Introduction to Glands and Connective Tissues
Glandular Structure and Mechanisms of Secretion
Goblet cells are specialized cells found in the intestinal lining that secrete mucin, which forms mucus.
The primary functions of goblet cells in the intestines are protection of epithelial surfaces and smoothing out the lining.
Multicellular exocrine glands are classified by three main criteria:
Structure of the ducts: Simple or compound.
Shape of the secretory portion: Tubular or alveolar (also known as acinar).
Relationship: Whether the gland is branched or unbranched.
Glandular structures include:
Simple tubular.
Simple coiled.
Simple branched.
Simple alveolar.
Simple branched alveolar.
Compound tubular (found in the mouth, bulbourethral glands of the male reproductive system, and the testes).
Compound alveolar or acinar (found in mammary glands).
Compound tubuloalveolar (found in salivary glands, respiratory passages, and the pancreas).
Categories of Glandular Secretions
Merocrine secretions (sometimes referred to as American Merofrin secretions): The product is released from secretory vesicles at the surface of the gland via exocytosis. Examples include sweat glands.
Apocrine secretions: The product is released by shedding portions of the cytoplasm. Examples include mammary glands.
Holocrine secretions: The product is produced and released as the entire cell bursts and dies. These gland cells are subsequently replaced by stem cells. Examples include sebaceous glands.
Secretions are further classified by their content:
Serous glands: Produce watery secretions containing enzymes.
Mucous glands: Secrete mucins that form mucus.
Mixed exocrine glands: Contain populations of both serous and mucous gland cells.
Connective Tissue Components and Functions
Connective tissue is a diverse group of supporting tissues comprising specialized cells, extracellular protein fibers, and a fluid called ground substance.
The matrix consists of extracellular components (protein fibers and ground substance) and makes up most of the tissue volume.
General functions of connective tissue include:
Providing a structural framework for the body.
Transporting fluids and dissolved materials.
Protecting delicate organs.
Supporting, surrounding, and interconnecting other tissue types.
Storing energy, specifically in the form of triglycerides.
Defending the body from invading microorganisms.
Major categories of connective tissue:
Connective tissue proper (viscous matrix rich in protein fibers).
Fluid connective tissue.
Supporting connective tissue.
Cells and Fibers of the Connective Tissue Proper
Matrix characteristics: Viscous, thick, and gel-like instead of watery.
Primary cell types:
Fibroblasts: The most abundant fixed cells.
Fibrocytes: The second most abundant cell type.
Adipocytes: Fat cells.
Mesenchymal cells: Stem cells that reproduce and specialize based on local signals (e.g., becoming bone in bone or cartilage in cartilage).
Melanocytes: Synthesize and store melanin, the pigment providing coloration to skins and eyes.
Macrophages: Phagocytic immune cells that engulf pathogens, damaged cells, and debris. They may be fixed or wandering.
Mast cells: Immune cells that release histamine (to stimulate inflammation after injury) and heparin (an anticoagulant to prevent clotting).
Lymphocytes: Immune cells that migrate through the body and develop into plasma cells to produce antibodies.
Microphages (Neutrophils and Eosinophils): Phagocytic cells attracted to sites of infection by signals from macrophages and mast cells.
Fiber types:
Collagen fibers: The most common type; long, straight, and unbranched. They are strong and flexible, providing strength for tendons and ligaments.
Reticular fibers: Thin, flexible branched networks that form a structure called stroma. They resist forces from many directions and stabilize cell positions.
Elastic fibers: Branched and wavy fibers containing the protein elastin. They return to their original length after being stretched.
Loose and Dense Connective Tissues
Loose connective tissue: Fills spaces between organs, stabilizes cells, and supports the epithelium.
Embryonic connective tissue: Found in embryos; gives rise to all other connective tissues. Consists of mesenchyme (tiny cells and fine protein fibers).
Mucus connective tissue: A type of loose embryonic connective tissue.
Areolar tissue: The least specialized adult connective tissue. Features an open framework, viscous ground substance, and high elasticity. Found in the deep dermis and epithelial linings of the digestive, respiratory, and urinary tracts.
Adipose tissue: Contains adipocytes that expand to store fat and shrink as it is released. Provides padding, shock absorption, insulation, and energy storage. White fat is the most common; brown fat is highly vascularized and produces heat in babies and young children.
Reticular tissue: Rich in reticular fibers; supports organs like the liver, kidney, spleen, lymph nodes, and bone marrow.
Dense connective tissue (collagenous tissue):
Dense regular connective tissue: Parallel, tightly packed collagen fibers. Provides strength to tendons (connecting muscle to bone), ligaments (connecting bone to bone), and aponeuroses (tendon sheaths).
Dense irregular connective tissue: Interwoven collagen fibers with no consistent pattern. Resists stress from many directions. Found in the dermis, perichondrium (around cartilage), periosteum (around bone), and capsules of joints and organs like the liver, kidney, and spleen.
Fascia and Fluid Connective Tissue
Fascia is connective tissue that wraps, supports, and surrounds internal organs to provide strength and stability through cohesion.
Layers of fascia include:
Superficial fascia: Separates skin from underlying tissues.
Deep fascia.
Subserous fascia.
Blood is a fluid connective tissue.
The fluid matrix contains cells and cell fragments known as formed elements.
Red blood cells are called erythrocytes.
Erythropoiesis is the creation of red blood cells.
White blood cells are called leukocytes and include monocytes (phagocytic), lymphocytes (dominant in lymph), eosinophils, neutrophils, and basophils.
Platelets are cell fragments involved in clotting.
Lymph is formed as interstitial fluid enters lymphatic vessels.
