The Anatomy of Hair Transplantation

Our scalp is divided into 5 layers. Each layer can be easily remembered using the mnemonic SCALP:

S = The skin on the head where hair grows. It contains the sebaceous glands and hair follicles.
C = The connective tissue, a dense subcutaneous layer of fat and fibrous tissue that is found underneath the skin. This is where the nerves and blood vessels of the scalp are.
A = The aponeurosis, also called epicranial aponeurosis, is the third layer. This is a tough layer of dense fibrous tissue that runs from the frontalis muscle located at the front of the scalp to the occipitalis located at the back of the scalp.
L = The loose areolar connective tissue layer provides a level of separation between the upper 3 layers and the pericranium. It also provides an area of entry during craniofacial surgery and neurosurgery. It is sometimes also referred to as the “danger zone” because of how easy it is for infectious agents to spread through it and on to emissary veins, which can then drain into the cranium. The loose areolar tissue is made up of random bundles of collagen type 1 and collagen type 3. It is also rich in glycosaminoglycans and will be made up of more matrix than fibres. This layer also allows the more superficial layers of the scalp to be able to shift in relation to the pericranium.
P = The pericranium is the dense layer of vascular connective tissue of the skull bones. It provides nutrients to the bones and the capacity for cellular repair. It can also be lifted from the bone to allow the creation of bone windows, like what is used in craniotomy.

The skin will contain all of the epidermal appendages. This includes hair follicles that will extend into the connective subcutaneous layer. In areas that have experienced hair loss, the thinning of the outer two layers is typically what has happened. This condition can be shown when the thickness of the scalp in recipient areas is compared to the scalp in donor areas.

The subcutaneous layer is very vascular and contains the main penetrating branches of the main arteries that travel primarily over the external surface of the galea. The importance of staying superficial when making the recipient sites along the connective subcutaneous tissue layer and avoiding doing any damage to the circulation in this area has only become apparent in recent years. The scalp has a very large blood supply. The primary vessels are the supraorbital, supratrochlear, superficial temporal, postauricular, and occipital arteries, and they typically all travel within the veins.

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The sensory stimulation of the scalp closely follows the blood supply. At the frontal area, the supraorbital and supratrochlear nerves are responsible for providing sensation to the front half of the scalp. In some cases, sensation to the frontal hairline may diminish for a few weeks when a large number of recipient sites have been made along the hairline. The occipital nerve serves the back half of the scalp, and the supra auricular and superficial temporal nerves are responsible for the sides.

The most important anatomic feature of the scalp with regard to hair transplantation is the microscopic re-distribution of hair. Scalp hairs typically do not grow individually. For most people, they often grow in tiny follicular-unit bundles with anywhere from 1 to 4 hairs in each graft. A follicular unit graft will contain these hairs, as well as a sebaceous gland element, and insertions of the arrector pili muscles, all wrapped in a tissue sheath. These follicular units are transplanted into the areas of hair loss. By transplanting these follicular units, the most natural-appearing results can be attained.

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The Anatomy of Hair Transplantation