Hair is a pigmented filamentous protein that grows from specialized structures in the skin called the hair follicles (Krause and Foitzik, 2006). Fascinatingly, hair growth starts right from the embryonic stage.
Hair Growth in the Fetus
The first hair is produced by the fetal hair follicles (Yasuyuki et al, 2010) between the twelfth and the twenty-second week of pregnancy (gestation), and called the lanugo hair. Lanugo hair is unpigmented and gradually replaced by vellus and terminal hair thereafter, through site-specific cellular differentiation usually by the thirty-sixth week of pregnancy (Hill, 2015).
Hair at Birth
The infants at birth have two types of hair, namely the vellus hair and terminal hair. The vellus and terminal hairs are produced by specific types of hair follicles. The hair follicles that give rise to vellus hair lack sebaceous glands, whereas, those that give rise to terminal hair have sebaceous glands (Hill, 2015). Vellus hair filaments are short, have diminutive or no pigmentation and are distributed throughout the body. Terminal hair filaments conversely, are pigmented, longer and present on specific locations like the scalp, eyelashes and eyebrows.
Hair Growth in the New Born
Hair growth in the new born thus, refers to terminal hair growth on the scalp and other regions of terminal hair distribution. Initially the hair growth begins at the frontal part of the head. Spontaneous shedding of the neonatal scalp hair, especially at the back may start by the twelfth week in some infants. Subsequent terminal hair growth occurs in a cyclic pattern as part of the normal development of the child into an adult.
The Hair Growth Cycle
Terminal hair growth occurs in a cyclic growth pattern in well-defined periods, namely the anagen, catagen, telogen and the final exogen phase. Anagen is the active growth phase and catagen marks the cessation of active growth. While telogen denotes the final resting phase of the terminal hair, exogen is the shedding phase (Milner et al, 2002).
The most intriguing aspect of the hair growth cycle is that every strand of hair is in a distinct growth phase of its own, independent of the other depending on the time of initiation of the anagen phase. A new anagen phase gets initiated after every shedding phase, resulting in growth of new hair strands (Hess et al, 2010).
Exogen Shedding – a Normal Event
In a healthy adult, since about five to fifteen percent of scalp-hairs remain in the telogen phase, periodic shedding of diminutive strands of hair is fairly normal and barely noticeable.
Excessive Exogen Shedding – a Matter of Concern
It is the frequent excessive exogen shedding of the final dead product of the hair growth cycle referred to as ‘club hair’ (Sato-Miyaoka et al, 2012), fairly visible in the bath towels, comb and pillows that calls for immediate attention. Dietary deficiency, hormonal imbalance, pathogenic infections, stress and immunological disorders can push almost seventy percent of the hair into the telogen phase and exogen shedding leading to alopecia.
Reference for Hair Growth Biology - Fabulous Facts
Dawber R, Comaish S. Scanning electron microscopy of normal and abnormal hair shafts. Arch Dermatol. 1970 Mar; 101(3):316-22.
Hess WM, Seegmiller RE, Gardner JS, Allen JV, Barendregt S (1990). Human hair morphology: a scanning electron microscopy study on a male Caucasoid and a computerized classification of regional differences. Scanning Microsc. Jun; 4(2):375-86.
Hill, M.A. (2015). Embryology Integumentary System - Hair Development. Retrieved March 25, 2015, from https://embryology.med.unsw.edu.au/embryology/index.php/Integumentary_System_-_Hair_Development
Krause, K and Foitzik, K (2006). "Biology of the Hair Follicle: The Basics". Seminars in Cutaneous Medicine and Surgery 25: 2.
Milner Y, Sudnik J, Filippi M, Kizoulis M, Kashgarian M, Stenn K (2002).Exogen, shedding phase of the hair growth cycle: characterization of a mouse model. J Invest Dermatol. Sep; 119 (3):639-44.
Pacini P, Zecchi S, Orlandini GE. [Statistical findings on scalp hair as observed by scanning electron microscopy] Boll Soc Ital Biol Sper. 1983 May 30; 59(5):603-7.
Sato-Miyaoka M, Hisatsune C, Ebisui E, Ogawa N, Takahashi-Iwanaga H, Mikoshiba K (2012).Regulation of hair shedding by the type 3 IP3 receptor. J Invest Dermatol. Sep; 132(9):2137-47.
Yasuyuki Amoh, Lingna Li, Kensei Katsuoka and Robert M Hoffman (2010). Embryonic development of hair follicle pluripotent stem (hfPS) cells. Med Mol Morphol. 43(2); 123-7.
Yutaka Shimomura, Angela M Christiano (2010). Biology and genetics of hair. Annu Rev Genomics Hum Genet: 11; 109-32.