Embryology: Development of Skin - 2024 PDF

XY2141 Embryology. Development of integumentary system Dr Viktoriia Yerokhina, Lecturer in Medical Sciences [email protected]  EMBR.06 - Development of integumentary and musculoskeletal system Learning EMBR.06.01 - Describe the cell types of the outcomes epi...

XY2141 Embryology. Development of integumentary system Dr Viktoriia Yerokhina, Lecturer in Medical Sciences [email protected]  EMBR.06 - Development of integumentary and musculoskeletal system Learning EMBR.06.01 - Describe the cell types of the outcomes epidermis and their developmental origins EMBR.06.02 - Development of the epidermis EMBR.06.03 - Describe the development of the dermis EMBR.06.04 - Describe the structures of the skin that form from ectoderm EMBR.06.05 - Describe the organization of the somites. 2 Highlights Skin is the largest organ in the body. Two layers of skin: 1) epidermis 2) dermis. Epidermis is derived from surface ectoderm. Dermis is formed by mesenchyme derived from dermatomes of somites. Nails develop from ectoderm at the tip of each digit. Later this ectoderm migrates to the dorsal aspect. Hairs are derived from surface ectoderm which is modified to form hair follicles. Sebaceous glands (ectoderm) arise as diverticula from hair follicles. Sweat glands develop as downgrowths from the epidermis that are later canalized. Mammary glands arise from surface ectoderm along a milk line extending from axilla to the inguinal region. 3  Overview: sources of skin development Skin is derived from three diverse components: 1. Surface ectoderm, 2. Underlying mesoderm, 3. Neural crest cells Presentation Title 4  Epidermis Epidermis is derived from the surface ectoderm. Epidermis, at first, a single layered. During 2nd month, it presents two layers: 1) superficial layer of flat cells - periderm/epitrichium 2) deep layer of cuboidal cells - basal/germinative layer. Later the cells of basal layer proliferate to form a third intermediate layer. Basal layer is known as stratum germinativum as these cells proliferate to form the various layers of epidermis. 5  Epidermis During 3rd to 5th month because of proliferation of cells, the epidermis becomes typical stratified squamous epithelium consisting of five layers or strata: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum. Up to the end of 5th month, there will be continuous keratinisation, desquamation and replacement of peridermal cells by those arising from basal layer. Periderm layer gradually sloughed into the amniotic fluid. Periderm shed off completely by the 21st week. Presentation Title 6  Vernix caseosa Many of the superficial layers of epidermis are shed off. These get mixed up with secretions of sebaceous glands and hair to form a whitish sticky substance (vernix caseosa) which covers the skin of the newborn infant. Vernix caseosa protects the skin from maceration by amniotic fluid. 7 Formation of epidermal ridges By 11th week, cells of the stratum basale proliferate and extend into the dermis to form epidermal ridges (become permanent by 18th week). Epidermal ridges form a specific pattern of fingerprints (also in the palm and sole) that is genetically determined. At birth, all the layers of the adult epidermis are present. Four basic types of dermatoglyphic patterns: Arch Whorl Loop Composite 8  Clinical correlation Fingerprints form the basis for many studies in medical genetics and criminal investigations (dermatoglyphics). In children with chromosomal abnormalities, the epidermal pattern on the hand and fingers is sometimes used as a diagnostic tool. Dr. Harrold Cummins (1893 –1976) achieved world recognition as the "Father of Dermatoglyphics" or the scientific study of skin ridge patterns found on the palms of human hands. 9 Melanoblasts Differentiated from neural crest cells migrate from the neural tube through skin dermis to epidermis and hair follicles. Invade the epidermis during 3rd month. In epidermis, melanoblasts differentiate into melanocyte precursors which resides in the basal layer of the epidermis. In hair follicles, the melanoblasts differentiate into melanocyte stem cells (McSCs) which are homed to the hair follicle bulge, the niche where hair follicle stem cells (HFSCs) reside. Melanocyte precursors and McSCs differentiate into mature melanocytes in the skin epidermis and hair matrix. Presentation Title 10  Melanoblasts Each mature melanocyte extends its dendrites and targets a defined population of epidermal or hair follicular