The skin is divided into two parts: the superficial part, the epidermis; and the deep part, the dermis (Fig. 1.4). The epidermis is a stratified epithelium whose cells become flat tened as they mature and rise to the surface. On the palms of the hands and the soles of the feet, the epidermis is extremely thick, to withstand the wear and tear that occurs in these regions. In other areas of the body, for example, on the ante rior surface of the arm and forearm, it is thin. The dermis is composed of dense connective tissue containing many blood vessels, lymphatic vessels, and nerves. It shows considerable variation in thickness in different parts of the body, tending to be thinner on the anterior than on the posterior surface. It is thinner in women than in men. The dermis of the skin is connected to the underlying deep fascia or bones by the superficial fascia, otherwise known as subcutaneous tissue. The skin over joints always folds in the same place, the SKIN CREASES (Fig. 1.5). At these sites, the skin is thinner than elsewhere and is firmly tethered to underlying struc tures by strong bands of fibrous tissue. The appendages of the skin are the nails, hair follicles, sebaceous glands, and sweat glands. The nails are keratinized plates on the dorsal surfaces of the tips of the fingers and toes. The proximal edge of the plate is the root of the nail (see Fig. 1.5). With the exception of the distal edge of the plate, the nail is surrounded and overlapped by folds of skin known as nail folds. The sur face of skin covered by the nail is the nail bed (see Fig. 1.5). Hairs grow out of follicles, which are invaginations of the epidermis into the dermis (see Fig. 1.4). The folli cles lie obliquely to the skin surface, and their expanded extremities, called hair bulbs, penetrate to the deeper part of the dermis. Each hair bulb is concave at its end, and the

1. EMBRYOLOGYOF
THE SKIN

2. Juxtaposition of two major embryonic elements. Prospective
epidermis from surface area of the early gastrula:
1 Prospective mesoderm, brought into contact with the
undersurface of the inner surface of the epidermis
during gastrulation.
2 Mesoderm is essential for differentiation of epidermal
structures such as hair follicles and maintenance of adult
epidermis.
Neural crest participates by pigment cell small bulk.

3. EPIDERMIS:
i) 3rd week of fetal life:
ingle layer of glycogen filled undifferentiated cells Periderm.
) 4 – 6 weeks old:
etus two layers are present:
A) Periderm or epithilium (purely embryonic structure
ultimately lost in utero).
Stratum germinativum.

4. EPIDERMIS (Continued):
iii) 8 – 11 weeks (crown –rump 26 -50 mm):
a middle layer is formed.Glycogen still abundant in all
layers. Few microvilli appear at the surface.

5. EPIDERMIS (Continued):
iv) 12- 16 weeks (crown –rump 69-102 mm):
ne or more intermediate layers of cell appear with:
Mitochondria
Gogli compleses
Few tonofilaments
Numerous microvilli
From this stage on dome shape blebs start to project from
the centre of the periderm.

6. EPIDERMIS (Continued):
16 – 26 weeks:
ermediate layers increase in number. 23
week:
ratohyaline granules appear in the higher layer 24
week:
vation of periderm cast off in amniotic fluid vernix caseosa.

7. EPIDERMIS (Continued):
Desmosomal proteins at basal layer distinguishable by
the10th week.
al keratins are expressed by 14th week.
agrin by the 15th week.
The periderm participates in the uptake of carbohydrates from
the amniotic fluid.

8. HAIR FOLLICLES & APOCRINE GLANDS:
Earliest appearance 9th week: Eyebrows, upper lip and
chin. Basal keratins are expressed by 14th week.
Crowding of nuclei at basal layer of epidermis hair germ or
Pregerm. This stage rapidly passes into germ stage
characterized by:
1.Basal cells become high.
2.Nuclei elongated
3. Down growth

9. HAIR FOLLICLES & APOCRINE GLANDS
(Continued):
At the same time:
1.Mesenchymal cells and fibroblasts increase in number to
form hair papilla beneath the germ.
2.Outer cells of hair peg become coloumner in shape and
arranged radially to the long axis.
3.Oblique growth downwards with the advancing axis
becoming bulbus gradually enveloping the mesodermal
papilla.

