Skin Lab

Learning Objectives

  • Name and distinguish the four layers of the epidermis in terms of structure and function.
  • Identify the two layers of the dermis and the hypodermis and explain their functional significance.
  • Describe the basic structure and function of several key epidermal derivatives.
  • Contrast the three modes of exocrine secretion and give examples of cells that exhibit each type.
  • Describe the structure of the mammary glands.
  • Recognize some key pathological examples affecting skin and epidermal derivatives.

Keywords

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Pre-Lab Reading

Introduction

The skin is the largest organ of the body and varies greatly in different regions. It has five main functions: protection, thermoregulation, sensation, metabolic functions (vitamin D, adipose metabolism), and sexual attraction.

Skin is composed of several subunits. The three main divisions are:

  • the epidermis, or surface epithelium, which is a self-regenerating stratified squamous epithelium that produces a protective protein layer of keratin.
  • the dermis, an underlying layer of dense collagenous connective tissue that contains hair follicles, sweat glands, blood and lymphatic vessels, sensory receptors and nerves, and connective tissue cells.
  • the hypodermis, another connective tissue layer that is rich in white adipose cells and contains large blood vessels that supply the smaller vessels of the dermis.

Epidermis

The epidermis is a stratified squamous epithelium that contains discrete layers of proliferating, differentiating, and differentiated cells called keratinocytes. It is divided into four layers that have different structural appearances:

  • Basal Cell Layer - Keratinocytes begin in the deepest layer of the epidermis, the stratum basale, which is a row of columnar cells resting on the basal lamina that separates the epidermis from the dermis. Mitosis occurs exclusively at the basal cell layer and allows for the replacement of cells lost from the surface.
  • Stratum Spinosum - After forming in the basal cell layer, keratinocytes migrate upwards into the stratum spinosum. In this layer, they develop short projections that attach via desmosomes to adjacent cells. The stratum spinosum is also known as the "prickly layer" because of these characteristic spines. The cells in this layer produce cytokeratin, an intermediate filament precursor to keratin.
  • Stratum Granulosum - The third layer is the stratum granulosum. In this layer, the keratinocytes have become squamous cells that contain granules of keratohyaline, a precursor to the extracellular keratin that protects the skin tissue from abrasion.
  • Stratum Corneum - The most superficial layer of the epidermis is the aceullar stratum corneum. It is the most functionally important layer of the skin and consists of flat, keratinized scales that are shed and replaced continuously. This is the layer that includes the final keratin product, which is a combination of cytokeratin and keratohyaline.

Recall from the Laboratory of Epithelia that epithelia differ in their degree of keratinization - those exposed to abrasion and desiccation are heavily keratinized, but those that form mucous membranes do not have much keratin. For example, the skin is highly keratinized, but the esophagus, anal canal,and vagina are not. Instead of protection by keratin, these mucous membranes are kept moist by glandular secretions. Mucous membranes lack a stratum granulosum and stratum corneum.

The epidermis contains several characteristic cell types:

  • Melanocytes occur at intervals among the basal keratinocytes and produce melanin pigment, which is most abundant in sun-exposed skin and in areas surrounding body openings. Melanin is synthesized from tyrosine and transferred as melanin granules to the surrounding epithelial cells. While the number of melanocytes is the same in light- and dark-skinned people, they are far more active in the latter.
  • Langerhans cells are typically located in the stratum spinosum and are the equivalent of macrophages in the skin tissue.
  • Merkel cells are attached to keratinocytes by desmosomes and are most commonly found in highly sensitive areas like the fingertips - these serve as touch receptors.

Dermis and Hypodermis

The dermis consists of two layers:

  • Papillary layer - The most superficial layer of the dermis is the papillary layer, which consists of loose connective tissue immediately beneath the epidermal basement membrane.
  • Reticular layer - The reticular layer is composed of dense, irregular collagenous connective tissue.

Most blood vessels, nerves, and sensory receptors occur in the papillary layer. This region also contains Meissner's corpuscles, which sense light touch.

The hypodermis is the fatty layer beneath the dermis. It is thickest in the abdominal wall and virtually absent in the eyelid, scrotum, penis, and the dorsal side of the hand. This layer contains a significant number of fibroblasts, which synthesize collagen and elastin. The hypodermis contains Pacinian corpuscles, which sense deep touch.

Epidermal Derivatives

Many structures are derived from epidermal tissue. Keep in mind that just because a structure is derived from the epidermis does not mean that it is located in the epidermis.

Hair follicles are encased by an invagination of the epidermis into the dermis known as the external root sheath. They contain specially organized keratin built into long tubular structures. Remember that hair follicles have generous blood and nerve supplies. There are three states of hair follicles:

  • Anagen - Growing follicles synthesize hair. They are long and most numerous in the scalp.
  • Catagen - Resorbing follicles are in a short phase of regression that signals the end of active hair growth.
  • Telogen - Resting follicles contain a fully formed hair.

Sebaceous glands are pear-shaped alveolar glands that secrete an oily substance called sebum, which moisturizes and waterproofs hair. They are usually attached to hair follicles near the arrector pili muscle, which allows the hair to "stand up." An extensive capillary plexus characterizes sebaceous glands.

Eccrine sweat glands occur throughout most of the skin. They consist of long tubules extending from the epidermis deep into the dermis or hypodermis. The secretory portion of each gland is tightly wound and appears as a collection of cross-sectioned tubules. It is encased by myoepithelial cells, which contain actin filaments. These cells receive input from nerve fibers, which cause them to contract and expulse the sweat from the gland. Keep in mind the organs that do not have eccrine sweat glands: the glans penis, the inner surface of the foreskin, the clitoris, and the labia minora.

