Δομές Ὀργανισμῶν ζῶντος

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.