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majority of the immune system.
There are five main types of WBCs: - the granular cells which include, Neutrophils, Eosinophils, Basophils, non-granular which includes Lymphocytes and Monocytes.

A. Neutrophils kills bacteria through the process of phagocytosis.
B. Eosinophils kills parasites and have a role in allergic reactions.
C. Basophils plays a major role in allergic reactions.
D. Lymphocytes function

T lymphocytes (T cells) are responsible for cell-mediated immunity.
B lymphocytes are responsible for humoral immunity or antibody production.
Destroying cancer cells.
They present antigens to activate other cells of the immune system.
E. Monocytes functions: -
Enters the tissue, where they become larger and turn into macrophages.
Destroy old, damaged and dead cells in the body.

3. Platelets (Thrombocytes)
   Nucleus Absent.
   Functions
   Platelets play large role in the process of blood clotting.
   They secrete vasoconstrictors which constrict blood vessels, causing vascular spasms in broken blood vessels.
   They secrete chemicals that attract neutrophils and monocytes to sites of inflammation.
   Dissolve blood clots when they are no longer needed.
   Nervous cells  
   The nervous tissues contain two types of cells, the  Neuron cells and Glial cells

The neuron is the basic functional unit of the nervous tissue. Neuron consists of the cell body with extensions called dendrites and axons.
The axons carry stimulus or signals from the cell towards other cells. Distally the axon branches and those branches are called axon terminals which make synaptic connections with other cells. The axon terminals contain various neurotransmitters which are released into the synapses to allow signal transmission from one neuron to the next cell.
The dendrites, bring signals or stimuli toward the cell body from other cells.
Neurons can generate electrical signals called action potentials which can pass from one neuron to another cell through synaptic junctions.
Types of Neurons by Functions
There are three major types of neurons: -
Sensory neurons

The sensory neurons get activated from the external stimulations such as touching a hot surface or hearing music or a chemical reaction. Once activated, the sensory neurons transmit the electrical impulses to the rest of the Central Nervous System (CNS) for processing.
Motor neurons

The motor neurons send impulses from the brain and spinal cord to skeletal, smooth, and cardiac muscles, and thus control all of the muscle movements.
There are two kinds of motor neurons: -
The upper motor neurons transmit impulses from the brain and brain stem to the spinal cord.
The lower motor neurons transmit impulses from the spinal cord to the muscles.
3- Interneurons: -

The interneurons are located in the CNC. They are the mediators between the sensory and motor neurons with other parts of CNC. They play vital role in the reflex actions.

Reflex Arc Diagram

Glial cells

Glial cells are supporting cells that aid in the maintenance and function of the neurons. There are different glial cells in the nervous system, such as Astrocytes, Ependymal cells, Oligodendrocytes, microglia, and Schwann cells. The first four types of glial cells are found in the central nervous system, and Schwann cells are found in the peripheral nervous system.
Astrocytes
Astrocytes store glycogen which can be broken down to glucose to be provided to neurons.
Regulation of extracellular ionic environment –Astrocytes remove excess potassium ions from the extracellular space following neuronal activation.
Neurotransmitter uptake – Astrocytes contain specific transporters for several neurotransmitters. A rapid removal of neurotransmitters from the extracellular space is required for the normal function of neurons.
Promotion of myelination by oligodendrocytes.
Oligodendrocytes
These cells are responsible for the insulation the axons in the central nervous system. They carry out this function by producing a myelin sheath that wraps around the axon.
A single oligodendrocyte has the capacity to myelinate up to 50 axonal segments. They are equivalent to the Schwann cells in the peripheral nervous system.

Microglia
These cells form the resident immune system of the brain. They are activated in response to tissue damage and have the capability to recognize foreign antigens and initiate phagocytosis to remove foreign materials.
Ependymal cells
The ependymal cells form a thin layer that line the ventricular system of the brain and spinal cord. The main function of these cells is the production of cerebrospinal fluid (CSF) as a part of the choroid plexus.
Their apical surfaces are covered with cilia and microvilli, which allow for the circulation and absorption of CSF respectively.

