Architecture and Vascular Component
The renal corpuscle has two functionally significant poles. At the vascular pole, the opening of the afferent arteriole and the origin of the efferent arteriole come close together. The afferent arteriole branches into 25–50 capillaries that anastomose with each other, forming a unified capillary network of the glomerulus, and then reunite into the efferent arteriole. The distal convoluted tubule of the exact same nephron is always adjacent to this same region.
The opposite edge is called the urinary pole. Here, the slit-like cavity of the capsule continues into the lumen of the proximal convoluted tubule, ensuring the outflow of filtered primary urine.
Glomerular Capsule (Bowman's Capsule)
The capillary glomerulus is surrounded by an epithelial capsule consisting of two layers, between which lies a slit-like space — the capsular space.
- Outer (parietal) layer is represented by simple squamous epithelium resting on its own thin basement membrane. At the urinary pole, its squamous cells transition abruptly into the cuboidal epithelium of the proximal convoluted tubule.
- Inner (visceral) layer is formed by specialized large cells — podocytes. They closely envelop each capillary, causing them to visually merge with the vascular wall under light microscopy. Podocytes feature a bulging cell body containing the nucleus (soma), from which large primary processes originate — cytotrabeculae.
Renal Filtration Barrier
This is a complex of structures separating the lumen of the blood capillary from the capsular space. Plasma filtration occurs precisely across this barrier. It includes three layers:
- Capillary endothelium. The cells possess thinned areas (fenestrae) and transcellular pores that provide high permeability.
- Glomerular basement membrane (GBM). Plays a central role in filtration. It is shared between endothelial cells and podocytes (both cell types participate in its synthesis). It consists of a dense middle layer (type IV collagen, performing a structural framework function) and two less dense peripheral layers (containing proteoglycans and hyaluronic acid). The membrane contains cylindrical pores several nanometers in diameter, allowing molecules up to mid-sized proteins to pass through.
- Podocyte layer. Podocytes attach to the GBM not with their entire cell body, but rather via numerous short processes — pedicels (foot processes). Free gaps (filtration slits) remain between adjacent pedicels, through which the filtrate enters the capsular space unhindered.
Mesangial Apparatus
In those areas of the glomerulus where capillaries are not covered by podocyte processes, mesangial cells are located. Depending on their structure and function, they are divided into two types:
- Smooth muscle type (mesangial cells / mesangiocytes). These cells synthesize the extracellular matrix filling the spaces between capillaries. In addition, they are capable of contraction, thereby regulating blood flow intensity within the glomerulus.
- Macrophage type. These are resident macrophages that perform phagocytosis and participate in local immune-inflammatory reactions.
Histological Features
When studying the renal corpuscle under a light microscope, it is important to understand the limitations of the method. Within the glomerulus, it is extremely difficult to reliably distinguish individual cell types (endothelial cells, podocytes, and mesangial cells). Due to dense packing, they form a unified conglomerate of nuclei.
However, two structures are always clearly identifiable in a slide:
- A clear slit-like space — the capsular space.
- A chain of flattened nuclei along the periphery — the outer layer of the capsule.