The Anatomy of Hair: Structure, Types, and Growth Cycles

Written by: Brilliant Staff

Fact checked by: Melissa Mitri, MS, RD

human hair growth cycle

Hair plays a significant role in our appearance, self-esteem, and overall well-being. Understanding hair growth and the science behind healthy hair growth can help us care for and maintain it better. 

This comprehensive article will delve into the hair's anatomy, types, textures, and growth cycles, along with some crucial factors affecting hair health.

A Closer Look at Hair's Composition and Structure

Hair Structure Overview

Our hair is composed of a protein called keratin, and it grows from the hair follicle located under the skin's surface. The hair follicle is crucial for healthy hair growth and receives nutrients through tiny blood vessels — each hair strand has its life cycle that is characterized by growth, rest, and shedding.

From Follicle to Shaft: A Comprehensive Breakdown

The story of hair starts beneath the skin, in a tiny organ called the hair follicle. Each follicle contains a bulb, where nutrients are received, and new cells are generated to create hair.

The root or lower part of the hair comprises protein cells wrapped within a strong protective tube of tissues known as the lower follicle or hair canal. The upper part of the hair exposed along your scalp is the hair shaft, which consists of dead cells and protein.

The hair shaft itself is divided into the following three layers:

  • Medulla: The innermost layer, which may be absent in fine or thin hair.
  • Cortex: The middle layer, comprising elongated cells and pigments, gives your hair its color.
  • Cuticle: The outermost layer, a protective layer of overlapping cells, gives hair shine and determines how manageable it can be.

Understanding Hair Types and Textures

Human hair varies dramatically, and it can be thick, thin, straight, wavy, curly, or coily. Different hair typing systems provide ways to understand the variety of hair characteristics. Below, we explore two prominent systems.

The Andre Walker Hair Typing System

Andre Walker, Oprah's hairstylist, created a hair typing system that's widely used today. It categorizes hair into four types, with subcategories describing the degree of curl.

  • Type 1 - Straight: This type is typically shiny and oily due to the lack of curls or waves, which allows natural oils to travel from the scalp down the hair shaft. Subcategories within Type 1 include fine (1A), medium (1B), and coarse (1C) hair.
  • Type 2 - Wavy: This hair type has a natural wave, with type 2C being the thickest and having a propensity towards frizz. Subcategories within Type 2 include 2A, 2B, and 2C, with increasing degrees of wave definition.
  • Type 3 - Curly: This hair type is often voluminous and climate-dependent and can be especially frizzy in damp conditions. Subcategories within Type 3 include 3A, 3B, and 3C, with increasing curl tightness and texture.
  • Type 4 - Coily: This type is tightly coiled and delicate and often experiences shrinkage, making it appear shorter. Subcategories within Type 4 include 4A, 4B, and 4C, with increasing degrees of coil definition and fragility.

The LOIS Hair Typing System

The LOIS hair typing system is another popular method that considers hair strand thickness, texture, and curl pattern. The system determines hair type using alphabetic categories L, O, I, and S, based on the shape your strand takes when you hold it against a white background.

  • L: If your hair forms the shape of an uppercase "L," you have angular or wavy hair.
  • O: If it forms a circular "O" shape, you have tightly coiled or curly hair.
  • I: If your hair is straight, like a lowercase "i," you have straight hair.
  • S: If your hair forms the shape of an uppercase "S," you have loose or spiraled curls.

The Hair Growth Cycle and Its Phases

Interestingly, your hair doesn't grow all at the same time. Each hair follicle cycles independently, moving through the following three key phases:

  • Anagen Phase (Growth Phase): Cells divide rapidly, creating new hair growth. This phase can last between two to seven years, with average hair growing at about six inches per year.
  • Catagen Phase (Transition Phase): A short phase lasting around ten days, where the hair follicle shrinks and growth stops.
  • Telogen Phase (Resting Phase): The final stage, lasting around three months, concludes with the hair falling out. Following the shedding, the follicle becomes active again, and new hair starts to grow.

On average, you shed 50 to 100 hairs per day. Don't worry, though — different hairs are at diverse growth cycle stages.

Factors Affecting the Hair Growth Cycle

Several factors influence your hair's growth cycle, such as genetics, age, health, hormones, and stress. While genetic factors may primarily dictate an individual's average hair growth rate, both internal and external factors can impact the hair's growth cycle.

One study noted that a nutritious diet rich in omega-3 fatty acids, biotin, vitamins A, C, D, and E, zinc, and iron supports overall hair health and growth. Scalp care, including gentle cleansing and lower heat styling, can also foster a healthier environment for hair growth.


Understanding our hair from the inside out helps us care for it more compassionately. Appreciating the intricate structure, the diversity of types and textures, and the fascinating growth cycle empowers us to make informed decisions for nurturing and maintaining our hair.

This article provides general information based on scientific and medical understanding and is not intended to replace personalized advice from a hair specialist and healthcare practitioner. We do not make definitive health claims or promote specific products, ensuring alignment with  FTC and FDA guidelines.