Here is a compilation of essays on ‘Endocrine Glands’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Endocrine Glands’ especially written for school and college students.
Essay on Endocrine Glands
- Essay on the Introduction to Endocrine Glands
- Essay on the Histology of Endocrine Glands
- Essay on Thyroid Gland
- Essay on Parathyroid Gland
- Essay on Pancreas
- Essay on Adrenal Cortex
- Essay on Pituitary Gland
- Essay on Thymus
- Essay on Pineal Body
- Essay on Testes
- Essay on Ovary
Essay # 1. Introduction to Endocrine Glands:
Endocrine glands are heterogeneous collection of glands distributed throughout the body, in the head, neck and abdomen. Their function is to produce and secrete hormones. A hormone is a chemical substance produced in an endocrine or ductless gland and secreted directly into the blood circulation to act on a second organ.
An organ has an endocrine function or not can be tested by demonstrating the changes produced by its removal, and their reversal on transplanting the organ or injection of an extract of the tissue. The active principle can be isolated, purified and the structure identified. Another test to ensure the function is to study the histology of the gland, which is characteristic of the activity of the secreting gland.
The gland cells possess numerous secretory granules and possess no ducts. Hormones are now established to be produced by organs other than endocrine glands, for example the intestine. Hypothalamic releasing hormones are synthesized in nerve cells and released into the hypothalamic portal system.
Hormones need not be released into the blood vessels. In gastrointestinal tract, the hormones act on surrounding cells. Thus, hormones may have a paracrine function, producing effects on neighbouring cells, or a neurocrine function, acting as neuromodulators and neurotransmitters, or even an autocrine function, being self-regulatory.
Endocrine glands show great variation in their histological architecture. This can be attributed to their diverse origins and developmental patterns. Some of the endocrine glands are derived from a single germ layer during their development while others arise from more than germ layer. Most of the endocrine organs are derived from the endoderm of the embryo. These include the thyroid, parathyroid and glands associated with the gastrointestinal tract such as pancreas.
The pituitary gland is derived from the ectoderm. It arises from the buccal roof as the Rathke’s pouch and from the floor of the brain, both being ectodermal in origin. The adrenal medulla is derived from the neural crest material, which is also ectodermal in origin. The adrenal cortex and the gonads including testes and ovary are derived from the mesoderm of the embryo. The pineal gland is nervous in nature and is therefore derived from the neural ectoderm.
The histological architecture of the endocrine glands is dependent upon the functions carried by these glands. For instance the thyroid and testes are tubular structures with central space. In thyroid the follicular lumen is used to store the thyroglobulin while the lumen of the seminiferous tubules of the testes helps in the transportation of spermatozoa.
One common feature exhibited by all the cells of the endocrine glands is that all these cells are well equipped with cellular organelles, which take part in the secretory activities of the cells. All the endocrine cells have well developed endoplasmic reticulum and Golgi complex required for the synthesis and packaging of the hormones.
Cells secreting steroid hormones have highly developed smooth endoplasmic reticulum while those secreting protein hormones have well developed rough endoplasmic reticulum. Mitochondria, which provide energy required for all the activities of the cell are well developed in all endocrine cells. The histochemical characteristics of the cytoplasm of the endocrine cells also reflects the functions carried upon by the endocrine cells.
Cells in the adrenal cortex reveal large number of refractile granules in the cytoplasm, which are the steroid secretory products of the cells. Adrenal medullary cells show chromaffin granules and these are also the hormones secreted by the medullary cells.
Essay # 3. Thyroid Gland:
Thyroid gland is a dumb-bell shaped, bilobed gland located in the thoracic region at the root of the throat. The two lobes are almost symmetrical and lie one on either side of the tracheal tube. Each lobe measures about 5 × 2 × 2 cm.
The two lobes are joined together by a narrow strip of tissue called isthmus or middle lobe, which crosses the 2 – 4th tracheal rings. In some cases the right lobe is bigger than the left lobe. A pyramidal lobe varying in size extends from the isthmus upward in the neck. Accessory thyroid bodies are located beneath the main thyroid gland.
