Epithelial and Blood Tissues: Structure, Functions and Cell Types

Epithelial Tissue

Functions

Epithelial tissue is responsible for secretion, absorption, protection and sensory perception.

Classification

Epithelia and glands are classified according to the number of cell layers and the shape of the cells in the outermost layer.

• Simple: a single layer of cells.
• Stratified: more than one layer of cells.

Types of Epithelium

• Simple cuboidal epithelium: a single layer of cube-shaped cells.
• Stratified cuboidal epithelium: several layers of cube-shaped cells.
• Simple squamous epithelium: a single layer of flattened cells.
• Stratified squamous epithelium: several layers of flattened cells, so tightly packed that the individual layers are hard to distinguish.
• Keratinized stratified squamous epithelium: contains keratin, a protein that makes the tissue waterproof and resistant. Cells in the surface layer are dead and keratinized, nothing enters or leaves them, and they form a protective layer (found in the lips and epidermis).
• Simple columnar epithelium: a single layer of column-shaped cells.
• Pseudostratified ciliated columnar epithelium: appears stratified but is actually simple, since all cells rest on the basement membrane. It has cilia that move mucus and particles.

Glands

Glands produce hormones. For example, GH (growth hormone) stimulates growth.

Connective Tissue

Connective tissue is a group of different structures and contains blood vessels.

Layers of the Skin

1. Stratum corneum (SC): the outermost layer, made of dead keratinized cells that are constantly shed and replaced.
2. Epidermis and dermis beneath it, including:
   • Stratum spinosum (SS): the spiny layer.
   • Stratum germinativum (SG): the germinal layer, where new cells are produced.
   • Dermal papillae (PD): projections of the dermis that increase the contact surface with the epidermis.
   • Dermis (D): the deeper connective tissue layer.

Blood Tissue

Whole blood contains blood cells produced by hematopoietic tissue, located within the bones in some adults.

Red Blood Cells (Erythrocytes)

• Contain hemoglobin, which carries O₂ from the lungs. Hemoglobin is formed from iron and other proteins.
• Approximately 5 million per cubic millimeter.
• Anucleate (no nucleus) and disc-shaped, thin in the center.

Platelets

Platelets are cytoplasmic fragments produced in the bone marrow by cells called megakaryocytes, which share the same origin as erythrocytes. They are essential for blood coagulation, inducing the formation of fibrin, a network that stops bleeding. When the body produces new skin beneath a scab, the fibrin falls off the skin. Fibrin is a whitish, insoluble protein.

White Blood Cells (Leukocytes)

White blood cells are classified as granulocytes (with granules) or agranulocytes (without granules).

Granulocytes

• Neutrophils: have a trilobed nucleus divided into 3 parts. They phagocytose bacteria and microorganisms that cause infection. They are checked in blood tests for infection and are highly mobile, leaving the blood vessels to reach infected areas. Pus is formed by dead neutrophils.
• Eosinophils: have a bilobed nucleus. They fight germs and parasitic invaders, releasing the contents of their granules. They are checked in blood tests, are mobile, and secrete anti-histamine substances.
• Basophils: have a bilobed nucleus. They have no specific function in active processes but release histamine, which increases vascular permeability, allowing neutrophils and eosinophils to leave the blood vessels. Histamine is responsible for itching, redness, runny nose, swelling and altered breathing in allergies.

Agranulocytes

• Monocytes: have a horseshoe-shaped nucleus. They show no activity while in the bloodstream, remaining there for a few hours before leaving it. Outside the circulation, they give rise to macrophages, which are part of the body's defense.
• Lymphocytes:
  • B cells: named after the bursa of Fabricius, an organ in birds where they mature. In mammals, they mature in the red bone marrow. They are responsible for the production of antibodies.
  • T cells: mature in the thymus and are divided into helper T cells (CD4+) and cytotoxic T cells (CD8+). Helper T cells stimulate B and T lymphocytes to produce antibodies. Cytotoxic T cells attack abnormal cells or destroy infected cells.

Diseases Affecting Lymphocytes

Lupus and AIDS are conditions that affect the immune system. AIDS destroys CD4+ and CD8+ cells.