Lesson 28 - Integumentary System (notes) SGL.pdf

Cytology and Histology LESSON 28 INTEGUMENTARY SYSTEM The integumentary system is made up of the skin (epidermis, dermis, hypodermis), skin appendages (glands, hair follicles, and hairs) and related structures (hooves, horns, claws and nails, comb, beards). I. SKIN The skin is the largest, most ob...

Cytology and Histology LESSON 28 INTEGUMENTARY SYSTEM The integumentary system is made up of the skin (epidermis, dermis, hypodermis), skin appendages (glands, hair follicles, and hairs) and related structures (hooves, horns, claws and nails, comb, beards). I. SKIN The skin is the largest, most obvious, and most extensive organ in the body. It covers the entire body surface and, in the lips, nose, eyelids, anus and vulva it is continued with the mucous membranes through the mucocutaneous junctions. Their gross appearance varies with species, breed, sex, age, and body area. In general, the thickness of the skin decreases dorso-ventrally and cranio-caudally. Thus, the areas with the greatest skin thickness are the forehead and the dorsal area of the neck, the thorax and the hip, and the thinnest are the ear pinna and the axillary, inguinal and perianal areas. Almost the entire skin surface of domestic animals is covered by hair, wool or feathers. The skin is made up of three layers: the epidermis, an epithelial tissue; the dermis, a connective tissue (Figure 1); and the hypodermis (subcutaneous tissue), which is a loose connective tissue and adipose tissue that attaches the dermis to the underlying structures (muscle or bone), and which can be considered either part of the skin or part of the underlying tissues. 1. The epidermis The epidermis has 4 different types of cells: 1) Keratinocytes represent 80-85% of the cells of the epidermis and are the cells of the keratinised squamous stratified epithelium. 2) Melanocytes represent 5% of the cells of the epidermis and are isolated cells. 3) Langerhans cells represent 5% of the cells of the epidermis and are isolated cells. 4) Merkel cells represent 5% of the cells of the epidermis and are isolated cells. Keratinocytes (Figure 1) Keratinocytes are organized into 4 layers: 1) Stratum basale or germinativum or basal layer: They form a layer of cuboidal to low columnar cells that rests on the basement membrane; they have a heterochromatic ovoid nucleus with frequent mitotic figures and scant cytoplasm. Cells attached to each other by desmosomes and to basement membrane by hemidesmosomes (express cytokeratin 5 and 14). They are the progenitor cells of the epidermis, which renew every 20-30 days. 2) Stratum spinosum: They form several layers of polyhedral cells with a rounded nucleus and wide, eosinophilic cytoplasm located above the stratum basale. They present "spines" or "intercellular bridges", filamentous structures between cells that in electron microscopy correspond to a large number of strong desmosomes located along the cell periphery. The keratinocytes of the stratum spinosum have already started the complex and long process of differentiation that will lead to their transformation into “squames” or “horny cells”. In addition, Cytology and Histology they have lost the capacity for mitotic division and initiate the expression of new cytokeratins, type I and type II cytokeratins (cytokeratins 1 and 10). Stratum granulosum: They form one or two layers of flattened nucleus and cytoplasm squamous cells located above the stratum spinosum. The stratum granulosum receives this name because the keratinocytes have fine granules in their cytoplasm made up of keratohyaline, which stain intensely with the basic dyes. From the functional point of view, the cells of the stratum granulosum are different from those of the stratum spinosum. They no longer synthesize keratin filaments but instead produce more operations aimed at the aggregation of these filaments. One of these molecules is the protein profilaggrin, present in keratohyaline granules and which is transformed into filaggrin, a protein that actsas a powerful glue for keratin filaments. 3) Stratum corneum: They form several layers of dead eosinophilic cells that lack a nucleus and cytoplasmic organelles (“squames” or “horny cells”). The plasma membrane also disappears, and the cells are surrounded by an insoluble, hydrophobic, lipid-rich mass called the "cornified cell envelope" formed by loricrin and involucrin. The stratum corneum has been compared to a wall in which the horny scales would be the bricks and the cornified cell envelope the cement. A B C Connective tissue Figure 1.