Skin Appendages Flashcards
Types of skin appendages
-Sebaceous gland
-Apocrine gland
-Eccrine glands
-Hair follicles
=Keratinocyte derived structures that geographically are mainly located mainly in the dermis.
=They are made up of specialised types of keratinocytes
=Structures develop by the end of the first trimester.
= If destroyed later in life, they cannot be regenerated
=Androgen end-organs
What are eccrine sweat glands?
-Innervated by cholinergic post-ganglionic fibres of the sympathetic nervous system.
-Eccrine sweating is essential for human viability, as humans are ‘designed’ to be able to run hot from exercise
=Sweating allows evaporative cooling and frictional grip
Anatomy of eccrine sweat glands
-Comprise a coiled secretory portion, a long duct that rises almost vertically through the dermis and then a tightly coiled duct within the epidermis
=The coiled structure in the epidermis is designed such that the duct doesn’t become occluded (i.e. blocked) due to pressure.
=When sweat is secreted into the coil (in the lower secretory portion), it is an isotonic fluid and gradually changes to a hypotonic fluid by the selective reabsorption of salts as the duct moves through the skin (‘salt saving’)
=Living in a hot climate, or prolonged physical activity, leads to changes in skin physiology such that eccrine sweating is increased, but the electrolyte loss from sweating is diminished (‘sweat gland training’).
Describe apocrine (sweat) glands
-Associated with hair follicles and are most common in the axillae, nipples and groin.
-Although freshly secreted oily apocrine gland material is odourless, bacterial decomposition produces the characteristic axillary (body) odour.
-The milk glands of the breast are closely related to apocrine glands.
-Apocrine glands are androgen sensitive, but details of their neural control are unclear
Describe sebaceous glands
-Sebaceous cells (sebocytes) specialised for production of various lipids that make up sebum.
=Sebum is a mixture of triglycerides, fatty acids, squalene and cholesterol
-Occasional sebocytes resting on the basement membrane divide, the daughter cells move up, differentiate and synthesise lipid.
=Eventually, they die and burst, releasing their contents into the hair follicle lumen.
-The highest density of sebaceous glands is found on the scalp, face and the upper chest and back
Describe the effect of androgens on sebaceous glands
-Androgen end-organs with androgens acting so as to increase sebum production: sebum rates are therefore higher in males than females.
-Progestogens increase sebum excretion too, and whereas oestrogens decrease sebum excretion.
-Once sebaceous glands have been exposed to high levels of androgens, even severe androgen blockade may only have a modest effect on sebum excretion rates.
-If, however, there is a congenital insensitivity to the effects of androgen, or the individual is eunuchoid, then sebum excretion rates will remain negligible.
-Females with severe PCOS may have worse acne because of skin virilism.
-Bodybuilders (of either sex) may also have worse acne if they take exogenous androgens
Anatomy of hair follicles
-Epidermal structures, comprised largely of a downgrowth of epidermal keratinocytes, although within the hair bulb there is a specialised dermal structure called the dermal papilla made up of specialised fibroblasts
-The archetypical hair comprises a central medulla, a cortex, and an outer cuticle, with the cortex being the main component.
Types of hair
-Lanugo hairs are fine hairs seen only on premature infants. They don’t have a medulla, are soft, and have no pigment.
-Vellus hair are normally short, (less than a few centimetres) and very thin. They have no medulla and lack significant pigment (i.e. they appear white). They are found all over the body postnatally, and comprise the majority of hairs (e.g. most facial hair in women)
-Terminal hairs are long, thick, pigmented hairs with a central medulla (e.g scalp, eyelashes)
=Sexual= specialised, appear during puberty as a response to androgens acting on vellus hairs in areas such as the genitalia and the beard area
Describe the hair cycle
-Hairs go through 3 growth phases that comprise the hair cycle.
-Anagen: a growth phase which typically might last 3 years
-Catagen: a 3 week transitory period in which each hair bulb undergoes apoptosis and regression and in which the lower part of the hair moves up close to the skin surface.
-Telogen: A period lasting approximately 3 months in which the hair is relatively quiescent. Then the new hair pushes out the old one which is shed. (Some refer to this shedding as exogen.)Almost 80% of hairs at any particular time, are in anagen with most of the remaining in telogen
Describe hair growth
-Differences in hair length on different body sites reflect variation in the components of the hair cycle rather than just variation in the absolute rates of growth.
=For instance, many scalp hairs grow at the rate of around 1.25cm per month. On the scalp, hairs will be in anagen (and hence grow) for 3 years or more and therefore if uncut, grow long.
=Whereas on areas such as the chest, anagen only continues for 3 to 4 months, resulting in shorter hairs.
-At birth, hair growth is synchronous, that is, all the follicles are at the same stage of development (as is common in many adult animals).
-However, postnatally hair cycling becomes asynchronous between hairs. This is why babies born with a head of hair may lose most of it after birth— before regaining it.
-Certain states, such as childbirth or severe illness, may force most hairs, at whatever stage of the hair cycle, through catagen into telogen.
=This leads to a clinical condition called telogen effluvium where most of your hair falls out at the same time (synchronous loss, rather than the usual asynchronous loss).The hair will usually reappear with time; asynchrony (‘disorder’ will reassert itself)
Components of nails
-Nail plate: This is the hard material that forms the bulk of the nail that is made up of dead keratinocytes (just like the stratum corneum).
-Nail bed: This is found under the nail plate and contributes slightly to the growth of the underside of the nail (known as ventral nail)
-Nail matrix: This is the main growth zone of the nail and extends beneath the proximal nail fold and as far as the end of the lunula (the crescentic pale part) of your nail. The matrix contributes most of the growth of the nail plate, but some of the ventral nail arises from the nail bed
Nail growth rate
-An average rate of growth for finger nails is around a tenth of a millimetre per day (3mm/month), but it varies between fingers, being fastest for the middle finger and less for the ring fingers
-Fingernails grow faster than toenails. A fingernail may grow completely out over 3-6months, whereas for toenails it might be 24 months (~1mm/month)