Supporting Connective Tissue: Cartilage and Bone
Cartilage: Provides shock absorption and protection via a firm gel matrix containing chondroitin sulfates.
Cells: Chondrocytes residing in chambers called lacunae.
Circulation: Avascular (no blood vessels). Nutrients and waste are exchanged via diffusion from surrounding connective tissue or adjacent bone endplates.
Growth mechanisms:
Interstitial growth: Lengthens the cartilage from within.
Appositional growth: Thickens the outer surface of the cartilage.
Types of cartilage:
Hyaline cartilage: Matrix of closely packed collagen fibers; tough but flexible. Found at synovial joints (ends of bones), the nose, and large respiratory passages.
Elastic cartilage: Contains elastic fibers; supportive and flexible. Found in the ear and larynx.
Fibrocartilage: Dominated by densely interwoven collagen fibers; extremely durable. Prevents bone-to-bone contact and limits movement. Found in the pubic symphysis and vertebral discs.
Bone (Osseous tissue):
Matrix: Calcified with calcium salts (providing hardness) and collagen fibers (providing flexibility).
Cells: Osteocytes residing in lacunae.
Structure: Organized in rings around a central canal containing blood vessels. Canaliculi allow for material exchange between cells.
Periosteum: A membrane covering the bone with outer fibrous and inner cellular layers.
Tissue Membranes
Membranes consist of epithelium supported by connective tissue and act as physical barriers.
Mucous membranes: Line passageways open to the exterior (digestive, respiratory, urinary, reproductive tracts). They must be moist for friction reduction and absorption. The underlying areolar tissue is called the lamina propria.
Serous membranes: Line internal trunk cavities and reduce friction. They consist of mesothelium supported by areolar tissue.
Parietal portion: Lines the cavity.
Visceral portion: Covers the organs.
Types: Peritoneum (peritoneal cavity), Pleura (pleural cavity), and Pericardium (pericardial cavity).
Cutaneous membrane: The skin; keratinized, stratified squamous epithelium with underlying areolar and dense irregular connective tissue. It is thick, waterproof, and dry.
Synovial membranes: Line synovial joint cavities. Made of areolar tissue with an incomplete epithelium. Movement stimulates synovial fluid production to lubricate joints and provide oxygen/nutrients to chondrocytes.
Muscle and Nervous Tissues
Muscle Tissues:
Skeletal muscle: Long, thin, multinucleated muscle fibers. They are striated and voluntary. The diaphragm is a skeletal muscle that also functions under autonomic control for breathing during sleep.
Cardiac muscle: Striated and involuntary. Features intercalated discs for signal transfer.
Smooth muscle: Non-striated and involuntary. Controls motion in the intestines and other areas not under conscious thought.
Nervous Tissue: Specialized for conducting electrical impulses, concentrated in the brain and spinal cord.
Neuron components: Cell body (nucleus, nucleolus, and organelles), dendrites (short branches receiving signals), and the axon (long process carrying outgoing signals).
Neuroglia: Supporting cells that maintain physical structure, repair tissue after injury, perform phagocytosis, provide nutrients, and regulate interstitial fluid.
Tissue Response to Injury and Regeneration
The four cardinal signs of acute inflammation (in Latin) are:
Rubor: Redness.
Tumor: Swelling.
Calor: Heat.
Dolor: Pain.
Inflammatory process:
Triggered by trauma or infection.
Damaged cells release prostaglandins, proteins, and potassium ions, which activate mast cells to release inflammatory chemicals.
Lysosomal enzymes from immune cells destroy pathogens and damaged cells.
This may lead to necrosis (tissue death) or an abscess (accumulation of pus containing debris, fluid, and dying cells).
Regeneration capacity varies by tissue:
Epithelial tissue, connective tissue proper, cartilage, and smooth muscle regenerate well.
Skeletal muscle, cardiac muscle, and nervous tissue regenerate poorly.
Fibrosis: The process where damaged cardiac muscle or other tissues are replaced by fibrous (scar) tissue, which does not restore normal function.
Questions and Discussion
Question: Why do you have goblet cells in your intestines?
Answer: For protection and to smooth out the surfaces, keeping the epithelial surfaces protected.
Question: What do mesenchymal cells become?
Answer: They are stem cells that become whatever they are signaled to be. If they are in bone, they become bone; if in cartilage, they become cartilage.
Question: Is skeletal muscle voluntary or involuntary?
Answer: Voluntary, although the diaphragm has autonomic functions for breathing.
Question: Is cardiac muscle voluntary or involuntary?
Answer: Involuntary.
Question: Is smooth muscle voluntary or involuntary?
Answer: Involuntary.
Question: Is blood a connective tissue?
Answer: Yes. This is a common test question and must not be missed.
Question: What are red blood cells called?
Answer: Erythrocytes.
Question: What are white blood cells called?
Answer: Leukocytes.
Question: How does avascular cartilage receive nutrients or remove waste?
Answer: Through adjacent structures, such as the end plates of bones for spinal discs.
Question: Does interstitial growth thicken or lengthen cartilage?
Answer: It lengthens it from within.
Question: Does appositional growth thicken or lengthen cartilage?
Answer: It thickens the outer surface.
Resources Discussed:
Textbook: Mancini book, version, "Fundamentals of Anatomy and Physiology."
Lab Book: Top Hat, edition (required, available at the bookstore).
Lab Safety and Practical Work:
No chemicals or fire in the first week.
Dissection requirements: Long pants (no shorts), closed-toed shoes, lab coats, goggles, and long hair tied back. These are required by OSHA.
Dissections for A&P : Cow eyeball and sheep brain.
Anecdote from A&P : A sheep uterus was dissected that contained a fetus (named "Lamb Chop"). Another contained twins (named "Peanut Butter and Jelly").