Pituitary gland

Embryonic origin –Anterior pituitary gland (adenohypophysis) Rathke’s pouch –Roof of the embryonic mouth –Glandular tissue containing secretory cells –Posterior pituitary gland (neurohypophysis) Infundibulum/brain floor) –Part of the CNS –Contains axons of the hypothalamic neurons –Housed in a bony capsle called sella turcica

Anterior lobe (predominant lobe) –Differentiation of cells Spatiotemporal regulation –Expression of cell- specific transcription factor(s) at the specific stage of development –Concentration gradient of soluble factors Sexually dimorphic –Gonadotrophs in male – predominantly LH secreting cells –Gonadotrophs in female- equal distribution of LH and FSH secreting cells

Pars tuberalis –Dorsal extension of anterior lobe Surrounds the infundibulum Pars Intermedia –Derived from anterior pituitary cells adjacent to infundibulum May not be clearly defined in many species

Pars nervosa –Posterior lobe –Extension of the CNS –Contains axons of hypothalamic neurons

Blood vessels –Portal plexus from hypothalamus Median eminence Forms hypothalamic-portal circulation Bidirectional flow of blood –Retrograde flow from the pituitary to hypothalamus (short-loop feedback system) –Systemic arteries Inferior hypophyseal arterial blanches –Posterior pituitary gland

Regulation of hormone secretion Hypothalamic level –Releasing/inhibitory factors Transcription of mRNA Secretion of hormone Peripheral hormones –Feedback system Autocrine/paracrine factors Net results –Pulstatile secretion of pituitary hormones

Role of transcription factors Determination of cell type lineage –Temporal regulation of transcription cascade Homeodomain transcription factors –Early differentiation Expression of Rpx and Ptx LIM homeodomain superfamily

Ptx superfamily –Universal regulator of transcription within the anterior pituitary Pit-1 –GH, PRL, TSH, and GHRH receptor mRNA transcription –Interaction with other factors to induce commitment of cells to differentiate (estrogen receptor, thyrotroph embryonic factor) –Transcription of its own mRNA GATA-2 –Developmental regulation of alpha subunit expression –Interaction with SF-1 and DAX-1 triggers differentiation into gonadotrophs

Endocrine cells Five types –Corticotrophs –Somatotrophs –Lactotrophs/mammotrophs –Thyrotrophs –Gonadotrophs Staining characteristics –Acidphils (stained with acidic dye) –Basophils (stained with basic dye)

Endocrine cells Corticotrophs –Basophils –20 % of functional anterior pituitary gland cells –Appears the earliest 8 weeks of gestation –Clustered mainly in central median pituitary wedge –Large, irregularly shaped cells

Lactotrophs –Acidophils –Same stem cells as somatotrophs Could give rise to mammosomatotrophs –Produce both GH and PRL –15-25 % of functional pituitary cells Two types –Large polyhedral cells (found throughout the gland) –Smaller angulated elongated cells (lateral wings and median wedge)

Gonadotrophs –Basophils –10-15 % of functional pituitary cells –Contains two types of secretory glanules Large (  m) Small (  m) –Secretion of two distinct hormones by the same cells GnRH pulsatility

Somatotrophs –Acidophils –35-45% of functional anterior pituitary cells –Same stem cells as lactotrophs Mammosomatotrophs Differentiation induced by TRH or dopamine along with estrogen –Large secretory granules (700  m)

Thyrotrophs –Basophils –5 % of functional anterior pituitary cells –Smaller in size Smaller secretory granules (  m)