Untitled Quiz

The Cell

• The cell, the fundamental unit of life, is responsible for the morphological and functional aspects of living organisms.
• Unicellular organisms consist of a single cell, while multicellular organisms are composed of numerous cells specialized for specific functions within the organism.
• Eukaryotic cells are more complex and evolved than prokaryotic cells, being larger in size with a distinct nucleus containing a more complex DNA structure. They also possess numerous organelles.

Human Cells

• Specialized in carrying out specific functions
• Grouped together to form tissues, which are organized into organs and systems

Animal Eukaryotic Cell Contents

• The cell membrane acts as a selective and permeable barrier, separating the internal environment from the external environment.
• It maintains the cell's integrity, regulates interactions with other cells, and controls the movement of substances in and out of the cell.
• The cytoplasm encompasses a vast array of organelles enabling the cell to carry out its functions.
• The nucleus is the command center, housing the genetic material of the cell.

Cell Membrane

• Consists of a phospholipid bilayer, a double layer of phospholipids arranged with their hydrophilic heads facing outwards and their hydrophobic tails facing inwards.
• The membrane's fluidity is influenced by the presence of cholesterol and glycolipids

Components of the Phospholipid Bilayer

• Lipids: (40-50%): Phospholipids, glycolipids, cholesterol
• Proteins: (60%): Integral or transmembrane proteins and extrinsic or peripheral proteins

Phospholipids

• Composed of a hydrophilic (polar) head and two hydrophobic (non-polar) tails made up of a fatty acid chain.
• The phospholipid bilayer forms with the hydrophobic tails facing inwards and the hydrophilic heads facing outwards.
• The arrangement of tails allows movement of molecules through the bilayer.

Glycolipids and Cholesterol

• Glycolipids: have two fatty acid tails linked to a carbohydrate. They are situated amongst the phospholipids in the membrane.
• Cholesterol: is located in the spaces between unsaturated tails. It regulates membrane fluidity and restricts molecular movement.

Membrane Proteins

• Integral or transmembrane proteins are embedded within the membrane.
• Extrinsic or peripheral proteins are associated with the membrane on the cytosolic side.
• Transport proteins: Facilitates the movement of molecules across the membrane by joining them or forming channels.
• Union proteins: Establish cell junctions with other cells and anchor internal cell structures.
• Signal reception proteins: Receive information from the environment and other cells to regulate cellular processes.
• Enzymes: Catalyze reactions within the membrane.

Protoplasm: The Living Substance of the Cell

• Divided into two compartments:
  • Cytoplasm: Primarily composed of water, containing dispersed organic and inorganic chemical substances, and housing organelles.
  • Carioplasm: The material composing the nucleus.

Cell Organelles

Ribosomes

• Small organelles composed of ribosomal RNA (rRNA).
• Responsible for protein synthesis.
• Each ribosome consists of a major and a minor subunit.
• The minor subunit binds to mRNA and tRNA.
• The ribosome reads the genetic information encoded by mRNA, adding amino acids in the sequence dictated by the mRNA.
• Found in the cytosol.

Endomembrane System

• Includes:
  • Endoplasmic reticulum
  • Golgi apparatus
  • Nuclear envelope

Endoplasmic Reticulum (ER)

• A network of tubules and vesicles with a lumen called a cistern.
• Two types:
  • Rough ER (RER): Contains ribosomes, involved in protein synthesis.
  • Smooth ER (SER): Lacks ribosomes, involved in lipid synthesis and calcium storage.

Rough Endoplasmic Reticulum

• The ribosomes attached to the RER synthesize proteins that may remain embedded in the ER membrane or be stored within the ER cisterns.
• Proteins exit the RER in vesicles, destined for other organelles or secretion outside the cell.

Smooth Endoplasmic Reticulum

• A network of interconnected tubes without ribosomes.
• Primarily involved in lipid synthesis and calcium storage.