keratinocytes at a ratio about 1:36 to create a pigmentary unit. Melanin is produced before birth and transported by melanocytes to the surrounding keratinocytes, leading to the formation of pigmented hair and skin. Melanoblast, McSCs and melanocyte express specific genes as shown in the figure. Red arrows indicate the developmental process of the melanocyte lineage. E, embryonic day; Epi, epidermis; HF, hair follicle; HM, hair matrix; ORS, outer root sheath; P, postnatal day; Presentation Title SG, sebaceous gland 11 Clinical correlation Albinism: clinical condition characterised by reduced or absence of pigmentation in the skin, hair, and eyes (retina lacks pigment; however, iris usually shows some pigmentation). In most cases, it occurs due to abnormal synthesis of melanin by the melanocytes. N.B. Generalized albinism is an autosomal recessive disorder, localised albinism is autosomal dominant disorder. 12 Clinical correlation Vitiligo: common skin condition in which a patchy loss of epidermal melanocytes results in depigmentation. Etiology is unknown but is thought to be multifactorial: Genetic predisposition Autoimmune destruction of melanocytes Oxidative stress (free radicals) Intrinsic defects of melanocytes (early apoptosis, defective cell- to-basement membrane adherence) May be triggered by stress or skin injury (e.g., sunburn). 13  Other epidermal cells Merkel cells are pressure-detecting mechanoreceptors in the skin (palms and soles). Their origin is not completely clear. They appear in 4th–6th months. Langerhans cells are the tissue macrophages. Primitive yolk sac and fetal liver hematopoiesis contribute to the initial formation of Langerhans’ cells; migrate into the skin from 7th week. 14 Mesoderm Intraembryonic mesoderm divides into: Paraxial, Intermediate, Lateral plate mesoderm. Paraxial mesoderm extends as a longitudinal column on either side of the notochord and the developing neural tube. 15  Somites Paraxial mesoderm becomes segmented to form 40–45 pairs of somites on either side of the developing neural tube and notochord. Somites appear between the 20th and 30th day of IUL → 4th week is known as somite period of development. A cross section through a somite shows that it is a triangular structure and has a cavity. 16 Somites subdivision 1. Sclerotome - ventromedial part; cell migrate medially, surround the neural tube and give rise to the vertebral column and ribs. 2. Dermatome - lateral part; cells migrate and come to line the deep surface of the ectoderm; cells give rise to some dermis and to subcutaneous tissue. 3. Myotome - intermediate part; gives rise to striated muscle. *Recently, it has been held that the dermatome only forms dermis on the back of the head and trunk, and that dermis elsewhere is derived from lateral plate mesoderm. 17  Sources of dermis Formed by condensation and differentiation of mesenchyme underlying the surface ectoderm. Three sources of origin of the mesenchyme: 1. Dermatomes (paraxial mesoderm) give rise only to the dermis on the dorsal aspect of the head and trunk. 2. Dermis of the limbs and that on the lateral and ventral aspects of the trunk arises from somatopleuric layer of lateral plate mesoderm. 3. Dermis over most of the head and over the anterior aspect of the neck is derived from the neural crest. 18 ATTENTION! Sensitive content!  Ichthyosis Ichthyosis (Gr. Ichthys - fish), excessive keratinisation of the skin, is characteristic of a group of hereditary disorders that are usually inherited as an autosomal recessive trait but may also be X- linked. In severe cases, ichthyosis may result in a grotesque appearance, as in the case of a harlequin fetus. 20 A 21-year-old woman comes to the physician because of a 2-month history of worsening dry and cracking skin on her legs. She says, “My mother told me that I've had dry skin since I was a baby. It's usually tolerable, but it's unbearable this winter because it's so dry!” She has been otherwise healthy and takes no medications. A photograph of the patient's skin is shown. The remainder of the physical examination shows similar findings on the trunk. There are prominent skin lines on the palms and soles. Which of the following is the most likely diagnosis? A. Ichthyosis B. Contact dermatitis C. Psoriasis D. Atopic dermatitis E. Pemphigus vulgaris 21  Dermis, or corium By 11th week mesenchymal cells begin to produce collagen and elastic fibers. Line of junction between dermis and epidermis is at first straight