Anatomy LE 1: Connective TIssue and the Skin and its Appendages

Connective Tissue Proper

• Myofibroblasts are a type of fibroblast involved in wound healing with well-developed contractile function, enriched with actin.

Macrophages

• Main function: uptake or phagocytosis of cellular debris for presentation to immune cells
• Turnover of protein fibers and removal of apoptotic cells, tissue debris, or other materials abundant at sites of inflammation
• Secrete growth factors for tissue repair and lymphocyte activation via antigens
• Histologic appearance: active macrophage has an irregular surface, eccentrically located, oval or kidney-shaped nucleus, deeply stained, and surrounded by lysosomal granules in the cytoplasm, which appears foamy under light microscopy.

Mast Cells

• Types of connective tissue proper
• Histologic appearance: spindle-shaped
• Display metachromasia when stained with toluidine blue

Plasma Cells

• Antibody-producing cells that arise from lymphocytes
• Migrate back to lymphoid organs after performing immune activity in the connective tissue
• Mostly seen in lymphoid tissues and the lamina propria of the gastrointestinal (GI) tract
• Histologic appearance: large, ovoid cells with basophilic cytoplasm and a spherical, eccentrically located nucleus containing alternating dark heterochromatin granules and light euchromatin granules arranged in a “spokes of wheel” or “clock face” appearance.

Leukocytes

• White Blood Cells (WBCs) include: Neutrophils, Eosinophils, Basophils, Monocytes, and Lymphocytes
• Migrate between the endothelial cells of venules to enter connective tissue via diapedesis
• First line of defense against antigens or antibodies that have invaded the CT
• Short-lived (hours-days), must be replaced continually via apoptosis.

Skin Appendages

• Hair follicles, sweat and sebaceous glands, arrector pili muscles
• Derived from the epidermis
• During development, grow into and reside within the dermis surrounded by connective tissue
• Embryonic origin: Ectodermal

Hair Pilosebaceous Unit

• Consists of: hair follicle, sebaceous gland, arrector pili muscle
• Arrector pili muscle contracts, causing hair strands to become erect (goosebumps) to trap warm air next to the skin.

Hair Bulb

• Found at the base of the hair follicle
• Also known as the “terminal dilation of the growing hair follicle”
• Composed of keratinocytes similar to the spinous and basal layer
• Divide rapidly and undergo keratinization, melanin accumulation, and terminal differentiation.

Hair Growth Process

• Keratinization
• Melanin accumulation
• Terminal differentiation
• Phases: anagen (long period of mitotic activity and growth), catagen (brief period of arrested growth and regression of hair bulb), and telogen (final long period of inactivity where hair may shed).

Thin Skin

• Found in most areas of the body
• Histologic appearance: stratum spinosum (spinous layer) with cytoplasm consisting of packed keratin filaments.

Friction Blisters

• Lymph-filled spaces between epidermis and dermis of thick skin due to excessive rubbing
• Can lead to protective thickening and hardening of outer cornified epidermal layers, forming corns and calluses.

Stratum Spinosum (Spinous Layer)

• Thickest layer
• Main function: actively synthesizing keratins (above basal layer)
• Cells in this layer have cytoplasmic projections called tonofibrils, which terminate at desmosomes and function to hold cell layers together.

Cutis Laxa (Elastolysis)

• Abnormal elastin metabolism leading to reduced skin elasticity
• Degenerative, rare, inherited or acquired CT disorder
• Characterized by loose, inelastic hanging folds of skin around the face and neck, giving a "bloodhound appearance".

Vitiligo

• Skin depigmentation due to loss or decreased activity of melanocytes or absence of melanocytes in the epidermis
• Discolored/depigmented patches in different areas of the body, hair, and mucous membranes
• Histologic appearance: lacks melanocytes at the basal cell layer.