3. TISSUES OF THE HUMAN BODY

TISSUES OF HUMAN BODY

Tissues are a group of cells having similar structure and function. The study of tissues is known as histology. Tissues are organised to form various body organs like heart, kidneys, lungs, etc. The body is composed of four major tissues which differ from each other in shape, size, type of cells, and in the amount and type of matrix present in extracellular space.

Classification of Human Tissues

1. Epithelial Tissue: It performs the function of covering and protecting.
2. Muscular Tissue: It is responsible for the movement of body.
3. Connective Tissue: It provides structural framework to the body.
4. Nervous Tissue: It controls different functions of the body.

Functions of Human Tissues

1. Secretion of Chemical Substances: Specialised and specific chemical substances, like enzymes, hormones, and lubricating fluids are secreted from the glandular tissues in the body.
2. Reduction of Friction: Human circulatory system is lined with smooth and tightly interlocked cells which help in reducing the friction between the walls of blood vessels and blood flow.
3. Production of Body Heat: Tissues produce body heat and maintains fluid balance in association with the muscles of whole body.
4. Acid-Base Balance: The fundamental acid-base balance of the body is maintained as the muscles aid in respiration and thus contributes to stipulate oxygen and CO2 elimination.
5. Structural Support: Tissues connects and binds the cells which provides support to body structure and also helps in resisting mechanical stresses.

EPITHELIAL TISSUES

The epithelium or epithelia covers the external body surface and lines the internal organs, tubules, vessels, and major body cavities. A thin sheet of connective tissues known as basement membrane separates the epithelial tissue from the underlying tissue. Basement membrane gives structural support to the epithelial layer and also helps in binding it to adjacent structures. Epithelial cells make up the surface layer of the skin, mucous membrane, and serous membranes. The epithelial tissues are avascular (do not have blood vessels). Therefore, the exchange of nutrients and oxygen from capillaries to the living epithelial cells takes place via the permeable basement membrane.

Sub-types of Epithelial Tissues

• Simple epithelium
  • Simple squamous epithelium
  • Simple cuboidal epithelium
  • Simple columnar epithelium
  • Pseudostratified columnar epithelium
• Stratified epithelium
  • Stratified squamous epithelium
  • Stratified cuboidal epithelium
  • Stratified columnar epithelium
  • Transitional epithelium
• Glandular epithelium

Simple Epithelium

Cells in simple epithelium are arranged in a single layer. Simple epithelium is further divided as:

1. Simple Squamous Epithelium: It comprises of a single layer of flat, scale-like cells, and allows diffusion of substances through tissues. Tissues having this type of epithelium include alveoli of the lungs, linings of blood, lymphatic vessels (termed as endothelium), surfaces of the pleura, pericardium, and peritoneum (termed as mesothelium).
2. Simple Cuboidal Epithelium: It comprises of one layer of cuboidal cells that rest on a basement membrane. Many types of glands (e.g., endocrine gland) and their ducts (e.g., the kidneys) have this type of epithelium.
3. Simple Columnar Epithelium: It comprises of a single layer of columnar cells. The structure of cells may be modified, and the three most commonly seen modifications include goblet cells, cilia, and microvilli. It is found on the surface of the mucous membrane lining the stomach, intestine, uterus, uterine tubes, and some parts of the respiratory tract.
4. Pseudostratified Columnar Epithelium: It is also known as pseudostratified ciliated columnar epithelium and comprises of only a single layer of irregularly shaped columnar cells touching the basement membrane. It is named so, as the cells in this epithelium appear to be stratified, though they are actually not. Also, the nuclei of these cells are located at odd and irregular levels, giving a false (pseudo) impression of stratification.

