Epithelial, Connective, and Muscle Tissues: Structure and Function
**Epithelial Tissue**
Epithelial tissue is characterized by cells that are tightly packed together, leaving no gaps between them. This means that there is minimal intercellular substance. Its cells have abundant cell junctions, including tight junctions, desmosomes, and hemidesmosomes. Epithelial tissue originates from the ectoderm, mesoderm, and endoderm, and it is renewed by stem cells.
**Types of Epithelial Tissue**
**Covering Epithelium**
This type of epithelium forms the outer covering of the animal’s body and lines the internal cavities of organs. It acts as a protective barrier and facilitates exchange. The latter usually occurs through cell membrane differentiations that increase surface area, such as microvilli and invaginations.
**Classification of Covering Epithelium**
- Simple Squamous: Found in pulmonary alveoli, the innermost layer of blood vessels (endothelium), and lining body cavities.
- Stratified Squamous:
- With a corneal layer (epidermis).
- Without a stratum corneum: Mucosa.
- Cuboidal: Lines duct walls.
- Columnar:
- Microvillous: Intestinal epithelium.
- Lines the fallopian tubes, larynx, trachea, bronchi, and urinary tract.
**Glandular Epithelium**
The cells of glandular epithelium are specialized in the secretion of substances and form glands. These cells have abundant rough endoplasmic reticulum (RER).
**Classification of Glands**
- Exocrine Glands: Discharge their products through ducts to the exterior of the body or into internal cavities. Examples include sweat, salivary, and gastric glands.
- Endocrine Glands: Do not have ducts, so they release their secretions directly into the surrounding capillaries. Examples include the thyroid and pituitary glands.
- Mixed Glands: Examples include the pancreas and liver.
**Connective Tissue**
Connective tissue consists of cells surrounded by an abundant intercellular substance known as the extracellular matrix. This matrix is composed of ground substance (proteoglycans) and fibers (collagen and elastic).
**Cells of Connective Tissue**
- Fibroblasts: Produce the matrix and fibers.
- Macrophages (Histiocytes): Have phagocytic function.
- Mast Cells: Contain and release histamine.
- Melanocytes: Contain melanin.
- Plasma Cells: Produce antibodies.
- Adipocytes: Accumulate fats.
**Fibers of Connective Tissue**
- Collagen Fibers: Can be identified and grouped into bundles. They are flexible and highly resistant to mechanical traction.
- Elastic Fibers: Formed by elastin.
- Reticular Fibers: Formed by mucopolysaccharides.
**Types of Connective Tissue**
**Adipose Tissue**
Similar to loose connective tissue but with fewer fibers and a greater abundance of adipocytes. These cells have abundant smooth endoplasmic reticulum (SER) and vesicles that mature and accumulate large amounts of triglycerides. Adipose tissue functions in protection, energy reserve, and thermal insulation.
**Cartilaginous Tissue**
The cells of cartilaginous tissue are chondroblasts, which produce the matrix and fibers. Chondrocytes are younger cells. Cartilaginous tissue provides support.
**Bone Tissue**
Osteoblasts are the cells involved in the formation of bone tissue. They produce a solid, rigid matrix containing collagen fibers. Bone tissue has skeletal, metabolic, and protective functions. It serves as a large deposit of calcium and phosphorus that the body can use to maintain normal levels of these elements in the blood. Osteoclasts physiologically destroy bone tissue.
**Hematopoietic Tissue**
Blood cells are formed from hematopoietic tissue. It is renewed through stem cells located in the bone marrow of long bones.
**Classification of Connective Tissue**
- Loose Connective Tissue: The most abundant cells are fibroblasts and macrophages. It is rich in elastin fibers, and collagen fibers are scattered. This tissue serves as padding and has a large number of blood capillaries. It appears under the lining of the gastrointestinal, respiratory, and urinary tracts, as well as in subcutaneous tissue and as interstitial filler in many organs.
- Fibrous or Dense Connective Tissue: Contains a large number of collagen fibers. Found in tendons, ligaments, and the dermis.
- Elastic Connective Tissue: Part of structures that can be deformed and return to their normal position after stretching. Found in the middle layer of blood vessels, trachea, and bronchi.
- Reticular Connective Tissue: Rich in reticular fibers produced by reticular cells. Found in lymphoid organs, allowing contact between antigens and lymphocytes.
- Hyaline Cartilage: Few cells and abundant matrix. Found in the nasal septum and the skeleton of the larynx.
- Elastic Cartilage: Abundant elastic fibers. Found in the earlobe and epiglottis.
- Fibrocartilage: Numerous collagen fibers. Found in intervertebral discs and some articular cartilages.
- Compact Bone: Formed by lamellae that leave no spaces between them. Found in the shaft of long bones.
- Spongy Bone: Lamellae leave gaps between them where red bone marrow is located. Found in flat bones and the heads of long bones.
**Functions of Bone Tissue**
- Mechanical: Insertion of tendons and ligaments, allowing muscles to attach to bone, which acts as a lever.
- Protective: Protects the central nervous system (brain and spinal cord) and thoracic organs.
- Hematopoietic: In the bone marrow of long bones, stem cells produce blood cells.
- Metabolic: Constitutes 99% of the body’s calcium reserves. Calcitonin promotes calcium deposition by activating osteoblasts, and parathormone promotes calcium release by activating osteoclasts. Calcium is essential for neurotransmission, muscle contraction, and blood clotting.
**Types of Bone Marrow**
- Myeloid (Red Bone Marrow): Contains large numbers of precursor cells for erythrocytes, leukocytes, and platelets. Found in the epiphyses of long bones.
- Lymphoid: Produces lymphocytes (T and B cells). Found in the thymus, spleen, lymph nodes, and tonsils.
**Muscle Tissue**
Muscle tissue cells are specialized for contraction. They contain cytoplasmic myofibrils made of contractile proteins, predominantly the cytoskeleton, and abundant mitochondria. They are spindle-shaped and elongated, enabling them to develop structures for their function. They usually have communication between cells.
**Classification of Muscle Tissue**
- Smooth Muscle: Cells with a single nucleus and a homogeneous appearance. Involuntary and slow contraction. Found in the musculature of invertebrates and the internal organs and blood vessels of vertebrates.
- Striated Muscle: Cells that have alternating light and dark bands across the long axis of the cell. This is due to the arrangement of myofilaments: actin (thin) and myosin (thick). The dark bands correspond to the overlap of actin and myosin, while the light bands contain only actin.