Animal Biology and Homeostasis

ANIMAL PRIMARY TISSUES

Overview of Tissues

A tissue is a group of closely associated, similar cells carrying out specific functions. There are four primary types of animal tissues:

• Epithelial
• Connective
• Muscle
• Nervous

Each of these tissues is composed of cells with characteristic sizes, shapes, and arrangements tailored for specific functions.

Evolution of Tissues

• Parazoa (Sponges): Lack defined tissues and organs, capable of disaggregating and aggregating their cells.
• Eumetazoa (All other animals): Possess distinct and well-defined tissues with irreversible differentiation for most cell types.

Types of Multicellular Animal Tissues

Epithelial Tissues

Line cavities, open spaces, and surfaces.

Connective Tissues

Connect tissues together and provide support.

Muscle Tissues

Generate movement.

Nervous Tissues

Generate and transmit electrical signals.

Epithelial Tissue Classification

Epithelial tissues are classified by the number of layers and the shape of the cells.

• Simple: Single layer.
• Stratified: Multiple layers.

Types of Epithelial Tissues

• Squamous Epithelial
  • Simple Squamous
  • Stratified Squamous
• Cuboidal Epithelial
  • Simple Cuboidal
• Simple Columnar
  • Columnar epithelial cells
  • Goblet cells
• Pseudostratified Ciliated Columnar Epithelial
  • Goblet cells
  • Pseudostratified epithelial cells

Connective Tissue

Used to connect different tissues or give the body structure. It consists of cells (fibroblasts) embedded in a non-cellular matrix. The matrix is composed of a ground substance, usually containing some combination of collagen, elastic, or reticular fibers.

• Collagen fibers: Tough fibrous proteins that remain intact when stretched.
• Elastic fibers: Found in structures that expand and contract, such as in lungs and arteries.
• Reticular fibers: Form a supporting framework in many organs, including the liver, spleen, and lymph nodes.

Types of Connective Tissue

• Loose/Areolar
  • Cells: fibroblasts, macrophages, some lymphocytes, some neutrophils
  • Fibers: few collagen, elastic, reticular
  • Location: around blood vessels; anchors epithelia
• Dense, Fibrous
  • Cells: fibroblasts, macrophages
  • Fibers: mostly collagen
  • Location:
    • Irregular: skin
    • Regular: tendons, ligaments
• Cartilage
  • Cells: chondrocytes, chondroblasts
  • Fibers:
    • Hyaline: few collagen
    • Fibrocartilage: large amount of collagen
  • Location: shark skeleton, fetal bones, human ears, intervertebral discs
• Bone
  • Cells: osteoblasts, osteocytes, osteoclasts
  • Fibers: some collagen, elastic
  • Location: vertebrate skeletons
• Adipose
  • Cells: adipocytes
  • Fibers: few
  • Location: adipose (fat)
• Blood
  • Cells: red blood cells, white blood cells
  • Fibers: none
  • Location: blood

Loose Connective Tissue

Composed of loosely woven collagen and elastic fibers. The fibers and other components of the connective tissue matrix are secreted by fibroblasts.

Dense Fibrous Connective Tissue

Contains large amounts of collagen fibers and few cells or matrix material. Found in areas of the body where stress occurs from all directions, such as the dermis of the skin, tendons, and ligaments.

Cartilage

A connective tissue with a large amount of matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae.

Bone

The predominant skeletal tissue of vertebrates other than sharks and rays. It consists mostly of calcium salts and collagen secreted by bone cells (osteocytes). It's a highly vascular tissue, with a blood supply.

• Canaliculi contain cytoplasmic extensions for communication between osteocytes.
• Compact bone is surrounded by spongy bone.
• A large, central marrow cavity contains yellow marrow (fat) and red marrow (produces blood cells).

Bone Structure:

• Osteon: Circular structure
• Lamellae: Ring units (concentric circles)
• Osteocytes: Located in lacunae

Adipose Tissue

A connective tissue made up of cells called adipocytes. Adipocytes have small nuclei localized at the cell edge and serve as insulation and energy source.

Blood

Consists of:

• Red blood cells (oxygen transport)
• White blood cells (defense)
• Platelets (clotting)
• All within non-cellular plasma (the liquid matrix, consisting of water, proteins, salts, and soluble chemical messengers).

Muscle Tissues

Three kinds: skeletal, smooth, and cardiac.

• Skeletal: voluntary; striated; multinucleated; mainly connected to the skeleton
• Smooth: involuntary; no striations; spindle-shaped cells; found in hollow muscular organs such as in the digestive, circulatory, and urogenital systems
• Cardiac: involuntary; striated; have intercalated discs between cells; found only in the heart

Muscle cells are also referred to as muscle fibers.

Nervous Tissue

Nervous tissue consists of neurons and glial cells (neuroglia).

• Neurons receive and transmit signals.
• Glial cells support and nourish the neurons, destroy pathogens, and modulate transmission of impulses.
• While some neurons transmit signals directly to spinal cord and brain, others relay, process, or store information.
• Some neurons transmit signals from the brain and spinal cord to the muscles and glands.

Neurons have a cell body containing a nucleus and two cytoplasmic extensions:

• Dendrites receive and transmit signals to the cell body.
• A single axon transmits signals away from the cell body.
• A nerve consists of many neurons bound together by connective tissue.

HOMEOSTASIS/THERMOREGULATION

Homeostasis

Aims to keep internal conditions around a set point. If conditions stray too far from the set point, homeostatic mechanisms kick in. The set point can potentially change over time (alteration), but homeostasis will still work towards the new set point.

• Acclimatization: Changes in one organ system to maintain a set point in another organ system (e.g., altitude acclimatization).

Negative Feedback Loop

Counteracts any internal changes (reverses the direction of the change). Most biological systems operate on negative feedback.

• Examples: Temperature, glucose, pH, blood calcium.

Positive Feedback Loop

Maintains and potentially strengthens the response to a stimulus. Not many biological systems are on positive feedback.

• Example: The birth of a human infant.

Thermoregulation

Must maintain a relatively constant internal temperature to keep enzymes efficient and avoid denaturation. Thermoregulatory control is managed by the hypothalamus of the brain.

Heat can be exchanged by four mechanisms:

• Radiation
• Evaporation
• Convection
• Conduction

Ectotherms

Animals that depend on the environment for their body heat. Compared with endotherms, ectotherms have a lower energetic cost to thermoregulation.

• Ectotherms thermoregulate using a combination of behavior and environmental heat.
  • Move between shade and sun.
  • Take advantage of radiant heat in the environment (e.g., hot rocks).
  • Turn the broad side of the body towards or away from the sun.

The term "cold-blooded" is a misnomer! Active reptiles are often warm to the touch, and much warmer than the environment.

Endotherms

Birds and mammals are endotherms. They generate body heat metabolically and have adaptations for thermoregulation.

• Structural adaptations, such as insulating feathers, hair, and fat.
• Physiological mechanisms: the regulation of heat production and heat exchange with the environment.
  • Production of heat: muscle contractions, shivering.
  • Loss of heat: panting, dilation of skin capillaries, and sweating.