Anatomy and Histology: Systems, Tissues, and Cell Junctions

Posted on Oct 2, 2025 in Environmental Biology

Integumentary System (IS)

  • Epidermis: Protects deeper tissues and prevents pathogen entry. Involved in Vitamin D (VD) production and preventing dehydration.
  • Dermis: Contains hair follicles, essential for detecting touch.
  • Sebaceous Glands (SG): Provide lubrication and moisture.
  • Sweat Glands: Crucial for thermoregulation.
  • Hypodermis: Functions primarily as fat storage.

Musculoskeletal and Skeletal Systems

Skeletal Muscles (MS) Functions

Skeletal muscles generate heat, provide support, and offer protection.

A.M.P.S. Functions of the Skeleton

The skeletal system provides:

  • Axial support (Posture)
  • Movement
  • Protection
  • Support

Skeletal System Components (SS)

Includes bone, cartilage, joints, and ligaments.

  • Axial Skeleton: Provides position and support. Protects vital structures (brain, spinal cord, sense organs, thoracic tissue). Offers weight support, especially over lower limbs (LL).
  • Appendicular Skeleton: Provides internal support, positioning of external limbs, mobility, balance, and movement of the axial skeleton.

Muscle Attachments

  • Tendons (Fibrous): Connect muscle to bone.
  • Aponeuroses (Sheets): Connect muscle to muscle or muscle to bone, facilitating attachment and force transmission.

Bone Marrow

  • Red Marrow: Produces red blood cells (RBCs).
  • Yellow Marrow: Primarily fat stores, convertible to red marrow during injury.

Nervous System (NS)

Composed of the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

  • Brain and Spinal Cord: The spinal cord acts as the primary communication channel between the body and the brain.

Endocrine System (ES)

Key glands and their primary hormones/functions:

  • Pineal Gland: Melatonin production.
  • Thyroid Gland
  • Pituitary Gland: Regulates growth (imbalances lead to gigantism or acromegaly).
  • Thymus: Essential for the maturation of T and B lymphocytes.
  • Adrenal Glands
  • Parathyroid Glands: Regulate calcium levels.
  • Gonads
  • Pancreas: Produces Insulin.

Lymphatic System (LS)

The lymphatic system manages excess interstitial fluid and plays a crucial role in immunity.

  1. Interstitial fluid (excess) enters lymphatic capillaries.
  2. Transported via lymphatic vessels as lymph to lymph nodes.
  3. Lymph Nodes: Filter lymph; lymphocytes initiate immune responses (e.g., swelling to attack pathogens).
  4. Filtered fluid returns via lymphatic ducts (L ducts) to the subclavian veins and back into the bloodstream.

Key Lymphatic Organs

  • Spleen: Monitors circulation, supports immune response, and recycles old RBCs.
  • Thymus: Crucial for lymphocyte maturation.

Major Organ Systems Components

Digestive System (DS)

Includes the oral cavity, salivary glands, pharynx, esophagus, stomach, small intestine, gallbladder, large intestine, and pancreas.

Urinary System (US)

Components include the kidneys, ureters, bladder, and urethra.

Erythropoietin (EPO) Function

EPO is a glycoprotein produced by the kidneys that promotes erythropoiesis (RBC stem cell production). This increases the RBC count (↑RBC) and enhances oxygen delivery (↑Delivery of O2).

Respiratory System (RS)

Includes the nasal cavity and paranasal sinuses, pharynx, larynx, trachea, bronchi, lungs, and diaphragm.

Female Reproductive System (FRS)

Ovaries, uterine tubes, uterus, vagina and external genitalia, and mammary glands.

Male Reproductive System (MRS)

Testes, accessory organs (epididymis, ductus deferens, seminal glands, prostate glands, urethra), and external genitalia.

Histology: Epithelial Tissue and Cell Junctions

Epithelial Tissue Characteristics

Epithelia line hollow organs, cavities, and ducts, and form glands. Functions include selective permeability, secretion, and protection.

  • Epithelial cells have apical, lateral, and basal surfaces, held together by cell junctions.
  • Epithelia are avascular; material exchange occurs via diffusion from vessels (V) in the underlying Connective Tissue (CT).

Cell Junctions

These junctions maintain structural integrity and polarity:

  1. Tight Junctions (TJ):
    • Formed by sealing strands (transmembrane proteins: Claudins & Occludins) linked via ZO-1 to actin in the cytoskeleton of adjacent cells.
    • Tighter junctions have more strands (e.g., stomach, intestines, bladder).
    • Function: Maintain polarity by stopping apical → basal mixing.
  2. Adherens Junctions (AJ):
    • Cadherins link across the gap to actin via catenins, forming a plaque.
    • Creates an adhesion belt that resists contraction tension and prevents cell separation.
  3. Desmosomes (DJ):
    • Cadherins link across the gap to desmoplakin.
    • Keratin intermediate filaments span from one desmosome to another, providing structural integrity (a “cable system”).
  4. Gap Junctions (GJ):
    • Six connexins form a connexon (hemichannel) on either side. Two connexons form the GJ.
    • Function: Communication and transport through the central tunnel.
  5. Hemidesmosomes:
    • Located on the basal surface, attaching the epithelia to the Basement Membrane (BM).
    • Integrin proteins link extracellularly to laminin (in the BM) and intracellularly to keratin, anchoring the basal surface (BS) to the BM.

Basement Membrane (BM)

The BM consists of two layers:

  • Basal Lamina: Produced by epithelial cells (contains collagen, proteoglycans, laminin).
  • Reticular Lamina: Produced by fibroblasts (contains fibronectin, collagen).

The BM serves as the interface between Epithelia (Epi) and Connective Tissue (CT).

Cytoskeleton Components

  • Actin (Microfilaments)
  • Keratin (Intermediate Filaments)

Covering and Lining Epithelia Types

Simple Squamous Epithelium

One layer, flat, and thin. Facilitates diffusion, secretion, and protection (delicate areas).

  1. Mesothelium: Lines pericardial, pleural, and peritoneal cavities, forming serous membranes (Mesothelium + CT). Produces serous fluid to reduce friction.
  2. Endothelium: Lines the heart, blood, and lymphatic vessels (LV). Regulates blood flow.

Simple Cuboidal Epithelium

Functions in absorption and secretion.

Locations: Pancreas ducts, kidney tubules, small ducts of glands, secretory chambers of the thyroid, anterior surface of the lens, and secretory parts of glands.

Simple Columnar Epithelium

Functions in secretion, protection, and absorption.

  • Ciliated Type: Often contains goblet cells. Found in bronchioles, fallopian tubes, sinuses, the central canal of the spinal cord, and brain ventricles. Cilia move substances.
  • Non-Ciliated Type: Often features microvilli to increase surface area for nutrient absorption.