Erythropoietin hormone which stimulates R.B.C. formation is produced by :

The correct answer is Juxtaglomerular cells of the kidney.

Explanation:

Erythropoietin (EPO) is a glycoprotein hormone that plays a crucial role in regulating the production of red blood cells (erythropoiesis) in the body. When oxygen levels in the blood decrease (a condition known as hypoxia), it triggers the synthesis and release of erythropoietin.

The primary source of erythropoietin in adult humans is the kidneys. Specifically, it is produced and secreted by the specialized interstitial cells, often associated with the juxtaglomerular apparatus, located in the renal cortex.

Let's review the other options to understand why they are incorrect:

1. Alpha cells of pancreas: These cells are endocrine cells located in the islets of Langerhans in the pancreas. They secrete the hormone glucagon, which raises blood glucose levels, and do not produce erythropoietin.
2. The cells of rostral adenohypophysis: The adenohypophysis (anterior pituitary gland) produces hormones such as growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. It is not responsible for producing erythropoietin.
3. The cells of bone marrow: While the bone marrow is the primary site where red blood cells are formed (erythropoiesis occurs here under the stimulation of erythropoietin), the hormone itself is produced elsewhere (principally by the kidneys) and travels through the bloodstream to act on the bone marrow receptors.

Therefore, the juxtaglomerular cells of the kidney are the key producers of the erythropoietin hormone that stimulates red blood cell formation.