Differentiation and Post-Cellular Structures
The specialization process begins with stem cells capable of division. As differentiation proceeds, the cell acquires specific organelles, inclusions, or processes while losing unneeded structures and often its capacity to divide. The end result is a mature specialized cell or the formation of post-cellular and supra-cellular structures.
Post-cellular structures originate from regular cells, but during development, they completely lose their nucleus and frequently their organelles as well. It is important to understand that these are not fragments of cellular destruction, but elements perfectly adapted for a specific function. All of them are surrounded by a plasmalemma.
In the human body, these include hair, nails, epidermal cornified scales (corneocytes), and blood elements such as erythrocytes and platelets. On a blood smear, erythrocytes appear as discs with a characteristic central pallor. This morphology is due to their biconcave disc shape: the structure is thinner in the center than at the periphery, which alters light transmission during microscopy.
Supra-Cellular Structures: Symplasts and Syncytia
The supra-cellular level of tissue organization is represented by structures that integrate cellular material.
A symplast is a continuous cytoplasmic space covered by a common plasmalemma and containing multiple or even numerous nuclei. Most commonly, symplasts are formed by the fusion of individual cells. Classic examples include skeletal muscle fibers, osteoclasts in bone tissue, and the outer layer of the placental trophoblast. Sometimes multinucleation arises from incomplete cell division, where the nucleus divides but the cytoplasm does not. In a skeletal muscle fiber (for example, in a longitudinal section of the tongue), multiple elongated nuclei are pushed to the periphery directly beneath the plasmalemma, and the fiber itself is elongated with cross-striations.
A syncytium is formed differently. It is an aggregate of cells resulting from incomplete division (cytotomy). The nuclei divide, but the cells do not completely separate, remaining connected via cytoplasmic bridges. In humans, germ cell precursors are organized this way: oogonia in embryos and spermatogenic cells in males. Historical note: Cardiac muscle, nervous tissue, and reticular tissue were previously misclassified as syncytia. Today, it is proven that cardiomyocytes and neurons make very close contact, but remain separate units without true cytoplasmic bridges.
Extracellular Matrix
The histological cell theory postulates that the extracellular matrix is a product of cellular activity. Chemical precursors are synthesized intracellularly, secreted via exocytosis into the extracellular space, where their final assembly (polymerization) takes place.
The extracellular matrix consists of fibers, ground substance, and complex structures (basement and elastic membranes, bone lamellae). In connective tissues, it predominates over the volume of cells, whereas in epithelial and nervous tissues, it is virtually absent.
Matrix morphology in histological preparations:
- Reticular dermis of the skin: Collagen fibers predominate. They form thick, tightly packed, multidirectional bundles. When stained with eosin, they exhibit oxyphilia (staining pink).
- Hyaline cartilage: The extracellular matrix occupies the bulk of the volume, but appears homogeneous (structureless). Fine collagen fibrils are masked by abundant ground substance and are indistinguishable with standard staining.
Cell Morphology in Histological Preparations
Cell morphology is extremely variable: ranging from spherical and polygonal to spindle-shaped and stellate. In cytology, the shape of epithelial cells is determined by the ratio of their height to width:
- Squamous: Height is markedly less than width.
- Cuboidal: Height and width are approximately equal.
- Columnar (prismatic): Height exceeds width.
This variety of shapes is clearly visible in a kidney section. Some tubules display cuboidal cells with round nuclei, while others show narrow, tall columnar cells whose rounded nuclei are shifted toward the basal portion (away from the lumen), and thin tubules are lined by squamous epithelium.
Blood cells on a smear (Romanowsky staining) demonstrate different features. Neutrophilic leukocytes are spherical, their nuclei are segmented (divided into parts connected by narrow strands), and pinkish-purple specific granulation is visible in the cytoplasm.
Process-bearing cells are prominently represented by neurons. In a preparation stained with nigrosin, a rounded cell body with a centrally located nucleus and numerous branching processes extending from it are clearly visualized.