Comprehensive Study Notes on Structural Organisation in Animals

Foundations of Animal Tissue and Early Evolutionary Development

Animal tissues are characterized by their structural organization, defined as a group of cells with a common origin and specific function. This level of organization first evolved in the belenterates. In contrast to plants, animal tissues are traditionally categorized into four fundamental types: epithelial, connective, muscular, and nervous. Epithelial tissue is characterized as being compactly packed and avascular, often resting on a basement nemh (membrane). Connective tissue originates from the mesoderm and is derived from mesochym (mesenchyme). The primary classifications of epithelial tissue include simple epithelium, which consists of a single layer, and compound epithelium, which is multi-layered. Connective tissue is further divided into loose, dense, and specialized categories, the latter including blood, lymph, bone, and cartilage.

Simple Epithelium and Its Specialized Histological Locations

Simple epithelium consists of several specific structural variations including squamous, cuboidal, columnar, pseudostratified, and ciliated types. Simple squamous epithelium is found in the lining of the Bowman's capsule, specifically within podocytes, and the alveolar lining of the lungs where pneumocytes are present. It also forms the endothelium of blood capillaries and the endocardium of the heart. Additionally, it is located in the thin part of the Henley is loop. Simple cuboidal epithelium is primarily found in secretory parts such as the acini of the pancreas and thyroid follicles, as well provided in the gonads. It is prominently located in the thick part of the Henley loop, the collecting tube, the Distal Convoluted Tubule (DCT), and the Proximal Convoluted Tubule (PCT).

Complex and Compound Epithelia: Structural Varieties and Locations

Simple columnar epithelium lines the stomach and intestine, and includes specialized goblet cells found in the respiratory tract and the intestinal mucosa. Ciliated columnar epithelium is located within the fallopian tube. Microvilli variations are found in the gallbladder. Pseidostratified epithelium is categorized into non-ciliated, found in the urembra of the human male and the parotid salivary gland, and ciliated, which is present in the trachea. Compound epithelium is divided into transitional and stratified types. Transitional epithelium, which can change shape (from polygonal to cuboidal to oval), is found in the urinary bladder and ureters. Stratified epithelium is categorized based on the presence or absence of keratin; keratinized stratified squamous epithelium is dry and found in hair and nails, while non-keratinized is moist and covers free surfaces like the pharynx and esophagus.

Cellular Junctions, Messaging, and Glandular Secretions

Cells within tissues are held together and communicate through specific junctions. Tight junctions, or $zonula\,occludens$, function to stop the leakage of fluid across a tissue layer. Adhering junctions, or desmosomes, provides cementing to hold neighboring cells together. Gap junctions allow for communication between cells through chemical signals. Epithelial cells also form glands. Glands are classified into exocrine, which possess ducts, endocrine, which are ductless like the pituitary, and heterocrine, which serve both functions as seen in the pancreas and gonads. Further classification is based on secretion mechanism: apocrine glands (axillary and mammary) lose a portion of the cell body; holocrine glands (sebaceous) involve the loss of the whole cell; and merocrine or enmoine glands (salivary and sweat $1$) release secretions via exocytosis.

Connective Tissue Cells and the Histiocytic Framework

Connective tissue consists of cells, a matrix, and fibers. The primary cells include fibroblasts, which are the largest in number and possess an irregular shape. Fibroblasts function as the chief matrix producers, secreting proteins and fibers. Mast cells serve immunological functions, releasing histamine as a vasodilator and for inflammatory responses, serotonin as a vasoconstrictor, and heparin as an anti-coagulant to prevent clotting. Macrophages, also known as histiocytes, are the second largest cell type and possess an amoeboid shape with a renal-shaped nucleus; they act as cellular scavengers through phagocytosis to clear cell debris and pathogens. Plasma cells, or cart wheel cells, are formed from lymphocytes and secretes antibodies. The matrix is composed of a ground substance consisting of jelly-like mucopolysaccharides of proteins.

Connective Tissue Fibers and the Areolar Matrix

There are three distinct types of fibers found in connective tissue. Collagen fibers are composed of collagen protein and are white, inelastic, and unbending; when boiled, they turn into gelatin. Elastin fibers are composed of elastin protein and are yellow, branched, and elastic; boiling has no effect on these fibers. Reticular fibers are composed of reticulin protein and are delicate and highly branched with no elasticity. Areolar tissue is the most widely distributed loose connective tissue, joining muscle to skin and filling spaces inside organs and around nerves and blood vessels. It provides the support framework for epithelium.

Adipose Tissue Characteristics and Dense Connective Types

Adipose tissue is a specialized loose connective tissue where fat globules push the cytoplasm and nucleus to the periphery of the cell; these cells also contain mitochondria and secrete leptin. White fat is monolocular and primarily found in adults, while brown fat is multilocular, contains a reduced nucleus, and is present in hibernating animals to produce heat. Dense connective tissue is categorized as regular or irregular. Dense regular connective tissue features collagen fibers arranged in rows between many parallel fiber bundles; tendons connect muscle to bone and are inelastic white collagen, while ligaments connect bone to bone and are elastic yellow with elastin. Dense irregular tissue features differently oriented fibers and is present in the dermis of the skin.