Golgi Apparatus (GA)

• A series of stacked cisterns, each surrounded by vesicles.
• The cis face, facing the RER and nucleus, receives vesicles from the RER.
• The trans face, facing the plasma membrane, releases vesicles.

Transport Control and Direction

• RER-synthesized and modified proteins are packaged into vesicles and transported to the GA through the cis face.
• These proteins undergo further modifications within the GA and are then released via the trans face for their final destinations.

Lysosomes

• Spherical organelles surrounded by a membrane and filled with enzymes.
• These enzymes degrade various molecules and require an acidic pH for their activity.
• The membrane isolates enzymes to maintain an acidic pH for proper function, protecting the cell's structures.
• Destroy macromolecules, waste products, and aged organelles.
• Degraded materials are then reused by the cell or released.

Types of Lysosomes

• Primary lysosome: A lighter circular structure on a picture, indicating it hasn't been involved in catabolic processes.
• Secondary lysosome: A circular structure with black formations on a picture, meaning it has participated in catabolic processes.
• Tertiary lysosome: The darkest circular structure on a picture, containing resistant compounds that remain stored within.

Peroxisomes

• Organelles surrounded by a membrane, containing oxidative enzymes.
• Involved in fatty acid degradation, during which hydrogen peroxide is produced.

Mitochondria

• Flexible, cane-shaped organelles responsible for ATP production.
• ATP (adenosine triphosphate), a key energy storage molecule, is crucial for cellular function.
• Mitochondria use oxygen and glucose to generate energy for the cell.
• Mitochondria have a smooth outer membrane and an inner membrane called the cristae.
• The cristae increase the surface area of the inner membrane, housing protein complexes that form ATP synthase and the electron transport chain, both involved in ATP generation.
• The matrix, a dense liquid within the inner membrane, contains enzymes of the Krebs cycle.
• Mitochondria contain their own DNA, distinct from nuclear DNA, which is more loosely packed, similar to prokaryotic organisms.

Nucleus

• The largest organelle in the cell, storing the genetic material (DNA) and assembling ribosomes.
• Located in the center of the cell and typically spherical, although shape, size, and location can vary according to the cell's function.
• Most cells possess a single nucleus, but some cells, such as osteoclasts, contain multiple nuclei, and others, such as erythrocytes, lack nuclei.

Components of the Nucleus

• Nuclear envelope: Composed of an outer and inner nuclear membrane, containing nuclear pores.
• Nucleoplasm: The liquid content of the nucleus, containing the nucleolus and chromatin.

Nucleoplasm

• The liquid portion of the nucleus, containing genetic material and the nucleolus.

Chromatin

• A DNA-protein complex, where proteins called histones help package and organize DNA for storage.
• Depending on its activity, chromatin can exist as heterochromatin (condensed) or euchromatin (loosely packed).

Chromosomes

• Humans have 46 chromosomes (23 pairs of homologous chromosomes), with one chromosome from each pair inherited from the father and the other from the mother.
• 22 pairs are autosomes, and 1 pair are sex chromosomes (X or Y).
• Cells with the complete set of chromosomes are called diploid (2n).
• Cells with one chromosome from each pair are called haploid (n).

Chromosome Structure

• Each DNA copy forms a chromatid, and two chromatids are joined together at a point called the centromere, holding them together to make up the chromosome.
• The centromere contains a structure called the kinetochore, where microtubules of the mitotic spindle bind during cell division.

Cytoskeleton

• A network of tubules and protein filaments, responsible for maintaining cell shape, organizing and moving organelles, and muscle contraction.
• Composed of three components:
  • Microfilaments: Involved in movement.
  • Intermediate filaments: Contribute to structural support.
  • Microtubules: Influence shape, transport processes, and cell division.

