Anatomical Division and Morphogenesis
The adenohypophysis forms during embryogenesis from the hypophyseal pouch (an epithelial evagination). During development, its anterior wall actively proliferates to form the massive anterior lobe, which constitutes the largest volume of the gland.
The posterior wall of the pouch remains thin and develops into the intermediate (middle) lobe. A remnant of the embryonic pouch cavity, the hypophyseal cleft, persists between these two structures. The third component of the epithelial part is the tuberal part, a small extension adjacent to the pituitary stalk.
The posterior lobe (neurohypophysis) develops separately from an evagination of the diencephalon floor and merely lies adjacent to the adenohypophysis. Unlike the epithelial part, it does not synthesize hormones.
General Histology and Endocrine Function
In histological preparations stained with Mallory-Heidenhain stain, collagen fibers appear blue, nuclei orange, and cell cytoplasm exhibits varying tinctorial properties (binding dyes differently).
Slide analysis typically begins with the intermediate lobe. It is easily recognized by the presence of follicles and broad connective tissue septa. Two hormones are synthesized here: melanocyte-stimulating hormone (MSH) and lipotropin.
The anterior lobe is characterized by high cellular density. Glandular cells (adenocytes) are organized into adenomeres that form branching cords. These cords are separated by a stroma of loose fibrous connective tissue. The anterior lobe synthesizes 6 hormones: growth hormone (GH / somatotropin), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
Blood supply is provided via the secondary capillary network of the portal system. Wide-lumen sinusoidal capillaries with fenestrated endothelium ensure rapid transport of hypothalamic regulators (releasing and inhibiting hormones) to target cells and efficient release of synthesized hormones into the bloodstream.
Classification of Anterior Lobe Cells
Based on their cytoplasmic affinity for stains (due to the presence or absence of protein secretory granules), anterior lobe cells are divided into three main groups and one specific type:
- Chromophobic cells (15–60%): Lightly stained elements lacking secretory granules. This group includes stem/reserve cells and transitional forms of chromophilic cells that have temporarily lost their granules following intense secretion.
- Acidophilic cells (30–65%): The most numerous group of stained cells with orange-red cytoplasm and a central rounded nucleus. They are subdivided into somatotrophs (small granules, GH synthesis) and lactotrophs (large granules, prolactin synthesis).
- Basophilic cells (approx. 15%): Cytoplasm stains purple with basic dyes. They are subdivided into two types:
- Type 1 (Thyrotrophs): Angular cells with a central nucleus that produce TSH.
- Type 2 (Gonadotrophs): Rounded cells with an eccentric nucleus. A pale Golgi zone, termed the macula, is located centrally. A single cell produces both FSH and LH.
- Cells with lobulated nuclei (Corticotrophs, 2–5%): Their tinctorial properties vary, but their primary marker is the lobulated nucleus. They produce ACTH, and their granules possess a dense protein core.
Cellular Morphology in Pathologies
Secretory activity of adenocytes is strictly controlled by feedback mechanisms. Removal of peripheral endocrine glands leads to characteristic pathomorphological changes in the pituitary:
- "Thyroidectomy cells": Appear during thyroid hormone deficiency. Thyrotrophs dramatically upregulate TSH production. Their endoplasmic reticulum expands drastically, turning into large vacuoles, which gives the cytoplasm a coarse reticular appearance.
- "Castration cells": Appear when sex hormones are deficient. In gonadotrophs (type II basophils), the macula area transforms into a giant vacuole that pushes the cytoplasm and nucleus to the periphery, giving the cell a characteristic signet-ring appearance.