Tissue Level of Organization and Genetic Basis

From Genotype to Phenotype

• The uniqueness of each newborn is determined by the DNA in the sperm and oocyte that combine to form the first diploid cell, the human zygote.

• Each human body cell contains a full complement of DNA stored in 23 pairs of chromosomes.

• A karyotype is a systematic arrangement of these chromosome pairs (refer to Figure 28.24).

• Sex chromosomes determine the sex of an individual:

  • XX in females.

  • XY in males.

• Autosomal chromosomes comprise the remaining 22 chromosome pairs.

• Each chromosome carries hundreds or thousands of genes, which are segments of DNA that code for the assembly of specific proteins; genes are "expressed" as proteins.

• An individual's genotype refers to their complete genetic makeup.

• A person's phenotype encompasses the characteristics expressed by their genes, whether physical, behavioral, or biochemical.

• In genetics, a "parent" describes the individual organism(s) contributing genetic material (gamete cells and chromosomes) to offspring, distinct from social and legal concepts of parenthood.

• An individual inherits one chromosome from each pair (a full complement of 23) from each parent during conception.

• Homologous chromosomes, which form a complementary pair, carry genes for the same characteristics at the same location on the chromosome.

• An allele is one copy of a gene, inherited from each parent, and alleles in complementary pairs may vary (e.g., an allele for dimples vs. an allele for smooth skin).

The Four Types of Tissues

• A tissue is defined as a group of cells found together in the body that share a common embryonic origin, exhibit similar morphological features, and are arranged in an orderly pattern to achieve specific functions.

• From an evolutionary standpoint, tissues are characteristic of more complex organisms; multicellular protists, for example, lack cells organized into tissues.

• The human body comprises 4 main categories of tissues, each with specific functions crucial for overall body health and maintenance:

  1. Epithelial tissue (epithelium): Sheets of cells covering exterior surfaces, lining internal cavities and passageways, and forming certain glands.

  2. Connective tissue: Binds cells and organs, providing protection, support, and integration throughout the body.

  3. Muscle tissue: Excitable, contracting to provide movement.

     • 3 major types: skeletal (voluntary), smooth, and cardiac (in the heart).

  4. Nervous tissue: Also excitable, propagating electrochemical signals (nerve impulses) for communication between different body regions (refer to Figure 4.2).

• Disruptions in tissue structure indicate injury or disease, which can be detected through histology, the microscopic study of tissue appearance, organization, and function.

• The next level of biological organization is the organ, where several tissue types combine to form a functional unit.

• This chapter focuses on epithelial and connective tissues, with muscle and nervous tissues discussed briefly.

Embryonic Origin of Tissues

• The zygote (fertilized egg) is a single cell resulting from the fusion of an egg and sperm.

• Post-fertilization, the zygote undergoes rapid mitotic cycles, generating numerous cells to form the embryo.

• The initial embryonic cells are totipotent, meaning they can differentiate into any cell type in the body and have the capacity to divide, differentiate, and develop into a new organism.

• As cell proliferation continues, 3 major cell lineages, or germ layers, are established within the embryo, from which all tissues and organs of the human body eventually form. These layers are identified by their relative positions:

  • Ectoderm (ecto- = “outer”)

  • Mesoderm (meso- = “middle”)

  • Endoderm (endo- = “inner”)

• Figure 4.3 illustrates the tissue and organ types associated with each germ layer.

  • Epithelial tissue originates from all 3 germ layers.

  • Nervous tissue primarily derives from the ectoderm.

  • Muscle tissue originates from the mesoderm.

Tissue Membranes

• A tissue membrane is a thin layer or sheet of cells that covers the body's exterior (e.g., skin), organs (e.g., pericardium), internal passageways leading to the exterior (e.g., stomach mucosa), and the lining of movable joint cavities.

• There are 2 basic types of tissue membranes: connective tissue membranes and epithelial membranes.

Connective Tissue Membranes

• Formed solely from connective tissue, these membranes encapsulate organs (e.g., kidneys) and line movable joints.

• A synovial membrane is a specific type of connective tissue membrane that lines the cavity of a freely movable joint (e.g., shoulder, elbow, knee).

  • Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity.