Microfilaments

• Formed by a protein called actin, organized into two chains forming a helix.
• Actin networks are positioned close to the plasma membrane and form the structural basis of the cellular cortex.
• In muscle cells, specific arrangements participate in muscle contraction.
• Actin filaments interact with another protein called myosin, which moves by sliding along the actin filaments.
• Involved in organelle and vesicle movements.
• Form structures like microvilli.

Microvilli

• Cellular membrane extensions, containing dense bundles of cross-linked actin filaments.
• Increase the cell's surface area for greater contact with the environment, facilitating absorption processes.

Intermediate Filaments

• Have a diameter between microfilaments and microtubules.
• Provide structural support for the cell's three-dimensional shape.
• Support the nucleus and anchor the cytoskeleton to the membrane.

Microtubules

• Hollow cylinders, assembled from a region near the nucleus called the centrosome.
• Constantly polymerize or depolymerize, changing their length according to cellular needs.
• Provide rigidity and maintain cell shape, regulating intracellular movement of organelles.

Complex Grouping of Microtubules

Centrioles

• Cylindrical structures, composed of nine triplets of microtubules.
• A pair of centrioles is surrounded by a dense matrix called pericentriolar material, forming the centrosome.
• Duplication of a centriole from another centriole: During cell division, each centriole duplicates, forming a new daughter centriole perpendicular to the mother centriole. These two pairs of centrioles contribute to the formation of the mitotic spindle, essential for cell division.

Cilia

• Membrane extensions containing a microtubule cytoskeleton.
• Structure: A central pair of microtubules is surrounded by nine microtubule doublets.
• The basal body, found at the base of the cilium, comprises nine triplets of microtubules.
• Cilia cover the cell surface, moving substances in the surrounding medium.

Flagella

• Only found in human sperm cells.
• Primarily responsible for movement.

Part 2 of CELL BIOLOGY: Cell Cycle

• The cell cycle, a series of events culminating in cell division into two daughter cells.
• Consists of mitosis and interphase.
• Interphase is the longest stage, during which the cell increases in size, accumulates essential components, and replicates its genetic material.

Interphase

• G1 phase: A period of cellular growth, enabling the cell to regain its original size after division. Synthesis of RNA, enzymes, and regulatory proteins necessary for cellular function.
• S phase: DNA replication phase of the cell cycle. The cell duplicates its DNA content.
• G2 phase: Synthesis of RNA and proteins required for cell division.

Cell Division

• Mitosis: Asexual reproduction without a reduction in genetic material.
• Meiosis: Sexual reproduction involving a reduction in genetic material.

Mitosis

• Phases: Prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis.

Interphase

• Chromosomes replicate, with the copies remaining connected.

Prophase

• Chromosomes condense and become visible in the nucleus.
• The spindle starts to form.

Prometaphase

• The nuclear membrane disintegrates.
• The spindle interacts with the chromosomes.

Metaphase

• Replicated chromosomes align at the spindle equator.

Anaphase

• Chromosomes separate into two genetically identical groups and move to opposite ends of the spindle.

Telophase

• Nuclear membranes form around each chromosome set.
• Chromosomes decondense.
• The spindle decomposes.

Cytokinesis

• The cell splits into two daughter cells, each containing the same number of chromosomes as the parent cell.
• Human cells are diploid, containing two copies of 23 chromosomes.
• During telophase, a segmentation groove forms in the cytoplasm.
• The ring contracts until two daughter cells are separated.

Meiosis

• A specialized cell division, producing gametes.
• Reduces the number of chromosomes from 2n to n.
• This reduction ensures that each gamete carries half of the chromosomes.
• Fusion with another gamete restores the diploid state (2n) specific to the species.