subsequently, during 3rd and 4th months the epidermis shows regularly spaced thickenings that project into the dermis. Portions of dermis intervening between these projections form the dermal papillae. Still later, surface elevations (epidermal ridges) are formed by further thickening of the epidermis in the same situation. Dermis differentiates into two layers: 1) superficial papillary layer 2) deep reticular layer. Presentation Title 22  Dermis, or corium Most of the dermal papillae of papillary layer contain a small loop of capillary plexus and a sensory receptor (sensory nerve end organ). Loops of capillary plexus provide nourishment to the epidermis and to the sensory nerve end organ. Reticular layer (deeper) contains large amount of collagen and adipose tissue. Blood vessels in the dermis develop initially as endothelium-lined structures that differentiate from the mesenchyme. Primordial vessels give rise to the capillaries (angiogenesis). Angiogenesis of the dermis is completed by the end of the first trimester. 23 Appendages of this skin Associated structures that are derived from the epidermis and dermis that usually are located adjacent to the skin and serve a specific function, e.g., prevention of heat loss, and sensation. Skin appendages include: nails, hairs glands. Presentation Title 24 Structure of the nail 1. Nail plate (body) consists of keratinised cells. Nail plate grows over underlying nail bed. 2. Nail bed is a highly vascular CT layer that lies deep to the nail plate. 3. Hyponychium is a part of epidermis that lies under free edge of the nail plate (Greek, onyx is nail). 4. Germinal matrix (nail root) is a growing area of nail. 5. Lunula is a proximal soft, half-moon shaped part of nail plate that overlies nail bed. 6. Cuticle is an overlapping epidermis around the base of nail (overlies matrix). 7. Nail walls are folds of skin that overlap sides of nail. 8. Perionychium includes nail wall and cuticle area. 9. Eponychium is an extension of base of nail plate under that nail plate emerges from the matrix. 25 Stages of nail development Nail field appears by the end of 10th week as a thickened area at the tip and adjacent sides of fingers and toes. At the base of nail field, U-shaped epidermal nail folds appear. Development of fingernails is followed by toenails. Underlayer of nailfold modifies to form a germinal matrix. In 5th month, a few cells of matrix get keratinised to form nail plate. Nail plate differentiates to form proximal lunula and remaining hard nail body. Adjacent epidermis forms nailwall and cuticle. 26 Development of nails With the formation of the nail plate, the part of epidermis overlapping proximal part of the developing nail is called eponychium. Eponychium degenerates and exposes the nail, except at its base where it persists as cuticle. Epidermis below free margin of the nail is called hyponychium. Part of the nail in the groove is called root of the nail. 27  Stages of nail development Fingernails reach the fingertips by 32 weeks Toenails reach toe tips by 36 weeks. After birth, nails grow about 0.5 mm a week. 28 Development of nails 29  Clinical correlation 1. Onychia is inflammation of nail folds that result in pus formation and shedding of nail. 2 2. Onycholysis (ingrown nails/unguis 1 incarnates) is painful condition that involves injury of nail bed by ingrowing nail plate. 3. Onychodystrophy is deformation of nail. 4. Onychogryposis / ram's horn nail is a hypertrophy of nail that produces ram's 3 4 horn. 5. Paronychia is bacterial or fungal infection of nail base. 6. Koilonychia/spoon-shaped nails are caused by iron deficiency or vitamin B12 deficiency. 5 6 30  Development of hair Hairs or pili (L. Pilus = hair) begin to develop early in the fetal life (7–12 weeks). Each hair develops from downgrowth of the epidermis into underlying dermis. Presentation Title 31  Development of hair Cells of the stratum germinativum proliferate to form a solid epithelial cord that extends obliquely downward in the dermis to form hair bud. Terminal part of the hair bud becomes club shaped and forms the hair bulb. Hair bulb is invaginated by the mesenchymal condensation of the dermis and the hair bulb now becomes inverted cup-shaped structure. Dermis within the cup-shaped hair bulb is called dermal papilla. Cells of the hair bulb at the summit of dermal papilla form the germinal matrix that later produces hair. 32 Development of hair Peripheral cells of developing hair follicle form epithelial root