detail regarding adrenal gland developement

anatomy adrenal gland

1. Adrenal Glands Anatomy & Development

2. The adrenal gland is a component of the hypothalamic-pituitary-adrenal axis that is responsible for coordinating stress response and metabolism . They are yellowish retroperitoneal organs that lie on the upper poles of the kidneys, just above the level of the last thoracic vertebra (T12). They are surrounded by renal fascia (but are separated from the kidneys by the perirenal fat ). Each gland has an outer yellow cortex and an inner dark brown medulla.

3. The adrenal gland is approximately one third the size of the kidney at birth. The adult adrenal gland is only one thirtieth the size of the kidney. This change in proportional size is a reflection of renal growth as well as the involution of the fetal adrenal cortex after birth. At the onset of puberty, the gland is at its adult size and only increases slightly in weight over the course of adult life, with the exception of stress and pregnancy or the development of pathology

4. A normal adrenal gland in an adult weighs approximately 4 to 6 grams. The left adrenal is larger and flatter than the right adrenal gland. The weight of each adrenal may increase by nearly 50 percent during times of stress and pregnancy . Pathologic glands may reach 700 grams

5. BLOOD SUPPLY   The blood flow to the normal adrenal gland is approximately 10  mL /minute. The blood supply to both the cortex and medulla increases during periods of stress. ACTH produces an immediate increase in blood flow to the adrenals. Arterial and venous capillaries within the adrenal gland integrate the function of the cortex and medulla.

6. BLOOD SUPPLY   Arteries : The arteries supplying each gland are three in number: superior, middle, and inferior suprarenal arteries arise from; inferior phrenic artery , abdominal aorta , and renal artery, respectively.

7. VENOUS DRAINAGE Venous drainage left adrenal gland  — The left adrenal vein emerges at the hilum of the gland and is approximately 2 to 3 cm in length . The left adrenal (suprarenal) vein drains into either the left renal vein or the left inferior phrenic vein. Venous drainage right adrenal gland  — The right adrenal (suprarenal) vein emerges from the hilum which is less than 1 cm and empties directly into the posterior inferior vena cava .

8. LYMPHATIC DRAINAGE The adrenal gland has two lymphatic plexuses : one in the medulla and one deep to the adrenal capsule. Most of the adrenal lymphatics terminate in the paraaortic nodes near the diaphragmatic crus and origin of the renal artery .. The remaining lymphatics traverse the diaphragm and drain toward the thoracic duct or the posterior mediastinum .

9. INNERVATION   visceral afferent fibers arising from the celiac, aorticorenal , and renal autonomic ganglia in the retroperitoneum . These fibers connect with the posterior vagus nerve, phrenic nerve, and greater and lesser splanchnic nerves. The nerve fibers provide sensory or indirect vasomotor innervation as they traverse the adrenal cortex, and terminate in the medulla as preganglionic sympathetic fibers . The adrenal medulla is equivalent to postsynaptic sympathetic nerve and functions as a neuroendocrine transducer.

10. Each adrenal gland has two distinct parts, each with a unique function, the outer  adrenal cortex  and the inner  medulla , both of which produce hormones . Cortex The adrenal cortex is the outermost layer of the adrenal gland. Within the cortex are three layers, called "zones ". Section of human adrenal gland under the microscope , showing its different layers. From the surface to the center: zona glomerulosa , zona fasciculata , zona reticularis , medulla.

11. Zona glomerulosa . The outermost layer of the adrenal cortex is the  zona glomerulosa . It lies immediately under the fibrous capsule of the gland. This layer is the main site for production of  aldosterone , a  mineralocorticoid , by the action of the enzyme  aldosterone synthase .

12. Zona fasciculata The  zona fasciculata  is situated between the zona glomerulosa and zona reticularis . Cells in this layer are responsible for producing glucocorticoids  such as  cortisol .  It is the largest of the three layers, accounting for nearly 80% of the volume of the cortex. In the zona fasciculata , cells are arranged in columns radially oriented towards the medulla.

13. Zona reticularis The innermost cortical layer, the  zona reticularis , lies directly adjacent to the medulla. It produces  androgens , mainly  dehydroepiandrosterone  (DHEA),  DHEA sulfate  (DHEA-S), and  androstenedione  (the precursor to  testosterone ).  Its small cells form irregular cords and clusters, separated by capillaries and connective tissue.

14. Medulla The  adrenal medulla  is at the centre of each adrenal gland, and is surrounded by the adrenal cortex. The  chromaffin cells  of the medulla are the body's main source of the catecholamines  adrenaline and noradrenaline , released by the medulla. Approximately 20% noradrenaline ( norepinephrine ) and 80% adrenaline (epinephrine) are secreted here. [18]

15. Development of the Adrenal Glands The two parts of the adrenal gland i.e. the cortex and the medulla develop from two different origins. Cortex is mesodermal in origin; develops from the celomic epithelium of the posterior abdominal wall. Medulla is ectodermal in origin; develops from the neural crest cells.

