The early days
The first outlines of the hair follicle and what is known as the pilo-sebaceous unit (comprising the hair shaft, follicle, sebaceous gland and erector pili muscle) appear on a foetus’s eyebrows, chin and upper lip from the eighth week of pregnancy. From the beginning of the fourth month they start to develop on the scalp. Progression occurs in a craniocaudal direction (from top to bottom). Hair only becomes visible on the surface of the scalp from the 28th week.
The origin of the pilo-sebaceous follicle is both ectodermal — due to its epithelial component formed from the epidermis (the surface layer of the skin) — and mesodermal — due to its connective tissue component, known as the the dermal papilla. This group of cells forms the structure directly below the hair follicle and is anchored four millimetres below the surface of the dermis (see diagram below).
The cellular formation of the hair follicle
The epithelial component starts out with the formation of a placode. This consists of cells with a specifically programmed future grouped in a particular area. Between the 8th and the 28th week of intra-uterine life, differentiation of the pilo-sebaceous follicle occurs. The ectodermal cells multiply to form first a bud, then a follicle and two associated structures: the sebaceous and apocrine glands.
During the third month, the ectoderm is formed from three or four cell strata: the germinal basal cells, the intermediary cells arranged in several layers and the superficial cells composing the periderm. The subjacent mesoderm acts as the inducer of the epidermal placode: two condensations of nucleated cells develop in the basal region of the ectoderm. These cellular nuclei become denser and elongate in the direction of the mesoderm, the cells of which condense to form the future dermal papilla.
As the placode gets bigger, a phase of invagination into the mesoderm forms a primitive bud. This then pushes slantwise into the dermis, form a column of epithelial cells. During this stage new molecular signals are emitted. The epithelial cells release messengers which cause the build-up of fibroblasts in the dermis into the dermal papilla, opposite the primitive hair bud. The two key elements of the hair follicle are thus in place : an epithelial part and a dermal part. The joining of these two components marks the true birth of the hair follicle which is in fact an epidermal cavity within the dermis.
Multiplication and differentiation
The final phase of differentiation and development of the pilosebaceous structure is controlled by the papilla: the germinal cells of the primitive bud multiply and give rise to daughter cells which will form the matrix and take part in various differentiation programmes. Some form the outer and inner root sheaths of the follicle, others undergo upward maturation, becoming keratinised to form the shaft. Finally, this complex biological machinery gives rise to the formation of the fibre and the inner root sheath.
From the beginnings of the hair follicle three bulges are formed:
- The lower bulge is invaded by mesenchymal cells (connective tissue derived from the mesoderm) and eventually becomes the arrector muscle
- The middle bulge forms the basis of the sebaceous gland
- the upper bulge produces the apocrine gland.
These dermo-epidermal interactions are accompanied by interactions with the nervous and vascular networks surrounding the terminal follicle and interactions with the immune system. One of the basic yet surprising facts which is unique to to hair is that throughout life under normal conditions it is these same embryonic processes that keep on recurring, thereby producing the growth cycle of hair.
*Sebum is produced in the sebaceous gland and formed mainly of lipids. It carries different scents such as the pheromones which allow sexual recognition. Sebum stops the growth of fungi and bacteria and is therefore fungistatic and bacteriostatic.
Hair, L’Oréal Research & Innovation