Human Tissues, Membranes, and the Integumentary System

All epithelial tissues are supported by a basement membrane, which serves as the connection point to the underlying connective tissue.

Exam Tip: When identifying epithelial tissue under a microscope, always look for the basement membrane and check the cell shape (squamous = flat, cuboidal = cube, columnar = tall) to determine the type.

Modes of Glandular Secretion

Glands are specialized epithelial tissues that produce and release products. There are three distinct modes of secretion based on how the cells release their contents:

1. Merocrine Secretion: The product is released from secretory vesicles at the apical surface of the gland cell via exocytosis. An example is the salivary gland.
2. Apocrine Secretion: The apical portion of the cytoplasm is lost along with the secretory product. The cell must then undergo a process of regrowth to replace the lost components. An example is the mammary gland.
3. Holocrine Secretion: The entire gland cell bursts to release its contents. Because the cell itself becomes the secretion, it must be replaced by the division of underlying stem cells. An example is the sebaceous (oil) gland of the hair follicle.

Connective and Muscle Tissues

Connective Tissue Overview

Connective tissues are diverse, ranging from fluid to solid matrices.

• Loose Connective Tissue: Includes areolar tissue (cushions organs), adipose tissue (stores energy and provides insulation), and reticular tissue (provides a supporting framework in organs like the liver and spleen).
• Dense Connective Tissue: Includes dense regular (found in tendons and ligaments) and dense irregular (found in the dermis and organ capsules).
• Cartilage and Bone: Cartilage types include hyaline (reduces friction), elastic (tolerates distortion), and fibrocartilage (resists compression). Bone provides a solid, crystalline matrix for support.

Muscle Tissue Types

Muscle tissue is categorized by its appearance and control:

• Skeletal Muscle: Long, cylindrical, striated, and multinucleate; it is responsible for moving the skeleton.
• Cardiac Muscle: Short, branched, and striated cells connected by intercalated discs; found only in the heart.
• Smooth Muscle: Short, spindle-shaped, and nonstriated; found in the walls of hollow organs like blood vessels and the digestive tract.

The Integumentary System

The integumentary system, or the skin, is the cutaneous membrane that covers the body. It consists of several layers and accessory structures.

Layers of the Epidermis

The epidermis is the superficial layer of the skin, composed of keratinocytes. In thick skin (like the palms and soles), there are five layers:

1. Stratum corneum: Multiple layers of dead, flattened cells.
2. Stratum lucidum: A glassy layer found only in thick skin.
3. Stratum granulosum: Where cells begin to flatten and die.
4. Stratum spinosum: Contains keratinocytes bound by desmosomes.
5. Stratum basale: The deepest layer, containing stem cells, melanocytes (which produce melanin pigment), and tactile cells.

Accessory Structures and Repair

The skin contains accessory structures such as hair follicles, sebaceous glands (which use holocrine secretion), and sweat glands (eccrine and apocrine).

When the skin is injured, it undergoes a multi-phase repair process:

1. Inflammatory Phase: Mast cells release chemicals, causing redness, heat, and swelling.
2. Migratory Phase: Cells from the stratum basale migrate toward the wound.
3. Proliferation Phase: Fibroblasts create a collagenous framework.
4. Scarring Phase: The scab is shed, and scar tissue may form.

Clinical Note: Skin cancers like basal cell carcinoma and the more dangerous melanoma are significant health concerns, often resulting from cumulative damage in the aging population.