Block 1 Learning Objectives

Describe the common features of epithelium.

• Close apposition of cells.

• Free surface of epithelial cells is adjacent to the space.

• Basal surface is adjacent to connective tissue.

• Sheets of epithelial cells may be modified into tubes forming glands.

• Absence of blood vessels within epithelial layer

Lining Epithelium

Sheet of closely apposed cells, separating space from underlying tissue, acts like a cover

Glandular epithelium

Invagination or aggregation of epithelium that forms a solid tissue structure

Tight Junctions

Hold cells together (one stich)

Gap junctions

Connection between cells which joins their cytoplasm. Allows the  passage of molecules freely between cells

Desmosomes

bind cells to other cells (stronger than tight junctions, multiple stiches)

Hemidesmosomes

(half a desmosome) bind cells to basement membrane

What are the functions of epithelium?

To protect the tissues that lie beneath it from injury such as desiccation, invasion by pathogens, and physical trauma

The regulation and exchange of molecules between the underlying tissues another compartment

The secretion of hormones into the blood vascular system, and/or the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts

basolateral

A surface not facing the lumen; faces the basement membrane or adjacent cells

How is epithelium classified in terms of cell layers?

One layer, simple. More than one layer, stratified. One cell with nuclei at different heights, but all cells are in contact with the basement membrane, pseudostratified epithelium.

Describe non-glandular epithelium and where it is located

Non-grandular, do produce/secrete extracellular products, cover outside of organ or inside structure (skin, UB, oral mucosa, esophagus, vasculature)

Describe Glandular Epithemlium

Glands are epithelial lines structures that produce a chemical that is secreted in an extracellular fashion, can be endocrine or exocrine

Exocrine glands

Secrete into lumen or space, excretion through merocrine, holocrine, or apocrine

Endocrine glands

Secrete product into extracellular space which enters bloodstream

Simple squamous function

can be continuous or discontinuous – This is particularly important inside of blood vessels. permeable and allows the passage of molecules

simple cuboidal function

typically provide secretory and absorptive functions

simple columnar function

often has a modification which enhances its function at a particular location (cilia and microvilli

stratified squamous

suited to regions where protection from abrasion is necessary

What is cornification?

The cells in the uppermost layer of stratified squamous epithelium can lose their nuclei and organelles leaving behind a dead cell with keratin and other inert structural elements

Composition of connective tissue

Fiber component, ground substance, and cellular component

Fiber component of connective tissue

Collagen, reticulin, and elastin

Reticulin is a form of collagen (type 3) but is usually classified differently because of its unique function

What is collagen and how is it composed?

Most abundant fiber component, made of fibrils

What are the types of collagen and what do they form?

Type I – Most abundant. Tendons, dermis (skin), arterial walls. #tough. Dense regular vs irregular.

Type II – Hyaline and elastic cartilage

Type III – Makes up reticulin fibers, granulation tissue

Type IV – Basal lamina, ocular lens, basement membrane of glomerular capillaries

 Type V – Interstitial connective tissue

reticulin fibers

Acts like a net or meshwork to hold cells of organ together. Silver stain. Pancreatic tissue, liver, spleen, kidney

elastic fibers

Derived from elastin and provides the ability to stretch. Found in tendons, ligaments, arteries.

Ground substance

Spongey space filler, extrafibrillar matrix. When an animal becomes dehydrated, water leaves this interstitial space.

cellular components of connective tissue

Fibroblasts, macrophages, leukocytes, adipose

Fibroblasts

Most common cells in CT, makes collagen, elongated with little cytoplasm,

Macrophages

Derived from blood monocytes, phagocytose debris and is the surveillance system for immune system

Adipocytes

Cells that store lipids between collagen fibers

Define morphologic characteristics of each connective tissue

Embryonic connective tissue - found in embryo and umbilical cord, loose collagen fibers, star shaped (stellate) cells, and LOTS of ground substance

Fibrous connective tissue – loose (areolar), dense fibrous, and reticulin

Loose (areolar) connective tissue

Network of type 1 collagen and reticulin fibers with ground substance, cells, and vasculature

Dense fibrous connective tissue

Closely packed collagen fibers with interspersed fibroblasts and fibrocytes

primary component of tissue that need to be tough yet pliable, tendons and ligaments

regular or irregular

Reticular tissue

Composed of reticulin fibers, skeleton for organs with high parenchymal component

What is cartilage and what makes it up?

flexible and compressible connective tissue found at specific locations

composed of chondrocytes embedded in extracellular matrix composed of collagen (type II), ground substance, and elastic fibers

What are the three major types of cartilage and what anatomical locations are they present?

