Cellular Biology: Epithelial and Connective Tissues
Epithelial Tissue
Formed by one or more layers of cells attached tightly to each other, epithelial tissue has little or no extracellular matrix. It is avascular, meaning its nutrition depends on the blood supply to neighboring tissue, and it is innervated with nerve endings.
Functions
- Protection
- Absorption
- Excretion
- Filtration
- Drainage
- Sensation
Types
- Lining epithelium
- Glandular epithelium
- Neuroepithelium (which forms the sense organs)
Epithelial Cell Junctions
- Tight junctions (occlusion zones)
- Areas of accession
- Desmosomes
- Interdigitations
- Gap junctions
According to the number of layers, epithelia can be classified as:
- Simple or monostratified (single layer of cells)
- Stratified or polystratified (more than one layer of cells)
Epithelial Outliers
- Pseudostratified: Named for inaccuracies, it appears to have several layers of cells, but in fact, they all reach the basal lamina. It is simple.
- Transitional or Polymorphic Epithelium: It has several layers where cells have different shapes (flat on the surface and cylindrical or cubic in deeper layers).
Connective Tissue
- Protection
- Provides nutrients to the surrounding tissue
- Support function (supports other tissues)
- Connects with other tissues
- Participates in movement (joints, etc.)
Structure: It is notable for having few cells and abundant intercellular matrix.
Inside the Cells
- Fibroblasts: Assist in the formation of the intercellular substance, mainly formed by a protein called collagen. They are also involved in the formation of some fibers that are placed differently in the intercellular matrix.
Macrophages are large cells responsible for various types of defense. They perform phagocytosis and may issue extensions called pseudopodia. Other connective tissue cells include prismatic cells, mast cells, and various white blood cells. Cartilage has chondrocytes, and bone tissue has osteocytes, osteoblasts, and osteoclasts.
Connective Tissue Classification
- Lax
- Reticular
- Adipose
- Cartilaginous
- Compact Bone
- Hematopoietic
Lax Connective Tissue
It is found throughout the body, forming multiple organs. It may also perform various types of extensions and holds as a special property, the extension. This is done by collagen and elastic fibers that form small fascicles, which in turn are mixed. It has a soft, viscous intercellular matrix where a variety of cells are located: fibroblasts, macrophages, plasma cells, adipocytes, white blood cells, etc.
Reticular Connective Tissue
Named for the presence of abundant reticular fibers that form a mesh network where you can also locate a few collagen fibers. These are formed by reticular cells that are characteristic of this tissue. It is found in lymph nodes, the spleen, and bone marrow, in those organs associated with hematopoietic tissue.
Adipose Connective Tissue
Formed by adipocytes or fat cells, which are characterized by lipid substances occupying the entire cytoplasm, displacing the nucleus. Fibroblasts and macrophages can also be found. Adipose tissue performs the protection of different organs, is a repository for excess lipids, and is an insulating material to conserve body heat.
Compact Connective Tissue
It has collagen and elastic fibers distributed in parallel rows within a soft or liquid matrix and contains fibroblasts. It forms tendons and ligaments.
Transportation Mechanisms
Mechanisms by which substances are transported inside or outside of the cell include:
- H+ (proton pump)
- Na+ and K+ (sodium-potassium pump)
- Amino acids and monosaccharides
Primary Active Transport
The energy derived from ATP pushes the substance across the membrane.
Secondary Active Transport
The sodium/potassium pump maintains a concentration difference of Na+ across the membrane. These ions tend to enter the cell through pores, and this potential energy is harnessed for other molecules. Glucose and amino acids can cross the membrane against a concentration gradient.
Passive Transport
Passive transport is the simple exchange of molecules across the plasma membrane. The cell does not expend energy because it favors the concentration gradient, i.e., from a place where there is a high concentration to one where there is less. The cellular process is by passive diffusion.
Simple Diffusion
Substances move from the compartment where there is a higher concentration to the one with the lower concentration until they match.
Facilitated Diffusion
Substances are transported through the membrane with the help of membrane proteins that facilitate their passage.