Epithelial 1 Tissue Notes

The Cell as a Basic Functional Unit

The cell is the basic functional unit of the body. Cells and extracellular components are characterized by a distinctive pattern of organization. This organized arrangement reflects the cooperative effort of cells performing a particular function. Cells that function in a collective manner are called tissue.

Four Fundamental Tissue Types

  1. Epithelial Tissue
  2. Connective Tissue: underlies or supports the other three basic tissues, both structurally and functionally.
  3. Muscle Tissue: is made up of contractile cells and is responsible for movement.
  4. Nerve Tissue: consists of nerve cells (neurons) and associated supporting cells of several types.

Tissues are characterized by different structural organization and physiologic properties and reflect different body organs.

Tissues Origin

In the early developing embryo during the gastrulation phase, a trilaminar germ disc is being formed, consisting of:

  • Ectoderm
  • Mesoderm
  • Endoderm

These three layers give rise to all the tissues and organs.

Epithelial Tissue

Body surface, lines body cavity, form glands.

Different Epithelial Tissues

  • Lining epithelium (skin, body cavities)
  • Secretory epithelium:
    • Exocrine glands
    • Endocrine glands

All 3 germ layers are involved:

  • Ectoderm: nasal and oral mucosa, epidermis, cornea, mammary and cutaneous glands.
  • Endoderm: epithelial lining of respiratory, intestinal tracts and glands (i.e., pancreas, liver).
  • Mesoderm: urogenital system, mesothelium.

Epithelial Tissue Function

  • Absorption
  • Secretory
  • Transportation
  • Mechanical protection
  • Receptor function (taste buds, retina, hair cells)

Main Features of Epithelial Tissue

  • Poor extracellular matrix
  • Numerous cell junctions
  • Presence of the basal lamina
  • Nourishment and oxygenation via diffusion

Cell Junctions

  • Tight junctions, zonula occludens
  • Adhering Junctions:
    • Cell-Cell contact: desmosomes, zonula and macula adherens
    • Cell-matrix contact: hemidesmosomes, focal contacts and podosomes
  • Gap Junctions, chemical synapses

The Basement Membrane

The basement membrane is a thin, well-defined layer between the epithelium and the connective tissue. Classically described as exclusively associated with epithelia, the basal lamina is the structural attachment site for overlying epithelial cells and underlying connective tissue.

  • It is also called lamina densa, made of laminins, a type IV collagen molecule, and various associated proteoglycans and glycoproteins.
  • Between the basal lamina and the cell is the lamina lucida, which contains extracellular portions of CAMs, mainly fibronectin and laminin receptors.

Classification of Epithelium

Classified by cell shape and number of layers. Cell shapes include:

  • Squamous: Width (W) > Height (H); example: Esophagus
  • Cuboidal: W = H; example: Kidney
  • Columnar: W < H; example: Stomach

Lining Epithelium

The lining epithelia are classified both on the number of cell layers and the shape of the cells.

Simple Epithelia

  • Squamous: flat or floor cells (pulmonary alveoli, end of capillaries, Loop of Henle)
  • Cuboidal: isoprismatic (lining of the ovaries, collecting tubule of the kidney, ducts)
  • Columnar: bathyprismatic (digestive tract lining, gallbladder)
    • Ciliated variant (uterine tube, small bronchi, paranasal sinuses)
  • Pseudostratified (nasal cavity)
    • Non-ciliated variant (epididymis, male urethra)

Stratified Epithelia

In stratified epithelia, the shape of the cells usually varies from layer to layer.

  • Squamous nonkeratinized: flat cells with nuclei (lips, cervix)
  • Keratinized squamous: flat cells without nuclei (skin)
  • Cuboidal: cubic cells (gland ducts)
  • Columnar: columnar cells (conjunctiva, large ducts)
  • Transitional: dome-shaped cells when the organ is relaxed, flat when the organ is tense (urinary system)

Squamous Nonkeratinized Epithelium

Only the top layer is in contact with the lumen. Not all the cells in this epithelium are squamous.

Keratinized Squamous Epithelium

The squamous keratinized epithelium is characterized by the outer cell layers made of cells transformed into squamous lamellae. This layer is made of keratinocytes which allow the protection of the inner layers.

