An Overview of Human Tissues
1. Types of Tissues
Tissues are distinct materials in animals and plants, made of specialized cells and their products. There are four main types:
1.1 Epithelial Tissue
These tissues cover body surfaces, line passageways and cavities, and form glands.
1.2 Connective Tissue
Found throughout the body, connective tissues primarily connect and support other structures.
1.3 Muscle Tissue
Located in skeletal muscles, hollow organs, the heart, and places like the iris, muscle tissue enables movement.
1.4 Nervous Tissue
This tissue forms the brain, spinal cord, and peripheral nerves, responsible for sending and receiving messages within the nervous system.
2. Cell Types within Tissues
2.1 Epithelial Tissue
- Squamous Cells: Uncontrolled growth of these cells, which make up the skin’s upper layers, can lead to cancer.
- Cuboidal Cells: Cube-shaped cells forming the epithelium.
- Columnar Cells: Column-shaped epithelial cells, some with cilia.
2.2 Connective Tissue
- Fibroblasts: Fixed, star-shaped cells that produce fibers.
- Macrophages: Wandering, phagocytic cells crucial for immunity.
- Adipocytes: Fat cells, either brown or white.
- Mast Cells: Fixed cells that release heparin and histamine.
- Mesenchymal Cells: Multipotent stem cells.
2.3 Muscle Tissue
- Skeletal Muscle Myoblasts: Long, tubular, striated cells attached to bones, enabling voluntary contraction.
- Cardiac Muscle Myoblasts: Short, wide, branched cells responsible for involuntary heart contraction.
- Smooth Muscle Myoblasts: Tapered cells involved in involuntary contraction.
2.4 Nervous Tissue
- Neurons and Neuroglial Cells: Specialized for transmitting and receiving impulses.
3. Detailed Look at Epithelial Tissue
3.1 Squamous Cells
A single layer of flat cells with a centrally located, flattened nucleus.
3.2 Cuboidal Cells
Short and spherical with a central nucleus, lining glands like the thyroid and kidneys.
3.3 Columnar Cells
Spherical nuclei located near the base, lining respiratory passages and much of the digestive tract.
4. Specialized Epithelial Tissues
4.1 Pseudostratified Ciliated Columnar Epithelium
Appears to have multiple layers but is actually a single layer of columnar cells with cilia, often called respiratory epithelium.
4.2 Transitional Epithelium
Stratified epithelium with cells that can change shape (dome-shaped to flattened), also known as urinary epithelium.
5. Epithelial and Connective Tissue Membranes
5.1 Epithelial Membranes
5.1.1 Mucous Membranes (Mucosae)
Line body cavities that open to the outside, such as the digestive, respiratory, excretory, and reproductive tracts.
5.1.2 Serous Membranes
Line body cavities that do not open to the outside, covering organs within those cavities. Examples include the pleura lining the thoracic cavity.
5.2 Connective Tissue Membranes
5.2.1 Synovial Membranes
Line the cavities of freely movable joints, secreting synovial fluid for lubrication.
5.2.2 Meninges
Connective tissue covering the brain and spinal cord within the dorsal cavity, providing protection.
6. Glands
Organs that secrete substances for use in the body or for discharge. Classified by cell number:
6.1 Unicellular Exocrine Glands
Composed of a single goblet cell.
6.2 Multicellular Exocrine Glands
Composed of many cells, such as sweat and salivary glands.
7. Types of Glands
7.1 Endocrine Glands
Secrete into tissue fluid or blood.
7.2 Exocrine Glands
Secrete into ducts, further classified by shape.
8. Modes of Glandular Secretion
- Merocrine: Cells remain intact (exocytosis).
- Apocrine: Apical part of the cell is released.
- Holocrine: Cells are destroyed as they release their products.
9. Junctions
- Tight Junctions: Fuse adjacent cells, creating a barrier to fluid passage.
- Gap Junctions: Allow direct communication between cells via intercellular channels.
- Anchoring Junctions (Desmosomes): Attach cells to each other and the extracellular matrix, providing structural cohesion.
10. Connective Tissue Proper
The most widespread connective tissue, consisting of fibroblasts (fibrocytes and adipocytes) that secrete an extensive extracellular matrix filled with protein fibers.