BIOL 4000 - LECTURE NOTES 11
TEXT - POWERPOINT
stratified squamous keratinized epithelium
stratum basale = stratum germinativum
Golgi vesicles (stages I, II, III, IV)
I. General considerations:
A. Skin is an organ that fulfills and/or
mediates many different functions. (i.e. excretion, temperature control,
communication with external environment, protection (insulation and protection
from abrasion and some dissolved substances), retards dehydration.)
B. It is the heaviest single organ in body (16%
C. Consists of two major tissue layers
1. epidermis (ectodermally derived) - stratified squamous
2. dermis (mesodermally derived) - connective tissue
D. Thickness and structure of these layers
varies according to whether we are dealing with thick or thin skin.
1. Most of the body is covered with thin skin.
2. Parts of the body that are subject to
frequent abrasion such as the palms of the hands and soles of the feet are
covered with thick skin.
E. Where epidermis and dermis meet, both are
thrown into a series of folds that interdigitate.
1. The dermal folds are called dermal papillae.
2. Epidermal folds are called epidermal
F. We can also consider a third layer that
is NOT actually part of the skin,
but that binds dermis to underlying tissues. This is the subcutaneous layer or hypodermis.
STRUCTURE AND FUNCTION OF SKIN
I. Epidermis - keratinized stratified
A. There are 4 cell types
1. Keratinocytes (skin cells that form the keratinized stratified
2. Melanocytes - pigment producing cells
3. Langerhans cells - found in the stratum spinosum, may have
4. Merkel cells - present in epidermis of thick skin of palms
and soles and may have sensory and/or hormonal function.
B. First and most abundant cell type - cells of the
keratinized stratified squamous epithelium.
1. These cells are called keratinocytes.
2. They form a protective outer layer that is
continuously shed and replaced from below.
3. The keratinized stratified squamous epithelium
can be divided into a series of sub-layers.
Sub-layers of the epidermis. Starting at
layer next to dermis.
a. Stratum basale = stratum germinativum
c. Stratum granulosum
d. Stratum lucidum
e. Stratum corneum
f. In thick skin such as palms of hands and
soles of feet, all layers can be easily identified. In thin skin that covers
rest of body the stratum lucidum may not be evident.
4. Stratum basale = stratum germinativum
a. A basophilic cuboidal to columnar epithelium
which lies on basement membrane
that attaches it to the underlying dermis.
b. Desmosomes bind the lateral surfaces of these cells so
they adhere strongly to each other.
c. Hemi-desmosomes are found in the basal cell membranes and are
involved in binding cells to the basal lamina.
e. There is lots of mitosis occurring in this
layer since it is responsible for providing cells to replace those shed as the
upper layer of keratinocytes is worn away.
f. These cells synthesize small amounts of keratin that is stored as intermediate filaments in their cytoplasm.
5. Stratum spinosum
a. Consists of cuboidal, polygonal, or slightly
flattened cells which have a plasmalemma that is thrown into a series of many
b. These processes are filled with intermediate
filaments and terminate with desmosomes formed between processes of adjacent cells or
with the apical survacees of cells in the stratum basale.
c. The processes of these cells give them a
spiny appearance when viewed with the light microscope. Thus the name "spinosum". Also sometimes called the "prickly" layer.
d. These interdigitating, tightly bound
processes are thought to give the skin some flexibility and thus impart
resistance to abrasion to the epidermis. This is supported by the fact that the
stratum spinosum is thicker in portions of skin subject to frequent abrasion
such as palms and soles.
e. The basal portion of the spinosum, directly
next to the stratum basale, also contains mitosing cells. This germinative area
is called the malpighian layer.
6. Stratum granulosum
a. This layer is sometimes difficult to discern
in thin skin
b. It consists of 3-5 layers of flattened
polygonal cells that are characterized by two types of non-membrane bound
* keratohyaline granules - contain a histidine rich protein.
** These granules are membrane bound and formed
by the golgi bodies.
**They move to the plasmalemma closest to the
external environment and are exocytosed into the intercellular space.