Epidermis Layers

In the epidermis, four distinct layers can be identified:

  • The stratum basale, also called the stratum germinativum because of the presence of mitotically active cells, the stem cells of the epidermis.
  • The stratum spinosum, also called the spinous layer or prickle cell layer because of the characteristic light microscopic appearance of short processes extending from cell to cell.
  • The stratum granulosum, which contains numerous intensely staining granules.
  • The stratum corneum, which is composed of keratinized cells.

Detailed Layer Description

  • The stratum basale is represented by a single layer of cells that rests on the basal lamina.
  • The stratum spinosum is at least several cells thick. Keratinocytes in this layer are larger than those of the stratum basale.
  • The stratum granulosum is the most superficial layer of the nonkeratinized portion of the epidermis.
  • The stratum lucidum, considered a subdivision of the stratum corneum, is not distinguishable throughout the epidermis. It is present in the epidermis of the palm of the hand and sole of the foot. It has flattened cells with large keratohyalin granules that contain Flaggrin. Superimposed on the granular layer, it appears as a thin, clear and refractive line. The cells are devoid of granules.

Cells of the Epidermis

  • Keratinocytes: highly specialized epithelial cells
  • Melanocyte: has long dendritic processes that contain accumulated melanosomes and extend between the cells of the epidermis.
  • Langerhans cell: a dendritic cell often confused with a melanocyte but is actually part of the mononuclear phagocytotic system and functions as an antigen-presenting cell of the immune system in the initiation of cutaneous hypersensitivity reactions (contact allergic dermatitis).
  • Merkel cells: sensitive mechanoreceptor cells associated with sensory nerve endings.

Melanocytes

The epidermal melanocytes produce and secrete the pigment melanin. Melanin is produced by the oxidation of 3,4-dihydroxyphenylalanine (DOPA), a reaction that occurs in lysosome-related organelles called premelanosomes (from the Golgi apparatus). As more melanin is produced, the premelanosome becomes obscured until the mature melanin granule, the melanosome, is formed. Premelanosomes are concentrated near the Golgi apparatus, while mature melanosomes are most commonly in and at the ends of the processes. Melanosomes and their melanin contents are transferred to neighboring keratinocytes by pigment donation. This process involves the phagocytosis of the tips of the melanocyte processes by keratinocytes.

Merkel Cells

They are dendritic cells located in the stratum basale and are most abundant in skin where sensory perception is acute, such as the fingertips. Merkel cells are bound to adjoining keratinocytes by desmosomes and contain intermediate (keratin) filaments in their cytoplasm. Merkel cells are best characterized by the presence of dense-cored neurosecretory granules that resemble those found in the adrenal medulla. Merkel cells are closely associated with the expanded terminal bulb of afferent myelinated nerve fibers. The neuron terminal loses its Schwann cell covering and immediately penetrates the basal lamina, where it expands into a plate-like ending called a disc receptor that lies in close apposition to the base of the Merkel cell. The combination of the neuron and epidermal cell, called a Merkel corpuscle, is a sensitive mechanoreceptor.

Dermis

The dermis is composed of:

  • The papillary layer, the more superficial layer, consists of loose connective tissue. The collagen fibers contain predominately type I and type III collagen molecules. The papillary layer is relatively thin and includes the substance of the dermal papillae and dermal ridges. It contains blood, nerve processes that either terminate in the dermis or penetrate the basal lamina to enter the epithelial compartment.
  • The reticular layer lies deep to the papillary layer. It is characterized by thick, irregular bundles of mostly type I collagen and by coarser elastic fibers.

Hypodermis

The hypodermis (subcutaneous layer) is made of:

  • Areolar layer, characterized by adipose tissue.
  • Lamellar layer, in which collagen is arranged in parallel bundles.

Cuboidal Stratified Epithelium

Columnar Stratified Epithelium

Transitional Epithelium (Urothelium)

The transitional epithelium (urothelium) is composed of at least three layers:

  1. The superficial layer contains single or multinucleated large, polyhedral cells that bulge into the lumen. They are frequently described as dome-shaped or umbrella cells because of their apical surface curvature.
  2. The intermediate cell layer contains pear-shaped cells that are connected to each other and the overlying dome-shaped cells by desmosomes. The thickness of this layer varies with the state of the urinary tract expansion.
  3. The basal cell layer consists of small cells containing a single nucleus that rests on the basement membrane. This layer contains stem cells for the urothelium.