Dermis & Hypodermis
The dermis is composed of dense irregular connective tissue. It contains nerves, vessels, sweat glands, and hair follicles.
The boundary between the epidermis and the dermis is a wavy layer called the dermal papillae. This irregular surface is the cause of your fingerprints (or epidermal ridges). These ridges increase friction allowing us to pick up objects more easily.
The dermis is composed of 2 layers:
Papillary layer: made of loose connective tissue
Reticular layer: made of bundles of collagen fibers
The arrangement of the collagen fibers cause lines of cleavage or lines of tension.
Why are lines of cleavage important for surgeons?
For quicker healing and less scar tissue formation.
Cuts should be made parallel to the lines of cleavage.
During exercise, the blood vessels in the dermis swell causing skin to appear red. This allows heat from the blood to dissipate, cooling the body. When blood supply to the skin is restricted for a prolonged time, decubitus ulcers (bedsores) can form.
Hypodermis is also known as the subcutaneous layer. Not actually a part of the skin. Composed of loose, fatty connective tissue that connects the skin to muscle or bone. Also insulates and stores nutrients.
Summary: The dermis is found just deep of the epidermis and contains blood vessels and nerves. It has a papillary layer and a reticular layer made of collagen that forms lines of cleavage. The hypodermis or subcutaneous layer attaches the dermis to the underlying organs.