- Epidermis and dermis or connective tissue (A); spiny keratinocytes (B); and melanocytes of light cytoplasm in the basal layer of the epidermis alternating with others of brown cytoplasm (C). On the foot pads and in the nasal plane there is a fifth layer of keratinocytes located between the stratum granulosum and the stratum corneum: the stratum lucidum. It is an intensely eosinophilic layer made up of a mixture of the phospholipid eleidin and proteins. This stratum is present only in very thick skin (foot pads and nasal plane) and is absent in all other skin areas and has not been described in species such as swine. Melanocytes (Figure 1) Melanocytes are cells that synthesize melanin, a pigment that gives to skin its colour and protects keratinocytes from the harmful effects of solar radiation. They are of neuroectodermal origin and migrate to the epidermis in the embryonic period. In routine histological preparations (haematoxylin and eosin), most of the time they appear as cells with rounded nuclei and clear cytoplasm situated between the keratinocytes of the stratum basale. On rare occasions, its cytoplasm appears brown and star-shaped, with numerous branching processes called dendrites that extend to the stratum spinosum between adjacent keratinocytes. The brown colour is due to the presence of melanin. Melanocytes do not form desmosomes with adjacent keratinocytes or hemidesmosomes with the basement membrane. Ultrastructurally, the cytoplasm contains numerous melanosomes, organoids that contain the enzyme tyrosinase, which catalyses the oxidation of the amino acid Cytology and Histology tyrosine first to dihydroxyphenylalanine (DOPA) and then to DOPA-quinone. DOPAquinone then undergoes a series of oxidation and polymerization reactions, leading to the formation of melanin. Therefore, melanin is a mixture of different DOPA-quinone polymers associated with proteins. Each melanocyte supplies melanin to a group of 20-30 keratinocytes and this group is called the epidermal melanin unit. The mechanism by which melanocytes transfer melanosomes to keratinocytes is called cytocrinia. Langerhans cells They are a type of macrophage, widely distributed throughout the epidermis and representing around 5% of the total cells of the epidermis. Their function is to act as antigen-presenting cells. They come from the bone marrow and are transported by the blood to the stratum spinosum. In conventional histological sections, they appear as clear cells located between the keratinocytes of the stratum spinosum. For the precise identification of these cells, histochemical (ATPase) and immunohistochemical (S100 protein expression) techniques can be performed. Ultrastructurally, they have a polymorphic nucleus, and the cytoplasm has characteristic “tennis-racket” shaped granules called Birbeck granules. Their position in the epidermis provides them with a privileged location to contact exogenous antigens that come into contact with the skin. These antigens are phagocytosed and partially degraded by Langerhans cells, which leave the epidermis to go, lymphatically, to the regional lymph node, where they present the antigen to specific lymphocytes. Thus, they stimulate the proliferation of helper T lymphocytes, induce cytotoxic T lymphocytes, and produce interleukins that mediate the immune response. Merkel cells They are cells of neuroendocrine origin located only in certain body regions such as the tilotric hairs (solitary, long, robust and rigid hairs such as whiskers and tactile hairs), and the footpads (in addition to oral mucosa) in animals. They are isolated cells located between the keratinocytes of the stratum basale with a rounded nucleus and clear cytoplasm. They can be differentiated from keratinocytes and melanocytes by immunohistochemical techniques because they have intermediate filaments of cytokeratins 8, 18, 19, and 20 (different from those of adjacent keratinocytes) and neuroendocrine lineage markers such as chromogranin A and neuronal-specific enolase (NSE). These cells are connected to the naked endings of myelinated nerve fibres forming the so-called Merkel cell-neurite complexes. These complexes are postulated to function as touch receptors. In the electron microscope they can be distinguished from other cells of the epidermis because they present in their cytoplasm electron dense granules and because, contrary to melanocytes and Langerhans cells, they are not stellate and connect with keratinocytes by means of desmosomes. Cytology and Histology 2. The dermis Fibrous connective tissue with a network of collagen fibres and elastic fibres that provide flexibility to the skin and allow its free movement on the underlying structures. The area closest to the epidermis is the papillary dermis, and the furthest is the reticular dermis. The papillary dermis is made up of dense, irregular fibrous connective tissue, and has blood vessels, Meissner's corpuscles, glands, hair follicles, and hairs. The reticular dermis is made of dense, irregular fibrous connective tissue and has bundles of smooth muscle, arrector pili muscles, and Pacinian corpuscles. Meissner´s corpuscle consists of an unmyelinated axon and is sensitive to light touch and Pacinian corpuscle consists of unmyelinated nerve terminal surrounded by layers of fibroblasts and is sensitive to vibration and pressure. The hypodermis (Figure 2) Fibrous connective tissue and adipose tissue (fibroadipose tissue). The adipocytes form small clusters or large masses that create a cushion or pad of fat called the adipose pad. A B Figure 2.