By: Dr. Mujahid Khan

The skin is a membranous protective covering of the body

Is a complex organ system

It consists of two layers derived from two different germ layers

Ectoderm and mesoderm

The epidermis is a superficial epithelial tissue

It is derived from surface ectoderm

The dermis is a deeper layer composed of dense irregularly arranged connective tissue

It is derived from mesoderm

The meshwork of embryonic connective tissue derived from mesoderm

It forms the connective tissues in the dermis

Skin structures vary from one part of the body to another

The skin of eyelids is thin and soft and has fine hairs

The skin of eyebrows is thick and has coarse hairs

The embryonic skin at 4-5 weeks consists of a single layer of surface ectoderm overlying the mesenchyme

During the first and second trimester, epidermal growth occurs in stages and result in an increase in epidermal thickness

Primordial of the epidermis is the layer of surface ectodermal cells

These cells proliferate and form a layer of squamous epithelium called periderm and a basal germinative layer

The cells of periderm continually undergo keratinization and desquamation and are replaced by cells arising from basal layer

The exfoliated peridermal cells form part of the white greasy substance called vernix caseosa which covers the fetal skin

Vernix caseosa protects the developing skin from constant exposure to amniotic fluid with its urine content during the fetal period

Vernix also facilitates birth of the fetus because of its slippery nature

The basal layer of the epidermis becomes the stratum germinativum

It produces new cells that are displaced into the layer superficial to it

By 11 weeks, the cells from stratum germinativum have formed an intermediate layer

Replacement of peridermal cells continues until about 21 weeks

Thereafter the periderm disappears and the stratum corneum forms

Proliferation of cells in the stratum germinativum also forms epidermal ridges which extend into the developing dermis

Epidermal ridges produce grooves on the surface of the palms and soles including digits

The type of pattern is determined genetically and constitute the basis for examining fingerprints

Abnormal chromosome complements affect the development of ridge patterns

Infants with Down syndrome have distinctive patterns on their hands and feet that are of diagnostic value

Late in the embryonic period the neural crest cells migrate into the mesenchyme of the developing dermis and differentiate into melanoblasts

Later these cells migrate to the dermoepidermal junction and differentiate into melanocytes

The differentiation of melanoblasts into melanocytes involves the formation of pigment granules

Melanocytes appear in the developing skin at 40 – 50 days, immediately after the migration of neural crest cells

In white races, the cell bodies of melanocytes are usually confined to basal layers of the epidermis

Melanocytes begin producing melanin before birth and distribute it to the epidermal cells

Pigment formation can be observed prenatally in the epidermis of dark-skinned races

There is a little evidence of such activity in light-skinned fetuses

Increased amount of melanin are produced in response to ultraviolet light

The relative content of melanin in the melanocytes accounts for the different colors of skin

It covers the palms and soles and lacks

Hair follicles

Arrector muscles

Sebaceous glands

But it has sweat glands

It covers most of the rest of the body and contains

Hair follicles

Arrector muscles of hairs

Sebaceous glands

Sweat glands

The dermis develops from mesenchyme, derived from the mesoderm underlying the surface ectoderm

Most of the mesenchyme that differentiates into the connective tissue of the dermis originates from the somatic layer of lateral mesoderm

Some of it is derived from the dermatomes of the somites

By 11 weeks the mesenchymal cells have begun to produce collagenous and elastic connective tissue fibers

As the epidermal ridges form, the dermis projects into the epidermis forming dermal ridges that interdigitate with the epidermal ridges

Capillary loops develop in some of these ridges and provide nourishment for the epidermis

Sensory nerve endings form in other ridges

The developing afferent nerve fibers play an important role in the spatial and temporal sequence of dermal ridge formation

Blood vessels in the dermis begin as simple, endothelium-lined structures that differentiate from mesenchyme

As the skin grow, new capillaries grow out from the primordial vessels

Such capillary-like vessels have been observed in the dermis by the end of 5th week

Some capillaries acquire muscular coats through differentiation of myoblasts developing in the surrounding mesenchyme and become arterioles and arteries

Others become venules and veins

By the end of 1st trimester, the major vascular organization of the fetal dermis is established

Most sebaceous glands develop as buds from the sides of developing epithelial root sheaths of hair follicles

