This photomicrograph of a Mallory-Azan–stained section shows the capsule (cap) and part of the underlying cortex. The outer layer of the capsule (OLC) is composed of dense connective tissue. The fibroblasts in this part of the capsule are relatively few in number; their nuclei appear as narrow, elongate, red-staining profiles against a blue background representing the stained collagen fibers. The inner layer of the capsule (ILC) consists of large numbers of myofibroblasts whose nuclei appear as round or elongate, red-staining profiles, depending on their orientation within the section. Note that the collagen fibers in this layer are relatively sparse and that the myofibroblast nuclei are more abundant than those of the fibroblasts in the outer layer of the capsule. 180.
vertical striations that appear to emanate from the medulla (see Fig 20.1). These striations are the medullary rays (of Ferrein). Their name reflects their appearance, as the striations seem to radiate from the medulla. Approximately 400 to 500 medullary rays project into the cortex from the medulla.
Each medullary ray is an aggregation of straight tubules and collecting ducts.
Each medullary ray contains straight tubules of the nephrons and collecting ducts. The regions between medullary rays contain the renal corpuscles, the convoluted tubules of the nephrons, and the collecting tubules. These areas are referred to as cortical labyrinths. Each nephron and its collecting tubule (which connects to a collecting duct in the medullary ray) form the uriniferous tubule.
The medulla is characterized by straight tubules, collecting ducts, and a special capillary network, the vasa recta.
The straight tubules of the nephrons and the collecting ducts continue from the cortex into the medulla. They are accompanied by a capillary network, the vasa recta, that runs in parallel with the various tubules. These vessels represent the vascular part of the countercurrent exchange system that regulates the concentration of the urine.
The tubules in the medulla, because of their arrangement and differences in length, collectively form a number of conical structures called pyramids. Usually 8 to 12 but as many as 18 pyramids may be present in the human kidney. The bases of the pyramids face the cortex, and the apices face the renal sinus. Each pyramid is divided into an outer medulla (adjacent to the cortex) and an inner medulla. The outer medulla is further subdivided into an inner stripe and an outer stripe. The zonation and stripes are readily recognized in a sagittal section through the pyramid of a fresh specimen. They reflect the location of distinct parts of the nephron at specific levels within the pyramid (Fig. 20.3).
The renal columns represent cortical tissue contained within the medulla.
The caps of cortical tissue that lie over the pyramids are sufficiently extensive that they extend peripherally around the lateral portion of the pyramid, forming the renal columns (of Bertin). Although renal columns contain the same components as the rest of the cortical tissue, they are regarded as part of the medulla. In effect, the amount of cortical tissue is so extensive that it “spills” over the side of the pyramid much as a large scoop of ice cream extends beyond and overlaps the sides of an ice cream cone.
The apical portion of each pyramid, which is known as the papilla, projects into a minor calyx, a cup-shaped structure that represents an extension of the renal pelvis. The tip of the papilla, also known as the area cribrosa, is perforated by the openings of the collecting ducts (Fig. 20.4). The minor calyces are branches of the two or three major calyces that in turn are major divisions of the renal pelvis (see Fig. 20.1).
The number of lobes in a kidney equals the number of medullary pyramids.