Week 10

• The skin as an endocrine organ is an essential part of the body's endocrine system.
• The skin senses hormones and is a crucial part of the body's hormonal response.
• Skin is an integral part of the body's immune system, protecting against biological pathogens, chemical and mechanical assaults, and UV radiation.
• The skin serves as a barrier to prevent dehydration and protect against environmental hazards.
• The skin plays a role in temperature regulation through sweating, horripilation (goose bumps), and vasodilation/vasoconstriction of blood vessels in skin.
• The skin epidermis consists of keratinocytes (85 - 95%), melanocytes, dendritic cells, langerhan cells, and merkel cells.
• The estimated turnover of the human epidermis is every 40 – 56 days.
• The skin epidermis is composed of five layers: Stratum corneum, Stratum lucidum, Stratum granulosum, Stratum spinosum, and Stratum basale.
• In postmenopausal women, almost all active sex steroids are made in the skin by an intracrine mechanism.
• The hypothalamic-pituitary-adrenal (HPA) axis in skin is organized in functional, cell type-specific regulatory loops, which imitate the signalling structural hierarchy of the central HPA axis.
• Keratinocytes, the most abundant cell type in the skin, express all elements of the HPA axis.
• The skin also expresses all elements of the HPA axis in the hair follicle.
• Skin expresses all elements of the HPA axis at the tissue level in dermal fibroblasts, melanocytes, and other dermal cells.
• The skin is our most important barrier, our largest endocrine organ, expresses almost every hormone receptor, is a key indicator of (endocrine) disease, and synthesizes steroids and many other hormones which can act on single cell, tissue and systemic level.
• The skin imitates the signalling structural hierarchy of the central HPA axis to regulate its homeostasis.
• Cortisol plays an important role in balancing glucose for skin maintenance and regeneration.
• Androgens control hair growth and can lead to androgenetic alopecia (AGA).
• Estrogens influence the loss of estrogen at the menopause on skin homeostasis and wound healing.
• Vitamin D, a secosteroid, plays a role in skin homeostasis and inflammatory skin disorders (atopic dermatitis, psoriasis).
• The dermal sheath consists of three layers: papillary, reticular and dermal white adipocyte tissue (DWAT) that ranges in thickness from 0.6mm in the eyelid to 3mm in the back or palm skin.
• The main component of the dermis is the extracellular matrix (ECM), which includes collagens, proteoglycans and elastins.
• Fibroblasts are the most abundant cells in the dermis, with many different types.
• Immune cells such as macrophages, leucocytes and mast cells are also present in the dermis.
• Blood and lymphatic vessels cells (endothelial cells, smooth muscle cells) are also found in the dermis.
• Cutaneous sensory nerve receptor cells are also present in the dermis.
• Hormone receptors are expressed in skin in different compartments or cell populations for peptide hormones and neurotransmitters, which are mostly aligned on the cell surface, and steroid and thyroid hormones, which are in the cytoplasm or nuclear compartments.
• All endocrine disorders are associated with characteristic dermatosis such as Cushing’s disease (hypercostisolism) which causes skin atrophy and abdominal striae, Addison’s Disease (hypocortisolism) which causes hyperpigmentation, PCOS (polycystic ovary syndrome) which causes male patterned hair growth in women, and Diabetes Mellitus (insulin deficiency/resistance) which causes skin atrophy, peripheral vascular necrosis, bullosis diabeticorum, candida-induced intertrigo, etc.
• Skin presentation in Cushing’s disease includes skin atrophy and abdominal striae.
• Medical uses of hormones on skin include topical steroids (e.g hydrocortisone) for mild/moderate hyperproliferative skin disorders such as psoriasis, dermatitis, eczema, insect stings, discoid lupus erthematosus, and intertrigo.
• Retinoids are used for acne, aging skin (wrinkles), psoriasis, warts.
• Vitamin D is used for vitiligo, scleroderma, psoriasis, actinic keratosis, and lupus vulgaris.
• Skin has the capacity to produce all classes of hormones and is also able to metabolise (activate or inactivate) them in different cell populations, indicating the endocrine autonomy of the skin.
• The molecular signalling modes of hormones in the skin include the Steroid Synthesis Pathway, which involves DHT and Aldosterone.
• Skin is expressing all crucial genes of glucocorticosteroidogenesis (except for aldosterone synthesis) and is emerging as an additional extra-adrenal organ with an endogenous steroidogenic capacity.