Integumentary System

Layers of the Epidermis

The epidermis is the outermost layer of the skin, composed of several distinct strata, each with specialized functions and cell types.

• Stratum Basale:

  • Mitotically active; responsible for generating new skin cells.

  • Most nourished layer, attached to the dermis.

  • Contains melanocytes (produce melanin for UV protection) and Merkel cells (touch receptors).

• Stratum Spinosum:

  • Metabolically active; consists of several cell layers.

  • Keratin is produced here.

  • Desmosomes first appear, providing cell adhesion.

  • Langerhans cells activate the immune system.

• Stratum Granulosum:

  • Cells die and become keratinized.

  • Appear squamous (flattened).

• Stratum Lucidum:

  • Only present in thick skin (palms, soles).

  • Provides extra protection against abrasion.

• Stratum Corneum:

  • All cells are dead, stratified squamous epithelium.

  • Desmosomes remain, providing a tough layer.

Desmosomes

Desmosomes are specialized cell junctions that provide strong adhesion between cells, especially in tissues subject to mechanical stress.

• Connect everything in the epidermis.

• Divide tension, preventing tissue separation.

Dermis

The dermis is the connective tissue layer beneath the epidermis, providing structural support and housing blood vessels, nerves, and glands.

• Reticular layer: ~80% of dermal thickness; dense, irregular connective tissue.

• Papillary layer: ~20% of dermal thickness; loose areolar connective tissue.

• Contains many contours that connect with the epidermis, increasing surface area for diffusion.

Skeletal System

Bone Formation: Ossification

Ossification is the process of bone tissue formation, occurring via two main mechanisms during development.

Intramembranous Ossification (Embryo Stage)

• Occurs in membranes because of osteoblasts.

• Stem cells differentiate into osteoblasts, which lay down collagen and minerals.

• Blood vessels enter, bringing osteoclasts for bone remodeling.

• Creates flat bones (skull, sternum, scapula, etc.).

Endochondral Ossification

• Starts with a cartilage (hyaline) template.

• Osteoblasts form a bony collar, starving chondrocytes.

• Chondrocytes secrete calcitonin, causing cartilage growth and eventual death.

• Cartilage grows in length; nutrients diffuse in the center.

• Periosteal bud invades, bringing osteoblasts and blood vessels.

• Epiphyseal growth plate remains for long bone growth at joints.

• Process continues after birth.

Long Bone Growth

Long bones grow in length through the activity of cartilage cells and hormonal regulation.

• Proliferation zone: Mitotically active cartilage cells.

• Hypertrophic zone: Cartilage cells enlarge and secrete calcium.

• Calcification zone: Cartilage cells die.

• Ossification zone: Osteoblasts lay down new bone and remodel.

• Growth ends when the epiphysis and diaphysis fuse.

• Regulated by growth hormone and thyroid hormone (requires iodine).

• Resting zone: Cartilage cells undergo mitosis.

Bone Repair

Bone repair after fracture occurs in several stages, restoring bone structure and function.

1. Hematoma forms: Blood clot forms, injured cells die, swelling and pain occur.

2. Fibrocartilaginous Callus: Soft callus forms (2-4 days post-break), fibroblasts and osteoblasts arrive, putting bones together.

3. Bony Callus: Spongy bone connects ends of bones (1 week post-break), osteoclasts kill cartilage, osteoblasts lay new bone.

4. Bone Remodeling: Thicker, scarred bone replaces bony callus.

Bone Resorption and Deposition

Bone tissue is constantly remodeled through deposition and resorption, maintaining strength and mineral balance.

• Deposition: Osteoblasts lay down osteon, deposit minerals (requires vitamin C, D, A).

• Resorption: Osteoclasts break down bone matrix, releasing minerals.

• Regulated by blood calcium levels (9-11 mg/dL), calcitonin (inhibits PTH), and parathyroid hormone (stimulates osteoclasts).

• Bones are mineralized by calcium phosphate.

Pathology of Bone

Paget's Disease

Paget's disease involves abnormal bone remodeling, resulting in an imbalance between spongy and compact bone.

• Abnormal ratio of spongy bone to compact bone.

• Poor mineralization of new bone.

• May be viral in origin.

Osteoporosis

Osteoporosis is characterized by weakened bones due to hormonal changes and loss of bone density.

• Common in postmenopausal women and older men with low testosterone.

• Lack of vitamin D, calcium, and protein.

• Spongy bone, femur neck, and spine are most affected.

• Hormonal role: HRT (hormone replacement therapy).

Osteomalacia

Osteomalacia is the softening of bones due to impaired bone metabolism and mineralization.

• Impaired bone metabolism and resorption of calcium.

• Inadequate bone mineralization (normal production of osteoid).

• Pain when weight-bearing, bowlegs, and deformities.

• Nutrition: vitamin D deficiency.

Hydroxyapatites

Hydroxyapatites are mineral compounds that provide resistance to compression in bone tissue.

• Essential for bone strength and durability.

Additional Key Concepts

Glands and Membranes

• Holocrine: Sebaceous glands (release entire cell contents).

• Merocrine: Exocytosis (eccrine sweat glands).

• Apocrine: Found in armpits/genitals; fatty substance in secretion.

• Mucous membrane: Lines closed ventral cavities.

• Serous membrane: Lines closed ventral cavities; contains hyaluronic acid for lubrication.

Bone Structure and Function

• Functions: Support, protection, anchorage, blood cell production, mineral storage.

• Periosteum covers the diaphysis; endosteum lines the medullary cavity.

• Osteons are organized in lamellae for strength.

• Canaliculi allow communication through gap junctions.

• Central canal (Volkmann's canal) provides a passage for blood vessels and nerves.

Table: Comparison of Bone Disorders

Key Formulas

• Blood Calcium Regulation:

• Bone Growth Equation:

Additional info: Some details inferred from standard Anatomy & Physiology curriculum to clarify fragmented points and ensure completeness.