Adenohypophysis

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 is the most voluminous part of the organ.

The posterior wall of the pouch remains thin and transforms into the intermediate (middle) lobe. Between these two structures, the remnant of the embryonic pouch cavity persists as the hypophyseal cleft. The third component of the epithelial part is the pars tuberalis, a small extension adjacent to the pituitary stalk.

The posterior lobe (neurohypophysis) develops separately from an evagination of the floor of the diencephalon and merely lies adjacent to the adenohypophysis. Unlike the epithelial part, it does not synthesize hormones.

General histology and endocrine function

In histological sections stained with Heidenhain's Mallory stain, collagen fibers appear blue, nuclei are orange, and cell cytoplasm exhibits various tinctorial properties (binds dyes differently).

Navigation across the specimen usually 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, forming branching cords. These cords are separated by a stroma of loose fibrous connective tissue. The anterior lobe synthesizes 6 hormones: growth hormone / somatotropin (GH / STG), 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. Sinusoidal capillaries with a wide lumen and 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 dyes (due to the presence or absence of protein secretory granules), anterior lobe cells are divided into three main groups and one specific group:

1. 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 intensive secretion.
2. Acidophilic cells (30–65%). The most numerous group of stained cells with orange-red cytoplasm and a central rounded nucleus. They are divided into somatotropes (small granules, GH synthesis) and lactotropes (large granules, prolactin synthesis).
3. Basophilic cells (approx. 15%). The cytoplasm stains purple with basic dyes. They are subdivided into two types:
4. Type 1 (thyrotropes): angular cells with a central nucleus, producing TSH.
5. Type 2 (gonadotropes): rounded cells with an eccentric nucleus. A pale zone of the Golgi complex — the macula — is located in the center. A single cell produces both FSH and LH.
6. Cells with a lobulated nucleus (corticotropes, 2–5%). Their tinctorial properties vary, but the main marker is the lobulated nucleus. They produce ACTH, and their granules possess a dense protein core.

Cell morphology in pathologies

The secretory activity of adenocytes is strictly controlled by a feedback mechanism. Following the removal of peripheral endocrine glands, characteristic pathomorphological changes occur in the hypophysis:

• "Thyroidectomy cells": develop in case of thyroid hormone deficiency. Thyrotropes dramatically increase TSH production. Their endoplasmic reticulum expands catastrophically, turning into large vacuoles, which gives the cytoplasm a coarse-meshed appearance.
• "Castration cells": appear in the event of sex hormone deficiency. In gonadotropes (Type II basophils), the macula area transforms into a giant vacuole that displaces the cytoplasm and nucleus to the periphery, giving the cell a characteristic signet-ring appearance.

Class and exam questions

What else to study on this topic

• Hemodynamic features: primary and secondary capillary networks of the portal system
• Ultrastructural differentiation of somatotropes and lactotropes
• Mechanisms of formation of thyroidectomy cells and their difference from castration cells
• Embryonic origin and transformation of the hypophyseal pouch
• Tinctorial properties of cells and chemical nature of hormone granules

Other topics in the "Endocrine System" section

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