10. HAIR FOLLICLES & APOCRINE GLANDS
(Continued):
At this stage( Hair peg ) Two epithelial swellings at the
posterior wall of the follicle appear.
1. The lower becomes the arrector pili muscle.
2.The upper becomes the rudiment of the sebaceous gland.
In many hair follicles a third pulp appear above the
sebaceous rudiment to form the apocrine gland found in scalp
,face chest, abdomen, legs, axillae, mons pubis, external
auditary meatus, eyelids.

11. HAIR FOLLICLES & APOCRINE GLANDS
(Continued):
As peg grows:
Cells of inner root sheath develop above the matrix.
Matrix grows down.
Inner cells grow upwards to form the hair canal.

12. HAIR FOLLICLES & APOCRINE GLANDS
(Continued):
Hair follicles are arranged in a specific pattern:
Groups of three at fixed interval 274-350
micrometer. As skin grows groups become separated
and new rudiments appear at a critical distance
dependant on the region of the body.
No large scale destruction of follicles during postnatal
development. Only increase in density as the body
surface increases.
No new follicles develop in adult skin.

13. SEBACEOUS GLANDS:
Solid hemispherical protuberance on the posterior
surface of the hair peg with moderate amount of
glycogen.
Cells in the centre soon accumulate droplets of fat.
SG become differentiated at 13—15 Gland is large and
functional------sebum and vernix caseosa
After birth size rapidly reduced.
Large and functional again at adult life.

14. ECCRINE GLANDS:
Start to develop about three months. Palms and soles
initially.
Rudiments identifiable as regularly spaced undulations of
stratum germinativum.
Cells are oblong palisading and lying close together.
14th to 15th week tips penetrate deeply into the dermis, while
in the epidermis coloumns of distinct cylindrical layer
elongated and curved pass outwards.
Intraepidermal ducts form by coalescence of groups of
intracytoplasmic Cavities.
Lumens form by dissolution of dismosomes.

15. NAILS:
Develop in the third month.
16—18 week (Cr R 120—150 mm) keratinizing cells
from both dorsal and ventral matrixes can be
distinguished.

16. MELANOCYTES:
Develop from the neural crest. Lose themselves in the
mesenchyme before 4 - 6 month of gestation and travel to
the basal layer. Some may get arrested in the dermis.
Langerhans cells: Derived from monocyte-macrophage
lineage, enter the epidermis about 12weeks.
Merkel Cells: Found in glaborous skin finger tips,
gingival lips, nailbeds and other regions by the 16th
week.

17. DERMIS:
Origin: Ventrolateral part of the somaite dermatome,
however; Most of the dermis come from cells
migrating from other parts of the mesenchyme.
Blood vessels, connective tissue, fibroblasts, mastcells
and fat cells also arise from mesonchyme.
Embryonic dermis is at first very celluar and up to 2nd
month is indistinguishable from subcutis.

18. DERMIS (Continued):
Soon regular bundles of collagen appear at the end of the
third month.
By the 5th month papillary and reticular dermis
become distinct. Hair root sheath appear.
By the 22nd week Elastic fibres and islands

19. DERMIS (Continued):
Cells of the dermis:
14th weeks: three types of cells, stallate, macrophage and
granular secretory (melanoblasts or mast cells).
14 - 21 Weeks: Numerous fibroblasts, perineural cells,
pericytes, merkel ,mast cells, langerhans and histiocyte.
Undersurface is smooth at first, At 4 month with the
appearance of hair follicles becomes irregular.

20. DERMIS (Continued):
Touch pads at fingers and toes reach maximum
development at 15 weeks. After that they flattem and
become indistinct.
In these areas papillary ridges which determine
dermatoglyphics take their place.