. Lecture [2] By Prof. dr. ashraf mahmoud m Epithelial tissue

Cells are the smallest functional unit of the organism. Tissues: 4 main types of tissues are distinguished: 1.Epithelial tissue. 2.Connective tissue. 3.Muscular tissue. 4.Nervous tissue.

Epithelial tissue: is a tissue that composed mainly of cells with little intercellular substance. These tissues exist in association with one another forming different organs and systems.

Characterstics of the Epithelial Tissue: 1.Cells are close to each other. 2.Little intercellular substances. 3.Tends to form junctions. 4.Lines surfaces or cavities to form the parenchyma of the organ.

Characterstics of the Epithelial Tissue: 4.Cells show polarity: apical (free) surfaces, lateral, basal surfaces. 5.The cells lie on a basement membrane (basal lamina). 6.No blood vessels enter in between the cells (Avascular), but nerves can pass inbetween the cells. 7.The epithelia are continuously renewed and replaced.

Types of Epithelium Covering epithelium (surface epithelium) sheet like tissues that cover or line the surfaces, cavities and organs of the body Glandular epithelium : perform secretory functions Neuroepithelium Myoepithelium

Classification of epithelial membranes Number of cell layers Simple: one layer Stratified: 2 layers or more. Shape of apical surface cells Squamous Cuboidal Columnar

I- Covering epithelium: Simple Epithelium: It is formed of one layer of cells resting on the basement membrane. Simple squamous Simple cubical (cuboidal) Simple columnar Psudostratified columnar

Simple squamous epithelium It is formed of one layer of flat cells. Each cell is formed of a thin cytoplasm and flat bulging nucleus. Sites: Lining the blood vessels endothelium Lining the serous membranes :pleura, pericardium and peritoneum mesothelium Lining the lung alveoli. Parietal layer of Bowman’s capsule of the kidney.

Simple cubical (cuboidal) epithelium It is formed of one layer of cubical cells (square cells), resting on the basement membrane. Each cell has a single rounded, central nucleus. Sites: Thyroid gland. Anterior surface of the lens of the eye. Surface of infantile ovary.

Simple columnar epithelium It is formed of one layer of columnar cells (height of the cell exceeds its width-rectangular). Each cell has a single, basal oval nucleus Types: Unmodified: in excretory ducts of the glands Modified: Secretory: e.g. lining epithelium of the stomach. Absorptive: e.g. absorptive columnar cells of the small intestine Ciliated: the free surface is supplied by cilia e.g. uterus fallopian tube and lung bronchioles.

The cells are overcrowded, but they all rest on the basement membrane. Some cells appear short and not reaching the surface. So appear as if it is formed of more than one layer. Sites : Trachea, bronchi (Ciliated with goblet cells). Large ducts of salivary glands (Non-ciliated). Pseudostratified columnar epithelium

Stratified epithelium The cells are arranged in two or more layers.

1. Stratified squamous non-keratinized The basal cells are columnar and lie on a basement membrane. This layer is responsible for generation of other cells. The intermediate cells are polygonal which gradually flattens to become thin squamous cells at the surface. -Sites: It is found in wet surfaces of all openings leading to the skin: Oral cavity, Tongue, Oesophagus, Cornea, Anal canal, Vagina, External auditory meatus. Types:

Stratified epithelium Stratified squamous non-keratinized

2. Stratified squamous keratinized like stratified squamous non- keratinized epithelium but The flat cells of the top layer are changed into non-living layer of keratin. This type is tough and resists friction and is impermeable to water. -Sites: epidermis of the skin. Stratified epithelium

3. Transitional epithelium Stratified cells that appear cuboidal when the organ or tube is not stretched, and squamous when the organ or tube is stretched by fluid. Sites: Lining the viscera which are subjected to distension e.g. urinary passages. The number of layers are nearly 4-8 layers in the empty bladder and become 2-3 layers in full bladder Stratified epithelium

Transitional Epithelium with rounded binucleated surface cells

Stratified epithelium 4. Stratified cubical epithelium The cells in uppermost layer are cubical in shape. Site: Cells lining the ducts of sweat glands of the skin. 5. Stratified columnar epithelium : The cells in the uppermost layer are columnar cells. Sites: - Large ducts of salivary glands. - Fornix of conjunctiva. - Penile part of male urethera. Stratified epithelium

II- Glandular Epithelium Classification of the glandular epithelium: According to:  Number of cells: Unicellular & multicellular.  Structure of the gland: Simple & branched.  Mode of secretion: Exocrine & endocrine.  Type of secretion: Serous & mucous …  Cellular changes after secretion (mode of secretion): Merocrine, apocrine, holocrine. Epithelium characterized by its secretory function.

Exocrine glands Unicellular: (Goblet cells) The gland is formed only of one cell. Sites: in tracheal and intestinal epithelium. L/M: Flask in shape; slender base and expanded apex due to presence of mucous granules. These granules are not stained by H&E.The nucleus is basal and flat.

Multicellular Exocrine glands

Classification: According to the type of secretion: Mucous glands: Secretes a viscous thick secretion containing mucin. Serous glands: Secretes a watery secretion containing proteins and rich with enzymes. Mixed glands :(Seromucous acinus): Secretes both mucous and serous secretions Mucous glands Serous glands:

Mucous aciniSerous acini Size Lumen In a section Cytoplasm Nucleus Secretion Relatively large in diameter Wide Formed of a few number of cuboidal cells that have well-defined boundaries Pale basophilic Each cell contains a single flattened and basal nucleus Mucous ( a viscous thick secretion) Relatively smaller than mucous acinus Narrow Formed of a relatively larger number of pyramidal cells Dark Basophilic Each cell contains a single rounded nucleus, shifted towards the base Watery whey-like containing enzymes

According to the mode of secretion:

1.Merocrine (eccrine): The secretory cells release their contents by exocytosis with no loss of the cytoplasm or cell membrane e.g. salivary glands. 2. Apocrine: Apical Part of the cytoplasm is released with the secretion e.g. lactating mammary glands. 3. Holocrine: The secretory cell together with its accumulated secretion is released into the duct e.g. the sebaceous glands of the skin

III- Neuroepithelium It is a special type of epithelium which is modified to receive sensations. Neuroepithelium has three main types of cells; 1)Sensory cells for receiving the stimuli 2)Supporting cells 3)Basal cells ( acts as stem cells) Sites: Taste buds for taste sensation Olfactory mucosa Organ of hearing

IV- Myoepithelium Epithelial cells which have the ability to contract. The cells contain myofilaments. They surround the secretory acini of glands They help in releasing the secretory products to the lumen of the acini.