Epithelial, Connective, Muscle, and Nervous Tissues: An Overview
Epithelial Tissue
Epithelial tissue lines the body’s exterior and internal surfaces of hollow organs, including those of the respiratory, digestive, urinary, and reproductive systems. Its primary function is protection and selective barrier formation, regulating substance passage. This non-vascularized tissue receives nutrients via diffusion from adjacent vascularized tissue.
Glandular Epithelial Tissue
During embryonic development, some epithelial cells specialize in secretion, forming glands. These glands are categorized based on their secretion method:
- Exocrine Glands: Possess ducts that release secretions onto external body surfaces or into hollow organs. Examples include salivary, sebaceous, and sweat glands, as well as gastric glands in the stomach.
- Endocrine Glands: Lack ducts and release hormones directly into the bloodstream. These hormones travel through the blood to affect distant target organs. Examples include the pituitary and thyroid glands.
Connective Tissue
Connective tissue, the most abundant and diverse tissue type, connects various tissues and organs, providing support and protection. Its key components include:
- Intercellular Substance (Matrix):
- Collagen: Grouped into fibrils, providing flexibility and resistance.
- Elastin: Extensible fibers providing elasticity.
- Reticulin: Isolated collagen fibers forming networks.
- Ground Substance: Composed of water, salts, and sugars attached to polypeptides, with varying consistency, from gelatinous to calcified.
- Cells: Vary depending on the connective tissue type.
Types of Connective Tissue
- Connective Tissue Proper:
- Loose Connective Tissue: Characterized by separated fibers in the intercellular substance. Found in the superficial dermis, it connects tissues without rigidity.
- Dense Connective Tissue: Contains more fibers, primarily collagen, making it resistant. Forms tendons, ligaments, and vocal cords.
- Reticular Connective Tissue: Predominantly reticulin fibers, forming a framework to support cells in organs like the liver and hematopoietic organs (spleen, lymph nodes, bone marrow).
- Adipose Tissue: Specialized cells called adipocytes store fat, serving as an energy reserve, thermal insulator, and organ protector.
- Cartilaginous Tissue: Provides support and flexibility.
- Bone Tissue: Provides structural support and protection.
Muscle Tissue
Muscle tissue consists of specialized, elongated cells called muscle fibers, containing contractile proteins (actin and myosin) responsible for movement.
Types of Muscle Tissue
- Smooth Muscle Tissue: Spindle-shaped cells with a single central nucleus. Involuntary contraction, found in the walls of organs like the digestive tract, respiratory system, bladder, blood vessels, and uterus.
- Striated Muscle Tissue: Cylindrical cells with multiple peripheral nuclei. Voluntary contraction, found in skeletal muscles. The striated appearance is due to the arrangement of contractile proteins.
- Cardiac Muscle Tissue: Found in the heart. Involuntary contraction. Uninucleate, striated cells connected by intercalated disks, facilitating contraction transmission.
Nervous Tissue
Nervous tissue captures, transmits, and processes impulses. It comprises two main cell types:
- Neurons: Responsible for irritability and conductivity. Composed of a cell body, dendrites (receiving impulses), and an axon (transmitting impulses). Axons may be myelinated for insulation.
- Glial Cells: Support and nourish neurons, remove dead cells, and line the central nervous system cavities.