Cytology | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction to Cytology

• Cell Types:

  • All living things are made up of cells.
  • Cells can be single or group together.
  • Cells are the building blocks and functional units of living organisms.

• Cell Size and Shapes:

  • Cells vary in size, ranging from 3 to 200 microns.
  • Shapes include circular, oval, cuboidal, columnar, or spindle-shaped.
  • The largest cell is the ovum, while the smallest are lymphocytes.

• Cell Structure:

  • Cells can be simple or complex internally.
  • The internal environment of a cell is protected by the plasma membrane.
  • Plant cells and some animal cells have an additional rigid cell wall.

• Energy and Metabolism:

  • Cells release and transfer energy for growth and metabolic processes.
  • Information retention and transfer are crucial for building and maintaining cells and their offspring.

• Cell Types:

  • Procaryotic cells: Simple structure, functions not isolated into separate units.
  • Eucaryotic cells: Highly organized with distinct functional units.

• Procaryotic Cells:

  • Functions like respiration and photosynthesis are associated with the plasma membrane.
  • DNA is distributed in the nucleoplasm without a membrane, found in bacteria and blue-green algae.

• Eucaryotic Cells:

  • Respiration and photosynthesis occur in specialized organelles (mitochondria and chloroplasts).
  • DNA is organized into chromosomes within a nucleus, separated by membranes.
  • Various organelles, like ribosomes and endoplasmic reticulum, are involved in different functions.

• Efficiency of Eucaryotic Cells:

  • Cells are highly efficient and adaptable for maintaining and evolving life.

Structure of the cell

• Common Features of Cells:

  • All cells, regardless of type, share common features: cell membrane and protoplasm.
  • Protoplasm includes cytoplasm, organelles, and inclusions.

• Protoplasm Composition:

  • Protoplasm is the living substance of a cell, consisting of protein, carbohydrate, lipid, nucleic acids, and inorganic materials in water.
  • The consistency of protoplasm varies among cells.

• Physiological Properties of Protoplasm:

  • Protoplasm exhibits several properties: irritability, conductivity, contractility, absorption, metabolism, secretion, growth, and reproduction.
  • Cells may emphasize certain properties more than others.

• Components of Protoplasm:

  • Protoplasm in each cell has two main components: nucleus and cytoplasm.
  • The nucleus contains hereditary material and is surrounded by cytoplasm with organelles and inclusions.

• Cytoplasmic Structure:

  • Cytoplasm contains organelles or inclusions, embedded in a translucent and homogeneous substance known as the ground substance.

• Plasma Membrane:

  • The cell is surrounded by a membrane called the plasma membrane.
  • This membrane separates the cell's contents from the external environment.
  • In eucaryotic cells, membranes compartmentalize the cytoplasm and are involved in complex biochemical processes.

• Membrane Functions:

  • Membranes are sites for various biochemical processes, including uptake of substances, ion selection, phosphorylation, photosynthesis, and nerve signal propagation.
  • The plasma membrane structure is referred to as the unit membrane, consisting of lipid and protein layers.

• Structure of Plasma Membrane:

  • The plasma membrane is about 100 angstroms thick.
  • It consists of a bimolecular lipid layer covered on each side by a protein layer.
  • The lipid layer faces outward, contacting the surface protein films.

These points highlight the key aspects of cell structure and function in simpler language for better comprehension.

The Organelles

Organelles are specialized structures within cells that perform specific functions related to cell activities. Examples include mitochondria, ribosomes, lysosomes, nucleus, endoplasmic reticulum, Golgi apparatus, centrosphere, and centriole.

Mitochondria:

• Known as the "Power House" of the cell, responsible for respiration and fatty acid metabolism.
• Consists of outer and inner membranes, with inner folds called cristae.
• Contains enzymes for the Krebs cycle, beta-oxidation, and phosphorylation.
• Has its own DNA and can increase in number through fission.

Lysosomes:

• Present only in animal cells, containing hydrolytic enzymes.
• Referred to as the "Suicidal bag."
• Surrounded by a single membrane and contains enzymes for breaking down substances.
• Release of lysosomal enzymes is linked to cell death.

Golgi Apparatus:

• Present in nerve and secretory tissues.
• Varied structure, consisting of smooth membranes, large vesicles, and small vesicles.
• Involved in secretion, particularly in plant and animal cells.

Nucleus:

• Found in all eucaryotic cells, surrounded by a double membrane.
• Numerous pores connect the nucleus to the cytoplasm.
• Contains nucleolus, chromatin, and nuclear sap.
• Chromatin is made up of DNA and protein.

Endoplasmic Reticulum (ER):

• Membrane sheets associated with ribosomes (rough ER) or without (smooth ER).
• Involved in transportation and storage within the cell.
• Accumulates large masses of proteins in some cells.

