Week 5

In Histology ALWAYS:

  • learn the unique structures of the cells

  • Learn the function of each cell

  • Be able to relate and make connection between each type of cell


Connective Tissue pdf

Movement of fluids through connective tissue (interstitium)

Types of Connective tissue:

  • differs in content and amount of each content: cell number, fibers, ECM, etc.

  • Connective Tissue Proper: Can be loose / areolar (fine fibers of collagen), or dense

    • Loose connective tissue is delicate and provide flexibility

    • Dense Connective tissue has a larger proportion of Type I collagen fibers and provides protection and strength to structures. Has 2 types:

      • Dense regular connective tissue (tendon) resists stress from one direction

      • Dense Irregular connective tissue (skin) resists stress from all directions

  • Reticular tissue: abundant in Type III collagen. Secreted by fibroblasts (called reticular cells)

    • Network mesh of tissue that allows for easy continuous passage of leukocytes and lymph.

    • Mostly found in hemopoietic tissues (bone marrow, spleen, etc.)

  • Mucoid tissue: very gelatinous and is mostly found in fetal umbilical cord, where is, is referred to as “Wharton’s Jelly”

    • Large fibroblasts surrounded by a large amount of very loose ECM that is mainly ground Substances very rich in Hyaluronan and wisps of collagen


Adipose Tissue pdf.

Fat tissue. Cells are called Adipocytes

A Connective tissue in which fat-storing cells are predominant

Can be stimulated by neuronal and hormonal stimuli

Functions:

  • storage of neutral fats (triglycerides), to avoid starvation and store nutrients

  • Secretes hormone Leptin, making us feel full, not hungry

  • Provides thermal insulation in the body

  • Acts to cushion areas including foot pads, palms, and behind eyes

Adipose tissue has rich blood supply and adipocytes are uniquely and individually surrounded by a thin external lamina containing collagen

2 types of adipose tissues:

  • White= more common large cells contain single large yellowish droplet of fat (referred to as unilocular)

    • The droplet flattens the nucleus (signet ring appearance)

    • Specialized for long-term energy storage

    • Fixative has osmium tetroxide, inch preserves lipid and stains it black

  • Brown= cells contain smaller, multiple droplets of fat (multilocular) with abundant mitochondria and blood vessels (makes it appear darker)

    • Main function is Heat production

    • Highly innervated by the sympathetic nervous system.

    • Regulates their metabolic activity in a process called non shivering thermogenesis (where mitochondria produces heat)

    • Mostly found in newborn babies on their neck, shoulder, and back.

    • Rarely found in adults, but if so, it is around the kidneys, adrenal glands, aorta and mediastinum

Fats in blood are hydrolyzed (degraded) by the enzyme: Lipoprotein Lipase into Free Fatty Acids (FFAs) and Glycerol.

  • FFAs enter fat cells and combine with glycerol to form triglycerides

    • When needed for energy, triglyceride droplets will be hydrolyzed to FFAs and Glycerol by hormone sensitive lipase.

      • Can be stimulated by various hormones or neurons


Skin pdf

Skin is the Integumentary system (or cutaneous layer)

Largest organ in the body (Takes up 15-20% of body weight)

3 layers of skin:

  • Epidermis (outermost layer)

    • Originates front he ectoderm and consists of epithelial layer and appendages

    • Components include: nail, hair, part of sweat and sebaceous glands

  • Dermis:

    • Mesodermal connective tissue

  • Subcutaneous layer (hypodermis)

    • Contains fat and lose connective tissues

    • Connects the skin loosely to the underlying tissues and corresponds to the superficial fascia

Dermal papillae: raised area of dermis layer that interdigitates with Epidermal ridges

  • where epidermis and dermis intertwine

Functions:

  • Protection: provides a physical barrier against thermal and mechanical insults

    • Langerhan’s cells protect from microorganisms

    • Melanin protects the cell nuclei form UV radiation

    • Skin is also a permeability barrier against excessive loss or uptake of water

  • Sensory:

    • many types of sensory receptors allow skin to constantly monitor the environment

    • Skin Mechanoreceptors help regulate the body’s interactions with physical objects or other stimuli

    • Merkel’s cell s in epidermis is a tactile receptor

  • Thermoregulation: Maintains body’s temperature because of the fatty layer and hair

    • Also contains mechanisms for accelerating heat loss if we are too hot

      • Sweat production and a dense superficial microvasculature

  • Metabolic:

    • Synthesizes Vitamin D3 for calcium metabolism and proper bone formation

    • Excess electrolytes are removed in sweat

    • Subcutaneous layer stores energy in the form of adipose tissue

  • Sexual signaling:

    • Features such as pigmentation and hair are visual indicators of health

    • This helps release sex pheromones, produced by apocrine sweat glands and other skin glands are also important for this attraction

Epidermis:

