Chapter 4 Histology Biol 2401

All cells (except blood) anchored to each other or their matrix by intercellular junctions Intercellular Junctions

Tight Junctions Encircle the cell joining it to surrounding cells – zipperlike complementary grooves and ridges Prevents passage between cells – GI and urinary tracts

Desmosomes Patch between cells holding them together – cells spanned by filaments terminating on protein plaque cytoplasmic intermediate filaments also attach to plaque Uterus, heart and epidermis

Gap Junctions Ring of transmembrane proteins form a water- filled channel – small solutes pass directly from cell to cell – in embryos, cardiac and smooth muscle

Glands A gland is a cell or organ that secretes substances in the body or releases them for elimination. Secrete substances – composed of epithelial tissue Exocrine glands connect to surface with a duct (epithelial tube) Endocrine glands secrete (hormones) directly into bloodstream Mixed organs do both – liver, gonads, pancreas Unicellular glands – endo or exocrine – goblet or intrinsic cells of stomach wall

Exocrine Gland Structure Stroma = capsule and septa divide gland into lobes and lobules Parenchyma = cells that secrete Acinus = cluster of cells surrounding the duct draining those cells

Types of Exocrine Glands Simple glands - unbranched duct Compound glands - branched duct Shape of gland – acinar - secretory cells form dilated sac – tubuloacinar - both tube and sacs

Types of Secretions Serous glands – produce thin, watery secretions sweat, milk, tears and digestive juices Mucous glands – produce mucin that absorbs water to form a sticky secretion called mucus Mixed glands contain both cell types Cytogenic glands release whole cells – sperm and egg cells

Holocrine Gland Secretory cells disintegrate to deliver their accumulated product – oil-producing glands of the scalp

Merocrine glands release their product by exocytosis – tears, gastric glands, pancreas, etc. Apocrine glands are merocrine glands with confusing appearance (apical cytoplasm not lost) – mammary and armpit sweat glands Merocrine and Apocrine Secretion

Mucous Membranes Epithelium, lamina propria and muscularis mucosae Lines passageways that open to the exterior: reproductive, respiratory, urinary and digestive – Mucous (movement of cilia) trap and remove foreign particles and bacteria from internal body surfaces

Membrane Types Cutaneous membrane = skin – stratified squamous epithelium over connective tissue – relatively dry layer serves protective function Synovial membrane lines joint cavities – connective tissue layer only, secretes synovial fluid Serous membrane (serosa) –internal membrane – simple squamous epithelium over areolar tissue, produces serous fluid – covers organs and lines walls of body cavities

Membranes Synovial Membrane Cutaneous Membrane Serous Membrane

Tissue Growth Hyperplasia = tissue growth through cell multiplication Hypertrophy = enlargement of preexisting cells – muscle grow through exercise Neoplasia = growth of a tumor (benign or malignant) through growth of abnormal tissue

Changes in Tissue Types Tissues can change types Differentiation – unspecialized tissues of embryo become specialized mature types mesenchyme to muscle Metaplasia – changing from one type of mature tissue to another simple cuboidal tissue before puberty changes to stratified squamous after puberty

Stem Cells Undifferentiated cells with developmental plasticity Embryonic stem cells – totipotent (any cell type possible) source = cells of very early embryo – Pluripotent (tissue types only possible) source = cells of inner cell mass of embryo Adult stem cells (undifferentiated cells in tissues of adults) – multipotent (bone marrow producing several blood cell types) – unipotent (only epidermal cells produced)

Tissue Shrinkage and Death Atrophy = loss of cell size or number – disuse atrophy from lack of use (leg in a cast) Necrosis = pathological death of tissue – gangrene - insufficient blood supply – gas gangrene - anaerobic bacterial infection – infarction - death of tissue from lack of blood – decubitus ulcer - bed sore or pressure sore Apoptosis = programmed cell death – cells shrink and are phagocytized (no inflammation)

Tissue Repair Regeneration – replacement of damaged cells with original cells – skin injuries and liver regenerate Fibrosis – replacement of damaged cells with scar tissue function is not restored – healing muscle injuries, scarring of lung tissue in TB or healing of severe cuts and burns of the skin – keloid is healing with excessive fibrosis (raised shiny scars)

Tissue Engineering Production of tissues and organs in the lab – framework of collagen or biodegradable polyester fibers – seeded with human cells – grown in “bioreactor” (inside of mouse) supplies nutrients and oxygen to growing tissue Skin grafts already available – research in progress on heart valves, coronary arteries, bone, liver, tendons

Wound Healing of a Laceration Damaged vessels leak blood Damaged cells and mast cells leak histamine – dilates blood vessels – increases blood flow – increases capillary permeability Plasma carries antibodies, clotting factors and WBCs into wound

Wound Healing of a Laceration Clot forms Scab forms on surface Macrophages start to clean up debris

Wound Healing of a Laceration New capillaries grow into wound Fibroblasts deposit new collagen to replace old material Fibroblastic phase begins in 3-4 days and lasts up to 2 weeks

Wound Healing of a Laceration Epithelial cells multiply and spread beneath scab Scab falls off Epithelium thickens Connective tissue forms only scar tissue (fibrosis) Remodeling phase may last 2 years