Process

1. In the S phase, DNA content doubles as in any cell division.
2. Prophase I: Chromosomes condense and become visible in the nucleus. Homologous chromosomes pair, exchanging genetic material.
3. Prometaphase I: The nuclear envelope disappears. The spindle begins to form, and homologous chromosomes remain together.
4. Metaphase I: Pairs of homologous chromosomes align at the spindle equator.
5. Anaphase I: Microtubules pull apart a complete chromosome from each pair.
6. Telophase I: Two daughter nuclei form, each with one chromosome from each pair.
7. Meiosis II: Follows the same steps as mitosis.
8. Meiosis I results in two daughter cells, and meiosis II produces two new daughter cells from each of these.
9. At the end of meiosis, four haploid cells are produced.

Cell Death

• Two mechanisms:
  • Apoptosis: Programmed cell death, a physiological process that can occur in cells destined for destruction during embryonic development.
  • Necrosis: Cell death due to injury or trauma. It is not a regulated process and can damage the organism.

Apoptosis

• Cell shape modification, reduction in cell size, loss of contact with neighboring cells, condensation of the cytoplasm, and no change in organelles.

Embryonic Development

• Cells programmed for apoptosis during embryonic development, for example, cells that form membranes between fingers and toes.

Maintaining Body Homeostasis

• Contributes to continuous remodeling and maturation of organs and tissues.
• Helps maintain the balance between cell proliferation and death.

Protection Against Damaged or Tutorial Cells

• When intracellular damage occurs and immune or inflammatory responses are inappropriate, apoptosis is activated.
• Examples of triggers: Physical agents (radiation), chemical agents (toxins), tumor cells.

Necrosis

• Cell swelling, mitochondrial damage, cell membrane breakdown, and organelle dissolution.
• Leads to tissue damage.

Topic 3: Epithelial Tissue

Epithelial Tissue

• Covers internal and external surfaces, lines cavities, and forms glands.
• Cells are tightly linked together by junctional complexes with minimal extracellular space.
• Separated from the underlying connective tissue by the basement membrane.

Functions

• Delineation
• Protection
• Diffusion
• Absorption
• Filtration
• Excretion
• Uptake of stimuli
• Secretion

Polarity

• Epithelial cells rest on a surface.
• Basal surface: Attached to the basement membrane.
• Apical surface: Faces the lumen of the cavity or the external environment.
• Lateral surfaces: Face the sides of adjacent cells.

Apical Membrane Specializations

• The apical domain faces the lumen.
• Highly concentrated with ion channels, transport proteins, and secretion release sites.
• Various structures related to epithelial function may appear in this region: microvilli, cilia, stereocilia, and keratin.

Stereocilia

• Extremely long microvilli.
• Increase the surface area for absorption and aid in medium movement.
• Found in the male reproductive system and the inner ear.

Keratin

• A protein produced by epithelial cells and located in the cytoplasm.
• Provides rigidity, enhancing the epithelium's resistance to external damage.

Lateral Membrane Specializations

• Structures involved in cell junctions within the lateral zones of epithelial cells, contributing to the cohesion of the epithelium:
  • Zonula occludens (tight junctions): Most apical junction between epithelial cells, forming a continuous band around the perimeter of the cell, completely sealing the space between cells and preventing the passage of membrane proteins.
  • Zonula adherens (adherens junctions): Located adjacent to tight junctions, forming a belt that surrounds the entire perimeter of the cell. The extracellular part of membrane proteins called cadherins fills the space between membranes, binding together between cells. The intracellular side of the cadherins is attached to actin filaments.
  • Macula adherens (desmosomes): Punctual junctions distributed randomly in the plasma membrane, located below the zonules. The cytosolic part of the junction contains a junction plate to which intermediate filaments of keratin are attached.

Basal Membrane Specializations

• Basement membrane: A membrane separating the epithelium from the underlying connective tissue.
• Acts as an anchor for the epithelium and filters substances passing from the epithelium to the interior of the body.
• Folds: Invaginations of the plasma membrane that increase the cell's surface area.

Hemidesmosomes

• Bind the cell membrane to the basement membrane.
• Located in the basal cells of the epithelia.

