Biological Classification and Human Tissue Systems

Developmental Biology: Protostomes vs. Deuterostomes

In protostomes, the mouth is formed from the opening of the gastrula. In deuterostomes, the anus is formed from the opening of the gastrula.

Marsupials (Metatheria)

Marsupials (Marsupialia) or metatherians (Metatheria) are an infraclass of mammals. They are characterized by a short gestation period in the mother's womb, completing much of their growth while clinging to the mammary glands inside a pouch, or marsupium.

Epithelial Tissues

Epithelial tissues are characterized by continuous layers of cells covering surfaces such as the skin epidermis and internal cavities. Their primary functions include protection, secretion, and absorption.

• Cell Shape: Flat, cylindrical, or cuboid.
• Layering: Simple or stratified.

Connective Tissue

Connective tissue is characterized by cells that secrete an abundant extracellular matrix. It is divided based on the type of matrix:

• Fluid Connective Tissue: Blood and lymph.
• Solid Connective Tissue: Fibrous tendons (muscle to bone), ligaments (bone to bone), cartilage (chondrocytes), bone, and adipose (fat) tissue.

Muscle Tissue Types

Muscle cells are specialized to contract to provide movement. There are three types:

• Skeletal (Striated): Long cylindrical cells attached to bones via tendons; function is voluntary.
• Visceral (Smooth): Spindle-shaped cells located in hollow viscera; function is involuntary.
• Cardiac: Located in the heart walls; function is involuntary.

Nervous Tissue

Nervous tissue coordinates body functions. The structural units are neurons. Key characteristics include:

• Irritability: Responding to internal and external stimuli.
• Conductivity: Carrying impulses over long distances.
• Transmission: Passing impulses to neurons, muscles, or glands.
• Plasticity: Adapting responses to repeated stimuli.

The Skin (Integumentary System)

The skin is a membrane and organ that protects the body. Its functions include:

• First line of defense for the body.
• Organ of the sense of touch.
• Temperature regulation.
• Synthesis of vitamin D.
• Determination of skin color via melanin and blood circulation.