Cell morphology and non-cellular structures

Differentiation and post-cellular structures

The specialization process begins with stem cells, which retain the ability to divide. As differentiation proceeds, the cell acquires specific organelles, inclusions, or processes, while discarding unnecessary structures and often losing the capacity to divide. The outcome is a mature specialized cell or the formation of post-cellular and supercellular structures.

Post-cellular structures originate from ordinary cells, but during development completely lose their nucleus and often their organelles as well. It is important to understand that these are not fragments of destruction, but elements ideally 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), as well as blood elements—erythrocytes and platelets. On a blood smear, erythrocytes appear as disks with a characteristic central pallor. This morphology is due to the biconcave disk shape: the structure is thinner in the center than at the periphery, which alters light transmission during microscopy.

Supercellular structures: symplasts and syncytia

The supercellular level of tissue organization is represented by structures that integrate cellular material.

A symplast is a unified cytoplasmic space covered by a common plasmalemma and containing several or even many nuclei. Most commonly, symplasts are formed by the fusion of individual cells. Classical examples include skeletal muscle fibers, osteoclasts of bone tissue, and the outer layer of the placental trophoblast. Sometimes multinucleation arises due to incomplete division, where the nucleus divides but the cytoplasm does not. In a skeletal muscle fiber (e.g., in a longitudinal section of the tongue), multiple elongated nuclei are pushed to the periphery, directly beneath the plasmalemma, and the fiber itself has an elongated shape 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 arranged this way: oogonia in embryos and spermatogenic cells in males.

Historical note: Previously, cardiac muscle, nervous tissue, and reticular tissue were mistakenly classified as syncytia. Today it is proven that cardiomyocytes and neurons contact very tightly, but are separate units without true cytoplasmic bridges.

Intercellular substance

The histological cell theory postulates that the intercellular substance is a product of cellular activity. Chemical precursors are synthesized intracellularly, secreted into the extracellular space via exocytosis, where their final assembly (polymerization) takes place.

The intercellular substance consists of fibers, ground amorphous substance, and complex structures (basal and elastic membranes, bone lamellae). In connective tissues, it predominates over the volume of cells, whereas in epithelial and nervous tissues, it is practically absent.

Matrix morphology on microscopic preparations:

• Reticular dermis of the skin: collagen fibers predominate. They form thick, tightly packed, multi-directional bundles. Upon eosin staining, they exhibit oxyphilia (stain pink).
• Hyaline cartilage: the intercellular substance occupies the main volume, but appears homogeneous (structureless). Fine collagen fibrils are masked by abundant ground amorphous substance and are indistinguishable with standard staining.

Cell morphology on microscopic 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.

The diversity of shapes is clearly visible in a kidney preparation. In some tubules, cuboidal cells with rounded nuclei are seen, while in others, narrow and tall columnar cells appear, whose rounded nucleus is displaced to the basal part (away from the lumen), and the thin tubules are lined with squamous epithelium.

Blood cells on a smear (Romanowsky staining) demonstrate other features. Neutrophilic leukocytes have a spherical shape, their nucleus is segmented (divided into parts connected by narrow bridges), 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 body with a centrally located nucleus and numerous branching processes extending from it are clearly visualized.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation and branching of nerve cell (neuron) processes
• Morphology of osteoclasts and the outer layer of the placental trophoblast
• Specific granulation of neutrophilic leukocytes
• The process of intercellular substance polymerization in the extracellular space
• Histogenesis of basal and elastic membranes

Other topics in «The Cell and Non-Cellular Structures. Cell Membranes»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.