Changes

[[image: Section of Foetal Cat Skin.jpg|thumb|200px|right|A section of foetal cat skin showing developing hair follicles. ©RVC2008]]

[[image: Section of Foetal Cat Skin.jpg|thumb|200px|right|A section of foetal cat skin showing developing hair follicles. ©RVC2008]]

Hair germs begin from an aggregation of keratinocytes in the stratum basale of the [[Skin - Anatomy & Physiology#Epidermis|epidermis]].  The initiating factor is the underlying dermal fibroblast cells.  The keratinocytes elongate, divide and relocate to the dermis.  Dermal fibroblasts then  form a dermal papilla beneath the hair germ.  This causes stimulation of the basal stem cells to up-regulate their cycle, producing cells that will keratinise and form the hair shaft.  Two swellings form on the shaft, one containing stem cells for follicle regeneration, the other becomes a sebaceous gland which will secrete sebum onto the hair shaft.

Hair germs begin from an aggregation of keratinocytes in the stratum basale of the [[Skin - Anatomy & Physiology#Epidermis|epidermis]].  The initiating factor is the underlying dermal fibroblast cells.  The keratinocytes elongate, divide and relocate to the dermis.  Dermal fibroblasts then  form a dermal papilla beneath the hair germ.  This causes stimulation of the basal stem cells to up-regulate their cycle, producing cells that will keratinise and form the hair shaft.  Two swellings form on the shaft, one containing stem cells for follicle regeneration, the other becomes a sebaceous gland which will secrete sebum onto the hair shaft.  The follicles develop from an ectodermal bud which invades the mesenchyme during embryonic development.  The mesoderm also condenses during the development creating an outer mesodermal component to the embedded part of the hair.

==Structure and Function==

==Structure and Function==