- Hypodermis (A) and sebaceous gland (B), HE, 10X. II. SKIN APPENDAGES 1. Sebaceous glands (Figure 2) They are exocrine glands which produce sebum, a substance of lipid nature (fatty acids and cholesterol), in a holocrine secretion and discharge their secretion to the hair follicle very close to the epidermis. They have two types of cells: basal or progenitor cells, and secretory cells, with pale foamy cytoplasm containing lipids and central nucleus. 2. Sweat glands They are of two types: 1) Apocrine Sweat glands, exocrine glands, of a simple coiled tubular type, serous secretion in apocrine form with a cooling function that consists mainly of slightly alkaline water, distributed over the entire skin surface. They are very abundant in canids and equids and their constitution is: Secretory unit (“adenómero”) with simple cuboidal to low columnar epithelium and duct with myoepithelial cells. In equids they have a cooling function, but not in canids (despite their abundance). 2) Eccrine Sweat glands, similar to apocrine except in the mode of secretion, which is merocrine, the epithelium of the secretory unit, which is cuboidal, and the location, since they are found in the pads of carnivores, the carpus of the pig, and the lips of ruminants and pigs. Cytology and Histology 3. Hepatoid glands Also called circumanal or perianal because they are located exclusively in that area (around the anus, in the perineum and in the buttock of the tail as well as in the prepuce), they are exocrine glands, of the simple branched alveolar type, and they do not produce any secretion. They have lumen-free secretory units with two types of cells: basal progenitor cells and differentiated polyhedral or columnar cells, with acidophilic and granular cytoplasm (proteinaceous granules). They are abortive sebaceous glands (there are lumenless excretory ducts that connect with the adjacent sebaceous glands), occur exclusively in canids, and have a tendency to neoplastic transformation in males. 4. Mammary gland It is a modified exocrine cutaneous sweat gland, of the tubuloalveolar type, compound, that produces milk, a substance rich in lipids and proteins, in an apocrine form that flows to the nipple and has secretory units and ducts of columnar to cuboidal epithelium with myoepithelial cells. Its morphology is highly variable with sex (rudimentary in the male), age and reproductive stage of the female. 5. Hair follicle and hair (Figure 3) The hair follicle is a tubular invagination of the epidermis and the basement membrane into the dermis where hair is formed. They are very numerous and cover practically the entire skin surface of domestic animals. They are not perpendicular but form 30-60º angles with the skin surface. Arrector pili muscles cause erection of hair shaft. From outside to inside, the components of the hair follicle are: 1) Thick (glassy) basement membrane. 2) External root sheath, made up of the stratum basale and spinosum of the keratinized squamous stratified epithelium. 3) Internal root sheath, with three layers in turn: a) Henle's layer (a layer of squamous cells); b) Huxley's layer (several layers of granular cells containing trichohyalin); c) Internal cuticle (a layer of horny squames). From the outside to the inside, the components of the hair, which are a continuation of the components of the hair follicle, are the cuticle, cortex and medulla and are made up of several compact layers of horny cells with hard keratin. The area where hair formation originates is called the hair bulb, and it is made up of the hair matrix, the deep part of the invagination that has an inverted cup shape and is made up of basal keratinocytes and melanocytes, and the dermal papilla, a richly vascularised loose connective tissue that maintains the viability of the hair follicle. The keratinisation of the cells of the hair matrix is different from the keratinisation that epidermal cells undergo because the keratin of the hair is very rich in sulphur amino acids and is known as hard keratin. A B C Figure 3.- Schemes of the hair follicle and hair in longitudinal section (A), dermal papilla (B) and hair in cross section (C). Cytology and Histology III. RELATED STRUCTURES The hoof of Equidae is a modified continuation of the epidermis, intensely cornified that lacks stratum granulosum and lucidum and presents a double system of ridges and papillae in the dermis. Ruminant horns consist of epidermis, dermis, and hypodermis that line the corneal processes of the frontal bone. The nails of carnivores are direct continuations of the epidermis and dermis that line the distal phalanges. IV. FUNCTIONS OF THE INTEGUMENTARY SYSTEM         Soft tissue covering Protection (two-way barrier) Regulation of body temperature Reception of sensations (touch, temperature, pain) Secretion (sweat, sebum, milk) Locomotion Synthesis of vitamin D Protection from ultraviolet rays from sunlight