BIOL 2401

The free surface of the skin has numerous ridges (as seen on the palms and soles)
and these hereditary patterns correspond to similar patterns on the dermis ( the dermal ridges).
You will note, however, that they are absent on the forehead and ear.

Skin classified as thick ( as on the palms and soles) or thin, which only refers to the epidermis (as the dermis can be thick).

Epidermis ----- 4 Cell Types

1. Keratinocytes :
primary cell, produces a fibrous protein, keratin, that replaces the majority of cytoplasm resulting in dead superficial cells that are lost in sheets since cells are connected by desomosomes. Old cells pushed upwards as new cells are formed and replaced by cells at the basal regions. Turnover rate is ~ 1 month.

2. Melanocytes :
produces melanin with granules called melanosomes
found in stratum germinativum and spinosum
all humans have the same number of melanocytes
Granules accumulate in the engulfing keratinocytes to protect the DNA of the epithelial cells in the first layers from UV radiation (sun) damage.

3. Langerhans :
dendritic cells involved with immune protection, sometimes considered fixed macrophages
Free macrophages are also present for the body's defense and migrate to areas to alert 2nd and 3rd line body defenses.
Immune cells are widely distributed in the stratum spinosum

4. Merkel cell :
encapsulated CT structure that is associated with nerve endings for discriminatory touch sensation
found in the stratum germinativum at the epidermal dermal junction.

As a review, the epidermal layers ...

Stratum germinativum and stratum spinosum contain the cells : Melanocytes, merkel, langerhans
along with the majority of cells that are Keratinocytes
These cells will also, under UV response, convert steroid precursors to Vitamin D.

III. Skin Color

depends on hereditary, hormonal, & enviromental factors

Three (3) other factors that are involved :

1. The color of skin is yellow due to Carotene that accumulates in the stratum corneum.

2. Blood (oxygen) showing from underlying dermis imparts a reddish hue due to hemoglobin

3. Melanin contained in granules in the cytoplasm of melanocytes, is a brown/black pigment.
The granules (melanosomes) vary in size, color, and number (genetically related).
The number of melanocytes does not differ but they can have random distributions.
The melanocytes inject melanosomes (granules) into keratinocytes as a response to : Hormonal and Environmental triggers such as:

Dermal cellular elements : Fibers :

1. Fibroblasts 1. collagen

2. Macrophages 2. elastic

3. Fat cells 3. reticular

4. Smooth muscle

and accessory organs ... Hair follicles, and sweat glands found in the reticular layer of the dermis

V. Hypodermis

NOT part of the skin and functions to anchor the skin to underlying organs

Also known as the Subcutaneous layer, and consists of loose connective tissue composed of superficial fascia with fat cells.

One-half of the body's fat stores are located here. Baby's have fat all over, where as adult`s have their fat redistributed.
The fat cells are involved in temperature, cushion , and energy reserve.

The connective tissue connects hypodermis to the dermis and provides a great mobility in most skin areas, except in the palm and sole, where there is considerable interlocking.

It is also the place for injection sites.

The follicle is tubular in appearance and associated with sebaceous glands and smooth muscle (the arrector pili m.)

The follicle base that was explanded into a hairbulb is indented by Connective Tissue papilla which provided blood supply, nutrients, and hormones to the modified epidermal sheath cells that permit hair growth and control hair growth cycles.

Free nerve endings wrap around the bulb to form the root hair plexus. Bending or plucking the hair stimulates these nerve endings, resulting in a painful perception.

Smooth muscle associated with the ahir follicle inserts at an angle. When the arrector pili muscle contracts, it pulls the follicle upright. Under ANS control in response to temperature or fight/flight reactions.

A. 2 Hair

Regions: Root in follicle and a shaft
Structure contains hard keratinized cells that have elastic keratinized threads

Three (3) concentric layers :: medulla, cortex and cuticle

1. Medulla

D. Ceruminous Glands

modified apocrine sweat glands that produce cerumen (ear wax) [see special sensory notes]

E. Mammary Glands

modified sweat glands that secrete milk [see female reproductive notes]

VI. Nails

Epidermal appendage that contains hard keratin

Regioins: Root, Body, Free Edge

Functions: to form a protective covering over the dorsal surface of the terminal phalanges.

Epidermal cells will form a groove and from this groove a nail plate will advance.
A bed of epidermis from the nail matrix will create the nail bed to support the nail plate and will be continuous with the finger tip, distally.
Growth is 0.5mm/week from the nail matrix pink area due to blood flow of the capillaries in the dermis.
Cuticle area are cells from the wall that extends on the nail plate and they will continue laterally as nail folds.