3A: ANIMAL HISTOLOGY AND ORGANOLOGY

• Tissues are aggregations of cells with similar shape, functional association, and intercellular component.
• Tissues are classified into four basic types:

  1. Epithelial Tissue
  2. Connective Tissue
  3. Muscle Tissue
  4. Nervous Tissue

  These basic types may combine to form larger functional units called organs with their own unique histologic pattern and properties.
• Epithelial Tissue

  • composed of tightly packed cells with very little intercellular substance
  • covers the outer surface and inner lining of the body
  • also the principal tissue of the glands
• Epithelial tissues are characterized by:

  1. Different shapes of cells ranging from squamous (flattened), cuboidal, columnar to intermediate (transitional)
  2. Types which are arranged in a single layer (simple, pseudostratified) or in 2 or more layers (stratified)
  3. Presence of a basement membrane and apical surface
  4. Presence of specializations on its apical surface such as cilia and microvilli
  5. Extremely coherent intercellular adhesions
• Simple Epithelium:

  • single layer of cells lying upon a basement membrane
• Simple Squamous

  • very thin and rectangular in profile
  • arrange closely together to form a sheet
  • seen in mesentery (mesothelium) and inner lining of the blood vessels (endothelium)
• Simple Cuboidal

  • lines each circular thyroid follicle
  • appear as box-like or cube-like cells with more or less equal height and width
• Simple Columnar:

  • observed from a section of the frog’s intestine
  • finger-like projections on the surface of the intestine that are called villi
  • tall and slender epithelial cells with darkly stained nuclei mor or less at the same level, near the bases of the cells
  • among these tall cells are “picher-like” mucus glands called goblet cells
• Stratified epithelium: two or more layers of cells
• Stratified squamous

  • observed a section of frog’s skin
  • shaped ranging from flattened superficial cells to cuboidal and columnar cells of the deeper layers
  • The cells of the deepest/basal most layer lying upon the basal membrane are thrown into folds showing considerable irregularity
• Transitional epithelium

  • observed a section of human urinary bladder
  • called transitional because the shape of the surface cells may vary depending on the degree of distention or stretching of the bladder
  • empty bladder = layer of superficial cells
  • while deeper layers have cuboidal to columnar shaped cells
  • filled bladder = large squamous superficial cells overlying several layers of cuboidal cells
• Ciliated pseudostratified columnar

  • observed from a human trachea
  • all in contact with the basement membrane but not all of them reach the surface
  • cells vary in shape; some with broad bases and others with tall wide parts
  • nuclei are found on wider part of both cell shapes (gives tissue false impression of stratification)
  • cilia on its apical surface
• Connective Tissue

  • comprises a diverse group of tissues that maintain and support the body
  • main component: extracellular matrix composed of protein fibers, an amorphous ground substance and tissue fluid
• Loose Connective Tissue/Areolar Tissue

  • composed of a great deal of ground substance
  • Has 3 types of protein fibers:
  • Collagen fibers: white thick and wavy fibers
  • Elastic fibers: yellow thin and taut/tight fibers
  • Reticular: extremely thin and tends to form branching networks; least in portion, may not be seen readily since they require special staining procedure
• Almost all type of cells (plasma, mast, macrophages) are found in connective tissue, but most numerous and easy to find cells are the spindle-shaped fibroblasts
• Dense Connective tissue

  • observed in tendon
  • predominant fibers are collagen which can arranged parallel to each other
  • has fewer cell types with fibroblasts as the most abundant cell type
  • fibroblast are seen in between collagen fibers
• Connective Tissue with special properties:

  1. Adipose Tissue
  2. Blood
  3. Agranulocytes
  4. Thrombocytes / Platelets
• Adipose Tissue

  • cells are called adipocytes
  • cells have a thin ring of cytoplasm surrounding a very large vacuole left by the dissolved lipids of droplets
  • nuclei are flattened and often seen at the sides of periphery (outside) of the cell
• Blood

  • specialized tissue is composed of elements and a fluid matrix, the plasma
  • formed elements composed of:
  • Erythrocytes / Red Blood cells
  • Leucocytes / White blood cells
• Erythrocytes / Red Blood Cells

  • cells in mammals have no nucleus but nucleated in other vertebrates
  • cells are biconcave (disk) and tend to form aggregates (clusters)
• Leucocytes / White blood cells

  • cells have nucleus
  • have different types depending on whether they have granulated cytoplasm (granulocytes) or non-granulated cytoplasm (agranulocytes)
• Granulocytes

  • Neutrophils
  • Eosinophils
  • Basophils
• Neutrophils:

  • nucleus has 3 or more lobes/segments; most abundant
• Eosinophils:

  • bilobed (2 lobes) nucleus; 1-3% of blood leucocytes
• Basophils:

  • U or J shaped nucleus; least numerous, only about 0.5%
• Agranulocytes

  • Lymphocytes
  • Monocytes
• Lymphocytes

  • small round cells with an indented nucleus and a thin rim of clear blue cytoplasm; 20-25% of the blood leucocytes
• Monocytes

  • larger than lymphocytes with round and eccentric nuclei
  • more cytoplasm than lymphocytes
  • 3-8% of the blood leucocytes
• Thrombocytes / Platelets

  • fragments of a large bone marrow cell called megakaryocytes
• Cartilage

  • cartilage cells are called chondrocytes
  • 1 or more cells can occupy a single cavity/lacuna which is filled with tissue fluid in life
  • this tissue does not have nerves or blood vessels; relies on simple diffusion for nourishment
• Chondrocytes are cartilage cells embedded in a firm, gel-like extracellular matrix
• Three forms of cartilage based on the amount of extracellular matrix and predominant fibers:

  1. Hyaline cartilage
  2. Elastic Cartilage
  3. Fibrocartilage
• Hyaline Cartilage

  • matrix is bluish-white and translucent with predominance of collage fibers
  • lacunae (gaps) with chondrocytes (cartilage tissue) may appear singly or in groups
  • the cartilage itself is in the middle, surrounded by a dense connective tissue capsule called the perichondrium
  • cartilage found in: nose, larynx, trachea, and the bronchi
• Elastic Cartilage

  • similar to hyaline but the numerous fine elastic fibers in its matrix
  • lacunae and chondrocytes appear more closely packed
  • found in external ear, epiglottis, larynx, and walls of the auditory tube
• Fibrocartilage

  • has irregular, dense bundles or collagen fibers
  • lacunae and chondrocytes appear in rows
  • matrix is less abundant
  • perichondrium is not identifiable
• Bone

  • hard calcified matrix deposited with inorganic salts makes it a supporting connective tissue
  • each osteocyte occupies a lacuna (filled with tissue fluid) with several canaliculi radiating from it
• Osteocyte

  • bone cell
• Haversian systems

  1. Haversian canal: central canal of an osteon
  2. Interstitial lamellae: concentrically arranged collagen bundles around the Haversian canal
  3. Lacunae: cavities found along the border of each lamellae; bone cells are no longer visible due to histologic preparations
  4. Canaliculi: fine lines or canals that radiate from the lacunae
• Skeletal /Striated Muscle

  • long cylindrical fibers
  • each muscle fiber contains 2 or more nuclei located in the plasma membrane also called sarcolemma
  • fiber shows alternating light and dark bands
  • striations are due to the arranged of myofilaments
• Cardiac Muscle

  • cross striations on long branching fibers
  • fiber may have 1 or 2 centrally located nucleus