  • Al skin consists of Stratified Squamous keratinized epithelium

    • Containing skin cells called Keratinocytes

    • The thickness of the layer depends on the location

    • All epithelial cells lack microvasculature

      • Only get oxygen and nutrients by diffusion form dermis

    • Thick skin is called Stratum Lucidum

      • By the time Keratinocytes are in the stratum lucidum, all organelles have been lost

    • Thin skin has only 4-5 layers. From deep to superficial:

      • 1. Stratum Basale

        • 1 cell thick

        • Cuboidal/columnar mitotic cells that contain keratin filaments

        • The layer that produces more skin cells

      • 2. Stratum Spinosum

        • Thickest layer

        • Synthesis of more keratin and other proteins

        • Keratin forms bundles called tonofibrils

        • Extensions of cells beyond the tonofibrils give an appearance of short “spines”

      • 3. Stratum Granulosum

        • Has flattened cells

        • Contains more keratohyalin granules

      • 4. Stratum Corneum

        • Consists of dead squames (Mostly composed of fully keratinized cells

  • Epidermis is replaced every 15-20 days

  • Most skin cancers originate form cells of the Basal and spinous layers

  • Melanocytes: located in Basal layer. (Creates darker pigment)

    • Synthesize melanin granules and transfer them through long branches/processes into neighboring keratinocytes

    • In keratinocytes, the melanin granules are transported to a region near and around the nuclei as a Supranuclear cap

      • This shades the DNA from the harmful effects of UV radiation

  • Langerhans Cells: Antigen Presenting Cells (APCs)

    • Cytoplasmic processes extend form these dendritic cells between keratinocytes of all the layers, forming a fairly dense network in the epidermis

    • Can bind, process, and present antigens to T lymphocytes

    • Also found in Dermis, where it plays a major role in the skin’s immunity

  • Tactile Cells (Merkel’s Cells): only mechanoreceptor in the epidermis

    • Low threshold and highly sensitive

    • Stimulated by gentle touch

    • Abundant is highly sensitive skin (like fingertips) and at bases of same hair follicles

    • Gentle touch initiates the release of granules (neurotransmitters) which will stimulate the neuronal fiber and carry touch sensation towards the central nervous system

    • Nerve ending it actually found in the Dermis

The Dermis:

  • a layer of connective tissue the supports the epidermis and binds it to the subcutaneous tissue

  • Nutrients (like keratinocytes) diffuse into Thea vascular epidermis from the dermal vasculature through the basement membrane

  • Dermis contains 2 layers:

    • Thin Papillary layer- loose connective tissue

      • Contains Dermal Papillae

      • This layer has anchoring fibrils of type VII collagen that inserts into the basal lamina, binding Dermis to Epidermis

    • Reticular layer - thicker, consists of dense irregular connective tissue

      • Provides elasticity to the skin

  • Richly innervated

    • Sensory neurons form a network in the papillary Eric’s and around hair follicles, ending at epithelial and dermal receptors

    • Autonomic effector nerves in dermal sweat glands and smooth muscle fibers into skin of some areas are postganglionic fibers of sympathetic ganglia

  • Functions of encapsulated skin receptors:

    • Meissner Corpuscles, detect light touch or deformation

      • Numerous in dermal papillae of fingertips, palms, and soles of feet

    • Lameliated Corpuscles, detect pressure and high-frequency vibrations.

      • Found deeply in skin among adipose tissue

    • Ruffini Corpuscles, detect tissue distortion (stretching or twisting)

Hair:

  • Elongated Keratinized structures that form around hair follicles (epidermal invaginations)

  • All skin has at least minimal hair except Glabrous skin (palms, lips, soles of feet, etc.)

Nails:

  • Hard, keratinized derivatives formed in a process similar to that of the stratum corneum and hair

  • Color of nail tells you about possible DX

Glands of skin:

  • Sebaceous glands: secrete oily sebum into the space around hair root

    • Holocrine secretion

    • Consists of large lipid-producing sebocytes, filled with small fat droplets

    • Undergo autophagy

    • Product released is called Sebum

    • Secretions increase at puberty

    • Stimulated primarily by testosterone in men and Androgens in women

  • Eccrine sweat glands: are Merocrine and empty their serous sweat onto the skin via sweat pores

    • Light sweat

  • Apocrine Sweat glands: secrete protein-rich sweat into the follicles of hair in skin of the axillae and perineum

    • Heavy/tick sweat

  • Dark Cells (D cells) = contain dense secretory granules the release bactericidal peptides for immunity

  • Clear Cells (C Cells) = produce sweat.

4 stages of wound repair:

  • Blood clots

  • Macrophages and Neutrophils enter the wound as the inflammation begins and remove debris and bacteria (epithelialization)

  • Fibroblasts proliferate and produce new collagen o form granulation tissue (filling in

  • Epidermis creates scab

    • Scar tissue is excessive collagen