Epithelia

• Epithelia are classified by cell layers, cell shapes and presence of specializations on the apical surface.
• Simple epithelia have a single layer of cells, stratified epithelia have multiple layers, and pseudostratified appear stratified but are a single layer with all cells attached to the basement membrane.
• Epithelial cell shapes include flat, cuboidal and columnar.
• Specializations on the apical surface include microvilli (increase surface area), cilia (move fluids), and stereocilia (sensory functions).
• Simple squamous epithelium is a single layer of thin, flattened cells that allows filtration and covers alveoli, blood vessels and lymphatic vessels.
• Simple cuboidal epithelium is a single layer of cube-shaped cells with a rounded nucleus that is associated with absorption and secretion and found in glands and duct lining.
• Simple columnar epithelium is a single layer of tall, rectangular cells with an oval nucleus near the base, involved in transport and absorption and found in the digestive tract and glands.
• Stratified squamous epithelium has multiple layers of cells with flattened apical cells and is involved in protection and lining the mouth, esophagus and vagina.
• Stratified squamous keratinized epithelium is similar to stratified squamous but with dead, keratin-filled apical cells, found in the epidermis of the skin.
• Stratified cuboidal epithelium contains multiple layers of cuboidal cells and lines ducts of some glands.
• Stratified columnar epithelium has a basal layer of polyhedral or cuboidal cells and a surface layer of columnar cells, found in some ducts and the male urethra.
• Pseudostratified columnar epithelium appears stratified but is a single layer where all cells touch the basement membrane; often ciliated and found in the respiratory tract.
• Urothelium is found only in the urinary tract and is one or more layers of cells with a pseudostratified appearance.
• Glands can be classified as exocrine (with ducts to the external environment) or endocrine (ductless, release hormones to the bloodstream).
• Exocrine glands have simple (one duct), compound (more than one duct) or branched (single duct for multiple secretory units) structures.
• Exocrine glands can also be classified by the shape of their secretory portion, such as tubular, acinar or alveolar.
• Merocrine glands release their products through exocytosis, holocrine glands release products with cell death, and apocrine glands release a portion of their cytoplasm with the secretion.

Connective Tissue

• Connective tissue is a supportive tissue that connects other tissues and organs.
• It consists of cells and an extracellular matrix (ECM).
• The ECM is made of ground substance (amorphous gel composed of GAGs, proteoglycans and glycoproteins) and fibers (collagen and elastin).
• GAGs are negatively charged long chains of repeating disaccharides that contribute to the matrix's viscosity and water content.
• Proteoglycans are GAGs bonded to a protein core, and hyaluronic acid can bind multiple proteoglycans to form aggregates.
• Structural glycoproteins bind to ECM components and anchor epithelia to the matrix.
• Collagen fibers are inelastic, tough, and resistant to traction.
• Collagen synthesis involves the formation of tropocollagen molecules, which assemble into microfibrils, then fibrils, and finally fibers.
• Vitamin C is essential for collagen synthesis.
• Elastic fibers are thin, branched fibers made of elastin and fibrillin, providing elasticity to tissues.
• Elastin biosynthesis involves the formation of fibrillin microfibrils, which create a hollow cylinder where tropoelastin is deposited.
• The basement membrane is a thin, specialized layer between epithelial and connective tissue.
• The basal lamina is synthesized by epithelial cells and contains structural glycoproteins and integrins.
• The reticular lamina is synthesized by connective tissue and contains reticular fibers (collagen type III), attaching the basal lamina to the connective tissue.