Ribosomes:

• Sites of protein synthesis in all cells, containing RNA.
• Smaller than mitochondria.
• Necessary for protein synthesis and energy production.
• Consists of two subunits with protein and RNA.

Centrosphere and Centriole:

• Centrioles are minute rods or granules near the nucleus in the centrosome.
• Two centrioles form the diplosome in non-dividing cells.
• Centrosphere surrounds the centriole and is important in mitosis.

Microtubules and Flagella:

• Microtubules present in mitotic spindles, plasma membrane, and may aid in cell movement.
• Flagella are structures for cell mobility.

Fibrilla:

• Prominent structures in muscle and nerve cells, associated with specialized functions.

Chloroplasts:

• Found in plant cells, responsible for photosynthesis.
• Surrounded by double membranes and contains DNA.
• Internal components involved in photosynthetic processes.
• Similar to mitochondria, resembling a prokaryotic organism within eukaryotic cells.

Inclusions

• Inclusions are substances within a cell that do not directly participate in its metabolic activities.
• They can be metabolic byproducts or substances taken in but not used immediately by the cell.
• Examples include fats, glycogen, pigments, and foreign bodies ingested by the cell.
• Pigments like melanin, hemosiderin, and lipofuscin may also be present.
• Inclusions are not essential for the cell's life or functioning.

This way, we highlight that inclusions are essentially stored substances in the cell, not directly involved in its day-to-day activities.

Fig: Diagram of a tybical cell based on studies made with the electron microscope

Cell Division

All multicellular organisms increase in size by increasing the number of cells. The zygote, formed at conception, divides repeatedly to give rise to all body cells. This division is achieved through a process called mitosis.

Mitosis:

• Definition: Division of the nucleus of a eukaryotic cell, involving the condensation of DNA into visible chromosomes.
• Mitosis lasts 30 to 60 minutes and includes two main steps: Karyokinesis (division of the nucleus) and Cytokinesis (division of the cell's cytoplasm).
• It is a continuous process but is conveniently divided into four phases: prophase, metaphase, anaphase, and telophase.

Interphase:

• The period between successive mitotic divisions.
• Replication of DNA occurs, and chromosomes become thin, extended threads.
• Replication begins at the ends of chromosomes and progresses towards the centromere.
• Centrioles separate, and the spindle starts to form.

Prophase:

• Chromosomes become visible and appear as thread-like structures.
• Each chromosome consists of two coiled subunits called chromatids.
• Chromatids are the functional units of chromosomes and contain a double strand of DNA.
• Nucleoli become smaller, the nuclear envelope breaks down, and chromosomes move to the cell's equator.

Metaphase:

• Marks the end of prophase and the beginning of metaphase.
• Mitotic spindle forms, aligning chromosomes along the equatorial plate.
• Spindle made of microtubules, with continuous fibers and chromosomal fibers.
• Centromere, a specialized DNA and protein region, holds chromatids together and forms an attachment site for chromosomal fibers.
• DNA duplication at the centromere results in the splitting of chromatids, which migrate to opposite poles.

Anaphase:

• Marks the beginning of chromatid separation.
• Chromatids (now called daughter chromosomes) move toward opposite poles.
• Centromeres split, and chromatids migrate to the poles of the spindle.
• By the end of anaphase, chromosomes have separated into two equal groups at each pole.

Fig : Diagram of cell livision by mitosis illustrated by a cell with two pairs chromosomes

Telophase

• Chromosome Movement:
  • Daughter chromosomes reach respective poles.
  • Endoplasmic reticulum surrounds each group, initiating nuclear envelope formation.
  • Chromosomes uncoil, becoming indistinct.
• Nuclear Envelope Formation:
  • Continuous portions of endoplasmic reticulum reform the nuclear envelope.
  • When complete, chromosomes become unclear, and nuclei assume interphase configuration.
  • Nucleoli reappear, marking the completion of Karyokinesis.

Cytokinesis

• Midway Formation:
  • During telophase, a constriction of the plasma membrane forms midway between the two nuclei.
  • This constriction, called a furrow, extends around the equator of the cell.
  • Mitotic spindle fibers in this region, often termed interzonal fibers, get stretched.
• Mid Body Formation:
  • The constriction deepens, separating daughter chromosomes.
  • Daughter chromosomes remain connected by a thin protoplasmic bridge called the midbody.
• Cell Separation:
  • Daughter cells move apart by amoeboid movement, completing separation.
  • This marks the end of cytokinesis, concluding the entire cell division process.

