A prefix is placed at the beginning of a word to modify its meaning by providing additional information. It usually indicates number, location, time, or status.

endo- [ˈɛndəʊ] – from Greek – within, inside, internal

Endo- is the opposite to exo-, which means out of, outside, external. Compare endoskeleton and exoskeleton. Endoskeleton is an internal skeleton, like our dogs or ourselves have. Exoskeleton is a rigid external covering of the body of some invertebrates.

Image Berkeley: Understanding Evolution

Endoskeleton of a dog. Image Wikimedia Commons

An endoscope, then, is an instrument that can be introduced into the body to view its internal parts.

Today these instruments can be used not only to view things, but also to take samples of tissues, remove foreign objects and even perform repairs, such as with arthroscopy.

Endoparasites are parasites that live inside your dog’s body, such as worms.

The endocrine system is a network of hormone producing glands and tissues with internal secretion, directly into bloodstream.

Endocrine system. Image Merck Manual for Pet Health

The organs of the endocrine system include hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, ovaries or testes  … and fat tissue.

Endothelium are the tissues lining various organs and cavities in the body, such as the blood vessels, heart and cornea.

Endothelial dystrophy is a inherited disorder of the inner lining of the cornea, leading to corneal swelling and degeneration.

An endocardium is a membrane that lines the inside of the heart chambers.

Endocarditis is an inflammation of the inner lining of the heart in response to an infection. The infection can originate elsewhere such as in you dog’s mouth. Yes, your vet nags you about your dog’s oral care for a reason. Periodontal disease was associated with cardiovascular-related conditions, such as endocarditis and cardiomyopathy.

Source:  http://dawgbusiness.blogspot.com/2013/07/tackling-veterinary-terminology_31.html