The weight of the thyroid gland of the adult varies between 20 to 25 gm and is influenced by diet, age, sex and reproductive state of the individual. Thyroid gland is highly vascular. Blood is supplied by the paired superior and inferior thyroid arteries and directly from the aorta. Venous blood is collected by internal jugular and innominate veins. Blood flows at the rate of 4 – 6 ml per gland per minute.
This high rate of blood flow ensures adequate supply of inorganic iodine to the gland. Thyroid gland is innervated by sympathetic fibers derived from the superior, middle and inferior cervical ganglia and parasympathetic fibers derived- from the superior and inferior laryngeal branches of the vagus nerve. These nerves control the blood supply to the gland.
Thyroid gland is endodermal in origin arising from the primitive foregut. During embryonic development, the primitive thyroid gland arises as an out-pushing from the middle of the neck infront of the thyroid cartilage. Thyroid gland is derived from the fourth pharyngeal pouch, which gives rise to the lateral lobes of the gland.
During intra-uterine development, thyroid gland can be recognized in the seventh week. The follicular structure of the gland is evident by 12 – 14th week of the foetus. Gradually, with the development of the gland, it acquires the capacity of concentrating iodine from the circulating blood and other functions of synthesis and secretion follow.
Microscopic examination of the sections of the thyroid gland reveals the presence of numerous acini or follicles about 200 µ in diameter. The number of the follicles in a normal gland is about 100, 000. The size of the follicles also varies. Large follicles are found near the periphery of the gland while smaller follicles are arranged at the center. Each follicle is made up of a cuboidal follicular epithelium.
In the resting condition, the epithelium is low but the height is dependent upon the extent of stimulation of the gland. Under stimulation by thyroid stimulating hormone the low cuboidal epithelium becomes converted into tall columnar epithelium. The thyroid follicles are usually spherical or oval in shape and measure about 20 – 150 µ m in diameter.
A single layer of cubical follicular epithelial cells lines each follicle. The follicular cells are surrounded by a basement membrane consisting of fine connective tissue fibers on which rest the bases of the follicular cells.
Follicles are surrounded by a highly vascular stroma containing lymph channels and nerve endings. The follicular lumen is filled with a colloidal material, the thyroglobulin. Thyroglobulin constitutes about 75% of the colloid material, which is the main storage form of the thyroid hormones (Fig. 6).
Each follicular cell is loaded with granular cytoplasm containing large number of mitochondria and distinct Golgi apparatus. The luminal end of each follicular cell facing the colloidal material is thrown into a number of microvilli. The nucleus is situated at the bases of the follicular cell and the cytoplasm is filled with well-developed rough endoplasmic reticulum and Golgi complex.
In addition to these, large number of cytoplasmic vesicles filled with the colloid material are also found in the cytoplasm. Based on the staining properties, the cytoplasmic vesicles can be distinguished into three types, eosinophilic or acidophilic, basophilic and mixed. Two types of follicular epithelial cells have been distinguished on the basis of electron microscopic and histochemical studies.
The first are the principal cells present in large number and contain small number of mitochondria, and different proteases and oxidases required for the synthesis of thyroid hormones. The second type of cells occurs in small number but contain large number of mitochondria and these are scattered in between the principal cells. These cells are known as parafollicular cells or C-cells those secrete thyrocalcitonin.
Parathyroid gland consists of two pairs of small bodies, oval in shape and lying embedded in the posterior surface of the thyroid gland (Fig. 7). Each gland is about 6 × 5 × 3 cm is size. The total weight of the gland varies between 23 to 500 mg.
Each gland is enclosed in a connective tissue envelope from which septa descend down into the glandular tissue partially dividing it to give a lobulated appearance. The gland consists of columns of epithelial cells interspersed with blood capillaries and nerve endings.
