Explore animal anatomy with our latest presentation! Discover animal tissues, organs, and organ systems in simple terms, perfect for NEET UG exam prep. Get ready to ace your exam with this easy-to-understand guide! #NEETUG #Biology

1. Dr. Shivankan Kakkar, MD Page 1

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STRUCTURAL ORGANISATION IN ANIMALS
Part I - Animal Tissues &Organ and Organ Systems
INTRODUCTION
- Unicellular organisms perform all functions, including digestion,
respiration, and reproduction, within a single cell.
- Multicellular animals organize these same functions among
different groups of cells within their complex bodies.
- Simple organisms like Hydra consist of various cell types,
with thousands of cells in each type.
- The human body comprises billions of cells, each contributing to
different functions.
- In multicellular animals, similar cells and intercellular
substances work together to perform specific functions,
forming tissues.
- Complex animals have four basic types of tissues, organized to
create organs such as the stomach, lung, heart, and kidney.
- Organs may cooperate to perform common functions through
physical and/or chemical interaction, forming organ systems like
the digestive and respiratory systems.
- The division of labor among cells, tissues, organs, and organ
systems contributes to the body's overall survival.

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ANIMAL TISSUES
- The structure of cells varies by function.
- Tissues differ, classified into four types:
(i) Epithelial
(ii) Connective
(iii) Muscular
(iv) Neural

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Epithelial Tissue
- Epithelial tissue, commonly called epithelium (pl.: epithelia), has
a free surface facing
body fluid
outside environment
providing covering or lining for body parts.
- Cells are compactly packed with little intercellular matrix.
- Two types of epithelial tissues:
simple epithelium
compound epithelium
- Simple epithelium: single layer of cells lining
body cavities
ducts
tubes
- Compound epithelium: two or more cell layers, providing
protective function, as in
skin.

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- Based on structural cell modifications, simple epithelium is
further divided into three types:
(i) Squamous
(ii) Cuboidal
(iii) Columnar

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- Squamous epithelium: single thin layer of flattened cells with
irregular boundaries, found in
blood vessel walls
lung air sacs
involved in forming diffusion boundary.
- Cuboidal epithelium: single layer of cube-like cells, commonly
found in
gland ducts
nephron tubular parts in kidneys
functions in secretion and absorption.
- Epithelium of proximal convoluted tubule (PCT) in kidney
nephron has microvilli.
- Columnar epithelium: single layer of tall, slender cells with
nuclei at base, may have microvilli on free surface, found in
stomach lining
intestine lining
aids in secretion and absorption.

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- If columnar or cuboidal cells bear cilia on free surface, called
ciliated epithelium, functions in moving particles or mucus
directionally over epithelium, mainly present in inner surface of
hollow organs like
bronchioles
fallopian tubes

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- Some columnar or cuboidal cells specialize for secretion,
called glandular epithelium.
- Two types:
unicellular
(e.g., goblet cells of alimentary canal)
multicellular
(e.g., salivary gland)
- Glands divided into exocrine and endocrine glands based on
secretion pouring mode.
- Exocrine glands secrete various substances through ducts or
tubes, e.g.,
mucus
saliva
digestive enzymes
- Endocrine (no ducts) glands secrete hormones directly into
surrounding fluid, without ducts.

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- Compound epithelium: multi-layered cells, limited role in
secretion and absorption, main function is protection against
chemical and mechanical stresses. Covers
dry skin surface
moist surfaces like buccal cavity and pharynx
inner linings of salivary gland and pancreatic ducts

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- Epithelial cells held together with little intercellular material.
- Specialized junctions provide structural and functional links
between cells.
- Three types of cell junctions:
tight
adhering
gap junctions
- Tight junctions prevent substances from leaking across tissue.
- Adhering junctions cement neighboring cells together.
- Gap junctions facilitate cell communication by connecting
cytoplasm of adjoining cells, allowing rapid transfer of ions and
molecules (both big and small).

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Connective Tissue
- Connective tissues are the most abundant and widely
distributed in complex animal bodies.
- Named for function of linking and supporting other
tissues/organs.
- Range from soft to specialized types like
Cartilage
Bone
Adipose
Blood
- In all connective tissues except blood, cells secrete collagen or
elastin fibers.
- Fibers provide strength, elasticity, and flexibility to tissue.
- Cells also secrete modified polysaccharides, acting as matrix
(ground substance) between cells and fibers.
- Connective tissues classified into three types:
(i) Loose connective tissue
(ii) Dense connective tissue
(iii) Specialized connective tissue

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- Loose connective tissue: cells and fibers loosely arranged in
semi-fluid ground substance, e.g.,
- Areolar tissue beneath skin, often supports epithelium.
- Contains
Fibroblasts
Macrophages
mast cells.
- Adipose tissue: specialized loose connective tissue
mainly beneath skin, cells store fats.
- Excess nutrients converted into fats and stored in
this tissue.

