Anatomic Structure and Cellular Composition
The adenohypophysis is formed from epithelial tissue and structurally divided into three components:
- Anterior lobe (pars anterior / distalis) — the largest part of the gland.
- Intermediate lobe (pars intermedia) — a narrow strip of tissue containing follicles. It is separated from the anterior lobe by the hypophyseal cleft.
- Tuberal part (pars tuberalis) — a small extension that directly envelops the pituitary stalk (infundibulum).
Secretory cells of the anterior lobe are classified based on their staining affinity:
- Acidophils (eosinophilic cells, $\alpha$-cells) — secrete growth hormone and prolactin.
- Basophils ($\beta$- and $\delta$-cells) — secrete tropic hormones (TSH, ACTH, gonadotropins).
- Chromophobes ($\gamma$-cells) — generally considered non-secretory, though some studies suggest a role in corticotropin synthesis.
Hormones Produced by the Adenohypophysis
Adenohypophyseal cells produce several major hormones. Tropic hormones enter the bloodstream, bind to specific receptors on target endocrine glands, activate second-messenger systems, and trigger peripheral effector hormone synthesis.
Adenohypophyseal hormones:
- Growth hormone (GH, somatotropin) — regulates growth, body composition, and cell metabolism.
- Prolactin (PRL) — stimulates milk production and lactation.
- Thyroid-stimulating hormone (TSH, thyrotropin) — regulates thyroid gland function.
- Adrenocorticotropic hormone (ACTH, corticotropin) — regulates adrenal cortex function.
- Gonadotropins — follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which regulate gonadal function.
- Melanocyte-stimulating hormone (MSH) — influences skin pigmentation.
- Lipotropin (LPH) — involved in lipid metabolism.
Mechanism of Hypothalamic Regulation
Unlike the neurohypophysis (posterior pituitary), the adenohypophysis lacks direct neuronal connections to the hypothalamus. Control is mediated humorally via the hypophyseal portal system.
- Small-cell neurosecretory nuclei of the medial hypothalamus (including periventricular areas) synthesize hypophysiotropic hormones: releasing hormones (liberins), which stimulate adenohypophyseal hormone synthesis and secretion, and inhibiting hormones (statins), which suppress it.
- These substances travel via axons to the median eminence.
- The neurosecretions are released into the primary capillary plexus.
- Blood carries the hormones through the portal vessels down to the secondary capillary plexus (sinusoids) located within the adenohypophysis.
This system operates via negative feedback: circulating levels of peripheral hormones regulate hypothalamic and pituitary hormone secretion. During puberty in females, a positive feedback mechanism also develops, where peak estradiol levels trigger the mid-cycle LH and FSH surge required for ovulation.
Clinical Significance and Pathology
Disorders of the adenohypophysis are often linked to neoplastic processes. Functionally active tumors (e.g., pituitary adenomas) lead to hormone hypersecretion, whereas non-functioning tumors cause compression and destruction of normal functional pituitary tissue.
- Panhypopituitarism (Simmonds disease, pituitary cachexia): A severe syndrome characterized by the progressive loss of all anterior pituitary functions. Manifestations include unintended weight loss, bradycardia, decreased libido, reduced skin pigmentation, and loss of body hair. Sudden loss of pituitary function can trigger a life-threatening addisonian (hypoadrenal) crisis.
- Infectious involvement: In hemorrhagic fever with renal syndrome (HFRS), the anterior pituitary can serve as a secondary site of viral replication.
- Pharmacological regulation: Gonadotropin-releasing hormone (GnRH) analogs such as goserelin are used in oncology. Continuous administration causes receptor desensitization and down-regulation, suppressing FSH and LH secretion and thereby reducing gonadal hormone stimulation.