The duration of mitosia in some animale

Cell Cycle Overview:

• Replication of nuclear DNA occurs during a specific part of interphase called the S phase (Synthesis).
• The cell cycle includes G1 phase (Gap before DNA synthesis), S phase, and G2 phase (Gap before mitosis).
• G phase allows the cell to monitor its environment, ensuring DNA replication is complete before entering mitosis.

Meiosis:

• Special cell division producing eggs and sperm with a reduction in genetic material.
• Involves two successive nuclear divisions with only one round of DNA replication.
• Results in four haploid daughter cells from an initial diploid cell.
• Unlike regular cell division, where each daughter cell inherits one copy of each chromosome from both parents, meiosis produces gametes with only one copy of each chromosome.

Meiosis Process:

• Sister chromatids line up on the spindle during mitosis, separating into individual chromosomes.
• Haploid gametes produced by meiosis must have half the original number of chromosomes.
• This demands a unique mechanism where homologous chromosomes recognize and pair up before lining up on the spindle.
• Unlike regular cell division, meiosis ensures that each gamete receives either the maternal or paternal copy of each gene, but not both.

Fig: Diagrammatic representation of meiosis, illustrated by two pairs of chromosomes.

Cell Types

1. Diversity of Cells:

   • Over 210 types of cells with distinct features exist.
   • Variations within a category, e.g., small eye muscle fibers vs. large leg-moving fibers.

2. Classification Basis:

   • Traditional classification based on:
     • Shape, structure, and chemical nature of cells.
     • Assessment through various stains.

3. Examples of Cell Variability:

   • Skeletal muscle fibers: Vary in size based on function (eye vs. leg movement).
   • Auditory hair cells: Tuned to different sound frequencies in different ear parts.

4. Histological Classification:

   • Based on cell appearance under the microscope.
   • Lymphocytes and smooth muscles have multiple distinct cell types.

5. Uterus Sensitivity:

   • Wall sensitivity varies to hormones like estrogen and oxytocin.
   • Gut wall reacts differently.

6. Non-Equivalent Cells:

   • Cells from different body regions are non-equivalent.
   • Differ in developmental capacities and effects on other cells.

Examples of Cell Types:

1. Keratinizing Epithelial Cells:

   • Differentiating epidermal cells (keratinocytes).
   • Basal cells of the epidermis.

2. Cells of Wet Stratified Barrier Epithelia:

   • Cells from various stratified squamous epithelia (cornea, tongue, oral cavity, anal canal, urethra, vagina).

3. Epithelial Cells Specialized for Exocrine Secretion:

   • Salivary gland cells, mammary gland cells, lacrimal gland cells, sweat gland cells, sebaceous gland cells.

4. Cells Specialized for Hormone Secretion:

   • Cells of the anterior pituitary, posterior pituitary, thyroid, parathyroid, adrenal glands, gonads.

5. Epithelial Absorptive Cells:

   • Brush border cells of the intestine, gall bladder epithelial cells, kidney tubule cells.

6. Cells Specialized for Metabolism and Storage:

   • Hepatocytes, fat cells.

7. Epithelial Cells Serving as Barriers:

   • Pancreatic duct cells, type I pneumocytes in the lungs, parietal cells, podocytes in kidney glomerulus.

8. Epithelial Cells Lining Internal Cavities:

   • Vascular endothelial cells, synovial cells, serosal cells lining body cavities, ciliary epithelium cells in the eye.

9. Ciliated Cells with Propulsive Function:

   • Respiratory tract cells, oviduct cells, endometrium cells in the uterus, central nervous system cells.

10. Cells Specialized for Extracellular Matrix Secretion:

    • Ameloblasts (enamel-secreting), fibroblasts, odontoblasts, chondrocytes.

11. Contractile Cells:

    • Skeletal muscle cells, heart muscle cells, smooth muscle cells, myoepithelial cells.

12. Cells of Blood and Immune System:

    • Red blood cells, macrophages, T-lymphocytes, B-lymphocytes, stem cells.

13. Sensory Transducers:

    • Photoreceptors, hearing cells, taste cells, blood pH sensors, temperature sensors.

14. Autonomic Neurons:

    • Cholinergic neurons, adrenergic neurons, peptidergic neurons.

15. Supporting Cells of Sense Organs and Peripheral Neurons:

    • Supporting cells in the organ of Corti, vestibular apparatus, Schwann cells, satellite cells.

16. Neurons and Glial Cells of Central Nervous System:

    • Neurons and glial cells.

17. Lens Cells.

18. Pigment Cells:

    • Melanocytes, retinal pigmented epithelial cells.

19. Germ Cells:

    • Oogonium/oocyte, spermatocyte.

20. Nurse Cells:

    • Ovarian follicle cells, Sertoli cells.