Connective Tissue Cells

• Connective tissue cells can be fixed (remain in the tissue) or migratory (move to the tissue).
• Fibroblasts are the most abundant fixed cells and synthesize most of the ECM; inactive fibroblasts are called fibrocytes.
• Reticular cells are specialized fibroblasts that secrete reticular fibers, forming a network that supports cellular organs.
• Myofibroblasts are fibroblasts with contractile properties that are involved in wound healing and found in the testes.
• Pericytes are pluripotent cells surrounding capillaries, able to differentiate into fibroblasts, endothelial cells and smooth muscle cells.
• Mast cells are large, fixed cells that store granules containing histamine and other substances involved in the immune response.
• Macrophages are large, irregular-shaped cells that can be fixed or migratory, phagocytizing cellular debris, and involved in immune defense.
• Leukocytes (neutrophils, eosinophils, basophils, monocytes, lymphocytes) are migratory cells involved in immune defense and inflammation.
• Plasma cells are migratory cells derived from B lymphocytes and secrete antibodies.

Connective Tissue Classification

• Embryonic connective tissue includes mesenchymal (undifferentiated cells) and mucoid (gelatinous matrix).
• Adult connective tissue includes loose (abundant ground substance and few fibers), dense (abundant fibers and few cells), elastic (abundant elastic fibers), reticular (reticular cells and fibers), and adipose (fat-storing).
• Loose connective tissue supports epithelial cells, found in the dermis.
• Dense connective tissue provides strength and is found in the dermis, tendons, and ligaments.
• Elastic connective tissue allows for stretching and is found in blood vessels and ligaments.
• Reticular connective tissue provides support for cellular organs and tissues, found in the liver, spleen and bone marrow.

Specialized Connective Tissues

• Adipose tissue is specialized for fat storage and made of adipocytes.
• White adipose tissue stores fat as a single large drop and provides insulation, mechanical protection, and trophic support.
• Brown adipose tissue stores fat in multiple small drops, has many mitochondria, and helps regulate body temperature.
• Cartilage tissue is a specialized connective tissue that is avascular, with cells called chondrocytes residing in lacunae.
• Chondrocytes divide and secrete matrix, leading to interstitial growth.
• Perichondrium is the connective tissue sheath surrounding cartilage, involved in apositional growth.
• Hyaline cartilage is the most common type, with type II collagen and is found in the embryonic skeleton.
• Elastic cartilage has the same composition as hyaline cartilage but also contains abundant elastic fibers, giving it more flexibility and found in the ear and epiglottis.
• Fibrocartilage is a combination of cartilage and dense connective tissue, with type I and II collagen and is highly resistant to tension and found in intervertebral discs.

Bone Tissue

• Bone tissue is highly specialized, providing support, protection, and sites for muscle attachment.
• Bone consists of cells (osteogenic, osteoblasts, osteocytes) and a matrix containing both inorganic (hydroxyapatite) and organic (collagen) components.
• Bone marrow is found within the central cavity of bones.
• Periosteum is the outer layer of bone tissue, containing osteogenic cells and osteoblasts.
• Endosteum is the inner layer lining the medullary cavity, containing osteogenic cells and osteoblasts.

Osteoblasts

• Parathormone (PTH) receptors are present on osteoblast membranes, stimulating osteoblast activation of osteoclasts for bone degradation.
• Mature osteoblasts trapped in lacunae become osteocytes, elongated cells with extensions that traverse the matrix through calcoforous canaliculi.

Osteocytes

• Osteocytes establish contact with other cells via their extensions in the canaliculi, facilitating nutrient transport through the fluid present within.

Osteoclasts

• Large, mobile, multinucleated cells originating from bone marrow precursors, responsible for bone resorption.
• Four distinct regions:
  • Basal zone: furthest from bone surface, containing nuclei and organelles.
  • Brush border: microvilli facing bone surface, involved in resorption.
  • Clear zone: surrounding the brush border, devoid of organelles, rich in actin filaments for adhesion and sealing.
  • Vesicular zone: between basal zone and brush border, containing vesicles for transporting resorption products.

Bone Resorption

• Osteoclasts break down bone tissue, releasing minerals, transferring calcium from bone to blood.
• Upon PTH stimulation, osteoblasts activate osteoclasts, leading to their migration to bone surfaces.
• Osteoclasts release protons into the subosteoclastic space, acidifying the environment to dissolve inorganic matrix components.
• Enzymes secreted by osteoclasts degrade the organic matrix, and the degradation products are endocytosed and released into the bloodstream.

