To understand its composition, it is first necessary to clarify some concepts related to tissue formation and types. By definition, a tissue is any group of cells that execute the same function in the body. There are four basic types of tissue: Epithelial, Connective, Muscular, and Nervous.
The functions of the Epithelial tissue range from structure and protection (Covering and Lining Epithelial tissue ) to substance secretion (Glandular Epithelium). Even though epithelial tissue has its own nerve supply, it is completely avascular (no blood) which makes it entirely dependent on the underneath tissues (Connective tissue) for nutrition through diffusion.
The Epithelial and Connective tissues are connected through the basement membrane which acts as an interface for nutrient exchange. This membrane can be divided into a basal and reticular laminae; the former located more superficially than the latter. The basal layer of epithelial cells connects to the basal laminae layer through hemidesmosome junctions. In this type of connection, Keratin filaments located in the intracellular space are bound to the protein Integrin. Another protein called Laminin is located in the basal lamina layer of the basement membrane. Laminin and Integrin are then bound together ensuring a very strong connection between the epithelial layer and the connective tissue below.
Epithelial and connective tissue differ in the amount of extracellular (matrix) space and the number of cells present. Connective tissues have scarce cells and much more matrix, which is filled with ground substance and fibers. On the other hand, epithelial tissues present a great number of densely packed cells with vestigially no interstitial space.
Cartilages are an important type of connective tissue. They can be of three basic types: Hyaline, Fibrocartilage, and Elastic. Hyaline cartilages are found lining the surfaces of articulations to reduce friction and prevent the onset of osteoarthritis. Elastic cartilage is found on the ears and epiglottis. Fibrocartilage is the toughest of the three types and is highly effective in dampening impacts and provide cushioning.
The collateral cartilages are an example of fibrocartilage. They are located palmar to the distal phalanx and are present on the medial and lateral aspects of the equine foot. This location ensures the dissipation of the forces coming down the leg, reducing their effects on the internal and more rigid structures of the hoof. They also play a crucial role in blood circulation. Every time the hoof is loaded, the collateral cartilages compress the venous plexuses axially located forcing the blood to be circulated. In some instances, ossification of the collateral cartilages happens which creates a condition called sidebone.
EVJ Podcast, No 16, Oct 16 – Subchondral bone thickness on MRI to identify racehorses at risk of lateral condylar fracture & Sesamoiditis, subclinical ultrasonographic SSLBC & injury in Thoroughbreds.
The neurons are the functional unit of the Nervous System. They have the ability to create and propagate action potentials (nerve impulses) and their shape varies considerably according to their location in the body. They not only connect all the regions of the body to the brain and spinal cord but also form complex processing networks within the Central Nervous System.
Through their extreme intricate networks, neurons provide most of the unique functions of the nervous system such as sensing, thinking, remembering, controlling muscle activity and regulating glandular secretions. Due to their specialization, most neurons have lost their ability to undergo mitotic divisions burdening their capability of regeneration, particularly if located in the Central Nervous System.
- Dendrites: It corresponds to the information receiving area. They contain numerous receptor sites for binding chemical messengers from other cells.
- Soma: It corresponds to the cell body where the organelles in charge of metabolic activity are located.
- Axons: Information carrying extensions of the cell membrane. Axons can be covered by a fatty coating called myelin sheaths for faster traveling of action potentials.
- Presynaptic terminal: It corresponds to the end of the axon where information is then transmitted to other cells with the aid of neurotransmitters.
The communication between neurons and effector cells (other neurons, muscles cells or gland cells) takes place through synapses. The tips of the axon terminals contain synaptic vesicles that store chemicals called neurotransmitters. Neurotransmitters are molecules that excite or inhibit effector cells. Many neurons contain different types of neurotransmitters that will act on different types of target cells.
Structural Classification of Neurons
Multipolar neurons have several dendrites and one axon. They are found in the many parts of the brain and spinal cord. Bipolar neurons have one main dendrite and one main axon. They are found in the retina of the eye, inner ear, and olfactory area. Unipolar neurons have dendrites and one axon that are fused to form a continuous process that emerges from the cell body. Pseudounipolar neurons start off as bipolar neurons; however, during development, the dendrites and axon fuse to form a single process. The dendrites of unipolar and pseudounipolar neurons work as sensory receptors. They detect a stimulus (touch, pressure, temperature, pain) and create an action potential is created in the trigger zone (area of connection between dendrites and axon).