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Levels of Organization of Living Matter

• In multicellular organisms, similar cells with similar morphological and functional characteristics group together to form tissues, a subject of histology.
• More complex organisms have tissues with different morphologies and functions that combine to create organs.
• Organs and organ systems, which are groups of diverse organs working together, create an intricate organism.
• Robert Hooke first used the term "cell" upon observing small squares (or "cells") in cork through a rudimentary microscope.
• Cellular theory (which developed in 1855):
  • All living organisms are composed of cells.
  • All cells come from pre-existing cells.
  • Cells are the fundamental units of life.
• All cells have certain morphological features:
  • A cell membrane separating internal and external environments.
  • A region containing DNA.
  • Cytoplasm (a semi-fluid region containing other cellular components).
• Cells exhibit variation in form, size, and function (differentiation).
• Differentiation begins with a single initial cell (zygote), which forms from the fusion of male and female gametes.
  • Totipotent = initial cells with potential to develop into any cell type
  • Pluripotent = cells can develop into some but not all cell types
  • Multipotent = cells can develop into different types of cells within a particular family
  • Unipotent = cells can develop only into one type of cell.
• Specialized tissues (such as epithelial, connective, muscle, and nervous tissue) emerge after differentiation.
• Embryonic development involves three primary germ layers:
  • Ectoderm (outermost layer)
  • Mesoderm (middle layer)
  • Endoderm (innermost layer)

Epithelial Tissue

• Epithelial tissues are formed from closely packed cells with minimal extracellular material.
• Epithelial tissues form external surfaces and lining of internal cavities.
• Scarcity of extracellular matrix contributes to structural features such as lack of vascularity and a supportive basement membrane.
• The epithelial tissue obtains oxygen and nutrients from underlying connective tissue via the basement membrane.
• Epithelial tissue is vital for protection, secretion, absorption, and transport.

Epithelial Specializations

• Apical surface
  • Exposed to external environment or inner lumen.
  • Specialized structures, such as microvilli and cilia, increase the surface area and efficiency of absorption.
• Lateral surface
  • Cells are bound tightly together by intercellular junctions.
  • Tight junctions are involved in controlling permeability.
  • Adherens, desmosomes, and gap junctions are involved in cellular adhesion.
• Basolateral surface
  • Area in contact with the basement membrane.
• Cilia for motility and absorption.
• Microvilli for absorption and secretion.

Types of Epithelial Tissue

• Simple (single layer) and stratified (multiple layers)
• Squamous (flat), cuboidal, columnar, pseudostratified.
• Based on number of layers and cell shape.

Glandular Epithelium

• Glands (secretory) are derived from epithelial cells.
• Endocrine glands secrete hormones directly into the bloodstream.
• Exocrine glands secrete their products into ducts or cavities, reaching an external or internal surface of an organ.
  • Unicellular glands (e.g., goblet cells)
• Multicellular glands have a secretory portion (adenomeres) and duct.
• Classification based on shape of adenomere and type of secretion.

Connective Tissue

• Connective tissues provide structural support and connect different tissues in an organism.
• Composed of cells and an extracellular matrix (a combination of structural proteins and ground substance).
  • Fibers (collagen, elastic, and reticular) give support and strength.
  • Ground substance fills the spaces between cells and fibers, and may be gelatinous, liquid or rigid.
• Consists of:
  • Fibroblast cells
  • Macrophage cells
  • Mast cells
  • Adipocytes
• Loose connective tissue and dense connective tissue
  • Loose is flexible and supports/protects
  • Dense gives strength and structure

Specialized Connective Tissues

• Cartilage:
  • Firm, flexible support
  • Contains chondrocytes embedded in a matrix of collagen and elastic fibers.
  • Hyaline, elastic, and fibrocartilage have different structural properties depending on function and location.
  • Supporting structures for bone and joints.
• Bone:
  • Hard, calcified, and rigid support
  • Provides a framework for the body. Made up of bone cells (osteocytes) in calcified matrix.
• Blood: -Specialized connective tissue that transports oxygen, nutrients, waste, and other substances
  • Is made up of plasma (the ground substance) and blood cells.

Nervous Tissue

• Nervous tissue forms the brain, spinal cord, and nerves that relay electrical signals throughout the body, governing actions and responses.
• Composed of neurons, which transmit the signals, and glial cells, which provide support and nutrients to the neurons.
  • Neurons exhibit excitability and conductivity: transmitting electrical signals.
  • Supporting cells are the neuroglial cells to facilitate signal transmission and neuronal survival.

Muscle Tissue

• Muscle tissue is responsible for movement in the body.
• There are three types of muscle tissue: skeletal, cardiac, and smooth.
• Skeletal muscle: voluntary control, large movements of the body.
  • Formed from fibers that are under somatic control to produce movement.
• Cardiac muscle: involuntary heart contractions.
  • Formed from fibers and has a spontaneous rhythmic contraction of the heart fibers. Coordinated contraction due to intercalated discs.
• Smooth muscle: involuntary action in internal organs.
  • Found in the walls of internal organs (e.g., stomach, intestines, blood vessels), involved in movement of substances through internal organs.

Adipose Tissue

• Stores energy in the form of fat.
• Composed of adipocytes containing fat droplets, that displace the nuclei and cytoplasm to the periphery.
• Two types: white and brown adipose tissue.