Ossification

• Bone formation during embryonic development occurs through two processes: intramembranous and endochondral ossification.
• Both processes result in primary bone, which is subsequently replaced by secondary bone.
• Secondary bone undergoes continuous but slower resorption throughout life.

Intramembranous Ossification

• Formation of flat bones.
• Mesenchymal cells differentiate into osteoblasts, secreting bone matrix to form trabeculae.
• The region where bone formation initiates is called the primary ossification center.
• Osteoblasts entrapped within the matrix become osteocytes.

Endochondral Ossification

• Involves cartilage as a template for bone formation.
• Chondrocytes proliferate and differentiate, facilitating bone growth in length.
• Growth ceases around 20 years of age when the epiphyseal plate cartilage is exhausted.

Bone Growth

• Chondrocytes in the epiphyseal plate proliferate, maintaining cartilage and enabling bone growth in length.

Bone Remodeling

• During growth, bone formation outweighs resorption.
• In adulthood, formation and resorption rates equilibrate, but remodeling continues in response to mechanical stress and calcium homeostasis needs.

Calcium Homeostasis

• 99% of body calcium is stored in bone.
• The remaining 1% circulates in plasma, crucial for muscle contraction, nerve impulse transmission, and blood clotting.
• Calcium deficiency triggers parathyroid hormone (PTH) secretion, stimulating bone resorption and releasing calcium into the bloodstream.
• Calcium excess prompts calcitonin secretion from the thyroid, inhibiting osteoclasts.

Morphological Classification of Bones

• Long bones: elongated structure with a diaphysis (shaft) and epiphyses (ends).
• Short bones: roughly cuboidal shape.
• Flat bones: thin and curved, often serving protective functions.
• Irregular bones: complex shapes with multiple projections and depressions.
• Sesamoid bones: small, round bones embedded within tendons.

Compact Bone

• Denser bone, found on the outer surface of all bone types and the diaphysis of long bones.
• Concentric sheets of collagen fibers surround Haversian canals containing blood vessels and nerves.
• Volkmann's canals connect Haversian canals, ensuring blood supply to the entire bone.

Spongy Bone

• Porous bone, found in the interior of flat, short, and irregular bones, as well as the epiphyses of long bones.
• Composed of trabeculae, containing collagen fiber bundles and housing bone marrow in the spaces between them.

Blood

• Dark, viscous, slightly alkaline liquid circulating in the circulatory system.
• Acts as a transport medium for various substances:
  • Nutrients from the digestive system to the entire body.
  • Hormones and signaling molecules between cells.
  • Helps regulate body temperature and osmotic balance of tissues.

Plasma

• Yellowish fluid composing the liquid matrix of blood, containing cells, platelets, organic compounds, and electrolytes.
• Composed of 90% water, 9% proteins, and 1% nutrients and respiratory gases.

Erythrocytes (Red Blood Cells)

• Anucleated and lack organelles.
• High enzyme content for ATP production.
• Lifespan of approximately 120 days.
• Biconcave disc shape, maximizing surface area for gas exchange and facilitating capillary passage.

Transport of O2 and CO2

• Erythrocytes contain hemoglobin, responsible for their red color and blood's overall red hue.
• In the lungs, hemoglobin binds four O2 molecules, forming oxyhemoglobin.
• In tissues, oxygen is released, and hemoglobin binds CO2, forming carbaminhemoglobin.

Leukocytes (White Blood Cells)

• Larger than erythrocytes, less numerous, with nuclei and organelles.

• Perform immune functions.

• Classified into two groups:

  • Granulocytes: possess specific granules in their cytoplasm:

    • Neutrophils: most numerous leukocytes, multilobed nucleus, granules containing antibacterial enzymes.
    • Eosinophils: bilobed nucleus, granules stain with acid dyes, involved in allergic, inflammatory, and parasitic reactions.
    • Basophils: S-shaped nucleus, granules stain with basic dyes, possess IgE receptors, initiate inflammatory responses.