Hyaline cartilage – joint surfaces

Fibrocartilage - Intervertebral disks, Symphysis (type of connection) between certain bones.

Elastic cartilage – ear and nose, similar to hyaline cartilage

Hyaline cartilage

Chondrocytes surrounded by chondroid matrix, chondrocytes arranged in clusters of linear arrays

Provides compressible surface, seen on joint surfaces

Elastic cartilage

Similar histology to hyaline, but found in ear and nose

Fibrocartilage

Higher collagen content than hyaline, so tougher yet flexible, but less cartilage than fibrous connective tissue

Not as many chondrocytes

Intervertebral disks, Symphysis (type of connection) between certain bones.

Describe the function of bone

Facilitate normal growth

Regulate mineral homeostasis

Adapt to stress/altered loading

Repair microinjury

What are the major organic and inorganic constituents of bone?

Organic – protein and non-collagenous proteins

Inorganic – mineral, not identifiable in histo

What are the structure and function of osteoblasts, osteocytes, and osteoclasts

Osteoblast – make bone

Osteocyte – osteoblasts trapped in bone/lacuna, maintain bone

Osteoclast – multinucleated cells that remove bone

Canaliculi

anatomic structures that facilitates communication between osteocytes

Explain the basic process of osteoclastic resorption, naming one important stimulus for resorption?

Physiologic stimuli that induce osteoclastic resorption generally operate through the osteoblast, the osteoblast secretes a molecule called ODF that binds RANK on osteoclasts, causing osteoclast activation

osteoblast initially releases collagenases that resorb the non-mineralized lamina limitans, osteoclast binds to the exposed mineralized bone, forming a tight seal. Carbonic acid/anhydrase secreted and the matrix is dissolved

What is the difference between osteoid and bone?

Osteoid – organic extracellular matrix of collagen that is secreted by osteoblast

Bone – mineralization of osteoid

What is the difference between cancellous and compact bone and their other names?

Compact/cortical – densely packed bone matrix in laminae

Cancellous/trabecular – spongey bone

What is the difference between woven and lamellar bone?

Woven – rapidly produced, immature, and fragile

Lamellar – mature, reorganized bone (normal adult bone)

osteon

Haversian system

Functional unit of bone containing blood vessels, nerves, and osteogenic cells

Modeling

Bone changes shape

Remodeling

Old bone is replaced by new bone

Remodeling/resorption steps

activation, osteoclastic resorption, reversal, formation

Periosteum (inactive)

Outer layer of tissue surrounding bone

Composed of fibrous layer (vessels and nerves) and osteogenic layer (where osteoblasts are) (may not be identifiable in normal adult bone)

Periosteum (active)

Can produce bone under certain conditions (woven bone)

Osteogenic layer easily identifiable during these periods

Endosteum

Lining on inside of bone

List two instances where bone remodeling plays an important role in NORMAL bone function

Normal Growth

Repair of ‘wear and tear’

Microcracks

To accommodate ‘moving vasculature’

Repositioning of osteons to accommodate stress

Mineral homeostasis

What does the process of membranous ossification involve? Where does it occur?

Primitive mesenchyme, Differentiation to osteoblastic phenotype, Production of bone laid on top of previous bone

Flat bones (skull, mandible) and bones growing in width, create cortical bone

process of endochondral ossification

Cartilage replaced with bone, create trabecular bone

Zone of reserve cartilage, zone of proliferation, zone of hypertrophy, zone of calcification, primary spongiosa, secondary spongiosa

difference between a primary and secondary ossification center

primary center of ossification  - begins in the middle of a long bone as blood vessels invade into the cartilage model

secondary center of ossification - vessels invade the periphery of the bone

 primary and secondary centers of ossification progressively expand with endochondral ossification occurring at the edges of an ossification center

Define the articular epiphyseal complex

Endochondral ossification that occurs at the edge of a secondary ossification center, immediately beneath an articular surface

List the three processes which are essential for endochondral ossification to occur normally

Calcification of mineral matrix, death of chondrocyte, Vascular ingrowth bringing osteogenic cells that deposit bone on mineralized cartilage

What is the physis and the significance of this finding in a histologic section?

Lengthens bone

Once the ossification centers get to a certain point, there becomes a thin line of growing cartilage between the primary and secondary ossification center

Articular cartilage

Hyaline cartilage

Avascular, nourished by synovial fluid

Synovium

Lines the innermost aspect of the joint capsule

Type a – macrophage like, phagocytic

Type b – fibroblast like, produce hyaluronate

Name the general functions of muscle.

