Animal Organization: From Cells to Systems

Posted on Nov 19, 2024 in Biology

Animal Organization Hierarchy

All animal cells originate from a zygote, the result of fertilization between an ovum and a spermatozoon. Fertilization triggers changes in the zygote, preparing it for segmentation, DNA and protein synthesis, and cytoplasmic reorganization during embryonic development. The zygote undergoes a series of mitotic divisions. This development is characterized by the formation of infrastructure and the determination of different cell functions. Cells specialize into tissues, organs, and organ systems.

Grades of Animal Organization

  • Protoplasmic Organization: Found in single-celled choanoflagellates.
  • Cellular Organization: A group of functionally differentiated cells that do not organize into tissues (sponges).
  • Cell-Tissue Organization: A tissue is formed when a group of similar cells, usually with a common embryonic origin, are adapted for specific roles (cnidarians).
  • Tissue-Organ Organization: Generally, the functional tissue performs the main function of the organ, and support tissues perform a support mission.
  • Organ-System Organization

Animal Tissues

Epithelial Tissue

Epithelial tissue is composed of tightly knit cells and covers exterior areas. Types:

  • Glandular: Formed by specialized cells that secrete substances. They can be isolated or grouped glands.
    • Exocrine: Secrete their products into ducts that empty outside the body (sweat glands) or into a cavity (salivary glands).
    • Endocrine: Ductless glands that discharge their secretions (hormones) into the blood (thyroid).
  • Covering: A tough, elastic tissue that covers external and internal surfaces. Types:
    • Simple: A single layer of flattened or polyhedral cells, lining internal ducts (respiratory, digestive, blood).
    • Stratified: Composed of multiple cell layers. The inner layers undergo constant division, and the outer layers shed more easily. It lines external surfaces or those communicating with the exterior (mouth, anus, vagina, tongue).
    • Pseudostratified: A single layer of ciliated mucosal cells (bronchi, bronchioles, urethra, bladder).
    • Endothelium: Simple, flat cells.

Connective Tissue

Connective tissues form a diverse group with the function of uniting, supporting, and protecting other tissues. Their cells are immersed in an abundant extracellular matrix. Types:

