Tissues, Connective Tissue Types, Membranes, and Muscle Tissue
Glandular Structure and Classification
Duct Structure: Glands are categorized based on the complexity of their duct system.
Simple Gland: Features a single, undivided tube.
Compound Gland: Features a system where a whole bunch of ducts fuse together into a single exit tube.
Secretory Shape: Glands are further differentiated by the shape of the area where secretions are produced.
Tubular: Shaped like a little tube, a worm, or fingers.
Alveolar or Acinar: Shaped like rounded blind pockets, similar to the alveoli found in the lungs.
Mechanisms of Exocrine Secretion
Exocrine glands do not send products directly into the blood; instead, they utilize a duct. The method by which substances are transferred from inside the cell to the outside defines the secretion type.
Merocrine (Eccrine) Secretion:
This is the most common method. The secretion is contained within a vesicle.
The vesicle originates within the cell, floats through the cytoplasm, fuses with the outside of the plasma membrane, and then dumps the contents into the duct.
Human Specialty: Humans possess eccrine sweat glands all over the body from fingers to toes. Most sweat glands are eccrine and produce a substance that is mostly water.
Comparison to Other Mammals: Dogs and cats have eccrine glands, but not in large quantities. They primarily have them on the hairless parts of their body, such as their paws and feet.
Thermoregulation Example: Because humans have eccrine glands across the body (except for the palms and plantar surfaces which lack hair cells and follicles for these secretions), humans can run marathons in heat reaching , , or even degrees. In contrast, dogs must pant to cool off. A zebra attempting a marathon in Africa would likely die by the th mile, whereas humans can complete a marathon in less than hours in the heat of the day. This evolutionary adaptation is thought to have evolved approximately million years ago.
Apocrine Secretion:
Unlike merocrine secretion, a vesicle does not fuse with the membrane. Instead, a physical chunk of the cytoplasm containing the secretions literally buds off from the exocrine cell.
The cell then repairs itself so it can repeat the process.
Locations: Found in areas with dense hair that appears after sexual maturity, such as the armpits, the pubic region, and near the anus.
Mammary Glands: These are a modified form of apocrine glands. Milk is a secretion produced via the apocrine method.
Holocrine Secretion:
This method involves the bursting of the superficial gland cell. The entire cell is killed in order to release the secretions.
The lost cells are replaced through constant replication or mitotic division of stem cells located underneath the most superficial glands.
Sebaceous Glands: These produce sebum, an oily substance that makes skin watertight and hair hydrophobic. They operate exclusively through holocrine secretion.
Connective Tissue Composition and Matrix
Connective tissues are defined by their shape and the orientation of their internal fibers. They share three core components known collectively as the matrix.
Specialized Cells: The specific cell types relative to the tissue.
Extracellular Protein Fibers: Fibers that provide structure and strength.
Ground Substance: A fluid that fills spaces between cells. It is typically clear, colorless, and viscous (thick). This viscosity helps slow the movement of pathogens, preventing them from moving immediately from one cell to another.
Categories of Connective Tissue
Connective tissues are differentiated primarily by the nature of their matrix.
Connective Tissue Proper: Characterized by a very viscous matrix and a high concentration of protein fibers. Examples include areolar tissue, ligaments, tendons, and adipose tissue (fat).
Fluid Connective Tissue: Characterized by a watery matrix where proteins are mostly dissolved. Examples include blood plasma and lymph.
Supporting Connective Tissue: Characterized by a matrix filled with extremely dense fibers. Examples include cartilage and bone.
Cells of Connective Tissue Proper
Fibroblasts and Fibrocytes: Responsible for secreting or maintaining connective tissue fibers. "Blast" refers to secretion, while "cyte" refers to a cell.
Adipocytes: Fat cells. They store energy in the form of single droplets. Visible as large drops, similar to those seen when washing dishes after cooking bacon.
Mesenchymal Cells: The stem cells of connective tissue proper. These are "unflavored" default cells that can differentiate into fibroblasts, fibrocytes, or immune cells.
Melanocytes: Cells responsible for creating and storing melanin, the pigment providing color to the skin, hair, and eyes.
Protein Fibers in Connective Tissue Proper
Collagen Fibers: Thick, strong, straight, and flexible. They provide strength to ligaments (attaching bone to bone) and tendons (attaching muscle to bone). They are the reason muscles stay glued to bones and joints stay articulated.
Reticular Fibers: Thinner than collagen but very flexible. They form a branching meshwork or net that resists forces from many directions. They form the base framework for organs like the liver.
Elastic Fibers: Composed of the protein elastin. These are thin, branching, and wavy. They allow for significant stretching and are found in structures that must expand and contract.
Loose Connective Tissues
Loose connective tissues fill spaces between organs, cushion cells, and support epithelia. Epithelial cells are anchored to a basement membrane, which is itself connected to loose connective tissue.
Areolar Tissue:
Fibrous and cellular components are loosely organized.
Contains significant viscous ground substance to cushion organs against force and stress.
The viscosity makes movement through it like "swimming through pudding or Jell-O" for pathogens.
Located in the deep dermis, the epithelial lining of the digestive, respiratory, and urinary tracts, between muscles, and around joints.
Adipose Tissue (Fat):
Consists mostly of adipocytes with very little matrix. The cells contain a large triglyceride droplet and a tiny nucleus. These cells expand or shrink based on energy storage needs.
White Fat: The standard fat found in adults (e.g., love handles, buttocks, saggier areas). It is found deep in the skin (subcutaneous) or between organs (visceral fat). It serves as energy storage for "feast or famine" scenarios.
Brown Fat: Highly vascularized (contains a blood supply) and rich in mitochondria, which gives it the brown color. Primarily found in babies and children to serve as an internal furnace for heat production. Because children have a higher surface area to volume ratio, they lose heat faster than adults.
Reticular Tissue:
Consists of a crosshatching meshwork of reticular fibers.
Forms the tough but flexible framework used to build organs such as the liver.
Dense Connective Tissues
Often called collagenous tissues because collagen is the dominant fiber type.
Dense Regular Connective Tissue:
Collagen fibers are oriented in parallel lines.
Tendons: Attach muscle to bone.
Ligaments: Attach bone to bone or organ to organ.
Aponeurosis: A big bundle of regular connective tissue that looks like a spider web, keeping tendons together (found in the palms of hands and soles of feet).
Dense Irregular Connective Tissue:
Fibers are arranged in interwoven networks with no specific symmetry or parallelism.
Found in the dermis (deep skin), around cartilages, around bones (the periosteum), and in joint capsules.
Dense Elastic Connective Tissue:
Made primarily of the protein elastin.
Found in large blood vessels like the descending aorta, respiratory pathways, and ligaments between vertebrae to allow for shock absorption and flexibility.
Fascia Types
Fascia consists of sheets of connective tissue that wrap around structures.
Superficial Fascia: Located very close to the skin; separates the skin from underlying muscle fibers.
Deep Fascia: Strong sheets that wrap around capsules, organs, and muscles. It acts like the thin plastic sheeting on a new phone, separating one muscle from another.
Subserous Fascia: Composed of areolar tissue. It lies deep to the deep fascia and lines the body cavities, forming the basis for membranes like the pericardium.
Supporting Connective Tissue: Cartilage and Bone
Cartilage:
Avascular (no blood supply), which makes it "cheaper" to maintain but difficult to repair. It is light and flexible.
Hyaline Cartilage: The most common. Found connecting ribs to the sternum, in joints, the larynx, trachea, and lungs. It is stiff but flexible due to closely packed collagen.
Elastic Cartilage: Extremely flexible. Found in the external ear, the nose, and the epiglottis (the flap that prevents food from entering the lungs).
Fibrocartilage: Very durable and tough with interwoven collagen fibers. Found in the discs of the vertebral column and the pubic symphysis. It limits movement between bones.
Growth: Occurs via appositional growth, adding new layers to the outside, similar to tree rings.
Bone (Osseous Tissue):
Calcified; made of calcium salts and some collagen fibers.
Highly vascular (has a blood supply) and is constantly remodeling.
Lacunae: Circular features surrounding osteocytes (bone cells).
Central Canal: A hole in the middle of lacunae containing blood vessels.
Canaliculi: Tiny channels that allow for the exchange of materials through the thick, inflexible matrix.
Periosteum: A membrane of dense irregular connective tissue that lines the outside of bones. It protects the bone and houses the vasculature and cells needed for growth and repair.
Body Membranes
Mucous Membranes: Line passageways and chambers (digestive, respiratory, urinary). They contain goblet cells that secrete mucus to protect against pathogens and stomach acid. The basement membrane of a mucous membrane is called the lamina propria.
Serous Membranes: Line the trunk cavities. They consist of two layers: the Parietal layer (contacts the body wall or bone) and the Visceral layer (contacts the organ). The space between them contains serous fluid to reduce friction.
Peritoneum: Lines the abdominal cavity.
Pleura: Lines the pleural cavities around the lungs.
Pericardium: Lines the heart within the mediastinum.
Cutaneous Membrane: The skin. A thick, stratified keratinized squamous epithelium anchored to areolar and dense irregular connective tissue.
Synovial Membranes: Line joint cavities. They lack a traditional basement membrane and produce synovial fluid to lubricate joints and prevent bone-on-bone grinding.
Muscle Tissue
Skeletal Muscle: Long, thin, and multinucleated. Having multiple nuclei allows for quicker replication and repair. These are striated (have perpendicular discs) and voluntary. Examples include biceps, triceps, deltoids, and quadriceps.
Cardiac Muscle: Located only in the heart. These are striated, branched, and shorter. They are involuntary and have a very limited ability to repair themselves naturally.
Smooth Muscle: Located in organs and the eye. They are involuntary, non-striated, and have a single nucleus. They are capable of dividing and regenerating easily after damage.
Questions & Discussion
Question: What would fat be categorized as?
Response: Fat is a type of connective tissue proper. It has a viscous matrix and viscous ground substance with protein fibers. It forms a structural framework, supports tissues, cushions organs, and stores energy in the form of triglycerides.
Question: Is the mediastinum a sheet of fascia?
Response: No, the mediastinum is a cavity. A sheet is a physical layer. The pericardium, which is found in the mediastinum, is a serous membrane. Fascia refers to the physical layer of tissue, such as the superficial fascia or deep fascia, which separates and wraps muscles. Subserous fascia is the specific tissue that makes up the serous membranes like the pericardium.