sheath. Dermis condenses around this sheath to form dermal root sheath. Cells of the germinal matrix proliferate to form root of the hair. As the root grows, it is pushed outside the hair follicle on the surface of skin as a shaft of the hair. Root and shaft of hair become keratinized. Melanoblasts migrate into the hair bulbs and differentiate into melanocytes. Melanin formed by these melanocytes is transferred to proliferating cells of the germinal matrix. Melanin is responsible for the color of hair. 33 Development of hair Presentation Title 34  Types of hair First hairs that appear – lanugo - fine, soft, lightly pigmented, and silky; help to hold the vernix caseosa on the skin surface. Lanugos (downy hairs) are replaced by coarser hairs during the perinatal period, which persists over most of the body except in axillary and pubic regions where they are replaced at puberty by even coarser terminal hair. In men, similar coarse hairs appear on the face (e.g., moustache and beard) and often on the chest. Definitive hair grows to a certain length and then cease to grow, e.g., hairs of eyelashes, eyebrows, pubic, and axillary regions. 35 Arrector muscle of hair (arrector pili muscle) Small bundle of smooth muscle fibers (SMF) differentiate from the mesenchyme surrounding the hair follicle usually on one side. SMF attach the dermal root sheath and papillary layer of the dermis to form arrector pili muscle. Contraction of arrector pili muscles causes goose bumps. Presentation Title 36 Glands of the skin Types of glands in the skin: sebaceous sweat mammary They derived from the epidermis and grow into the dermis. Presentation Title 37 Sebaceous glands Develops as a bud from the epithelial root sheath of hair follicle. Bud grows into the surrounding dermis and divides into a number of branches to form primordia of several alveoli and their ducts. Cells in the center of alveoli (acini) degenerate to produce an oily secretion called sebum. Sebum is released into the hair follicle and from here it passes to the surface of the skin. Presentation Title 38 Sebaceous glands Sebaceous gland is holocrine in nature (whole cell rupture to become secretion). N.B. At few places (e.g., glans penis and labia minora), the sebaceous glands develop independent of the hair follicles as buds from the epidermis. 39 Types of sweat glands Eccrine - found in the skin of most parts of the body. Apocrine - found in the axilla, pubic and perineal regions, and areolae of nipple. 40 Development of eccrine sweat glands Develops from downgrowth of the epidermis into the underlying dermis. Cells of stratum germinativum proliferate to form a solid mass of epithelial cells that extend downward in the underlying dermis to form the bud of sweat gland. 41 Eccrine sweat glands Downgrowth elongates and its terminal part becomes coiled. Later this solid downgrowth central cells degenerate to form a gland lumen. Peripheral cells differentiate into an inner layer of secretory cells and an outer layer of myoepithelial cells, which contract to expel sweat from the gland. Terminal coiled part of down-growth forms secretory part of the sweat gland, while the proximal straight part forms duct of the sweat gland. Site of beginning of downgrowth from the surface epithelium forms pore of the duct of the sweat gland. 42 Eccrine sweat glands Eccrine sweat glands start functioning shortly after birth. Presentation Title 43 Apocrine sweat glands of the skin Begin to develop during puberty. They develop from same epidermal buds that form the hair follicles. These glands open into the hair follicles instead of opening on the skin surface. N.B. Sweat produced by apocrine sweat glands contains lipids, proteins, and pheromones. Odor produced from these sweat glands is due to breakdown of these products. It is thought to act as a sexual attractant. 44 Breast development Mammary glands are modified and highly specialised sweat glands and therefore develop from the surface ectoderm. 4th week: surface ectoderm thickens on either side of the ventral aspect of trunk of embryo along the line extending from the axilla to the inguinal region to form mammary ridge or line. About 15–20 mammary buds develop as solid down-growths of the epidermis into the underlying mesenchyme along the mammary ridge on each side. A. Mammary ridge (left side) and positions of accessory nipples (right side). B. Stages of development of mammary glands.  MCQ for self-control https://forms.gle/KrNxLLLTxVN HVhUy6 46 References