Stratified Epithelium

Stratified epithelium is characterised by the arrangement of cells over one another. It is further divided into:

1. Stratified Squamous Epithelium: It comprises of multiple layers of flattened squamous cells at the free surface (or outer surface) of the epithelial cell sheet. It is further categorised into keratinised and non-keratinised types.
2. Stratified Cuboidal Epithelium: It consists of two or more rows of cuboidal cells that are arranged randomly over a basement membrane. It is found in the pharynx, ducts of sweat gland, and over some portions of the epiglottis.
3. Stratified Columnar Epithelium: It comprises of multiple layers of columnar cells, however, only the superficial layer of cells appears columnar.
4. Transitional Epithelium: It is a type of stratified epithelium which lines the areas that are subjected to changes in stress and tension, e.g., walls of the urinary bladder. It is also known as urothelium, as it is found lining some parts of the urinary tract.

Glandular Epithelium

Glandular epithelium is specialised for performing secretory activity. Cells of glandular epithelium may be present as a single unicellular gland or as a multicellular gland in the form of hollow follicles, clusters, or solid cords. Secretions of glandular tissues may be secreted into ducts, lumen of hollow visceral structures, onto the surface of the body, or directly into the blood.

Functions of Epithelial Tissues

1. Protection: They protect the body from mechanical injury, excessive loss of water, harmful chemicals, and from invading bacteria.
2. Sensation: Stimulus is received by specialised epithelial cells, which have sensory nerve endings and are present on eyes, ears, nose, tongue, and skin.
3. Secretion: Epithelial tissues of glands are specialised to secrete specific chemical substances (e.g., enzymes, hormones and lubricating fluids).
4. Absorption: Cells of epithelial tissue of small intestine are specialised to absorb nutrients from the digested food.
5. Excretion: Epithelial tissues of kidney are specialised to excrete waste products from the body. Sweat glands excrete sweat via epithelial tissues.
6. Diffusion: Simple epithelium forms a thin lining that helps in diffusion of nutrients, gases (e.g., walls of capillaries and lungs), and liquids.
7. Cleaning: Dust particles and foreign bodies that enter the air passage are removed by ciliated epithelium.

MUSCULAR TISSUES

Muscular tissues are present in all parts of the body. These tissues assist the skeletal system in movement and locomotion of the body. Contraction and relaxation are characteristics of this tissue. The pumping of blood by the heart, peristaltic movement of stomach, movement of food in GIT, etc. are brought by the contraction of muscles. The muscular tissue is formed by the aggregation of muscle cells.

Sub-types of Muscle Tissues

1. Skeletal muscles
2. Smooth or visceral muscles
3. Cardiac muscles

Skeletal Muscles

Muscle fibres of skeletal muscles are cylindrical-shaped, multinucleated cell, having a group of muscle fibrils. These are joined to bones by collagen fibre bundles called tendons. Skeletal muscles are voluntary in nature and are supplied by the cerebrospinal nerves. Skeletal muscles (a type of striated muscle tissue) are controlled by somatic nervous system.

Smooth Muscles

Smooth muscle fibres are thin and spindle-shaped, and consist of actin (thin) and myosin (thick) filaments sliding over each other, in order to bring about cell’s contraction. They assist in peristalsis, diameter change of pupil, lens and vessels, and also in hair erection. They are unstriated, involuntary, and controlled by the autonomic nervous system.

Cardiac Muscles

Cardiac muscles are cross striated involuntary muscles found in myocardium of heart and extend up to the basal membrane of the large blood vessels to join the inner surface of the heart. Cardiac muscles are similar to smooth muscles in terms of their functions, and to skeletal muscles in terms of their structure. These muscles help in generating automatic and rhythmic contractions.

Functions of Muscular Tissues

1. Osseous Tissue Movement: Osseous muscle tissue allows movement at joints.
2. Body Heat Production: These tissues help in the production of a large amount of heat and maintenance of fluid balance of the whole body.
3. Body Posture Maintenance: Posture maintenance, formation of walls of body cavities, and organ support within the cavities are all carried out with the help of muscular tissues.
4. Protection: Muscles form a protective layer around different organs, tissues, and blood vessels.
5. Expressions: Expressions of feelings, thoughts, etc., are processed by the brain and perceived by the muscle.