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- Dense Connective tissue: Fibers and fibroblasts densely
packed.
- Fibers' orientation determines pattern:
dense regular
dense irregular tissues
- In dense regular connective tissues, collagen fibers arranged
between parallel bundles, found in
tendons (attach skeletal muscles to bones)
ligaments (attach one bone to another)
- Dense irregular connective tissue has fibroblasts and collagen
fibers oriented differently, found in
skin

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- Specialized connective tissue:
cartilage
bones
blood

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- Cartilage's intercellular material solid and pliable, resists
compression.
- Chondrocytes (cartilage cells) enclosed in small cavities within
secreted matrix.
- Most cartilages in vertebrate embryos replaced by bones in
adults.
- Cartilage present in
nose tip
outer ear
joints
between vertebral column bones
limbs
hands
(in adults)

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- Bones consist of a hard and non-pliable ground substance rich
in calcium salts and collagen fibers, providing strength.
- They serve as the main structural frame for the body and
support/protect softer tissues and organs.
- Bone cells (osteocytes) reside in lacunae.
- Limb bones, like the long bones of the legs, bear weight and
facilitate movement through interaction with attached skeletal
muscles.
- Some bones contain bone marrow, where blood cell
production occurs.

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- Blood is a fluid connective tissue comprising
Plasma
red blood cells (RBC)
white blood cells (WBC)
platelets
- It serves as the primary circulating fluid, aiding in the
transportation of various substances.

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Muscle Tissue
- Each muscle comprises long, cylindrical fibers arranged in
parallel arrays.
- These fibers consist of numerous fine fibrils known as
myofibrils.
- Muscle fibers contract in response to stimulation, then relax and
return to their uncontracted state in a coordinated manner.
- Their action facilitates body movement to adapt to environmental
changes and maintain body part positions.
- Generally, muscles actively participate in all body movements.
- There are three types of muscles:
skeletal
smooth
cardiac

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- Skeletal muscle tissue is closely attached to skeletal bones.
- In a typical muscle, such as the biceps, striated skeletal muscle
fibers are bundled together in a parallel fashion.
- Several bundles of muscle fibers are enclosed within a sheath of
tough connective tissue.

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- Smooth muscle fibers taper at both ends (fusiform) and lack
striations.
- Cell junctions hold them together, and they are bundled within
a connective tissue sheath.
- This type of muscle tissue forms the walls of internal organs
like
blood vessels
stomach
intestine
- Smooth muscles are involuntary; their functioning cannot be
directly controlled (unlike skeletal muscles).
- Contractions of smooth muscles cannot be initiated merely by
thought.

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- Cardiac muscle tissue is found exclusively in the heart.
- Plasma membrane fusion through cell junctions maintains
cohesion among cardiac muscle cells.
- Intercalated discs, serving as communication junctions, allow
coordinated contraction of cardiac muscle cells; when one cell
contracts, neighboring cells are also stimulated to contract.

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Neural Tissue
- Neural tissue plays a significant role in regulating the body's
response to changing conditions.
- Neurons, the basic unit of the neural system, are excitable cells.
- Neuroglial cells, comprising the remainder of the neural system,
protect and support neurons.
- Neuroglia account for more than half the volume of neural
tissue in the body.
- When a neuron receives appropriate stimulation, an electrical
disturbance is generated, swiftly traveling along its plasma
membrane.
- Arrival of this disturbance at the neuron's endings, or output
zone, initiates events that may stimulate or inhibit adjacent
neurons and other cells.

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ORGAN AND ORGAN SYSTEM
- Basic tissues organize to form organs, which in turn combine to
create organ systems in multicellular organisms.
- This organization is vital for the efficient coordination of millions
of cells within an organism.
- Each organ comprises one or more types of tissues.
- For instance, the heart consists of
epithelial
connective
muscular
neural tissues
- The complexity of organ and organ systems follows
discernible evolutionary trends.
- Morphology refers to the study of form or externally visible
features, specifically in plants or microbes.
- In animals, morphology pertains to the external appearance of
organs or body parts.
- Anatomy traditionally involves the study of internal organ
morphology in animals.
- The morphology and anatomy of earthworms, cockroaches, and
frogs, representing invertebrates and vertebrates, will be
explored in Part II to illustrate organization and function.