Epithelial cells
The epithelial cells are closely bound to each other and are supported by a connective tissue called the basement membrane.
Functions of Epithelial cells

Absorption, secretion and movement of substances.
The epithelial cells of small intestine have microvilli on their surface to increase the surface area for absorption.
Epithelial cells in the bronchioles of the lungs contain cilia that move mucus and foreign bodies up to be expelled out.
Epithelial cells are classified in to the followings: -
Squamous
Cuboidal
Columnar
Transitional
Squamous epithelial cells are flat and are connected to each other by cell junctions or disc. The nucleus is flattened and located near the center of the cell. The cytoplasm is pale and contains few organelles. These cells are found in the epithelial tissues of the skin and GIT.
The cuboidal epithelial cells are almost of the same tall and width. They contain a large spherical nucleus that are centrally located. They are rich with mitochondria and other organelles. Their major functions are absorption and secretion.

Columnar epithelial cell has an oval nucleus usually located towards the basal of the cell. They are found in the female reproductive system and in the digestive tract. There are two functionally different types of columnar epithelial cells ciliated and non-ciliated. The ciliated columnar epithelium bears cilia (finger-like projections of the plasma membrane) on the apical surfaces of cells. The cells in the fallopian tubes are ciliated and involved in the movement of the ovum towards the uterus. Those in the digestive tract have no cilia but contain microvilli. Microvilli are small cytoplasmic projections on the apical surface of cells. Microvilli serve to increase the surface area of the epithelium for secretion of mucin and enzymes and to increase the surface area for absorption of nutrients.
Tissues
Tissue is a group of cells that share a common embryonic origin and share the same morphological features.
Embryonic Origin of Tissues
The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm. After fertilization the zygote gives rise to rapid mitotic cycles and generating three distinct germ layers from which all the tissues and organs of the human body are form and these are: ectoderm, mesoderm, and endoderm.  The epithelial tissue originates in all three layers, whereas nervous tissue derives from the ectoderm and muscle tissue from mesoderm.
There are four types of tissues: -
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue
Epithelial tissue, it consists of sheets of cells that cover exterior surfaces of the body, lines internal cavities, lumens of internal organs, and forms certain glands. 
Connective tissue, binds the cells and organs of the body together and helps in the protection, support, and integration of all parts of the body. Connective tissue includes the bones, cartilages, blood, CSF, and lymph tissues.
Muscle tissue is excitable and contract in response to electrical stimulus which helps in movement. There are three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. 
Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.

Bone tissues
Bone is living tissue that makes up the body's skeleton. It provides shape and support for the body, as well as protection for some organs. Bone also serves as a storage site for minerals and provides the medium for the development and storage of blood cells.
Bone tissues are classified into the following types: -
Compact tissue. The harder, outer tissue of bones.
Cancellous tissue. The sponge-like tissue inside bones.
Subchondral tissue. The smooth tissue at the ends of bones, which is covered by tissue cartilage. 
The outer membrane covering the bones is called the periosteum. Beneath the periosteum are tunnels and canals through which blood and lymphatic vessels run to carry nourishment for the bone. Muscles, ligaments (رباط), and tendons (وتر ) may attach to the periosteum.

Cartilage tissue
It is a connective tissue that forms the skeleton of embryos before bone formation begins
and persists in parts of the human skeleton into adulthood. It is composed of a dense network of collagen fibers embedded in a firm, gelatinous ground substance. Cartilage tissue contains no blood vessels or nerves.
There are three types of cartilage tissues: -
Hyaline cartilage, it makes up the embryonic skeleton. It persists in human adults at the ends of bones as an articular cartilage, at the ends of the ribs, and in the nose, larynx, trachea, and bronchi.

Fibrocartilage is very strong tissue found predominantly in the intervertebral disks and at the insertions of ligaments and tendons.
 Elastic cartilage, which is yellow in appearance, is more flexible than the other two forms because it contains elastic fibers in addition to collagen. In humans it makes up the external ear and the epiglottis.