Cartilage Structure, Development, and Specialized Types

Cartilage is an avascular skeletal tissue with a solid matrix. It is surrounded by a membrane called the perichondrium. The cells involved are chondroblasts (which produce the matrix), chondrocytes (mature cells found in lacunae), and chondroclasts (which destroy cartilage). Hyaline cartilage is the weakest cartilage, appearing fiberless and translucent; it is found in the larynx, tracheal rings, nasal septum, and articulating surfaces of joints (costal cartilage). Fibrous cartilage is divided into white fibrous (collagen fibers) and yellow fibrous (elastin fibers, found in the epiglottis, pinna, and Eustachian tube). Calcified cartilage is initially hyaline but becomes hardened by the deposition of calcium salts, as seen in the suprascapsula and pectoral girdle.

Detailed Anatomy and Physiology of Bone

Osteology is the study of bones, and ossification is the process of bone formation. Bone matrix is approximately $70\%$ inorganic and $30\%$ organic. The inorganic portion consists of calcium phosphate ($80\%$), calcium carbonate ($10\%$), magnesium phosphate, and fluorides. The organic portion consists of ossein and collagen. Bone producing cells are osteoblasts, while mature cells are osteocytes, and bone destroying cells are osteoclasts. Structurally, long bones consist of the epiphysis (ends), metaphysis (growth area), and diaphysis (shaft). The periosteum is the outer membrane and the endosteum is the inner membrane. Yellow bone marrow is found in the shaft (white fat) and red bone marrow is found in spongy or cancellous bone areas like the epiphysis. Haversion canals and Volkman’s canals are essential features of mammalian bone structure.

Classification and Types of Bones

Bones are classified based on their development and location. Cartilaginous or replacing bone, such as the humerous and femur, replace a cartilage model. Investing bone, or dermal bone, forms as thin plates in the dermis of the skin, such as skull bones. Sesamoid bone forms from the ossification of a tendon, with the patella (kneecap) being the primary example. Visceral bone forms in soft organs, including the $os-cordis$ in the heart of deer, $os-penis$ in bats, and $os-palpeber$ in eyelids. Decalcified bone is bone treated with dilute HCl to remove minerals, while dried bone is bone treated with high temperature to remove organic matter.

Fluid Connective Tissue and Muscular Tissue Structure

Fluid connective tissue includes blood and lymph. Blood is composed of plasma ($55\%$) and corpuscles ($45\%$). The corpuscles include erythrocytes ($RBC$), leukocytes ($WBC$), and thrombocytes (platelets). Leukocytes are divided into granulocytes (neutrophil, basophil, eosinophil) and agranulocytes (monocyte, lymphocyte). Lymph, also called tissue or interstitial fluid, consists of plasma and WBCs but lacks RBCs and platelets. Muscular tissue is enclosed by a plasma membrane called the sarcolemm and contains sacroplasma and sacroplasmic reticulum. It is composed of fibers containing contractile proteins actin and myosin. The four essential properties of muscle are contractibility, excitability, extensibility, and elasticity.

Functional Types of Muscular Tissue

Muscles are categorized into three types: skeletal, smooth, and cardiac. Skeletal muscle is associated with the skeletal system, consisting of unbranched, cylindrical, multinucleated (syncytion) fibers that are voluntary and striated. It is surrounded by layers called the epimysium (whole muscle), perimysium (muscle fascicle), and endomysium (muscle fiber). Smooth muscle is associated with visceral organs, consisting of spindle-shaped, unbranched, uninucleated fibers that are involuntary and non-striated; they may be single-unit (linked by gap junctions in the stomach and intestine) or multi-unit (independent fibers at the base of hair roots and large blood vessels). Cardiac muscle is found only in the heart and consists of branched, cylindrical, uninucleated, involuntary striated fibers with intercalated discs and gap junctions for coordinated contraction.

Anatomy and Components of Nervous Tissue

Nervous tissue consists of neurons and supporting neuroglial cells. A neuron's structure includes a cell body ($cyton$, $perikaryon$, or $soma$) from which dendrites and an axon extend. The cyton contains Nissl’s granules, composed of RER and ribosomes for protein synthesis, but lacks centrioles, preventing cell division. The axon arises from the axon hillock and contains axoplasm covered by the axolemm. The myelin sheath may surround the axon, with the gaps between sheath segments known as nodes of Ranvier. The axon terminates in telodendria and synaptic knobs containing neurotransmitters.

Classification of Neurons and Specialized Neuroglial Cells

Neurons are classified based on structure: apolar neurons (lacking axons and dendrites, found in belenterates), unipolar (simple process, embryonic stage of vertebrates), pseudounipolar (found in the dorsal root ganglion of spinal nerves), bipolar (retina of eye and olfactory epithelium), and multipolar (most common in the CNS). Functional categories include sensory (organ to CNS), motor (CNS to organ), and relay (connecting the two). Neuroglial cells provide support: astrocytes in the CNS maintain the blood-brain barrier; oligodendrocytes in the CNS myelinate multiple axons and cannot be repaired; microglial cells are the smallest glial cells and are phagocytic; ependynd cells release cerebrospinal fluid (CSF); and Schwann cells in the PNS provide myelination and can be repaired.

Functional Categories and Germ Layer Origins of Tissues

The organizational matter of the nervous system is categorized into gray matter (collection of cytons) and white matter (collection of myelinated axons). In the CNS, these are called nuclei and tracts, while in the PNS, they are called ganglia and nerves. In terms of embryonic development, epithelial tissue can be derived from all three germ layers. Connective tissue and muscular tissue are primarily mesodermal, with the exception of the ciliary muscle which is ectodermals. Nervous tissue is ectodermal, with the exception of microglial cells which are mesodermal.