature. Sarcoplasm has a large number of myofibrils which are tightly packed. Each myofibril shows dark bands (= A bands) and light bands (= I bands) alternating with each other, hence they are called striated muscles.
These are found in the limbs, body walls, tongue, pharynx and beginning of the oesophagus.
2. Smooth muscles Non-striated muscles:
The muscle fibres are elongate and spindle-shaped. They are found in visceral organs like the stomach, intestine, lungs, urinogenital tract, blood vessels, iris of the eye, etc.
The actions of these muscles are controlled by the autonomic nervous system i.e., they are involuntary.
3. Cardiac muscles:
They are contractile in nature present only in the heart. Cardiac muscles are involuntary and have a rich blood supply, thus, never get fatigued.
These are striped, but having dark intercalated discs which function as boosters of contractions wave and permit the wave of muscle contraction to be transmitted from one cardiac fibre to another.
Neural tissue is meant to receive stimuli and conduct impulses for controlling and coordinating body functions.
These tissues form the central nervous system (CNS), peripheral nervous system (PNS) and the autonomic nervous system (ANS).
The special properties of the cells of the neural tissue are excitability and conductivity. It is devoid of the power of division and regeneration.
- Neurons are structural and functional units of the neural system. Each neuron consists of the cell body called cyton or soma. The processes of neurons are of two types- dendrites and axon.
- The axon ends in a group of branches, the terminal arborizations. The axon or dendrite covered with sheaths is called nerve fibre.
- Modulated nerve fibres are covered with a myelin sheath. Myelin sheath is absent at certain points called the nodes of Ranvier.
- Non-medullated nerve fibres do not have myelin sheath and are found in autonomic nerves. They conduct nerve impulses much slower than the medullated nerve fibres.
- The neuroglial cells, which constitute more than 50% of the neural tissue, protect and support the neurons.