  • Connective Tissue Proper: Serves to join other tissues and organs. Its principal cells are fibroblasts, which secrete extracellular matrix proteins. Other cells include histiocytes, mast cells, and lymphocytes with anticoagulant and antibody production functions.
    • Loose or Areolar Connective Tissue: Filler material in the body, found beneath the skin and separating organs. Its extracellular matrix contains elastin.
    • Fibrous or Dense Connective Tissue: Compact tissue located in tendons and ligaments, covering bones and muscles. Its matrix contains collagen.
    • Elastic Connective Tissue: Characterized by its elasticity, located in the lungs, pleura, and blood vessels. Its matrix contains elastin.
    • Reticular Connective Tissue: Protects soft organs and lymphoid organs like the spleen, tonsils, and red bone marrow. Its matrix contains reticulin.
  • Cartilaginous Tissue: Performs a skeletal mission. Chondroblasts form the extracellular matrix, a dense network of elastic and collagen fibers impregnated with a gelatinous substance, providing flexibility and resistance. Chondrocytes are isolated in cavities called lacunae. Cartilage lacks blood and lymph vessels and is surrounded by perichondrium, which provides nutrients. Types:
    • Hyaline: Most abundant in the organism, provides flexibility. Contains few collagen fibers and extracellular matrix. Found in the fetal skeleton, trachea, larynx, bronchi, nasal gristle, and rib cartilage connecting to the sternum.
    • Fibrous: Provides strength and stiffness due to numerous collagen fibers. Forms meniscus cartilage and intervertebral discs.
    • Elastic: Provides flexibility and elasticity, maintaining the shape of organs. Contains many elastin fibers. Found in the pinna, larynx, epiglottis, and bronchioles.
  • Adipose Tissue: Performs the mission of energy reserves, acts as a thermal insulator, and has a protective mechanical function. Specialized fat cells store fats. The matrix contains collagen fibers and elastin. Located in the subcutaneous area, covering organs like the heart, kidneys, and forming the stomach and yellow bone marrow.
  • Bone Tissue: The toughest tissue, its function is skeletal support and protection of soft organs. Osteoblasts are responsible for forming the bone matrix and subsequent mineralization. When osteoblasts are surrounded by calcified fibers, they are called osteocytes. Osteoclasts destroy bone tissue for remodeling. Bone tissue forms bones, which can be long, short, or flat. Types:
    • Spongy: Located in the epiphysis of long bones and in short and flat bones. Formed by plates called trabeculae, containing lamellae, osteocytes, and lacunae.
    • Compact: Located in the diaphysis of all bones, mainly the shaft of long bones, which contains bone marrow. Each unit is formed by a long, hollow cylinder containing concentric bony lamellae. The cylinder’s center is the Haversian canal, containing blood vessels and nerves. Haversian canals communicate through Volkmann’s canals.
  • Blood Tissue: Blood is a liquid with a liquid extracellular matrix (plasma), red blood cells, white blood cells, and platelets. It maintains the internal environment’s equilibrium, transports substances, and defends against foreign substances. The matrix is formed by water, salts, minerals, ions, and abundant proteins. Blood cell proteins include fibrinogen (structural, forms clots), albumin, and immunoglobulins (antibodies).
    • Erythrocytes (Red Blood Cells): Characteristic biconcave discs, lack a nucleus, and contain hemoglobin for oxygen transport and CO2 elimination. Damaged or aged erythrocytes are removed from circulation by the spleen.
    • Leukocytes (White Blood Cells): Spherical, lack hemoglobin, and constantly pass through capillary circulation into connective tissues. They acquire amoeboid capabilities and are phagocytic. Their main function is defense against infectious agents.
    • Thrombocytes (Platelets): Discs lacking a nucleus, originating from the fragmentation of giant cells (megakaryocytes) in red marrow. They contain enzymes and chemicals for repairing blood vessels and initiating blood coagulation.
  • Lymphoid Tissue: Function is to collect interstitial fluid not collected by blood vessels and return it to the blood. Components include lymphocytes and a liquid matrix similar to plasma but with less protein.

Muscle Tissue

Responsible for movement, composed of myocytes or muscle fibers containing actin and myosin myofibrils. Types:

  • Smooth: Elongated, mononuclear cells producing slow, sustained, involuntary contractions. Located in the walls of hollow internal organs.
  • Striated: Moves the skeleton. The functional unit is the sarcomere, composed of ordered actin and myosin filaments.
  • Cardiac: Forms the heart. Cells are short, branched, and mononuclear, with light and dark bands connected by intercalated discs, allowing contraction transmission.

Nervous Tissue

Responsible for producing, carrying, and receiving messages from the exterior and the organism.

Neurons

Specialized cells for nerve impulse transmission, the functional and structural units of nervous tissue. Composed of:

  • Cell Body or Soma: Contains the nucleus and cytoplasmic structures, Nissl granules (rough endoplasmic reticulum portions where neurotransmitters are produced), and the axon.
  • Dendrites: Short, branched extensions capturing nerve impulses and sending them to the cell body.
  • Axon: Long, single cytoplasmic extension connecting with other neurons through synapses. Types of neurons:
    • Motor: Carry impulses from the central nervous system to muscles and glands.
    • Sensory: Receive information from sensory organs and transmit it to nerve centers.
    • Association: Connect with other neurons.

Glial Cells

Provide protection, support, and nourishment to neurons. Types:

  • Astrocytes: Star-shaped with many extensions, communicate with neurons and blood vessels, providing support.
  • Microglia: Small, branched cells phagocytosing cellular debris and microorganisms, performing cleaning and defense functions.
  • Oligodendrocytes: Cells with few extensions, forming myelin sheaths around axons in the central nervous system.
  • Schwann Cells: Form myelin sheaths around axons in peripheral nerves.

Animal Organs and Systems

Apparatus

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