16. Embryology First detected at 6 weeks' gestation, the adrenal cortex is derived from the mesoderm of the posterior abdominal wall . Steroid secretion from the fetal cortex begins shortly thereafter. Adult-type zona glomerulosa and fasciculata are detected in fetal life but make up only a small proportion of the gland, and the zona reticularis is not present at all. The fetal cortex predominates throughout fetal life. The adrenal medulla is of ectodermal origin, arising from neural crest cells that migrate to the medial aspect of the developing cortex

17. Cortex Adrenal cortex tissue is derived from the  intermediate mesoderm . It first appears 33 days after  fertilisation , shows  steroid hormone production  capabilities by the eighth week and undergoes rapid growth during the first trimester of pregnancy.

18. The fetal adrenal cortex is different from its adult counterpart, as it is composed of two distinct zones: the inner " fetal " zone(FZ),which carries most of the hormone-producing activity, and the outer "definitive" zone(DZ), which is in a  proliferative  phase. After birth, the FZ regresses and the DZ, which contains an inner zona fasciculata (ZF) and an outer zona glomerulosa (ZG), proliferates. The fetal zone produces large amounts of adrenal  androgens  (male sex hormones) that are used by the  placenta  for  estrogen  biosynthesis.  Cortical development of the adrenal gland is regulated mostly by  ACTH , that stimulates  cortisol  synthesis.

19. During midgestation , the fetal zone occupies most of the cortical volume and produces 100–200 mg/day of  DHEA-S , an  androgen  and precursor of both androgens and  estrogens  (female sex hormones). Adrenal hormones, especially  glucocorticoids  such as cortisol , are essential for prenatal development of organs, particularly for the maturation of the  lungs . The adrenal gland decreases in size after birth because of the rapid disappearance of the fetal zone, with a corresponding decrease in androgen secretion.

20. During the fetal period, the morphology of the adrenal cortex remains relatively constant. The DZ is composed of a narrow band of small (10–20 mm) cells that exhibit structural characteristics typical of cells in a proliferative state. Inner layers of the DZ form arched cords with finger-like columns of cells reaching the outer rim of the FZ. Cells in the DZ are lipid-poor during midgestation . As gestation advances, the cells accumulate cytoplasmic lipid and begin to resemble steroidogenically active cells .

21. The FZ consists of large (20–50 mm) cells with ultrastructural characteristics typical of steroidogenic cells. In the outer regions of the FZ, the cells are arranged in tightly packed cords. In the central portion, the FZ forms a reticular pattern, with cells spaced more widely and separated by numerous vascular sinusoids.

22. Ultrastructural studies also have revealed a third zone between the DZ and FZ, which is named as the transitional zone (TZ). Cells in this zone show intermediate characteristics . TZ cells have the capacity to synthesize cortisol , being analogous to cells of the zona fasciculata of the adult adrenal cortex. By the 30th week of gestation, the Fetal adrenal cortex manifests a rudimentary form of the adult adrenal cortex; the DZ and TZ begin to resemble the zona glomerulosa and the zona fasciculata , respectively

23. Medulla The adrenal medulla is derived from  neural crest cells , which come from the  ectoderm  layer of the  embryo . These cells  migrate  from their initial position and aggregate in the vicinity of the  dorsal aorta , a primitive blood vessel, which activates the differentiation of these cells through the release of proteins known as  BMPs . These cells then undergo a second migration from the dorsal aorta to form the adrenal medulla and other organs of the  sympathetic nervous system .

24. Cells of the adrenal medulla are called  chromaffin cells  because they contain granules that stain with  chromium  salts, a characteristic not present in all sympathetic organs.  Glucocorticoids  produced in the adrenal cortex were once thought to be responsible for the differentiation of chromaffin cells. More recent research suggests that  BMP-4  secreted in adrenal tissue is the main responsible for this, and that glucocorticoids only play a role in the subsequent development of the cells.

25. P450scc  - cytochrome P450 side chain cleavage Pregn .  - pregnenolone P450C17  - cytochrome P450 17a-hydroxylase, 17-20 lyase 17OHP5  - 17-hydroxy-pregnenolone DHEA/S  - dehydroepiandrosterone-sulfate S- Tfase  - DHEA sulfotransferase 3ß-HSD  - 3ß-hydroxysteroid dehydrogenase Prog .  - progesterone 17OHP4  - 17-hydroxyprogesterone P450C21  - cytochrome P450 21-hydroxylase P450C11  - cytochrome P450 11ß-hydroxylase P450aldo  - cytochrome P450 aldosterone synthase .

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