  • Hyaluronan effectively traps available water to form synovial fluid.

  • Synovial fluid is a natural lubricant that allows bones in a joint to move freely with minimal friction.

  • This fluid readily exchanges water and nutrients with blood, similar to other body fluids.

Epithelial Membranes

• Composed of epithelium attached to an underlying layer of connective tissue (e.g., skin).

• Mucous membranes (mucosae):

  • Composite of connective and epithelial tissues.

  • Line body cavities and hollow passageways that open to the external environment (e.g., digestive, respiratory, excretory, and reproductive tracts).

  • Mucus, produced by epithelial exocrine glands, covers the epithelial layer.

  • The underlying connective tissue is called the lamina propria (literally “own layer”), which supports the fragile epithelial layer.

• Serous membranes:

  • An epithelial membrane composed of mesodermally derived epithelium called mesothelium, supported by connective tissue.

  • Line the coelomic cavities of the body (cavities that do not open to the outside) and cover the organs within these cavities.

  • Essentially membranous bags with mesothelium lining the inside and connective tissue on the outside.

  • Serous fluid, secreted by the cells of the thin squamous mesothelium, lubricates the membrane and reduces abrasion and friction between organs.

  • Identified by location:

    • 3 serous membranes line the thoracic cavity: the 2 pleura (covering the lungs) and the pericardium (covering the heart).

    • A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many digestive organs.

• Skin (cutaneous membrane):

  • An epithelial membrane that is a stratified squamous epithelial membrane resting on top of connective tissue.

  • Its apical surface is exposed to the external environment and covered with dead, keratinized cells that protect the body from desiccation and pathogens.

Epithelial Tissue: Structure and Function

• Most epithelial tissues form large sheets of cells covering all body surfaces exposed to the outside world and lining the exterior of organs.

• Epithelium also constitutes a significant portion of the body's glandular tissue.

• Areas lined by epithelium include airways, the digestive tract, and the urinary and reproductive systems.

• Endothelium (plural = endothelia) is a type of epithelium that lines hollow organs and body cavities not connected to the exterior, such as blood vessels and serous membranes.

• Embryonic origins of epithelia:

  • Epithelia lining the skin, parts of the mouth and nose, and the anus develop from the ectoderm.

  • Cells lining the airways and most of the digestive system originate in the endoderm.

  • The endothelium lining vessels in the lymphatic and cardiovascular system derives from the mesoderm.

• Shared structural and functional features of all epithelia:

  • Highly cellular with minimal or no extracellular material between cells.

  • Adjacent cells form specialized cell junctions between their cell membranes.

  • Exhibit polarity, meaning structural and functional differences between the exposed apical (facing surface) and basal (close to underlying structures) surfaces.

  • The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium and separates it from underlying connective tissue.

  • The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue, forming a basement membrane that anchors the epithelium.

  • Epithelial tissues are nearly completely avascular; no blood vessels cross the basement membrane to enter the tissue, so nutrients are obtained by diffusion or absorption from underlying tissues or the surface.

  • Many epithelial tissues can rapidly replace damaged and dead cells, a characteristic of surface epithelium, allowing quick repair in airways and digestive tracts.

Generalized Functions of Epithelial Tissue

• Protection: Provides the body's first line of protection against physical, chemical, and biological wear and tear.

• Control Permeability: Epithelial cells act as gatekeepers, controlling permeability and allowing selective transfer of materials across a physical barrier; all substances entering the body must cross an epithelium.

• Selective Transport: Some epithelia possess structural features allowing selective transport of molecules and ions across cell membranes.

• Secretion: Many epithelial cells are capable of secretion, releasing mucus and specific chemical compounds onto their apical surfaces (e.g., digestive enzymes from small intestine epithelium, mucus in the respiratory tract to trap microorganisms and particles).

• A glandular epithelium contains many secretory cells.

The Epithelial Cell

• Typically characterized by a polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces.

• Specific structures, like cilia, are adaptations for particular functions.

• Cilia: Microscopic extensions of the apical cell membrane, supported by microtubules.

  • Beat in unison to move fluids and trapped particles.

  • Line the ventricles of the brain, helping circulate cerebrospinal fluid.

