Cellular organization of domains of life
The modern phylogenetic tree divides all organisms into three domains, which differ in their level of cellular organization:
- Bacteria (Bacteria). Prokaryotes lacking membrane-bound organelles (mitochondria, Golgi apparatus) and possessing 70S-type ribosomes. Their cell wall contains a unique polymer — peptidoglycan (including N-acetylmuramic acid). Based on envelope structure, they are divided into Gram-positive (with a thick layer of peptidoglycan, e.g., Staphylococcus) and Gram-negative bacteria (with a thin layer and an outer lipid membrane, e.g., Salmonella). There are also wall-less forms — mycoplasmas. Some bacteria are capable of forming intracellular protective endospores to survive in extreme conditions.
- Archaea (Archaea). Ancient prokaryotes, often inhabiting extreme environments (thermophiles, halophiles). Unlike bacteria, their cell wall lacks peptidoglycan, and their genetic apparatus has a different structure. There are no human pathogens among archaea.
- Eukaryotes (Eukarya). Organisms with a distinct nucleus and a complex system of membrane-bound organelles. These include protozoa (Protozoa), whose cytoplasm is often divided into a dense ectoplasm and a granular endoplasm, as well as true fungi (Eumycota) with a thick cell wall lacking chlorophyll, chromists (Chromista), plants, and animals.
Cytological features of the egg cell
Female germ cells possess a number of unique structural characteristics necessary for successful fertilization and early embryogenesis:
- Protein-synthesizing apparatus. The oocyte cytoplasm contains a vast reserve of mRNA, tRNA, and ribosomes. This ensures robust and rapid protein synthesis immediately after fertilization, even before the zygote genome is activated.
- Multivesicular bodies. Specific membranous structures in the cytoplasm formed as a result of the digestion of engulfed (phagocytosed) particles.
- Absence of the centrosome. Upon completion of the first meiotic division, the secondary oocyte loses its centrioles. Because of this, independent cell division is impossible. The capacity for mitosis is restored only when the spermatozoon contributes its centrioles.
- Envelopes and surface. The oocyte plasmalemma is covered with microvilli, which increase the surface area for metabolic exchange. Externally, it is surrounded by the zona pellucida, secreted by the oocyte itself and follicular cells. It consists of glycosaminoglycans and glycoproteins. The Zp3 fraction performs a crucial function — it serves as a species-specific receptor for sperm binding.
Motility structures and methods of cytological research
The study of the cytoskeleton occupies an important place in cell biology. One of its specialized structures is the axoneme — the core of eukaryotic flagella and cilia. It is built from microtubules, motor proteins (dyneins), and nexin, and ensures the mobility of cellular structures.
A complex of methods is used to study cells in medicine and pathological anatomy:
- Cytological examination. Allows for the assessment of the morphology of individual cells, their number, and arrangement in a smear or punctate. At the same time, the general tissue architectonics is not preserved.
- Electron microscopy. Reveals changes at the subcellular level: the ultrastructure of organelles, membranes, and intracellular inclusions.
- Immunohistochemistry (IHC). Used at the molecular-cellular level to identify specific marker proteins and for cell phenotyping (e.g., in the diagnosis of breast cancer).
- Light microscopy. Evaluates cellular composition and the spatial arrangement of elements within tissue. Often correlated with the macroscopic picture (e.g., detection of optically empty vacuoles in hepatocytes in "fatty liver").
To memorize the structure of the bacterial cell wall: G+ (plus) — Positive, Dense Peptidoglycan (blue-violet). G− (minus) — thin layer, but with an additional membrane (red).
Class and exam questions
Why are cell wall-destroying antibiotics safe for humans?
The bacterial cell wall consists of peptidoglycan containing N-acetylmuramic acid and D-isomers of amino acids. These substances are completely absent in the human body, which is why the enzymes involved in their synthesis serve as a selective and safe target for drugs.
Can an egg cell begin division without the participation of a sperm cell?
No, because the mature egg cell lacks the centrioles of the centrosome. They are introduced exclusively by the spermatozoon at the moment of fertilization, which restores the zygote's ability to undergo mitosis.
How do archaea differ from true bacteria?
Archaea lack peptidoglycan in their cell wall, possess a unique structure of ribosomes and genetic apparatus. In addition, no pathogens of human infectious diseases have been found among them.
What is an axoneme?
This is a specialized cytoskeletal structure of eukaryotic cells that provides movement for cilia and flagella. It is built from microtubules, dynein motor proteins, and nexin.
What else to study on this topic
- Immunohistochemical examination (IHC) and cell phenotyping
- Morphology of protozoa: differentiation into ectoplasm and endoplasm
- Bacterial endospores: mechanism of formation and biological role
- Classification of bacteria by respiration type and mode of nutrition
- Macroscopic and microscopic methods in pathological anatomy
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