The glandular buds grow into the surrounding embryonic connective tissue and branch to form the primordia of several alveoli and their associated ducts

The central cells of alveoli break down forming an oily secretion called sebum

Sebum is released into the hair follicle and passes to the surface of the skin

In skin it mixes with desquamated peridermal cells to form vernix caseosa

Sebaceous glands independent of hair follicles develop in a similar manner to buds from the epidermis

Eccrine sweat glands are located in the skin throughout most of the body

They develop as epidermal down growths into the underlying mesenchyme

As bud elongates, its end coils to form the primordium of the secretory part of the gland

The epithelial attachment of the developing gland to the epidermis forms the primordium of the duct

The central cells of the primordial ducts degenerate, forming the lumen

Peripheral cells of the secretory part of the gland differentiate into myoepithelial and secretory cells

Myoepithelial cells are thought to be specialized smooth muscle cells that assist to expel sweat from the glands

Eccrine sweat glands begin to function shortly after birth

The distribution of large apocrine sweat glands in humans is mostly confined to axilla, pubic and perineal regions and areola of the nipples

They develop from the stratum germinativum of the epidermis that give rise to hair follicles

The ducts of these glands do not open into the skin surface

They open into the upper part of hair follicles superficial to the opening of sebaceous glands

They secrete during puberty

They are a modified and highly specialized type of sweat glands

Mammary buds begin to develop during the 6th week as a solid downgrowths of the epidermis into the underlying mesenchyme

These changes occur due to the inductive influence from the mesenchyme

The mammary buds develop as downgrowths from thickened mammary crest or ridges

Mammary crest are thickened strips of ectoderm extending from the axillary to the inguinal regions

The mammary crests appear during the 4th week but normally persist in humans only in pectoral area

Each primary bud soon gives rise to several secondary mammary buds that develop into lactiferous ducts and their branches

Canalization of these buds is induced by placental sex hormones entering the fetal circulation

By the term, 15 to 20 lactiferous ducts are formed

The fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme

During the late fetal period the epidermis at the site of origin of the mammary gland becomes depressed forming a shallow pit

The nipples are poorly formed and depressed in newborn infants

Soon after birth the nipples usually rise from the mammary pits

The smooth muscle fibers of the nipple and areola differentiate from surrounding mesenchymal cells

The rudimentary mammary glands of newborn males and females are identical

Some secretion, often called witch’s milk may be produced

This is caused by maternal hormones passing through the placental membrane into the fetal circulation

Only main lactiferous ducts are formed at birth and mammary glands remain underdeveloped until puberty

In females the breasts enlarge rapidly during puberty due to development of mammary glands and accumulation of fat

Growth of the duct system also occurs because of the raised levels of circulating estrogens

Progestogens, prolactin, corticoids and growth hormone also play a role

If pregnancy occurs, the mammary glands complete their development owing to the raised estrogen and progesterone level

The intralobular ducts undergo rapid development forming buds that become alveoli

The breasts become hemispherical in shape largely because of fat deposition

Full development occurs at about 20 years

It refers to excessive development of the male mammary tissue

The rudimentary mammary glands in males normally undergo no postnatal development

It occurs in most newborn males because of stimulation of the glandular tissue by maternal sex hormones

The effect disappears in few weeks

During midpuberty about two-thirds of boys develop varying degree of hyperplasia of breasts

The subareolar hyperplasia may persist for a few months to 2 years

A decreased ratio of testosterone to estradiol is found in boys with gynecomastia

About 80% of males with Klinefelter syndrome have gynecomastia

An extra nipple occurs in about 1% of females

It usually develops just inferior to the normal breast

It is also relatively common in males

Often they are mistaken for moles

Less commonly they appear in the axillary or abdominal regions of females

In these positions the nipples develop from extra mammary buds that develop along the mammary crests

It usually becomes obvious in women when pregnancy occurs

About one-third of affected persons have two extra nipples

Supernumerary mammary tissue very rarely occurs in a location other than along the course of the mammary crests

It probably develops from tissue that was displaced from these crests

Sometimes nipples fail to elevate above the skin surface

They remain in their newborn location

Inverted nipples may make breast feeding of an infant difficult

A special exercise can be used to prepare the nipple for feeding an infant