IA MVST HISTOLOGY
TEXT FOR CAL MODULE ON THE
THYROID, PARATHYROID AND PITUITARY GLANDS
This module will focus on the structure-function relations of the thyroid, parathyroid and
pituitary glands in mammals.
This CAL module is designed to run alongside the activities scheduled for this class.
Supplementary information may be obtained from your Histology handbook, Demonstration
material, and from your Homeostasis lecture notes.
1. THYROID AND PARATHYROID
LM of cat thyroid and parathyroid glands:
The thyroid gland is a lobulated gland lying adjacent to both sides of the trachea. It is
composed of numerous follicles (F) of various sizes.
The follicles contain thyroid hormones stored in the form of a proteinaceous material called
colloid.
Colloid is stained red, purple or blue in this LM. It is shown filling the follicular spaces.
Parathyroid glands are small discoid glands (6 x 3 x 2 mm in humans), characterized by a
fairly homogeneous appearance.
There are usually four parathyroid glands, located behind the thyroid gland.
Parathyroid glands are divided into poorly defined lobules by septa (S). Septa (stained blue)
are thin extensions of the supporting tissue capsule (cap), and carry blood vessels, nerves and
lymphatics.
Parathyroid glands are primarily concerned with the regulation of plasma calcium and
phosphate concentrations.
LM of cat parathyroid gland:
Two major types of cells are present in the parathyroid: chief or principal cells, and oxyphil
cells.
Chief cells, the most common type, secrete parathyroid hormone (PTH) and have a large
round nucleus and a pale-staining cytoplasm. Under EM, their cytoplasm shows a prominent
Golgi, rER and secretory granules.
PTH secretion is under direct negative feedback control through plasma ionized calcium levels.
Oxyphil cells (not shown) represent a minor portion of the parathyroid cells, and their function is
unknown. Their cytoplasm is eosinophilic (oxyphilic) and contains numerous mitochondria. In
humans, few are seen before puberty.
LM of cat thyroid gland:
This micrograph shows the typical structure of the thyroid gland consisting of numerous
follicles (F) containing the characteristic colloid material (stained red).
The main component of colloid is thyroglobulin, a glycoprotein to which the thyroid hormones
are bound.
Thyroxine (T4) and triiodothyronine (T3) are the hormones produced by the thyroid gland.
Supporting tissue septa (S) carry blood vessels, lymphatics and nerves throughout the thyroid
gland parenchyma, and divide it into lobules. (Note: the space occurring between the colloid and
the follicular edge is an artifact, caused by shrinkage during tissue preparation).
LM of rat thyroid gland – Resin section:
Thyroid follicles are lined by a single layer of cuboidal cells (f), which are involved in the
synthesis and secretion of thyroid hormones.
A second type of cell, called parafollicular, clear, or C cells (c), are scattered between the
follicles, either in clumps or as single cells. These cells synthesize and secrete calcitonin, which
lowers circulating calcium levels. Its precise physiological role is uncertain.
Numerous fenestrated capillaries (not seen at this magnification) are found close to the thyroid
follicles. During active secretion, thyroid hormones are transported from the follicular cells to
the interstitial space and thence to the circulation to reach their targets.
The morphology of thyroid follicles changes with the degree of activity of the thyroid gland.
In glands with little secretory activity, as shown in this LM, the follicular cells (f) appear
flattened or small cuboidal. The follicles are distended by the large amount of colloid (C)
stored inside them.
LM of rat thyroid gland, treated with exogenous TSH:
This LM shows an actively-secreting gland, under the effect of TSH (thyroid stimulating
hormone).
Under stimulation, the follicular cells (f) enlarge and become columnar in shape, with their
nuclei at the base.
The follicles become smaller and the amount of colloid (C) decreases as it is taken up by the
follicular cells.
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2. PITUITARY GLAND
LM of cat pituitary gland - anatomical features:
The pituitary gland, or hypophysis, secretes a variety of hormones, some of which regulate the
activity of other endocrine glands.
It lies beneath the hypothalamus and is divided into two major areas, anterior or
adenohypophysis, and posterior or neurohypophysis, each with a different embryological
origin.
The intermediate lobe, considered to be part of the adenohypophysis, is well developed in
certain mammals (not in humans).
The hypothalamus and pituitary gland form a complex functional unit which regulates body
growth, water metabolism, milk secretion, lactation, and the functions of the thyroid gland,
adrenal gland and gonads.
The posterior pituitary is connected to the hypothalamus by the pituitary stalk.
The posterior pituitary consists of axons from nerve cell bodies in the hypothalamus.
The anterior pituitary consists mainly of a glandular epithelium.
The anterior pituitary has intimate vascular connections with the hypothalamus.
The hypothalamus is a main regulator of the secretory activities of the pituitary gland.
LM of cat anterior pituitary:
The anterior pituitary consists of cords or clumps of cells surrounded by sinusoid capillaries
(S), which are supported by a network of collagen and reticulin fibres.
On the basis of their staining reactions, the cells of the anterior pituitary have been traditionally
divided into chromophobes (C) and chromophils (A, B).
Chromophobes (C) are the smallest in size, are poorly stained and contain few cytoplasmic
granules.
Chromophils contain numerous cytoplasmic granules and are subdivided into basophils (B,
light blue), which stain with basic dyes, and acidophils (A, red), which stain with acidic dyes.
EM of anterior pituitary cell - Somatotroph
Immunochemical methods and EM have enabled the identification of five types of secretory
cells: somatotrophs (~50%, secrete growth hormone), corticotrophs (~20%, secrete ACTH),
gonadotrophs (~5%, secrete both LH and FSH), lactotrophs (~20%, secrete prolactin) and
thyrotrophs (~5%, secrete thyrotrophin).
Under EM, the anterior pituitary secretory cells are characterized by numerous cytoplasmic
secretory granules, which contain the hormones. Each cell type can be identified on the basis
of the electron density, size and shape of their secretory granules.
This EM shows a somatotroph, filled with secretory granules (S) of moderate size. Note the
mitochondria (m), ER and Golgi (G).
--------------------------------------------------------------------------------------------------------------------LM of cat posterior pituitary
This micrograph shows the non-myelinated axons of the posterior pituitary, supported by
pituicytes, a type of glial cell (most of the nuclei in this LM are pituicyte nuclei).
The cell bodies of the axons are in the hypothalamus where the two peptide hormones,
antidiuretic hormone (ADH) and oxytocin, are synthesized.
The posterior pituitary is highly vascularised with fenestrated capillaries (cap). These is not
clearly shown in this micrograph.
EM of posterior pituitary cell
This EM of the posterior pituitary shows pituicytes (P) which contain some organelles in the
cytoplasm, but few, if any, secretory granules.
The non-myelinated axon terminals, on the other hand, are filled with numerous secretory
granules. The nerve cell bodies of these axons are located in the hypothalamus and are
responsible for the synthesis of ADH and oxytocin, secreted by the posterior pituitary.
ADH and oxytocin are synthesize as precursor molecules. These are packaged into secretory
granules in the Golgi and transported down the axons to the terminals in the posterior pituitary.
Cleavage of the precursor molecules into free hormones occurs during transport.