Hair follicle

1. Hair follicle

2. Papilla The   papilla  is a large structure at the base of the hair follicle. The papilla is made up mainly of  connective tissue  and a capillary loop. Cell division in the papilla is either rare or non-existent.

3. Matrix Around the papilla is the  hair matrix , a collection of  epithelial  cells often interspersed with the pigment-producing cells,  melanocytes . Cell division in the hair matrix produces the cells that form the major structures of the hair fiber and the inner root sheath.

4. The hair matrix epithelium is one of the fastest growing cell populations in the human body, which is why some forms of  chemotherapy   or radiotherapy , that kill dividing cells, may lead to temporary  hair loss . The papilla is usually ovoid or pear shaped.

5. The matrix wraps completely around it, except for a short stalk-like connection to the surrounding connective tissue. This provides access for the capillary .

6. Root sheath The root sheath is composed of an external and internal root sheath. The external root sheath appears empty with cuboid cells when stained with  H&E stain . The internal root sheath is composed of three layers,  Henle's layer , Huxley's layer , and an internal cuticle that is continuous with the outermost layer of the hair fiber.

7. Lamellar corpuscles , or  Pacinian corpuscles , are   nerve  endings in the  skin  responsible for sensitivity to vibration  and  pressure . The vibrational role may be used to detect surface texture, e.g., rough vs. smooth.

8. Hair fiber The hair fiber is composed of  keratin .

9. arrector pili muscles These   are small muscles attached to  hair follicles  in  mammals . Contraction of these muscles causes the hairs to stand on end. Each arrector pili is composed of a bundle of  smooth muscle   fibres which attach to several follicles (a follicular unit), and is innervated by the sympathetic branch of the  autonomic nervous system .

10. The contraction of the muscle is therefore involuntary - stresses such as cold, fear etc. may stimulate the sympathetic nervous system  and thus cause contraction, but the muscle is not under conscious control.

11. sebaceous glands These secrete an oily/waxy matter, called  sebum , to lubricate and waterproof the skin and hair They are found in greatest abundance on the  face  and  scalp , though they are distributed throughout all skin sites except the palms and soles .  

12. Sweat glands There are two main types of sweat glands: Eccrine sweat glands  are distributed almost all over the body, though their density varies from region to region. Humans utilize eccrine sweat glands as a primary form of  cooling .

13. Apocrine sweat glands  are larger, have a different secretion mechanism, and are mostly limited to the  axilla (armpits)  and perianal areas in humans .   Although apocrine glands contribute little to cooling in humans, they are the only effective sweat glands in hoofed animals such as the  camel ,  donkey ,  horse , and cattle

14. Apocrine  is a term used to classify  exocrine glands  in the study of histology . Cells which are classified as apocrine bud their secretions off through the plasma membrane producing membrane-bound vesicles in the lumen .

15. This method is also called  decapitation secretion . The apical portion of the secretory cell of the gland pinches off and enters the lumen.

16. Apocrine secretion is less damaging to the gland than  holocrine  secretion (which destroys a cell) but more damaging than  merocrine  secretion ( exocytosis ). An example of true apocrine glands are mammary glands, responsible for secreting breast milk