LAB 4

HISTOLOGY OF THE ENDOCRINE SYSTEM

 I. Introduction

 The endocrine glands are the ductless glands. The epithelial cells of endocrine glands are not oriented around ducts as are the cells of exocrine glands. Instead, the cells are in close association with capillaries, and the cell products (hormones) are secreted into the connective tissue compartment where they are rapidly absorbed by the circulatory system. The liver is both an endocrine and exocrine gland. It secretes products such as proteins, lipoproteins and glucose (not hormones) into the blood, but it has recently been shown to produce the hormone somatomedin and a prohormone angiotensinogen.  

The pancreas, kidneys, testis, ovaries, placenta, brain, lungs, and gastrointestinal tract are known to produce hormones, but they have other functions as well. The traditional endocrine glands are the pituitary, thyroid, pineal, parathyroid and adrenal glands as well as the pancreatic islets of Langerhans.

  

II. Procedure 

A. Thyroid Gland 

Slide 27 (human thyroid) 

When you are looking for the thyroid, also examine slide 21, it often has a section of the thyroid gland on it. The thyroid is composed of two lobes containing many follicles, the basic histological unit. The wall of each follicle is composed of squamous to columnar epithelial cells. The center of each follicle contains colloid secreted by the follicle cells. The follicles are surrounded by connective tissue elements and a rich capillary network. 

The quantity and consistency of the colloid and the shape of the cells in the epithelium depends on the functional state of the gland. In the presence of low levels of thyroid stimulating hormone (THS) the follicle cells are squamous and the colloid is dense and abundant. When stimulated by high levels of THS the cells of the epithelium become more columnar in shape and the amount of colloid is reduced, stains less intensely and may appear vacuolated or "foamy" around the periphery. 

 

 

 

 

 

 

 

 

 In addition to the follicle cells, the thyroid contains "light cells" sometimes called parafollicular or C- cells. These cells may be hard to find on our slides. These C-cells originated in the ultimobranchial body, and only in mammals do they migrate into the thyroid. The parafollicular cells produce calcitonin, a hormone that regulates plasma calcium levels by lowering the plasma concentration. Note the capsule, the arrangement of the  follicles and the and the vascularized connective tissue.

 

B. Parathyroid Gland

 Slide 27 (parathyroid and thyroid gland)

 Examine slide 27 and identify the principle cells (chief cells) which are the most common cell type. These cells secrete PTH (also called parahormone) which acts to raise plasma calcium levels. PTH acts by increasing the rate of breakdown of bone by osteoclasts, increasing the reabsorption of calcium, inhibiting the reabsorption of phosphate ions (kidney) and promotion absorption of calcium from the small intestine. 

Another type of parenchymal cell is the oxyphil which are larger cells with dark (may be lighter)  acidophilic cytoplasm. They usually occur in groups or nests, but may be isolated. These cells have been identified in primates and cattle but not in other animals. The oxyphils, in common with the parietal cells of the stomach contain large numbers of mitochondria. The function of oxyphils is unknown. Note that the cells of the parathyroid are arranged in irregular anastomosing cords.

 

C. Adrenal Gland

 Slide 65 (adrenal gland)

 Examine slide 65 and with the aid of your text and atlases identify the connective tissue capsule  which sends septa into the organ, the cortex and the medulla. Within the cortex find the following zones:

         1. zona glomerulosa

         2. zona fasciculata

         3. zona reticularis 

 

The zona glomerulosa secretes mainly aldosterone, a mineralocorticoid. The zona fasciculata secretes mainly cortisol. The zona reticularis may secrets some to the sex hormones (androgenic hormones). Over 30 different hormones have been isolated from the adrenal cortex but aldosterone and cortisol are of greatest importance. Note the arrangement of cells in each zone. In the medulla, note that the cells are arranged in elongated strands, which differ from the cell arrangements of the cortex. The main cellular components of the medulla are the chromaffin cells which are arranged in irregular groups.

