Simply put, we define tissue as a group of cells with similar shape and function, which we call tissues. These tissues create an intermediate organizational level between cells and the organ system, with organs formed by combining these functional groups of tissues. Histology focuses on studying tissue, while the analysis of diseases related to tissue is referred to as histopathology. Embedding and sectioning using the paraffin block are the standard techniques for studying tissues.
The human body consists of four primary types of tissues:
1. Epithelial,
2. Connective,
3. Muscular, and
4. Nervous tissues
Each of these tissue types has distinct subtypes and characteristics:
Types of tissues
Epithelial Tissue:
Cells forming these tissues cover the external parts of body organs and line the surfaces of organs like the skin, the reproductive tract, the airways, and the inner lining of the digestive tract. They perform vital functions like protection, excretion, glandular secretion, and absorption. Epithelial cells have minimal extracellular material, but they arrange themselves on a basement membrane.
Epithelium is further classified as
1. Simple epithelium
2. Stratified epithelium
1. Simple epithelium:
It consists of a single layer of cells. It is further divided into
1. Squamous 2.Cuboidal
3.Columnar 4. Ciliated
5. Glandular.
1. Squamous epithelial tissue:
1. Epithelium tissues
consists of flat cells arranged edge to edge, as in a mosaic. This type of epithelium covers the alveoli of the lungs and the pericardial, pleural, and peritoneal cavities.
2. Cuboidal epithelium:
In the distal convoluted tubules of the kidney, cubical cells arrange themselves over a basement membrane.
3. Columnar epithelial tissue:
It consists of tall, pillar-like cells arranged on the basement membrane. It is found in the lungs and ducts of glands.
4. Ciliated epithelium tissue:
The cells may be cuboid or columnar. But they contain hair-like structures called cilia present in the free border, e.g., cells lining the trachea.
5. Glandular epithelium tissue:
It consists of cylindrical or columnar cells. It is present in secretory glands like salivary glands and breasts. The glands are of two types :
1. Uunicellular
2. Multicellular
2. Stratified epithelium
This is also called the compound epithelium. It consists of several cells. It is present on surfaces where there is considerable wear and tear. Depending on the types of cells present, the stratified epithelium is further classified into:
1.Stratified squamous epithelium
2.Stratified cuboidal epithelium
3.Stratified columnar epithelium
Sometimes, the superficial cells of stratified epithelium become dry and hard. They develop a protein called keratin. The epithelium earns the name “keratinized stratified epithelium” when its superficial cells contain keratin. On the other hand, when the superficial cells lack keratin, it is referred to as the “non-keratinized stratified epithelium.”
Connective tissues
A group of tissues consists of cells separated by a non-living material known as an extracellular matrix.This tissue provides shape to the different organs and maintains their positions. For example, blood, bone, tendon, adipose, ligament, and areolar tissues.
Connective tissue is classified as:
1. Areolar tissue (loose connective tissue):
The most widely distributed tissue in the body. It is a loose irregular connective tissue. It connects the skin to the underlying structures. Also, it fills unoccupied space between organs. It is found between muscles, blood vessels, and nerves.
2. Dense fibrous tissue:
This tissue has compactly arranged fibers with greatly reduced interspace. The cells are fewer in number than areolar tissue. This type of tissue is present in the dermis of the skin.
3. Elastic tissue:
In areas where both strength and elasticity are needed, such as in the trachea and bronchi, you’ll find this yellow-colored tissue, which is rich in elastic fibers.
4. Adipose tissue (fatty tissue):
it contains large, rounded cells and cytoplasm loaded with fat. The nucleus of the cells is pushed toward the periphery e.g. subcutaneous tissue of the skin.
5. Cartilage:
IT has a tough, gelatinous, and elastic matrix formed of chondrin. The cells are more or less round and lie in groups of four embedded in the matrix. Cartilage is of three types:
A. Hyaline cartilage
B. Fibrocartilage
C.Elastic cartilage
6. Bone:
It is the hardest connective tissue. Bones contain a high concentration of salt like calcium phosphate and calcium carbonate. A bone generally consists of:
1.Periosteum: It is a membrane of fibrous tissue covering the surface of the bone.
2.Compact bone: It is a hard, dense substance found below the periosteum
3.Cancellous or spongy bone: It is a spongy, porous tissue. It forms the interior of the mature bone.
4.Bone marrow: It is a soft material that fills the hollow interior of the mature bone. It is of two types:
Muscles tissues
Muscle is a tissue that has the power of contraction. Contraction of muscles produces movements. All muscles contain bundles of fibers that run in the same direction.
Types of muscle tissues
Muscles are classified into three types. They are:
- Voluntary muscles
- Involuntary muscles
- Cardiac muscles
Voluntary muscle (striated or skeletal muscles)
It is under the control of will that forms the flesh of limb and trunk. It contains unbranched fibers called myofibrils. These fibers are cylindrical in shape containing
A) Distinct striations
B) A number of nuclei. Each fiber is surrounded by a thin membrane called sarcolemma.
2. Involuntary muscles (non-striated, plain, or smooth muscles)
This not controlled by will. It is present in internal organs like the stomach, intestine, and uterus. It contains long, spindle-shaped fibers with a central nucleus. The fibers in this type of muscle tissue are unstriated and lack any membrane covering, but they are interconnected by loose connective tissue. Control of these muscles is regulated by the autonomic nervous system.
3. Cardiac muscles:
Cardiac muscle is exclusively located in the heart. It is composed of short, cylindrical, and branched fibers with centrally placed nuclei. The striations in cardiac muscle are indistinct. Unlike skeletal muscle, cardiac muscle is not under voluntary control; instead, it contracts automatically and rhythmically.
Nervous tissue
Nervous tissue is found in the brain, spinal cord and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses. The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendritis the cell body, and one axon.
Nervous tissue is composed of three kinds of matter.
1. Gray matter: which forms the nerve cell
2. White matter: which forms the nerve fibers
3. Neurolgia: which is supporting tissue. It holds together and supports nerve cells and fibers.
Neuron
It is the unit with which the nervous system is composed. It consists of
1. Nerve cell.
2. Axon
3. Dendrite.
Structure of the neuron
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- Nerve cell: Each nerve cell has a nucleus and a highly specialized protoplasm. The body of the nerve cell gives rise to nerve fibers.
- Axon: It is the main fiber that arises from the body of each nerve cell. Impulses pass through the axon in one direction only.
- Dendrities: These are short fibers arising from the body of each nerve cell. They conduct impulses toward the cell body. Synapse is the junction between the axon of one neuron and the dendrite of another
- Myelin sheath: It is the sheath that envelops and safeguards the axon of certain nerves. Nerves with this sheath are referred to as medullated nerves, while those lacking it are known as non-medullated nerves. Within this medullated nerve, nuclei are present between the myelin sheath and the outer covering known as neurolemma. The interruptions in the myelin sheath found at regular intervals are called nodes of Ranvier.
