Study Guide: Cells, Tissues, and the Integumentary System (Chapters 3–5)

Cells: The Living Units

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• G1 phase (Gap 1): Cell grows and carries out normal functions.

• S phase (Synthesis): DNA replication occurs.

• G2 phase (Gap 2): Cell prepares for division; organelles duplicate.

• M phase (Mitosis): Division of the nucleus and cytoplasm.

Example: Skin cells rapidly progress through the cell cycle to replace lost or damaged cells.

Phases of Mitosis and Meiosis

Mitosis and meiosis are processes of nuclear division. Mitosis produces two identical daughter cells, while meiosis produces four genetically unique gametes.

• Mitosis Phases: Prophase, Metaphase, Anaphase, Telophase

• Meiosis Phases: Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate)

• Similarities: Both involve chromosome duplication and division.

• Differences: Meiosis has two divisions and results in haploid cells; mitosis has one division and results in diploid cells.

Example: Mitosis occurs in somatic cells; meiosis occurs in gamete formation.

Cell Growth and Death

• Hyperplasia: Increase in cell number (e.g., callus formation on skin).

• Hypertrophy: Increase in cell size (e.g., muscle growth from exercise).

• Atrophy: Decrease in cell size or number (e.g., muscle wasting from disuse).

• Apoptosis: Programmed cell death, important for development and tissue homeostasis.

• Necrosis: Uncontrolled cell death due to injury or disease, often causing inflammation.

Example: Apoptosis shapes fingers during embryonic development; necrosis occurs in tissue after a heart attack.

Tissue: The Living Fabric

Characteristics of Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is characterized by closely packed cells, polarity, avascularity, and high regenerative capacity.

• Polarity: Has an apical (top) and basal (bottom) surface.

• Specialized contacts: Cells are joined by tight junctions and desmosomes.

• Supported by connective tissue: Basement membrane anchors epithelium.

• Avascular but innervated: No blood vessels, but has nerves.

• Regeneration: Rapid cell division to replace lost cells.

Types of Epithelia

• Simple epithelium: Single cell layer (e.g., simple squamous in lungs for gas exchange).

• Stratified epithelium: Multiple layers (e.g., stratified squamous in skin for protection).

• Shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

Example: Simple columnar epithelium lines the digestive tract for absorption.

Glands and Their Types

• Gland: One or more cells that secrete a product.

• Exocrine glands: Secrete onto body surfaces or into ducts (e.g., sweat glands).

• Endocrine glands: Secrete hormones into the bloodstream (e.g., thyroid gland).

• Unicellular glands: Single cells (e.g., goblet cells).

• Multicellular glands: Composed of many cells (e.g., salivary glands).

Classification of Multicellular Exocrine Glands:

• Structural: Simple (unbranched duct) vs. compound (branched duct); tubular, alveolar, or tubuloalveolar shape.

• Functional: Merocrine (secrete by exocytosis), holocrine (entire cell ruptures), apocrine (apex of cell pinches off).

Connective Tissue

• Common characteristics: Originates from mesenchyme, varying degrees of vascularity, extracellular matrix.

• Structural elements: Ground substance, fibers (collagen, elastic, reticular), and cells (fibroblasts, chondrocytes, osteocytes, etc.).

Types of Connective Tissue:

• Connective tissue proper: Loose (areolar, adipose, reticular) and dense (regular, irregular, elastic).

• Cartilage: Hyaline, elastic, fibrocartilage.

• Bone: Compact and spongy.

• Blood: Fluid connective tissue.

Example: Adipose tissue stores energy; bone supports and protects organs.

The Integumentary System

Layers of the Skin and Subcutaneous Tissue

• Epidermis: Superficial layer, composed of epithelial tissue.

• Dermis: Deeper layer, composed of connective tissue.

• Subcutaneous tissue (hypodermis): Not part of skin; consists mainly of adipose tissue, anchors skin to underlying structures.

Tissue Types in the Skin

• Epidermis: Stratified squamous epithelium.

• Dermis: Connective tissue (areolar in papillary layer, dense irregular in reticular layer).

Major Layers of the Epidermis

• Stratum basale: Deepest, mitotically active layer.

• Stratum spinosum: Several layers of keratinocytes, provides strength.

• Stratum granulosum: Cells flatten, keratinization begins.

• Stratum lucidum: Only in thick skin (palms, soles), clear layer.

• Stratum corneum: Outermost, dead keratinized cells, protective barrier.

Major Layers of the Dermis

• Papillary layer: Areolar connective tissue, contains dermal papillae (fingerprints).

• Reticular layer: Dense irregular connective tissue, provides strength and elasticity.

Factors Contributing to Skin Color

• Melanin: Pigment produced by melanocytes; main determinant of skin color.

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red pigment in blood, visible in fair skin.

Hair Follicle Structure and Function

• Parts: Bulb (growth zone), root, shaft, follicle wall, dermal papilla, matrix.

• Arrector pili muscle: Smooth muscle that causes hair to stand (goosebumps).

Hair Regions, Color, and Life Span Changes

• Regions: Medulla (core), cortex (middle), cuticle (outer layer).

• Hair color: Determined by type and amount of melanin.

• Growth: Cycles of growth and rest; varies with age and location.

Nail Structure

• Nail plate: Visible part.

• Nail bed: Skin beneath the nail plate.

• Matrix: Growth area at nail base.

Sweat and Oil Glands

• Sweat (sudoriferous) glands: Eccrine (widely distributed, thermoregulation) and apocrine (axillary/genital, odor).

• Oil (sebaceous) glands: Secrete sebum, lubricate skin and hair.

• Secretions: Sweat is mostly water, some salts; sebum is oily and contains lipids.

Eccrine vs. Apocrine Sweat Glands

Feature	Eccrine	Apocrine
Location	All over body	Axillary, genital areas
Secretion	Watery, clear	Thicker, milky
Function	Thermoregulation	Activated at puberty, odor

Functions of the Skin

• Protection: Barrier against pathogens, chemicals, and physical injury.

• Body temperature regulation: Sweat and blood flow adjustments.

• Sensation: Contains sensory receptors for touch, pain, temperature.

• Metabolic functions: Synthesis of vitamin D.

• Blood reservoir: Stores blood in dermal vessels.

• Excretion: Eliminates wastes via sweat.