**The contents of these granules are glycosaminoglycans and phospholipids.
**These two substances help to cement cells
together and also act as a barrier to penetration of the skin by foreign
materials. (i.e. they seal the
skin and decrease its permeability).
7. Stratum lucidum
a. This layer is most apparent in thick skin.
b. It consists of a thin layer of extremely
c. Organelles and nuclei are no longer
evident in the cytoplasm.
d. The cytoplasm is rich in densely packed
keratin filaments in an
electron-dense matrix called eleidin.
8. Stratum corneum
a. This is the outermost layer of the skin.
b. Consists of flattened, non-nucleated, dead,
c. The cytoplasm of these cells is filled with
keratin filaments embedded in matrix.
d. These keratinized cells are called horny
e. The stratum corneum is very thick in areas
subject to frequent abrasion such as the palms and soles (e.g. 3mm on soles).
In thin skin, this layer is relatively thin and delicate.
9. Since the upper layer of skin is heavily
keratinized and the lower layer (stratum germinativum) is not, it is clear that
as cells of the epidermis progress toward the external surface, one of the
things that is happening is that higher and higher concentrations of keratin
are being synthesized and deposited in the cytoplasm.
10. Also, since cells of the stratum corneum
lack organelles, something is occurring to rid the cells of these structures.
a. So, what we find is that as keratinization
proceeds, more and more autophagosomes and lysosomes are present in the cytoplasm of epidermal cells.
b. The autophagosomes are responsible for engulfing
and degrading cytoplasmic organelles.
C. Second skin cell type - melanocytes - pigment producing cells
1. Melanocytes are derived from neural crest
cells (which you'll recall are
also involved in the formation of the adrenal medulla).
2. Cell bodies of melanocytes are found between
or beneath those of the stratum basale and stratum spinosum.
3. They extend cytoplasmic processes between
cells of these two layers. These processes end in invaginations in the
plasmalemma of the cells of the stratum spinosum and stratum basale.
4. Melanin is synthesized by melanocytes via the
action of an enzyme called tyrosinase on the amino acid tyrosine.
5. Melanin synthesis occurs in vesicles formed
by the Golgi body and consists of 4 stages.
a. Tyrosinase precursors are synthesized on
ribosomes of endoplasmic reticulum and accumulate in vesicles formed by the
* Stage I vesicle - slight tyrosinase activity, so either the
golgi packaged precursor is undergoing further processing, or tyrosinase in
being added to Golgi vesicles as these vesicles fuse with each other.
* Stage II vesicle - vesicle contains intermediate filaments which
melanin is deposited on.
* Stage III vesicle - concentration of melanin increases obscuring
* Stage IV vesicle - mature melanin granule, no filaments visible.
6. These vesicles are transported into the
processes of the melanocytes and are transferred to epidermal cells in the
stratum basale and stratum spinosum.
7. Pigmentation of the skin is due to
accumulations of melanin in epidermal cells rather than in the melanocytes.
Darker skin is due to greater accumulation of melanin in these cells and not to
a change in the number of melanocytes.
8. Thus, differences in skin color in people of
the same race and of different races are due to differences in the
concentration of melanin granules in epidermal cells and not to differences in
the number of melanocytes present.
9. As skin cells mature and become keratinized,
melanin is enzymatically broken down such that dead cells of the stratum
corneum have lost their pigmentation by time they are shed.
D. Third skin cell type - Langerhans cells
1. These cells are found in the stratum spinosum
2. They may have an immune function and are a
specialized type of macrophage
that "entraps" foreign antigens that enter the skin.
E. Fourth type of skin cell - Merkel cells
1. Present in hair follicles, thick skin of
palms and soles, and oral mucous membrane
2. Large, oval, poorly stained
3. Unmyelinated axons penetrate the stratum
basale and terminate as expanded disks on the base of Merkel cells - suggests sensory
4. Granules resembling neurosecretory vesicles
sugget a possible hormonal function
for these cells.
A. This is the connective tissue that supports
epidermis and binds it to subjacent tissue
B. The dermis is composed of two layers:
1. Papillary layer
2. Reticular layer
C. Papillary layer
1. Composed of loose connective tissue, mostly fibroblasts, but also mast cells,
macrophages, and extravasated leukocytes.
2. This portion of the dermis interdigitates
with the epidermis forming folds of the dermis that are called dermal
a. Most papillae contain a capillary bed that
provides nutrients to overlying and surrounding cells.
b. Some palillae are "tactile
papillae" and house sensory structures for touch called Meissner's
3. From this layer, collagen fibrils penetrate
the dermal papillae and insert into the basal lamina of the epidermis, just
beneath the stratum germinativum.
a. This binds the two layers together.
b. The collagen fibrils are called anchoring
D. Reticular layer
1. This layer is thicker than the papillary
layer and is composed of dense irregular connective tissue that contains
collagen and elastic fibers.
a. Fibers are the major component.
b. There are fewer cells in the reticular layer
than in the papillary layer.
2. Thick elastic fibers emerge from this layer
and penetrate the papillary layer.
a. These thick fibers end in the fine
fibrotubular component of the elastic fibers.
b. As is the case with the anchoring fibers
mentioned above, the ends of these thick fibers penetrate the papillary layer
and insert on the basal lamina below the stratum basale.
c. This elastic network is responsible for the
elasticity of the skin.
3. The functional cells of hair follicles,
sweat glands, and oil glands are found in the reticular layer of the dermis. Hair follicles may be considered specializations
of the epidermis that penetrate into the reticular layer of the dermis.
E. While the epidermis contains no blood or
lymph vessels, the dermis contains many.
IV. Underlying the dermis is a fatty,
loose connective tissue layer called the hypodermis.
V. Appendages of the skin
A. There are a number of structures that can be
defined as appendages of the skin:
2. Finger and toe nails
3. Sebaceous glands
4. Sweat glands
B. Hair follicles
1. The hair follicle is a deep invagination of
the epidermis that involves the stratum basale and stratum spinosum.
2. The hair itself is essentially a specialized
form of the keratinized layers of the epidermis.
3. The hair follicle extends deep into the
reticular layer of the dermis and terminates at a vascularized "dermal
* Note that the dermal papilla is close to the
base of the reticular layer and is different from the "dermal
papilla" that forms as an interdigitation with the epidermis in the
papillary layer of the dermis.
* The basal region of the follicle, including
the dermal papilla, is called the bulb.
* The epidermal cells covering the dermal
papilla in the lower half of the bulb are called matrix cells and are responsible for producing the shaft of
2. The hair shaft itself, consists of 3 regions,
* 2-3 layers of vacuolated cuboidal cells
* present near base of hair, doe not extend
through entire shaft
b. The Cortex
* several layers of elongated, heavily keratinized
* Melanin is sequestered between or within these
* These cells become more flattened as you move
toward the tip of the hair.
c. The Hair Cuticle
* a layer of clear, flat, enucleate, cells
* these cells overlap one and other in a scale
3. In the root of the follicle the cuticle is
surrounded by an inner sheath consisting of 3 layers. These are,
a. The innermost cuticle - innermost layer
* Flat overlapping cells that abut the cuticle
of the hair shaft
* The edges of the cells of the innermost zone
interdigitate with those of the hair cuticle forming a robust bond between the
hair and the follicle wall.
b. Huxley's layer - the middle layer
* Several layers of elongated cells
* Contain trichohyalin granules (a type of kerotohyalin granule)
c. Henle's layer - the outermost layer
* Single layer of flat, clear cells that contain
4. Henle's layer is surrounded by the outer
(or external) root sheath.
* This is continuous with the stratum spinosum
and stratum basale
* A thin, hyalin, modified basement membrane
called the glassy membrane,
surrounds the outer root sheath
* Outside the glassy membrne is a poorly
defined, connective tissue investment.
READ ABOUT SWEAT GLANDS, AND
SEBACEOUS GLANDS IN YOUR TEXT.