The Integumentary System
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
The main structural features of the skin epidermis.
Photomicrograph of the four major epidermal layers (200
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.
Most superficial layer; 20–30 layers of dead
cells, essentially flat membranous sacs filled
with keratin. Glycolipids in extracellular space.
Typically five layers of flattened cells,
organelles deteriorating; cytoplasm full of
lamellar granules (release lipids) and
Several layers of keratinocytes unified by
desmosomes. Cells contain thick bundles of
intermediate filaments made of pre-keratin.
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
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-
sid-um; “light”), visible only
in thick skin, is a thin translucent band just above the stratum