Chapter 5
The Integumentary System
153
5
granulosum. Considered by some to be a subdivision of the su-
perficial stratum corneum, it consists of two or three rows of
clear, flat, dead keratinocytes with indistinct boundaries. Here,
or in the stratum corneum above, the gummy substance of the
keratohyaline granules clings to the keratin filaments in the
cells, causing them to aggregate in large, cable-like, parallel ar-
rays of intermediate filaments called
tonofilaments
.
Figure 5.2
The main structural features of the skin epidermis.
(a)
Photomicrograph of the four major epidermal layers (200
3
).
(b)
Diagram
showing these four layers and the distribution of different cell types. The four cell
types are keratinocytes (orange), melanocytes (gray), dendritic cells (purple), and
tactile cells (blue). A sensory nerve ending (yellow), extending from the dermis
(pink), is shown associated with the tactile cell forming a tactile disc (touch
receptor). Notice that numerous desmosomes join the keratinocytes together.
The stratum lucidum, found only in thick skin, is not illustrated here.
Melanocyte
Melanin
granule
Tactile
(Merkel)
cell
Sensory
nerve
ending
Dendritic cell
Dermis
Dermis
Keratinocytes
Desmosomes
(b)
(a)
Stratum corneum
Most superficial layer; 20–30 layers of dead
cells, essentially flat membranous sacs filled
with keratin. Glycolipids in extracellular space.
Stratum granulosum
Typically five layers of flattened cells,
organelles deteriorating; cytoplasm full of
lamellar granules (release lipids) and
keratohyaline granules.
Stratum spinosum
Several layers of keratinocytes unified by
desmosomes. Cells contain thick bundles of
intermediate filaments made of pre-keratin.
Stratum basale
Deepest epidermal layer; one row of actively
mitotic stem cells; some newly formed cells
become part of the more superficial layers.
See occasional melanocytes and dendritic
cells.
too far from the dermal capillaries and the glycolipids coating
their external surfaces cut them off from nutrients, so they die.
Tis is a normal sequence of events.
Stratum Lucidum (Clear Layer)
Trough the light micro-
scope, the
stratum lucidum
(loo
9
sid-um; “light”), visible only
in thick skin, is a thin translucent band just above the stratum
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