Cartilage tissue forms a model for later growth of the bony skeleton. In the embryo, cartilage gradually calcifies, and chondrocytes are replaced by osteocytes. After birth a thin plate of cartilage, called the epiphyseal plate, persists at the ends of growing bones, finally becoming ossified itself when the bone behind it has completed its growth. At the growing edge of the plate, chondrocytes continue to grow and divide, while on the trailing edge they are replaced by osteocytes as new bone is laid down. The cartilage plate thus remains of a constant thickness while the bone grows behind it. Once this plate disappears, no further longitudinal bone growth is possible.

Epithelial tissues
Epithelial tissues are thin tissues that cover the skin, line the oral cavity, digestive tract, the urogenital tract, the hollow parts of every organ such as the heart, lungs, eyes, ears, as well as the ventricular system of the brain and central canals of the spinal cord.
The epithelial tissue consists of closely bound cells and is supported by a connective tissue called the basement membrane.
Functions of epithelial tissues include: -
Protection against abrasion, radiation, chemical substance, and invasion by pathogens. Protective tissue tends to be thicker and made of multiple layers such as the skin and the esophagus.
Absorption, secretion, and movement of substances. These tissues are usually thin, containing cilia or microvilli and are often made of one layer of cells. With the exception of the mouth and esophagus, the rest of digestive tract which include the stomach, small and large intestine are covered by these kinds of thin epithelial tissue.

Epithelial tissues are classified according to the number of cell layers and the shape of the epithelial cells.
Classification according to the number of cell layers: -
Simple epithelial tissue, it is composed of a single layer of epithelial cells.
Stratified epithelial tissue, it is composed of two or more layers of epithelial cells.

Classification according to the shape of cell composing cells: -
Squamous epithelial tissue: consists of squamous epithelial cells.
Cuboidal epithelial tissue: consists of cuboidal epithelial cells.
Columnar epithelial tissue: consists of columnar epithelial cells.
Considering the number of cell layers and cell types together results in 6 epithelial tissues’ types.
Simple squamous epithelia
Simple cuboidal epithelia
Simple columnar epithelia
Stratified squamous epithelia
Stratified cuboidal epithelia
Stratified columnar epithelia
Additionally, there are pseudostratified epithelia and transitional epithelia.

Examples of Epithelial tissues
Simple squamous epithelial tissue is composed of a single layer of squamous cells that are in direct contact with the basement membrane. Example of this tissue is the tissue that lines the alveoli of lungs.
Simple cuboidal epithelial tissue is composed of a single layer of cuboidal cells. This tissue lines the collecting ducts of the kidney and thyroid follicles that secrete thyroid hormones.
Simple columnar epithelial tissue is composed of a single layer of columnar epithelial cells and it lines the digestive tract and the female reproductive system.
Pseudostratified epithelial tissue is composed of cells that have varying heights and therefore it looks as if it is a stratified tissue. Every cell in this tissue makes contact with the basement membrane.
Stratified Epithelial tissue
It is composed of more than one layer of epithelial cells and only one layer is in direct contact with the basement membrane. These tissues often have a protective role, and the extent of friction or abrasion often determines the number of layers of cells.

Stratified squamous epithelial tissue is found in the epidermis of the skin. The upper layer of this tissue is keratinized and sloughed out continuously. The keratinized cells provide protection against water and nutrient loss.
Stratified cuboidal epithelial tissue can be found surrounding the ducts of many glands, including mammary glands in the breast and salivary glands in the mouth.
Stratified columnar epithelial tissue is found predominantly in some organs of the reproductive system, and in the conjunctiva of the eye.
Transitional epithelial tissue is a stratified epithelial tissue that consist of ovoid cells that can stretch based on the pressure of liquids inside the organ. They are found in the urinary and gall bladder lining epithelial tissues.

majority of the immune system.
There are five main types of WBCs: - the granular cells which include, Neutrophils, Eosinophils, Basophils, non-granular which includes Lymphocytes and Monocytes.

A. Neutrophils kills bacteria through the process of phagocytosis.
B. Eosinophils kills parasites and have a role in allergic reactions.
C. Basophils plays a major role in allergic reactions.
D. Lymphocytes function

T lymphocytes (T cells) are responsible for cell-mediated immunity.
B lymphocytes are responsible for humoral immunity or antibody production.
Destroying cancer cells.
They present antigens to activate other cells of the immune system.
E. Monocytes functions: -
Enters the tissue, where they become larger and turn into macrophages.
Destroy old, damaged and dead cells in the body.