 The vascular system of the cortex consists of a network of sinusoids supplied by branches of a capsular plexus of vessels. The sinusoids pass between "cords" of epithelial cells. The sinusoids are fenestrated and are bordered by a basal lamina.  The arrangement of sinusoids and cells is similar in many ways to that in the liver. A perisinusoidal space separates the lining cells form the parenchymal cells and microvilli or the parenchymal cells extend into this space.

  

D. Pituitary Gland

 Slide 26  (hypophysis)

 The pituitary gland is composed of several glandular epithelial components lumped under the name  adenohypophysis (derived from Rathke's pouch) and nervous parts collectively called the neurohypophysis (derived from the floor of the diencephalon). The adenohypophysis can be  subdivided into three regions: pars distalis, pars tuberalis and pars intermedia. The neurohypophysis also has three parts: pars nervosa, infundibular stem and median eminence. The whole gland is covered by a connective tissue capsule. 

 There is a great difference in the gross appearance of the pituitary gland in different vertebrates. Identify under low power the adenohypophysis, neurohypophysis and pars intermedia. In the adenohypophysis note three types of cells:

         1.                         Chromophobes or chief cells which are the most numerous. They are small cells which stain rather lightly and often possess little cytoplasm. 

         2.                         Eosinophilic or acidophilic cells.

         3.                         Basophilic or basophils which are the least numerous.

  

 

Note that the distribution of the cell types is irregular within the adenohypophysis and made up of individual cords. Pay attention to the arrangement of cells in this network of anastomosing cords separated by sinusoidal capillaries. The pars intermedia consists of only one type of epithelial cell. The pars nervosa is composed of many neuroglial cells and fibers but no true nerve cells are present.

E. Testis

 Slide 79, 80 (mammalian testis)

 The testis is a compound tubular gland enveloped by a layer of mesothelium and a connective tissue capsule called the tunica albuginea. The parenchyma contains convoluted seminiferous tubules, blood vessels, nerves, and interstitial connective tissue. Groups of tubules are separated by connective tissue septa into compartments called lobules. In humans there are 1-4 tubules per lobule. The Leydig cells which lie outside the tubules are normally located near blood vessels and are eosinophilic. 

 Each seminiferous tubule is lined with a seminiferous epithelium made up of Sertoli cells and germinal cells in various stages of maturation. The sertoli cells envelop and nourish spermatocytes and spermatids. The Sertoli cells occupy the spaces between the germinal cells. The Sertoli cells can be identified with their large variably shaped nuclei with a dense spherical nucleolus. The nuclei of the germinal cells are nearly always round near the margins of the tubules where the Sertoli cell  nuclei are located. The extent of the cytoplasm of Sertoli cells cannot be discerned in ordinary H &  E sections). 

  

F. Ovary

 Slide 53, 54 (ovary)

 The surface of each ovary is covered by a layer of squamous to cuboidal cells called the germinal epithelium. Under this membrane is a connective tissue layer called the tunica albuginea. The ovary is roughly divided into a cortex and medulla. The medulla contains many blood vessels and a  connective tissue stroma (matrix). The cortex is composed of primordial follicles, developing follicles, maturing follicles, degenerating follicles and corpora lutea. 

 During maturation the oocytes and follicles enlarge. The follicle cells form a multilayered granulosum layer. A space develops within the follicle and becomes filled with an albuminous fluid called the liquor folliculi. Two layers or sheaths develop around the follicles the inner glandular theca interna and the fibrous theca externa. The oocyte becomes surrounded by a mucopolysaccharide acellular layer called the zona pellucida.   

At ovulation the germinal epithelium ruptures and the oocyte along with some follicular cells is released into the paraovarian space. The follicles are transformed into corpora lutea after ovulation. As the epithelium heals the granulosa cells hypertrophy (increase in size) and are called granulosa lutein cells. The granulosa lutein cells are large and acidophilic. They become arranged in cords surrounded by sinusoids. Cells around the outside of the granulosa lutein cells form the theca interna and are called theca lutein cells. Identify the major structures of the ovary.

Identify glands in fetal pigs: