Case 8

Glands

• Glands are formed by indentations of surface epithelium into connective tissue.
• They can be simple or branched, with the end of the indentation forming a tubule or acinus, which secretes products.
• The remaining indentations form the duct.
• Exocrine glands have ducts, while endocrine glands do not.
• Endocrine glands secrete their products directly into the blood.
• The thyroid gland is an endocrine gland that lacks a duct and stores thyroid hormone.
• The thyroid gland has a richly vascularized lamina propria surrounding the follicles.
• The stored thyroid hormone is a colloid.
• Active thyroid glands have cuboidal to columnar epithelial cells, while inactive glands have flat epithelial cells.

Connective Tissue

• Connective tissue supports and binds other tissues, facilitates transport and cell interaction, protects against microorganisms, and has storage functions.
• Connective tissue consists of cells (e.g., fibroblasts) and extracellular matrix.
• The extracellular matrix is secreted by fibroblasts and consists of ground substance and fibers.
• The ground substance contains glycosaminoglycans, proteoglycans, hyaluronic acid, and water.
• The fibrous components of connective tissue determine its physical properties.
• The types of fibers include collagen, elastin, and reticular fibers.

Collagen Fibers

• Consists of different polypeptides that form fibers of 2-20 µm (type I).
• They have high tensile strength and form the supporting tissue of the dermis, tendons, ligaments, and bones.
• Rich in glycine and proline, also contains hydroxyproline and hydroxylysine.
• Forms a helical structure, with three collagen strands winding around each other.

Elastic Fibers

• Consist of tropoelastin, which is stretchable, and fibrous fibrillin.
• Often found in membranes and can retract to their original form after stretching.
• Found in the lung, aorta, elastic ligaments, and cartilage.

Reticular Fibers

• Very thin (1 µm) collagen type III fibers with bound glycoproteins.
• Visualized with special stains like PAS and silver.
• Slightly stretchable, reinforcing organs.
• Form a delicate branched meshwork in highly cellular organs like liver, bone marrow, and lymphoid tissue.

Cartilage and Bones

• Special forms of connective tissue with collagen fibers (types I-III) in their extracellular matrix.
• Different fiber types result in different physical properties of cartilage.
• Bone tissue's physical properties are due to the interaction of hydroxyapatite and collagen.
• Bone development begins with a connective tissue template, which is replaced by cartilage and then ossified.

Muscle Tissue

• Muscle tissue is responsible for movement.
• The three types of muscle tissue are skeletal, smooth, and cardiac.

Skeletal Muscle

• Voluntary, striated, and multinucleated.
• Responsible for movement of bones and other structures.
• Has a syncytium structure.

Smooth Muscle

• Involuntary, non-striated, and has one nucleus.
• Moves material through organs.
• Can behave as a syncytium through direct contact with neurons or gap junctions.
• Can stretch more than skeletal muscle.

Cardiac Muscle

• Involuntary, striated, and branched.
• Forms meshes connected by intercalated discs and gap junctions.
• Functionally forms a syncytium with one nucleus in the cell center.
• Moves blood through the body.

Nervous Tissue

• Forms the central and peripheral nervous systems.
• Consists of neurons, glial cells, and derivatives of sympathetic neurons.
• All components originate from neuronal stem cells.
• Responsible for communication within the nervous system, to muscles and glands, and from sensory receptors.
• Communication occurs through electrical impulses and chemical signals at the synapse.
• Three main types of neurons: sensory, motor, and interneurons.
• Neurons contain a perikaryon (cell body), dendrites, and an axon.
• The axon is surrounded by a myelin sheath, formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.
• The tissue containing glia cells and neuron perikarya is called neuropil.

Tissue Organization

• Tissues are organized in a hierarchy: cells, tissues, organs, organ systems, and organisms.
• This hierarchy allows for efficient functioning and adaptation to various physiological needs.