3. Platelets (Thrombocytes)
   Nucleus Absent.
   Functions
   Platelets play large role in the process of blood clotting.
   They secrete vasoconstrictors which constrict blood vessels, causing vascular spasms in broken blood vessels.
   They secrete chemicals that attract neutrophils and monocytes to sites of inflammation.
   Dissolve blood clots when they are no longer needed.
   Nervous cells  
   The nervous tissues contain two types of cells, the  Neuron cells and Glial cells

The neuron is the basic functional unit of the nervous tissue. Neuron consists of the cell body with extensions called dendrites and axons.
The axons carry stimulus or signals from the cell towards other cells. Distally the axon branches and those branches are called axon terminals which make synaptic connections with other cells. The axon terminals contain various neurotransmitters which are released into the synapses to allow signal transmission from one neuron to the next cell.
The dendrites, bring signals or stimuli toward the cell body from other cells.
Neurons can generate electrical signals called action potentials which can pass from one neuron to another cell through synaptic junctions.
Types of Neurons by Functions
There are three major types of neurons: -
Sensory neurons

The sensory neurons get activated from the external stimulations such as touching a hot surface or hearing music or a chemical reaction. Once activated, the sensory neurons transmit the electrical impulses to the rest of the Central Nervous System (CNS) for processing.
Motor neurons

The motor neurons send impulses from the brain and spinal cord to skeletal, smooth, and cardiac muscles, and thus control all of the muscle movements.
There are two kinds of motor neurons: -
The upper motor neurons transmit impulses from the brain and brain stem to the spinal cord.
The lower motor neurons transmit impulses from the spinal cord to the muscles.
3- Interneurons: -

The interneurons are located in the CNC. They are the mediators between the sensory and motor neurons with other parts of CNC. They play vital role in the reflex actions.

Reflex Arc Diagram

Glial cells

Glial cells are supporting cells that aid in the maintenance and function of the neurons. There are different glial cells in the nervous system, such as Astrocytes, Ependymal cells, Oligodendrocytes, microglia, and Schwann cells. The first four types of glial cells are found in the central nervous system, and Schwann cells are found in the peripheral nervous system.
Astrocytes
Astrocytes store glycogen which can be broken down to glucose to be provided to neurons.
Regulation of extracellular ionic environment –Astrocytes remove excess potassium ions from the extracellular space following neuronal activation.
Neurotransmitter uptake – Astrocytes contain specific transporters for several neurotransmitters. A rapid removal of neurotransmitters from the extracellular space is required for the normal function of neurons.
Promotion of myelination by oligodendrocytes.
Oligodendrocytes
These cells are responsible for the insulation the axons in the central nervous system. They carry out this function by producing a myelin sheath that wraps around the axon.
A single oligodendrocyte has the capacity to myelinate up to 50 axonal segments. They are equivalent to the Schwann cells in the peripheral nervous system.

Microglia
These cells form the resident immune system of the brain. They are activated in response to tissue damage and have the capability to recognize foreign antigens and initiate phagocytosis to remove foreign materials.
Ependymal cells
The ependymal cells form a thin layer that line the ventricular system of the brain and spinal cord. The main function of these cells is the production of cerebrospinal fluid (CSF) as a part of the choroid plexus.
Their apical surfaces are covered with cilia and microvilli, which allow for the circulation and absorption of CSF respectively.

Epithelial cells
The epithelial cells are closely bound to each other and are supported by a connective tissue called the basement membrane.
Functions of Epithelial cells

Absorption, secretion and movement of substances.
The epithelial cells of small intestine have microvilli on their surface to increase the surface area for absorption.
Epithelial cells in the bronchioles of the lungs contain cilia that move mucus and foreign bodies up to be expelled out.
Epithelial cells are classified in to the followings: -
Squamous
Cuboidal
Columnar
Transitional
Squamous epithelial cells are flat and are connected to each other by cell junctions or disc. The nucleus is flattened and located near the center of the cell. The cytoplasm is pale and contains few organelles. These cells are found in the epithelial tissues of the skin and GIT.
The cuboidal epithelial cells are almost of the same tall and width. They contain a large spherical nucleus that are centrally located. They are rich with mitochondria and other organelles. Their major functions are absorption and secretion.

