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