  • In the airway, ciliated epithelium forms a mucociliary escalator that sweeps dust particles and pathogens trapped in secreted mucus towards the throat.

  • Nasal cilia sweep the mucus blanket downwards toward the throat.

  • Transported materials are usually swallowed and end up in the acidic environment of the stomach.

Cell to Cell Junctions

• Epithelial cells are closely connected with no intracellular material separating them.

• 3 basic types of connections allow varying degrees of interaction between cells:

  1. Tight junctions:

     • Separate cells into apical and basal compartments.

     • Eliminate extracellular space between adjacent epithelial cells, blocking the movement of substances through this space.

     • Enable epithelia to act as selective barriers.

  2. Anchoring junctions:

     • Help stabilize epithelial tissues by providing strong and flexible connections, common on lateral and basal surfaces.

     • 3 types:

       • Desmosomes: Occur in patches on cell membranes, composed of structural proteins on the inner surface. The adhesion molecule cadherin is embedded in these patches and links with cadherin molecules of adjacent cells, crucial for holding cells together.

       • Hemidesmosomes: Resemble half a desmosome, linking cells to the extracellular matrix (e.g., basal lamina). They use adhesion proteins called integrins instead of cadherins.

       • Adherens junctions: Use either cadherins (for cell-to-cell linking) or integrins (for cell-to-matrix linking). Characterized by the contractile protein actin on the cytoplasmic surface, which can connect isolated patches or form a belt-like structure, influencing epithelial tissue shape and folding.

  3. Gap junctions:

     • Form an intercellular passageway between the membranes of adjacent cells.

     • Facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells.

     • Enable electrical and metabolic coupling, coordinating function in large groups of cells.

Classification of Epithelial Tissues

• Epithelial tissues are classified based on the shape of their cells and the number of cell layers (refer to Figure 4.6).

Cell Shapes

• Squamous: Flattened and thin.

• Cuboidal: Boxy, with width approximately equal to height.

• Columnar: Rectangular, taller than wide.

Number of Cell Layers

• Simple epithelium: Consists of a single layer of cells, where every cell rests on the basal lamina.

• Stratified epithelium: Composed of more than one layer, with only the basal layer of cells resting on the basal lamina.

• Pseudostratified (pseudo- = “false”): Describes tissue with a single layer of irregularly shaped cells that gives the appearance of more than one layer because nuclei appear at different levels, though all cells contact the basal lamina (some may not reach the apical surface).

• Transitional: A specialized form of stratified epithelium where the shape of the cells can vary, found exclusively in the urinary system.

Simple Epithelium

• The shape of a simple epithelium's cells reflects their function.

• Simple squamous epithelium:

  • Cells have a thin, scale-like appearance with flat, horizontal, and elliptical nuclei.

  • Endothelium: The epithelial tissue lining vessels of the lymphatic and cardiovascular system, made of a single layer of squamous cells.

  • Found where rapid passage of chemical compounds occurs, such as the alveoli of the lungs (gas diffusion), segments of kidney tubules, and the lining of capillaries.

  • Mesothelium: A simple squamous epithelium forming the surface layer of serous membranes that line body cavities and internal organs, primarily providing a smooth, protective surface and secreting lubricating fluid.

• Simple cuboidal epithelium:

  • Box-like cells with round nuclei generally located near the center of the cell.

  • Active in secretion and absorption of molecules.

  • Observed in the lining of kidney tubules and in the ducts of glands.

• Simple columnar epithelium:

  • Tall, column-like cells with elongated nuclei typically located in the basal end.

  • Active in absorption and secretion of molecules.

  • Forms the lining of some sections of the digestive system and parts of the female reproductive tract.

  • Ciliated columnar epithelium: Composed of simple columnar epithelial cells with cilia on their apical surfaces, found in the lining of the fallopian tubes and parts of the respiratory system (cilia aid in removing particulate matter).

• Pseudostratified columnar epithelium:

  • Appears stratified due to irregularly shaped and differently sized columnar cells with nuclei at varying levels, but is actually a single layer where all cells contact the basal lamina (though not all reach the apical surface).

  • Found in the respiratory tract, where some cells are ciliated.

  • Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional cell types, such as goblet cells, which are mucus-secreting unicellular