Columnar epithelial cell has an oval nucleus usually located towards the basal of the cell. They are found in the female reproductive system and in the digestive tract. There are two functionally different types of columnar epithelial cells ciliated and non-ciliated. The ciliated columnar epithelium bears cilia (finger-like projections of the plasma membrane) on the apical surfaces of cells. The cells in the fallopian tubes are ciliated and involved in the movement of the ovum towards the uterus. Those in the digestive tract have no cilia but contain microvilli. Microvilli are small cytoplasmic projections on the apical surface of cells. Microvilli serve to increase the surface area of the epithelium for secretion of mucin and enzymes and to increase the surface area for absorption of nutrients.
Tissues
Tissue is a group of cells that share a common embryonic origin and share the same morphological features.
Embryonic Origin of Tissues
The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm. After fertilization the zygote gives rise to rapid mitotic cycles and generating three distinct germ layers from which all the tissues and organs of the human body are form and these are: ectoderm, mesoderm, and endoderm.  The epithelial tissue originates in all three layers, whereas nervous tissue derives from the ectoderm and muscle tissue from mesoderm.
There are four types of tissues: -
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue
Epithelial tissue, it consists of sheets of cells that cover exterior surfaces of the body, lines internal cavities, lumens of internal organs, and forms certain glands. 
Connective tissue, binds the cells and organs of the body together and helps in the protection, support, and integration of all parts of the body. Connective tissue includes the bones, cartilages, blood, CSF, and lymph tissues.
Muscle tissue is excitable and contract in response to electrical stimulus which helps in movement. There are three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. 
Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body.

Bone tissues
Bone is living tissue that makes up the body's skeleton. It provides shape and support for the body, as well as protection for some organs. Bone also serves as a storage site for minerals and provides the medium for the development and storage of blood cells.
Bone tissues are classified into the following types: -
Compact tissue. The harder, outer tissue of bones.
Cancellous tissue. The sponge-like tissue inside bones.
Subchondral tissue. The smooth tissue at the ends of bones, which is covered by tissue cartilage. 
The outer membrane covering the bones is called the periosteum. Beneath the periosteum are tunnels and canals through which blood and lymphatic vessels run to carry nourishment for the bone. Muscles, ligaments (رباط), and tendons (وتر ) may attach to the periosteum.

Cartilage tissue
It is a connective tissue that forms the skeleton of embryos before bone formation begins
and persists in parts of the human skeleton into adulthood. It is composed of a dense network of collagen fibers embedded in a firm, gelatinous ground substance. Cartilage tissue contains no blood vessels or nerves.
There are three types of cartilage tissues: -
Hyaline cartilage, it makes up the embryonic skeleton. It persists in human adults at the ends of bones as an articular cartilage, at the ends of the ribs, and in the nose, larynx, trachea, and bronchi.

Fibrocartilage is very strong tissue found predominantly in the intervertebral disks and at the insertions of ligaments and tendons.
 Elastic cartilage, which is yellow in appearance, is more flexible than the other two forms because it contains elastic fibers in addition to collagen. In humans it makes up the external ear and the epiglottis.

Cartilage tissue forms a model for later growth of the bony skeleton. In the embryo, cartilage gradually calcifies, and chondrocytes are replaced by osteocytes. After birth a thin plate of cartilage, called the epiphyseal plate, persists at the ends of growing bones, finally becoming ossified itself when the bone behind it has completed its growth. At the growing edge of the plate, chondrocytes continue to grow and divide, while on the trailing edge they are replaced by osteocytes as new bone is laid down. The cartilage plate thus remains of a constant thickness while the bone grows behind it. Once this plate disappears, no further longitudinal bone growth is possible.

