Integumentary System
SkinThe skin has a dual origin
A superficial layer, the epidermis, develops from the surface ectoderm deep layer, the dermis, develops from the underlying mesenchyme.
Epidermis
Epidermis
The basal layer (germinative layer)
is responsible for production of new cells. This layer later forms ridges and hollows, which are reflected on the surface of the skin in the fingerprint.
spinous layer
thick , consists of large polyhedral cells containing fine tonofibrils.
The granular layer
contains small keratohyalin granules in its cells.
is made up of closely packed dead cells containing keratin.
Cells of the periderm
are usually cast off during the second part of intrauterine life and can be found in the amniotic fluid.
Melanocytes
During the first 3 months of development, the epidermis is invaded by cells arising from the neural crest.
Clinical Correlates
Pigmentary DisordersDiseases of melanocyte function include the various forms of albinism
Vitiligo
Fingerprints
Dermis
Dermis is derived from mesenchyme that has three sources
(a) lateral plate mesoderm supplying cells for dermis in the limbs and body wall, (b) paraxial mesoderm supplying cells for dermis in the back, and (c) neural crest cells supplying cells for dermis in the face and neck.
Dermis
vernix caseosa
At birth, the skin is covered by a whitish paste, the vernix caseosa, formed by secretions from sebaceous glands and degenerated epidermal cells and hairs. It protects the skin against the macerating action of amniotic fluid.
Hair
solid epidermal proliferations from the germinative layer that penetrates the underlying dermis.
hair papillae
At their terminal ends, hair buds invaginate. The invaginations, the hair papillae, are rapidly filled with mesoderm in which vessels and nerve endings develop
The hair shaft the hair sheath
Cells in the center of the hair buds become spindle-shaped and keratinized, forming the hair shaft, while peripheral cells become cuboidal, giving rise to the epithelial hair sheath
The dermal root sheath arrector pili muscle
Lanugo hair
Sebaceous glands
Mammary Glands
The mammary line or mammary ridge
Here it forms 16 to 24 sprouts, which in turn give rise to small, solid buds. By the end of prenatal life, the epithelial sprouts are canalized and form the lactiferous ducts, and the buds form small ducts and alveoli of the gland. .
Initially, the lactiferous ducts open into a small epithelial pit Shortly after birth, this pit is transformed into the nipple by proliferation of the underlying mesenchyme
Clinical Correlates
Mammary Gland Abnormalities
Polythelia
is a condition where accessory nipples have formed due to the persistence of fragments of the mammary line. Accessory nipples may develop anywhere along the original mammary line, but usually appear in the axillary region.
Mammary Gland Abnormalities
Polymastia occurs when a remnant of the mammary line develops into a complete breast.
Inverted nipple is a condition in which the lactiferous ducts open into the original epithelial pit that has failed to evert.
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