Movement - Both movement of the skeleton as well as internal organs.

Maintenance of posture

Joint stabilization

Heat generation

Three major types of muscle and their locations

Skeletal muscle – Attachments to skeletal structures (bones), Abdominal wall, Esophagus

Cardiac muscle – heart

Smooth muscle – Tubular organs (GI tract, repro tract), hair follicles, eye, vasculature

Skeletal muscle

Attach and move skeleton, 40% of body weight, Cells with obvious striations, Voluntary contractions

Cardiac muscle

Function is to reduce size of the chambers of the heart (atria and ventricles)

System of contractile proteins and smooth ER is similar to skeletal muscle

Smooth muscle

Muscle cells are spindle shaped, one nucleus

Grouped into sheets: often running perpendicular to each other

Peristalsis

No striations (no sarcomeres)

Contractions are slow, sustained and resistant to fatigue

Does not always require a nervous signal: can be stimulated by stretching or hormones

Smooth muscle locations

6 major locations: 1. inside the eye 2. walls of vessels 3. respiratory tubes 4. digestive tubes 5. urinary organs 6. reproductive organs

Myofiber

Muscle cells, have striations, called fibers because they are elongated

Multinucleated in skeletal, 1-2 nuclei cardiac

Myofibril

made of consecutive linked sarcomeres/long rows of repeated sarcomeres, make up the myofibers

Sarcomere

Basic unit of contraction made of myofilaments

Boundaries: Z discs

Made of myosin (thick), actin (thin), titin (elastic)

Myosin is anchored to m-line, actin to z-line

Epimysium

Connective around an entire muscle

Allows muscle to move independent from other muscles

Perimysium

Connective tissue surrounding an entire group of myofibers

Endomysium

Connective tissue surrounding a single myofiber

What structures are unique  to skeletal and cardiac muscle?

Sarcomere structure (actin, myosin, troponin, tropomyosin)

How does a sarcomere function?

Myosin pulls actin along its length

When muscle is relaxed, tropomyosin blocks binding sites on actin

Calcium attaches to troponin, displacing tropomyosin; they roll away, exposing the active site on actin

Bound ATP on myosin is hydrolyzed (ADP+P), allowing myosin to attach to a binding site on actin, pulling actin toward the m-line, shortening the sarcomere

Myosin is bound to actin until ATP until a new ATP binds

T tubules

Invagination of sarcolemma (cell membrane) into the cell, forming a tubular network INSIDE of the muscle cell at the junction of the A and I bands

This network allows for the conduction of depolarization impulses from the outside of the cell to around the sarcomeres

Simultaneous contraction of sarcomeres

Sarcoplasmic reticulum (smooth ER)

Tubules of SR surround the myofibrils

Cross-channels called “terminal cisternae”

Stores Ca++ and releases when muscle stimulated to contract

Types of skeletal muscle fibers

Fast - “white fibers” large, predominantly anaerobic, fatigue rapidly (rely on glycogen reserves); most of the skeletal muscle fibers are fast

Slow - “red fibers” – half the diameter, 3X slower, but can continue contracting; aerobic, more mitochondria, myoglobin

Intermediate

Myocardium

Thick layer of muscle that forms bulk of heart

Compare and contrast cardiac muscle and skeletal muscle

Cardiac muscle cells are single cells joined together (unlike skeletal muscle)

Cells branch (unlike skeletal muscle)

1-2 nuclei in center (unlike skeletal muscle)

Each cell beats separately without any stimulation (inherent rhythmicity)

Intercalated discs

Where cardiac muscle cells join, allows for simultaneous contraction

Purkinje Fibers

Highly modified cardiomyocytes that are part of the conduction system of the heart

These cells do not have T tubules

They are connected by gap junctions and desmosomes instead of intercalated discs

Purkinje fibers are easiest observed on the endocardial surface

What is the function of smooth muscle?

sustained involuntary contraction

Smooth muscle contraction

• Contractile proteins are anchored within the cell and are arranged in a criss-cross manner

• Contractile proteins of smooth muscle include Actin, Tropomyosin and Myosin

• With membrane excitation, calcium is released by the SR.

  • Calcium binds to calmodulin

  • Calcium calmodulin complex activates myosin light chain kinase (MLCK)

  • MLCK phosphorylates myosin and allows it to bind to actin

Myoepithelium

located in and around glands where extracellular product produced by the epithelial cells needs to be moved by contractile action towards the duct system

Myofibroblasts

seen in healing wounds and assist in maturation and contraction of the granulation tissue (immature fibrous connective tissue) as it matures and contracts

Fasciculi

Bundle of muscle fibers