The two upper parathyroid glands are formed from the fourth branchial pouches while the lower pair is formed from the third branchial pouch.
Examination of the sections of the gland under microscope reveals the presence of masses or columns of epithelial cells with blood sinuses in between them.
The epithelial cells can be distinguished into two types:
(i) Chief or Principal Cells:
These cells are small in size and the cytoplasm is clear without any granules but contains glycogen. Nucleus is large and vesicular in shape (Fig. 7). They constitute the majority of the cell population and sustain through-out life. Occasionally, these cells become enlarged and their cytoplasm becomes vacuolated. Under such conditions, chief cells are known as water clear cells.
Cytochemical studies have shown that the chief cells can be further differentiated into two types:
(a) Light chief cells containing glycogen and few secretory granules, and
(b) Dark chief cells poor in glycogen and rich in secretory granules.
(ii) Oxyphil or Eosinophil Cells:
In contrast to the chief cells, oxyphil cells are larger in size and fewer in number. They are polyhedral in shape and the cytoplasm is granular and acidophilic. Granules can be stained with acidic dyes like eosin and electron microscopically, these granules have been shown to be mitochondria.
Cytoplasm of both the types of cells contains fatty granules or globules whose number goes on increasing with age. Adipose tissue is interspersed in the glandular tissue and sometimes-small colloid vesicles are also noticed.
Pancreas is a leaf shaped gland situated between the stomach and duodenum. It is one of the most important glands of the body, both exocrine and endocrine in function. It measures about 12 to 15 cm in length. The endocrine part of the gland constitutes only 1-2 % of the total weight of the gland.
The endocrine pancreas consists of scattered groups of cells throughout the organ called as Islets of Langerhans discovered by Langerhans in 1869.
Islets of Langerhans constitute groups of epithelial cells distributed among the exocrine pancreatic acini. Their number varies from 200, 000 to 2,000, 000. The cells of the islets can be stained intravitally with neutral red. In 1972 Lacy and Greider have demonstrated the presence of three different types of cells in the islets.
(i) α-cells- or A2 cells,
(ii) β cells and
These cells constitute 10 -15% of the total cells. The cells are larger in size, and contain granular and oxyphilic cytoplasm. The granules are opaque and spherical, uniform in size and distributed throughout the cytoplasm. They are enclosed in a smooth membranous sac and take a deep red color with Mallory-Azan stain.
Golgi complex is well developed while granular endoplasmic reticulum is moderate. Free ribosomes are also found frequently in the cytoplasm. Mitochondria are few in number and filamentous. Nucleus is deeply indented or lobulated. Alpha cells secrete the hormone glucagon.
Beta cells form 30 -40% of the islet cells. They are smaller in size, granular, basophilic and distributed in the periphery of the islets. Cytoplasmic granules are uniform in size and soluble in alcohol. A double membrane covers the granules and a large clear space exists between the granules and the overlying membrane.
Golgi complex is better developed than the alpha cells and the endoplasmic reticulum is more prominent with free ribosomes. The nucleus is round or ovoid, and the space between the nucleus and cell membrane is filled with granules. The granules containing insulin are released to the exterior by the rupture of the cell membrane.
These cells form 5% of the total cell types. The cytoplasm of these cells is characterized by the presence of numerous, small; membrane bound granules containing the hormone gastrin. In addition to these cell types, ϒ_Cells with very few granules in the cytoplasm constituting about 5% of the islet cells have also been reported. They are supposed to be the precursors of the alpha and beta cells. The function of these cells is not known but it is believed that they secrete lipocolin that controls the metabolism of lipids.
There are two adrenal glands. Each gland is a triangular, flattened, cap-like structure situated on the dorsal surface of the kidney. The name Supra renal is derived from its position occupied above the kidneys. Each gland is about 5 x 3 x 1 cm in size and 8 – 10 gm in weight.
The gland is covered over by a delicate but firm connective tissue capsule that separates it from the underlying kidney. The position occupied by the gland, size and shape vary from species to species. The right gland is smaller than the left.