  • Agranulocytes: lack specific granules:

    • Monocytes: largest blood cells, kidney-shaped nucleus, differentiate into macrophages in tissues.
    • Lymphocytes: smallest leukocytes, round nucleus, scarce cytoplasm, composed of T, B, and NK cells.

Neutrophils

• Most abundant leukocyte, multilobed nuclei, specific granules that don't stain with standard dyes.
• Granules contain enzymes for antibacterial functions.
• Chemotactic substances attract them to infection sites, where they leave blood vessels via selectins.
• Phagocytize bacteria, release hydrolytic enzymes and leukotrienes, initiating inflammation.
• Limited capacity for enzyme and lysosome regeneration, leading to death after bacterial elimination.
• Accumulation of leukocytes, dead bacteria, and tissue fluid forms pus.

Eosinophils

• Bilobed nucleus, specific granules stained with acid dyes.
• Histamine, leukotrienes, and eosinophil chemotactic factor attract them to allergic, inflammatory, or parasitic sites.
• Degrade antigen-antibody complexes, moderating inflammatory/allergic responses.

Basophils

• S-shaped nucleus masked by specific granules that stain with basic dyes.
• Possess IgE receptors.
• Upon initial antigen exposure, IgE specific for that antigen binds to the basophil membrane.
• Re-exposure to the antigen triggers antigen-IgE binding, releasing granular contents.
• Initiates an inflammatory response, attracting neutrophils and eosinophils, producing vasodilation and increased capillary permeability through histamine, and enhancing blood flow through heparin.

Monocytes

• Largest blood cells, with a large, eccentric, kidney-shaped nucleus.
• Remain in blood for a few days, then migrate to tissues to become macrophages.

Lymphocytes

• Smallest leukocytes, rounded with a round nucleus and limited cytoplasm.
• Three types: T, B, and NK (natural killer) cells.
• T lymphocytes: originate in bone marrow, mature in the thymus, acquire immune competence.
• B lymphocytes and NK cells: develop in bone marrow, directly migrate to tissues.
• Upon antigen contact, T and B lymphocytes proliferate, generating two populations:
  • Memory lymphocytes: persist in the body, enabling rapid immune responses upon re-exposure to the same antigen.
  • Effector cells: carry out the immune response.

Platelets

• Smallest formed blood elements, disc-shaped fragments of larger cells produced in bone marrow.
• Lack a nucleus but contain numerous granules.
• Lighter zone: hyalomere.
• Darker central zone with granules: granulomere.
• Three types of granules:
  • Alpha granules: most abundant, containing fibrinogen and coagulation factors.
  • Dense granules: contain Ca2+, ATP, and ADP.
  • Lambda granules: lysosomes.

Clotting Process

• Platelets participate in hemostasis, a series of processes preventing blood loss, repairing damaged vessels, and maintaining blood fluidity.
• Vessel endothelium secretes prostacyclins, inhibiting platelet aggregation, and thrombomodulin, blocking coagulation.
• Platelet aggregation forms a plug, stopping bleeding.
• Coagulation factors activate a cascade, culminating in fibrinogen activation and fibrin formation, creating a denser, more stable clot.
• Once the vessel is repaired, endothelial cells release enzymes to initiate clot removal.

Hematopoiesis

• The process of blood cell formation, essential for renewal due to their limited lifespans.
• Begins in the second week of embryonic development, initially occurring in the liver and spleen.
• After six months, hematopoiesis shifts to bone marrow as the skeletal system develops.
• Stem cells differentiate into either:
  • Common lymphoid progenitors, giving rise to all lymphocyte types.
  • Common myeloid progenitors, generating erythrocytes, granulocytes, and monocytes.