Apocrine sweat glands are much larger than eccrine glands and produce a thicker secretion. They have straight, narrow ducts that run parallel to hair follicles and frequently open into the pilosebaceous canal. These sweat glands occur in the axilla, the areola of the nipple, the labia majora, and the circumanal region.

Mammary Glands

Mammary glands are one of the most complex epidermal derivatives. These glands are present in both sexes, but only develop fully in females after parturition. They begin to undergo dramatic structural changes at puberty.

The basic structure of the mammary glands involves alveoli that contain two layers of cells: an inner cuboidal epithelium and an outer layer of myoepithelial cells. The alveoli make up tubuloalveolar glands, or lobes, which connect via lactiferous ducts to the base of the nipple. After milk is produced, it is secreted and travels through the ducts into spindle-shaped enlargements beneath the areola known as lactiferous sinuses.

Important structural changes occur in the mammary glands over the course of the female's lifetime:

  • In a nonpregnant, sexually mature female, the glandular tissue consists of ducts with small terminal alveoli embedded in an abundant connective tissue stroma that contains many adipose cells.
  • During pregnancy, hormonal stimulation results in the proliferation of the intralobar ducts and terminal alveoli. The epithelial cells become enlarged and vacuolated, as milk fat production increases.
  • After parturition, the gland enters its active secretory phase and produces watery milk containing membrane-bound lipid droplets, as well as milk proteins, lactose, and cellular debris.
  • Suckling causes release of prolactin from the anterior pituitary and oxytocin from the posterior pituitary (processes that you will study in detail during Physiology). Prolactin maintains milk production and oxytocin causes the contraction of myoepithelial cells and ejection of milk.

Types of Exocrine Secretion

In your study of Histology, you may hear three different terms to describe exocrine cells. These can often be confusing.

  • Merocrine, or eccrine, secretion occurs by exocytosis. This is the mode of secretion of both eccrine and apocrine glands, which can be very confusing.
  • Apocrine secretion occurs when a portion of the plasma membrane containing the secretion buds off from the cell. This is the mode of secretion of the mammary glands and mucous-producing cells, but not the apocrine sweat glands.
  • Holocrine secretion occurs when the entire cell disintegrates in order to release its secretion. Sebaceous glands exhibit holocrine secretion, as the sebum is released with remnants of dead cells.

Pre-Lab Quiz

  1. Name the four layers of the epidermis and the state of keratin associated with each.
  2. Answer:
  3. What are the important differences between sebaceous glands, eccrine sweat glands, and apocrine sweat glands?
  4. Answer:
  5. What is the structure of the mammary gland, and what key differences do you expect to see between active and inactive mammary tissue?
  6. Answer:
  7. Name the three types of exocrine secretion, their key characteristics, and an example of a cell that demonstrates each one.
  8. Answer:

Virtual Microscope Slides

  1. Layers of the Skin
  2. At low magnification, distinguish the epidermis, dermis an hypodermis. In which layers do you find blood vessels? Zoom in to identify the four layers of the epidermis. Which layer appears granulated and why? Which layer appears prickly and why?
  3. Hair Follices
  4. Focus on a single hair follicle at high magnification. Can you identify the medulla and cortex? Try to find the hair papilla and hair bulb.
  5. Mammary Gland
  6. At low magnification, differentiate between glandular lobules and connective tissue. Is this sample from an inactive or active breast?

Pathology

Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden.

  1. Basal Cell Carcinoma
  2. Melanoma
  3. Epidermolysis Bullosa

Quiz

  1. Name the layer on the left.
  2. Answer: Stratum corneum
  3. What cell type synthesizes the brown material? In what cell type does most of the melanin reside?
  4. Answer: Melanocytes. Keratinocytes.
  5. Identify this tissue.
  6. Answer: Mammary gland
  7. Identify the structure on the left and its primary function?
  8. Answer: Desmosome - promote adhesion between cells.
  9. What type of glands are these, what is their function, and what is their mode of secretion?
  10. Answer: These are sebaceous glands, which secrete sebum onto hair follicles to moisturize and protect them. They secrete through the holocrine mode, in which the cells burst.
  11. This slide shows a sweat gland. Identify A and B, and explain their different functions.
  12. Answer: A is the myoepithelium, B is the cuboidal epithelium. The cuboidal epithelium secretes sweat through the eccrine method, the myoepithelium causes contraction of the gland and expulsion of sweat through the duct.
  13. Is there anything abnormal in the epidermis? If so, what might it indicate.
  14. Answer: Hint: Look for cells with pale cytoplasm in the epidermis. These cells are melanocytes and the large number of melanocytes in upper layers of epidermis is abnormal. In normal skin, melanocytes are found only in the basal layer of the epidermis. The condition suggests the early stages in the development of melanoma.
  15. This image is of skin with a tattoo. Why is the tattoo permanent?
  16. Answer: The ink of the tattoo is in the dermal layer which doesn't not get turned over like the cells in the epidermal layer.
  17. This image is an example of psoriasis that is an inflammatory disorder that results in increased scaling and shedding of the skin. What differences from normal skin do you observe?
  18. Answer: Elongation of the rete ridges and mitotic cells above the basal layer that is a sign of increased cell turnover in the epidermis. Increased number of cells in the dermal region that is a sign of an inflammatory response.
  19. It is thought that acne results from alterations in hair follicles, and particularly their secretions. If this were the case, what type of glands would be responsible for acne?
  20. Answer: Sebaceous glands empty directly onto hair follicles; overactivity may cause acne.