Epithelial tissues
Epithelial tissues are thin tissues that cover the skin, line the oral cavity, digestive tract, the urogenital tract, the hollow parts of every organ such as the heart, lungs, eyes, ears, as well as the ventricular system of the brain and central canals of the spinal cord.
The epithelial tissue consists of closely bound cells and is supported by a connective tissue called the basement membrane.
Functions of epithelial tissues include: -
Protection against abrasion, radiation, chemical substance, and invasion by pathogens. Protective tissue tends to be thicker and made of multiple layers such as the skin and the esophagus.
Absorption, secretion, and movement of substances. These tissues are usually thin, containing cilia or microvilli and are often made of one layer of cells. With the exception of the mouth and esophagus, the rest of digestive tract which include the stomach, small and large intestine are covered by these kinds of thin epithelial tissue.

Epithelial tissues are classified according to the number of cell layers and the shape of the epithelial cells.
Classification according to the number of cell layers: -
Simple epithelial tissue, it is composed of a single layer of epithelial cells.
Stratified epithelial tissue, it is composed of two or more layers of epithelial cells.

Classification according to the shape of cell composing cells: -
Squamous epithelial tissue: consists of squamous epithelial cells.
Cuboidal epithelial tissue: consists of cuboidal epithelial cells.
Columnar epithelial tissue: consists of columnar epithelial cells.
Considering the number of cell layers and cell types together results in 6 epithelial tissues’ types.
Simple squamous epithelia
Simple cuboidal epithelia
Simple columnar epithelia
Stratified squamous epithelia
Stratified cuboidal epithelia
Stratified columnar epithelia
Additionally, there are pseudostratified epithelia and transitional epithelia.

Examples of Epithelial tissues
Simple squamous epithelial tissue is composed of a single layer of squamous cells that are in direct contact with the basement membrane. Example of this tissue is the tissue that lines the alveoli of lungs.
Simple cuboidal epithelial tissue is composed of a single layer of cuboidal cells. This tissue lines the collecting ducts of the kidney and thyroid follicles that secrete thyroid hormones.
Simple columnar epithelial tissue is composed of a single layer of columnar epithelial cells and it lines the digestive tract and the female reproductive system.
Pseudostratified epithelial tissue is composed of cells that have varying heights and therefore it looks as if it is a stratified tissue. Every cell in this tissue makes contact with the basement membrane.
Stratified Epithelial tissue
It is composed of more than one layer of epithelial cells and only one layer is in direct contact with the basement membrane. These tissues often have a protective role, and the extent of friction or abrasion often determines the number of layers of cells.

Stratified squamous epithelial tissue is found in the epidermis of the skin. The upper layer of this tissue is keratinized and sloughed out continuously. The keratinized cells provide protection against water and nutrient loss.
Stratified cuboidal epithelial tissue can be found surrounding the ducts of many glands, including mammary glands in the breast and salivary glands in the mouth.
Stratified columnar epithelial tissue is found predominantly in some organs of the reproductive system, and in the conjunctiva of the eye.
Transitional epithelial tissue is a stratified epithelial tissue that consist of ovoid cells that can stretch based on the pressure of liquids inside the organ. They are found in the urinary and gall bladder lining epithelial tissues.

Cells
A cell is the smallest structural, functional, and biological unit of living organisms and can replicate itself independently.
The cell has three main parts, the cell membrane, the cytoplasm and the nucleus.
The cell membrane is a thin flexible barrier that separates the inside of the cell from the environment outside the cell and regulates what can pass in and out of the cell.
The cytoplasm is where most functions of the cell are carried out. It consists of watery jelly like substance called the cytosol; and the different organelles.
The nucleus is separated from the cytoplasm by a nuclear membrane. It contains the DNA (genes) that code for proteins necessary for the cell to function.
The inside environment of a cell is called the intracellular fluid (ICF) and the environment outside a cell is called the extracellular fluid (ECF).  Plasma, the fluid are part of blood which constitutes the ECF compartment that links all cells in the body.

Types of cells:
Stem cell
Muscle cell
Bone cell
Cartilage cell
Blood cell
Sex cell
Nervous cell
Fat cell