The gland consists of two parts:
(i) Cortex, and
The two parts differ in their embryological origin, structure and function (Fig. 9).
Adrenal gland is highly vascularized receiving about 6-7ml of blood per gm of tissue per minute. Blood is supplied by the branches of phrenic, middle adrenal and inferior adrenal arteries while right and left adrenal veins collect blood from the adrenal glands. Blood vessels enter from the surface to form a rich vascular plexus in the cortex. From here, blood enters the dilated sinuses in the medulla.
Adrenal cortex is mesodermal in origin. It is derived from the coelomic epithelium covering the anterior part of the mesonephros. During embryonic development, an inner layer of provisional cortex is formed from groups of eosinophilic, large, granular cells. At this stage, the permanent cortex is not well developed. After birth, the provisional cortex undergoes degeneration while the permanent cortex starts to develop and envelop the medulla.
Is made up of polyhedral parenchyma cells forming two cells thick cords running radially from the cortex to the medulla (Fig. 10). Blood capillaries form a .close network around the cords ensuring blood supply to each cell. The cells are characterized by well-defined nuclei, mitochondria, and Golgi.
Cytoplasm contains refractile lipid granules. Adrenal cortex can be divided into three layers as follows (Fig. 11):
a. Zona Glomerulosa:
This forms the outermost layer. It consists of groups of columnar cells with their long axis running parallel to the surface. Cells are smaller in size and thickly set. This layer is generally irregular and ill defined (Fig. 12).
b. Zona Fasciculate:
This forms the middle and widest layer. The polyhedral cells forming this layer are larger in size and arranged in radiating columns lying perpendicular to the surface. The cytoplasm of the cells contains brownish yellow pigment granules and lipid droplets. Nuclei are larger and less dense.
The lipid droplets dissolve in normal staining with hematoxylin-eosin giving vacuolated appearance to the cytoplasm and spongy texture to the layer. Cells are rich in mitochondria having tubular villi. Smooth endoplasmic reticulum is highly developed indicating secretion of steroid hormones. This layer constituting the largest part secretes glucocorticoids.
c. Zona Reticularis:
This is the innermost layer of the cortex. In this zone, cells are arranged irregularly leaving wide blood spaces (Fig. 13). The cytoplasm of the cells contains few lipid droplets. Zona reticularis secretes sex steroids and minute quantity of glucocorticoids.
Essay # 7. Pituitary Gland:
Pituitary gland is located at the base of the brain behind the optic chiasma. It lies in a groove, the sella tunica of the sphenoid bone at the floor of the skull. The pituitary gland is a reddish, small, rounded or oval, unpaired structure about the size of a large pea. It weighs 0.5 to 0.6 gm in adult human beings. In women it is larger in size.
Pituitary gland continues to grow upto the age of forty years, but later starts to decrease in size. In pregnant females, it is usually larger. Its dimensions are 10-mm (antero-posterior) × 6 -mm (dorso-ventral) × 13-mm lateral.
The gland is divisible into two parts:
i. Adenohypophysis or anterior lobe and
ii. Neurohypophysis or posterior lobe.
However, based on its microscopic structure pituitary gland can be divided into six pans.
(i) Pars distalis or Pars anterior,
(ii) Pars tuberalis,
(iii) Pars intermedia,
(iv) Pars nervosa,
(v) Median eminence, and
(vi) Infundibulum or Pituitary stalk.
The interglandular cleft separates the pars intermedia from the anterior and posterior lobes. Adenohypophysis includes the first three parts while the neurohypophysis includes the last three parts (Fig. 16).
Pituitary gland is derived from the ectoderm of the embryo. An up-growth arises from the roof of the embryonic buccal roof to meet the down-growth from the floor of the third ventricle (Fig. 16). The outgrowth from the buccal roof is known as the Rathke’s pouch. The stalk of Rathke’s pouch soon disappears but the rest of it forms the adenohypophysis. The neural down-growth becomes the hypothalamus.