CONNECTIVE TISSUES

Connective tissue is the most diverse and widespread tissue in the human body. During embryonic development, connective tissue arises from mesenchyme (stem cell tissue), which itself originates from the mesoderm (primary germ layer). Connective tissues have extensive amount of extracellular matrix, embedded with cells, fluid, and different kinds and numbers of fibres. These substances are known as ground substances and make up the extracellular matrix or intracellular matrix.

Sub-types of Connective Tissues

• Fibrous (connective tissue proper)
  • Loose fibrous (areolar)
  • Adipose
  • Reticular
  • Dense
• Cartilage
  • Hyaline
  • Fibrocartilage
  • Elastic
• Bone
  • Compact
  • Cancellous (spongy)
• Blood

Fibrous Connective Tissues

1. Loose Fibrous Connective Tissue (Areolar): Out of all the connective tissues, it is one of the most widely distributed tissue which connects several adjacent body structures by acting as elastic glue thus allows movement.
2. Adipose Tissue: It primarily comprises of fat cells which are also referred to as adipocytes. White fat is present more abundantly which stores energy for the body. Brown fat is also present in small amounts.
3. Reticular Tissue: The term reticular means like a net. Reticular tissue is named so as this connective tissue forms a reticular network. Found in spleen, lymph nodes, and bone marrow.
4. Dense Fibrous Tissues: Dense tissue is characterised by the presence of densely packed fibres in the matrix, further divided into dense irregular and dense regular fibrous tissue.

Cartilage Tissues

1. Hyaline Cartilage Tissue: It appears shiny and translucent due to collagen in the matrix. Provides firm but flexible support.
2. Fibrocartilage Tissue: It is the strongest and most durable cartilage of the body, densely packed with white collagen fibres.
3. Elastic Cartilage Tissue: Characterised by fewer collagen fibres and a large number of fine elastic fibres providing flexibility. Present in external ear, eustachian tube, and larynx.

Bone Tissues

Bone is a hard connective tissue and is also known as osseous tissue. Matrix of bone mainly consists of collagen fibres and mineral salt crystals; osteocytes are embedded in this matrix. Two types: Compact bone tissue (cortical bone) and Cancellous (spongy) bone tissue.

Blood Tissues

Blood is a liquid connective tissue. It comprises plasma (55%) and blood cells (45%) including RBCs (erythrocytes), WBCs (leukocytes), and platelets (thrombocytes).

Functions of Connective Tissues

1. It connects different tissues of the body.
2. It supports various tissues, organs, and structures of the body.
3. It binds together various organs of the body.
4. It helps in defending the body against foreign substances.
5. It transports substances and respiratory gases from one body part to the other.

NERVOUS TISSUES

Nervous Tissues are mainly situated in the brain, spinal cord, and nerves. It helps in organising and regulating body activities. It promotes muscular contraction, develops environmental awareness, and is crucial for emotions, memory, and thinking. Neurons or nerve cells are the cells that produce and conduct impulses.

Neurons

A typical neuron has a cell body (soma), axon (transmits impulses away), dendrites (carry signals toward the body), myelin sheath, and Nodes of Ranvier.

Types of Neurons

1. Sensory Neurons: Convert signals from external environment into internal stimuli.
2. Motor Neurons: Carry signals from the brain to the body’s muscles.
3. Interneurons: Connect sensory and motor neurons and aid in signal transmission between two neurons.

Glial Cells

1. Astrocytes: Present in CNS, regulate neuron function and protect from toxins.
2. Microglia Cells: Found in brain, destroy pathogens and discard damaged tissue cells.
3. Schwann Cells and Oligodendrocytes: Present in PNS, electrically insulate axons.

Functions of Nervous Tissues

1. Sense stimuli in internal and external environment.
2. Coordinate appropriate responses by stimuli analysis and integration.
3. Conduct nerve impulses through afferent/sensory neurons.
4. Supply impulses between efferent and afferent neurons and to different parts of CNS.
5. Transmit impulses from CNS to muscles (effector organs).
6. Transmit impulses to involuntary glands and muscles.