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Collateral Cartilages

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.

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