Definitive skin is divided into an outer layer, the epidermis, and the adjacent deep layer, the dermis. The epidermis is derived from the single cell layer of ectoderm on the surface. By the end of the second month the ectoderm (basal layer) produces at the surface a layer of flattened cells called the periderm. During the third month the basal layer also forms an intermediate layer. Four definitive layers of the epidermis can be identified by the fourth month and are produced by the basal and intermediate layers. From deep to superficial they are the stratum basal, stratum spinosum, stratum granulosum and stratum corneum. Neural crest cells are thought to invade the epidermis during the third month and give rise to the melanoblasts that produce melanin (skin pigment). The periderm is shed during the second half of pregnancy when the epidermis produces a protective, white, cheesy substance called vernix caseosa
The dermis (corium) develops from the underlying mesenchymal tissue that is probably derived for the most part from dermatomes. Connective tissue fibers differentiate in the mesenchymal tissue during the third and fourth month. The superficial layer or stratum papillare extends into the epidermis as dermal papillae that contain capillaries and sensory receptors. The deep layer, stratum reticulare, is composed primarily of connective tissue in which fat accumulates before term.
NAILS
Each nail on the fingers and toes begins during the third month as an epidermal thickening at the tip of the digit called the nail field. As the digits lengthen, the nail field migrates to the dorsal side bringing along the palmar or plantar nerves. An epithelial nail fold develops at the periphery of the nail field and extends proximally beneath the epidermis to form the nail matrix. During the fifth month the nail matrix produces special, keratinized cells that compact to form the nail plate. As the nail plate is produced, it moves distally over the nail bed reaching the tip of the digit by term.
HAIR
Fine fetal hair (lanugo) begins to develop during the early part of the third month beginning in the face. By the end of the third month it is scattered over much of the body. Fetal hair is shed around term and is replaced by coarser infant hair.
Each hair begins as a bud or cylindrical downgrowth of the basal layer of the epidermis into the underlying dermis. Each bud forms a hair follicle, the growing tip of which is invaginated by mesenchymal tissue. The mesenchyme produces a papilla containing capillaries and nerve terminals. The cells in the center of the follicle become keratinized forming the hair shaft. The peripheral cells give rise to the epidermal sheaths. Sebaceous glands arise as outpocketings of the peripheral follicle cells. The cells in the center of such outpocketing break down to produce sebum that is secreted into the follicle.
Sweat glands begin similar to hair but their downgrowth is not invaginated with mesenchymal tissue. Later they become coiled and canalized.
MAMMARY GLAND
During the fetal period (third month to term) the mammary gland primordium (primary bud) in both sexes penetrates the underlying tissue where it gives rise to 16 to 24 sprouts. Near term each sprout canalizes and becomes a lactiferous duct. Tertiary sprouts arise from the lactiferous duct and invade the underlying fat to become the ducts and alveoli of the gland.
Initially the lactiferous ducts open onto the surface into a depression called the mammary pit. After birth the surrounding tissue proliferates to produce the nipple.
TEETH
The labiodental lamina produces two additional laminae: an outer labial lamina that invades the maxillary and mandibular processes to form the labiogingival groove and an inner dental lamina that gives rise to buds called enamel organs. The labiogingival groove will enlarge to become the vestibule of the mouth between the lips and the jaws. Ten enamel organs are produced in each jaw and represent the primordia of the deciduous teeth.
During the third month the deep surface of each enamel organ is invaginated by the underlying mesenchyme. As a result of the invagination, the enamel organ becomes bell shaped and differentiates into three layers, an outer and an inner enamel epithelium separated by the enamel reticulum. The invaginated mesenchyme proliferates to form a condensed mass called the dental papilla.
The inner enamel epithelium gives rise to ameloblasts that produce enamel. After the tooth erupts, the enamel organ is shed. The cells of the dental papilla adjacent to the inner enamel epithelium become odontoblasts that form dentine deep to the enamel. The odontoblasts persist throughout the life of the tooth. Blood vessels and nerves invade the remaining part of the dental papilla forming the pulp.
By the end of the third month the enamel organs for the permanent teeth begin to develop on the lingual side of the deciduous teeth. The deciduous teeth start to erupt approximately 6 months after birth; the permanent teeth begin to erupt approximately 6 years after birth.
Source: Atlas of Human Embryos.