1. Skin

2. General outline barrier and protection
skin is the heaviest organ of the human body, it constitutes about 16% of body weight
is composed of epidermis , dermis and its appendages
the function of the skin:
barrier and protection
sensory reception
excretion
body temperature regulation
absorbing some materials

3. Layers of skin
Epidermis – outer layer composed of stratified squamous epithelium.
Dermis – layer mainly composed of dense and loose irregular connective tissue with lots of appendages in it.
*Hypodermis –Loose connective tissue between skin and deeper structures, lots of adipose cells form adipose tissue

4. Epidermis: stratified squamous epithelium of keratinized type
Dermis: dense connective tissue

5. Epidermis Contains many layers of stratified squamous epithelium.
Depending on the thickness of the skin, it is classified as thick or thin skin.
The cells of thick skin are arranged in five layers
There are no blood vessels in the epidermis
Abundant never ending

6. Layers of Thick Skin From basal to surface: Stratum basale
Stratum spinosum
Stratum granulosum
Stratum lucidum
Stratum corneum

7. stratum corneum
stratum lucidum
stratum granulosum
stratum spinosum
stratum basale

8. Stratum basale A layer of cuboidal or columnar cells
Cells are actively undergoing mitosis and push previous cells outward. Keeps constant renewal of EC.
These cells receive nutrients from the blood supply that is found in the dermis next to them.

9. stratum granulosum
stratum spinosum
stratum basale

10. Stratum spinosum
Stratum spinosum are polyhedral cells connected to each other by intercellular bridges (called spinous cells)
A few lamellated granules are found in the cytoplasm, containing glycolipid and steroid

11. Stratum granulosum
Relatively flattened cells, the unclei are undergoing disintegration
Containing irregularly keratohyalin granules which fill the cell destroying nuclei and other organelles.

12. Stratum lucidum
is made up of several layers of homogeneous and translucent cells, the nuclei and organelles of the cells no longer exist.

13. Stratum corneum is the outermost or surface layer.
It constituted of a series of tightly-connected, flattened, cornified cells
Cells are filled with keratin, intercellular spaces are filled of material discharged by lamellated granules
Thickness varies on different parts of body

14. Cell Types in Epidermis
Keratinocytes – responsible for production of keratin- makes skin waterproof
Non-keratinised cells
Melanocytes- responsible for the production of melanin.
Langerhans cells – specialized cells believed to process antigens and deliver to lymph nodes.
Merkel cells – function as mechanoreceptors

15. Melanocyte: pigment-producing cell
large cells with long branched processes, located among the
stratum basale cells
derived from neural crest cell
abundant melanosome, containing tyrosinase which turns
tyrosine into melanin
The melanin granule content of the skin is responsible for skin
color
Melanin granule absorb ultraviolet light, protect the body
against ultraviolet irradiation

18. Langerhans Cell:
dendritic type, located among the spinous cells, presents antigens,
-take up antigens in the skin and transport to lymph nodes
-reservoir for the HIV virus

19. Merkel Cell:
- Have short processes, distributed in the basal cell layer
- Their functional significance is not clear
It is presumed that they are sensitive
Mechanoreceptor because they are in
close relation to nerve endings

20. Dermis: located beneath the epidermis Papillary layer:
a thin layer of LCT which increase the total contact surface between the dermis and epidermis, contain capillary network and nerve ending (tactile corpuscle)
Reticular layer:
under the papillary layer; consists of irregular DCT; contain large blood vessels, appendages of skin, nerves (lamellated corpuscle). The elastic network is responsible for the elasticity of the skin.

21. Papillary layer
Reticular layer
Sweat glands
Lamellated corpuscle

22. Tactile Corpuscle in papillary layer

23. Meissner’s corpuscle

24. Hypodermis (subcutaneous tissue)
A layer of loose connective tissue between the skin and deeper structures
the fat content is more significant in the hypodermis which not only serves as a reservoir of fat but also protects deeper tissue against pressure or impact

25. Skin appendages
hair
sebaceous gland
sweat gland

26. Skin appendages

27. Hair Hair is a thorny thread-like structure.
Hair shaft is the part that extends above the surface and hair root is part that is embedded in the skin
Hair resides in a hair follicle.
Hairs differ in length, thickness and color according to their position on the body

30. Hair root
Hair follicle

31. Hair root
Hair follicle

32. Hair papilla

33. Sebaceous Glands Associated with the hair follicles
Produce oily substance, which is act as a lubricant to make the skin soft and protects both skin and the hair from drying out.
Oil released to outside via the hair follicle. If passage becomes blocked, cause acne.

34. Arrector pili – smooth muscle of hair
this muscle contracts– “goose pimple”

35. Sebaceous gland
Arrector pili
The secretory potions are round sacs, the cells at the periphery are smaller, toward the centre the cells grow large and become mature which fill with fatty droplet

36. Arrector pili

37. Sweat Glands Sweat glands are distributed over most of the body.
Simple tubular glands that transverse from the dermis to the surface of the epidermis.
Merocrine sweat glands are found on most of the body and produce a thin watery solution and function in heat regulation.
Apocrine sweat glands are found in axilla, mammary areolae, and circumanal region produce much thicker secretion that is odor producing. Often become activated at puberty

38. A sweat gland is divided into two parts:
Secretory portion: is composed of pyramidal secretory cells
Duct: lined by two layers of cuboidal epithelial cells
Secretory portion
duct