DISEÑO Y SIMULACIÓN DEL DUCTO

The main epithelial component of the thyroid, the follicular cell, is of endodermal origin and is a derivative of the primitive buccopharyngeal cavity (Thomas and Williams, 1994). The thyroid gland begins as a median, ventral downgrowth in the region of the first pharyngeal (branchial) pouch (Fig. 1.4). It is attached to the pharyngeal pouch epithelium by a stalk, the thyroglossal duct. As the thyroid gland develops, it separates from the floor of the mouth cavity by involution of the thyroglossal duct and migrates caudally into the neck.

The ultimobranchial body develops from the caudal surface of the third pharyngeal pouch in the rat (Rogers, 1927). It loses its connection with the endodermal surface and becomes embedded in the thyroid lobes. The ultimobranchial bodies contain the precursors of the C-cells. These cells are thought to originate from the primordial cells of the neural crest ectoderm before infiltrating the ultimobranchial bodies (Pearse and Polak, 1976). Thus, the concept has arisen that follicular cells and C-cells of the thyroid gland are derived from

different germ layers: namely endoderm and neuroectoderm respectively. This is also believed to be true in the human thyroid gland.

CRANI AL F o r a m e n c a e c u m T h y r o g l o s s a l d u c t P a r a t h y r o i d g l a n d

m

T h y m us U l t i m o b r a n c h l a l bo dy R i g h t t h y r o i d l o b e L e f t t h y r o i d l o b e CAUDAL

Fig. 1.4: Embryology of the thyroid gland and ultimobranchlal body in relation to the primitive pharyngeal pouches (labelled I, II and III) in the rat. (Based on Capen, 1985.)

1.5.2 Structure and function of the thyroid gland

The follicular cells of the thyroid gland are arranged in closed, epithelial-lined spherical structures known as follicles (Fig. 1.5). The follicles are lined by a single layer of epithelial cells and possess a central lumen that contains colloid material (Thomas and Williams, 1994). There is considerable variation in follicular size within the normal rat thyroid gland. Typically, small follicles lined by tall cuboidal cells are present in the central areas, and large follicles lined by flattened epithelium are seen at the periphery.

This height variation in follicular cells is dependent on the activity of the gland, as taller cells are usually engaged in active hormone synthesis (Hardisty and Boorman, 1990). Formation of thyroid hormones depends on the exogenous supply of iodide, and involves a number of complex events. The thyroid specific protein, thyroglobulin (TG), is synthesised by the follicular epithelium and, following iodination catalysed by thyroid peroxidase (TPO), is stored in the lumen of the follicle as a proteinaceous colloid. During the secretory phase, droplets of colloid are resorbed into the follicular cell by endocytosis (Thomas and Williams, 1994). Once within the follicular cell cytoplasm, the colloid droplet fuses with a lysosome. TG is then broken down to release the active hormones (T3 and T4) which are then released into the circulation. Thyroid function is regulated by TSH, which increases transcription of both the TG and TPO genes, and by increasing the uptake of iodide into the follicular cell.

C -cell

Follicular cell

C olloid

Fig. 1.5; Normal arrangement of follicles and C-cells in the thyroid gland of the young adult rat. The C-cells are parafollicular in location and lie between the follicular cells and the basement membrane of the follicle. They are not in contact with the lumenal colloid.

In the rat, C-cells are not distributed uniformly throughout the thyroid gland but tend to be concentrated in the central part of each lobe apposed to the smaller follicles. C-cells are located within the follicle between the epithelium and the basement membrane and occur singly or clustered in small groups (Biddinger and Ray, 1993). In haematoxylin and eosin (H&E) stained sections, most C-cells have a polygonal or spindle shape, with pale granular cytoplasm and centrally placed nuclei that are slightly larger than the nuclei of adjacent follicular cells (DeLellis, 1994).

C-cells are classified as part of the diffuse neuroendocrine system, along with others such as pancreatic islet cells, pituitary cells and gastroenteric endocrine cells (Sano, 2000). All these cells express biosynthetic functions characteristic of neurones and classic endocrine cells. CT is the major secretory product of the normal C-cell. Cloning of the CT gene has led to the discovery and characterization of several other related peptides including calcitonin gene-related peptide (CGRP). CGRP is produced by alternative splicing of the primary RNA transcript of the CT gene (Rosenfeld et al., 1983). The process is tissue- specific such that normal C-cells splice the primary transcript to produce CT mRNA primarily, whereas neural cells use the same transcript to produce CGRP. Both CT and CGRP mRNA transcripts are produced in normal C-cells at a ratio of about 95:1. A variety of other peptides has been localised in rat C-cells by IHC, including somatostatin (SS), gastrin-releasing peptide (GRP) and thyrotropin-releasing hormone (TRH) (Zabel et al., 1987; DeLellis, 1994). However, a degree of heterogeneity within the C-cell population appears to exist as only a small subset of C-cells are capable of producing and storing SS (Thomas et al., 1994).

The interaction between C-cells and follicular cells has long been of interest to workers in the field, and the storage of regulatory peptides by C-cells would support the existence of a paracrine relationship. Further evidence is provided by ultrastructural observations where C-cells form specific structural complexes with follicular cells, and the two cell types appear to adhere tightly to each other (Sawicki, 1995). The exact role of many of the regulatory peptides still remains uncertain, however SS is of particular interest because it has been shown to regulate follicular cell secretion and growth in vitro (Sawicki, 1995). This last effect is due to the ability of SS to inhibit TSH drive on follicular cells, and also it is believed that SS can exert this cytostatic effect on follicular cells in vivo (Medina et al.,

1999). This has led to the use of SS analogues in the treatment of thyroid tumours in humans, where they have been shown to have a beneficial effect on prognosis in some cases.

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