Topography and cellular composition of the JGA
The juxtaglomerular apparatus is located in a strategically vital area of the kidney — between the afferent and efferent arterioles, at the site of contact between the renal corpuscle and the distal convoluted tubule. It consists of three key elements:
- Macula densa (macula densa). This is a modified segment of the distal tubule wall. Its cells lack the typical basal striations, their borders are indistinct, and their nuclei are located so close together that they create the appearance of a "dense" cluster. It acts as an osmoreceptor: it responds to an increase in sodium ions ($Na^+$) in the primary urine and transmits a signal for renin production.
- Juxtaglomerular cells. Modified smooth myocytes localized in the media of arterioles (predominantly afferent). They have a rounded nucleus, a polygonal shape unusual for myocytes, and a cytoplasm filled with large renin granules. These cells perform a dual function: secretory (secreting renin) and baroreceptor (excited by a drop in blood pressure within the vessel).
- Juxtavascular cells (Goormaghtigh cells). Located in the space between the arterioles and the macula densa. Characterized by the presence of long processes. This is the reserve pool of the JGA: they are capable of synthesizing renin under the influence of the same stimuli, but are recruited only during functional insufficiency of the main juxtaglomerular cells.
Mechanisms of hemodynamic regulation
The JGA's response to homeostasis disruption is the release of renin. There are three classical scenarios for this complex's operation:
- Response to changes in urine composition. If the concentration of $Na^+$ ions in the primary urine increases, the osmoreceptors of the macula densa are stimulated. A cascade is triggered: renin $\rightarrow$ angiotensin II $\rightarrow$ aldosterone. Aldosterone forces the tubules to actively reabsorb sodium back into the blood, reducing its loss in the urine.
- Response to acute blood loss. With a systemic drop in blood pressure, the baroreceptors of the afferent arterioles are stimulated. Renin synthesis leads to the formation of angiotensin II and aldosterone. Sodium reabsorption increases the osmotic pressure of the blood, which stimulates the hypothalamus to release antidiuretic hormone (vasopressin). Vasopressin retains water, increasing circulating blood volume. Concurrently, angiotensin II causes potent vasoconstriction (blood vessel narrowing), which collectively restores blood pressure.
- Renal hypertension. With pathological narrowing of the renal artery (due to a tumor or atherosclerosis), blood pressure in the afferent arterioles drops locally. This results in false triggering of baroreceptors and uncontrolled stimulation of renin synthesis. This leads to the development of malignant hypertension, which responds poorly to standard drug treatment because its root cause is renal ischemia.
Medullary interstitial cells
The endocrine function of the kidney is not limited to the JGA. Interstitial cells are localized in the stroma of the medullary pyramids (between the loops of Henle, collecting ducts, and vasa recta).
They possess processes that envelop epithelial tubules and blood capillaries, forming a connecting network. Their cytoplasm contains granules that store important biologically active substances:
- Prostaglandins (of the $E_2$ class) — act as local vasodilators (expand blood vessels).
- Erythropoietin — a hormone that stimulates hematopoiesis (erythropoiesis).
Briefly: answers to topical questions
From which embryological source do the elements of the juxtaglomerular apparatus develop?
Sources explicitly indicate that the macula densa is a segment of the distal convoluted tubule wall; the epithelium of the renal tubules is of mesodermal origin (celonephroderma type). The embryological source of juxtaglomerular and juxtavascular cells is not specified in the cited sources.
Which enzymes and proteins participate in the complete RAS cascade from renin production to angiotensin II formation?
The following proteins and enzymes participate in the cascade:
- Renin — an enzyme (protease) produced by the kidneys.
- Angiotensinogen — an inactive peptide of hepatic origin, the substrate for renin.
- Angiotensin I — a 10-amino-acid peptide, the product of renin action.
- Angiotensin-converting enzyme (ACE, or carboxydipeptidyl peptidase) — cleaves two amino acids from the C-terminus.
- Angiotensin II — an active 8-amino-acid peptide, the product of the reaction with ACE.
Which factors and hormones inhibit renin production by juxtaglomerular cells?
Renin production by juxtaglomerular cells is inhibited by:
- Atrial natriuretic peptide (ANP / ANF / sodium diuretic hormone) — inhibits the formation and secretion of renin.
- Beta-blockers (BB) — reduce renin secretion by blocking $\beta_1$-receptors of the juxtaglomerular apparatus.
Class and exam questions
Why is the "macula densa" named as such?
In this segment of the distal tubule, cell borders are not visible, and their nuclei are located very close to each other. Under a microscope, this appears as a solid, dark, "dense" cluster of nuclei.
What happens during false triggering of JGA baroreceptors?
When the renal artery narrows, blood flow drops locally, even though overall systemic pressure may be normal. The JGA perceives this as general blood loss, massively releases renin, and triggers mechanisms to raise blood pressure, establishing persistent renal hypertension.
Why are Goormaghtigh cells needed if renin is secreted by juxtaglomerular cells?
Juxtavascular cells with long processes serve as a reserve. They activate renin synthesis only when the primary arteriolar cells are depleted or failing to perform their function.
What else to study on this topic
- Morphological differences between Goormaghtigh cells and myocytes
- Molecular mechanism of macula densa osmoreceptors
- The cascade of transformations in the renin-angiotensin-aldosterone system
- Effect of prostaglandins on microcirculation in the renal medulla
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Juxtaglomerular Apparatus" and 3,485 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Urinary System" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.