1. The skin develops from the embryonic ectoderm and mesoderm. The mesoderm is essential for differentiation of epidermal structures like hair follicles and maintenance of the adult epidermis. 2. During the third week of fetal life, the epidermis consists of a single layer of cells. Between 4-6 weeks, it develops into two layers. By 8-11 weeks, a middle layer forms and the epidermis takes on more differentiated features through the rest of gestation. 3. Hair follicles begin developing between 9-12 weeks, starting as a thickening of basal epidermal cells. Sebaceous glands and arrector pili muscles

1. EMBRYOLOGY OF
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:
Fetus 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):
Mitochondria
Gogli compleses
Few tonofilaments
Numerous microvilli
From this stage on dome shape blebs start to project
from the centre of the periderm.
One or more intermediate layers of cell appear with:

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

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

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

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.
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.
2. The upper becomes the rudiment of the sebaceous
gland.
1. The lower becomes the arrector pili muscle.

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:
No new follicles develop in adult skin.
No large scale destruction of follicles during
postnatal development. Only increase in density as
the body surface increases.
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.

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

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

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.
Merkel Cells: Found in glaborous skin finger tips,
gingival lips, nailbeds and other regions by the 16th
week.
Langerhans cells: Derived from monocyte-macrophage
lineage, enter the epidermis about 12weeks.

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

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

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

20. DERMIS (Continued):
Hemidesmosomes 3rd month
DEJ; Lamina densa 2nd month
In these areas papillary ridges which determine
dermatoglyphics take their place.
Touch pads at fingers and toes reach maximum
development at 15 weeks. After that they flattem and
become indistinct.

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