The part of the gland that grows up from the Rathke’s pouch gives rise to the pars distalis pars and tuberalis while the posterior wall gives rise to pars intermedia. The neural down-growth persists and forms the pituitary stalk or infundibulum. The pars intermedia invests the pars nervosa.
The interglandular cleft is formed by the original cavity of the Rathke’s pouch. The neural down-growth forms the posterior lobe and stem of the gland. The two parts are combinedly known as neurohypophysis.
Histology of Adenohypophysis:
Pars anterior is the largest part of the pituitary gland constituting 75% of the gland. It consists of masses of epithelial cells interspersed with numerous blood sinuses.
Based on their staining properties, cells in the pars anterior can be broadly divided into two types:
Chromophobes do not accept any stain. Therefore, they are not stained by either acid or basic dyes. They lack secretory granules in the cytoplasm and constitute the reserve cells. They contain less cytoplasm than chromophils.
Chromophils are larger in size and accept stains. The distinction between these two cell types is not very sharp as chromophobes can gain granules and become converted into chromophils while the chromophils may lose their granules to get converted into chromophobes.
Chromophils can be further differentiated into acidophils and basophils. Those cells stained by acid dyes like eosin or acid Fuchsin are acidophils while those which accept basic dyes such as hematoxylin or basic fuchsin are known as the basophils. Acidophils constitute 75% and basophils 25% of the chromophils.
Acidiphils can be further divided into the following specific types of cells:
These cells constitute 4-10 % of the total weight of the pituitary gland. They are oval or round cells filled with dense granules measuring about 350 – 400 nm in size. Somatotrophs secrete somatotrophs or growth hormone and are located in the lateral part of the anterior lobe.
Lactotrophs are oval or elliptical cells scattered randomly in the anterior lobe. Cytoplasmic granules are fewer and larger measuring 600 to 900 nm. Their number is dependent upon the reproductive stage. They increase during pregnancy and lactation. Lactotroph cells secrete the hormone prolactin.
Basophils can be further divided on the basis of differential staining into the following types:
Thyrotrops synthesize thyroid stimulating hormone. These cells are large, polygonal in shape and contain small nuclei. They can be stained with periodic acid-Schiff reagent due to the glycoprotein hormone secreted by them.
Granules in the cytoplasm are small, measuring 50 to 100 nm. Thyrotrophs occupy the anteromedial and anterolateral portions of the anterior lobe. Their number increases during thyroid failure or removal of the thyroid gland.
Corticotrophs secrete the hormone Adreno Corticotrophic Hormone (ACTH) and are embryologically derived from the intermediate lobe. They also produce lipotropins and endorphins. The secretory granules in the cytoplasm are 360 nm in size. Under conditions of glucocorticoid excess, corticotrophs degenerate losing their granules.
Gonadotrophs are located in the lateral parts of the anterior lobe. They are positive to periodic acid – Schiff stain. Cells secreting the hormone Follicle Stimulating Hormone (FSH) are known as Folliculotrophs. These cells are larger and round with spherical secretory granules measuring 150 – 300 nm in size.
Leutenizing hormone (LH) and Interstitial Cell Stimulating Hormone are produced by Luterotrophs and Interstitiotrophs. These cells are comparatively smaller and have scanty cytoplasm. They may be round or polygonal in shape and are located around the sinusoidal capillaries. The secretory granules are spherical measuring 100 – 300 nm in diameter.
Blood Supply to Adenohypophysis:
The anterior lobe of the pituitary gland derives its blood supply from the superior hypohyseal artery originating from the internal carotid artery. The superior hypophyseal artery gives rise to two types of branches. The first set supply directly to the gland by forming sinusoids while the second set reaches the capillary plexus of the median eminence and the stem of the infundibulum.
This capillary plexus is drained by a long portal vessel that finally divides into sinuses of the anterior lobe. The second set of capillary plexus constitutes the hypothalamo-hypophyseal system that controls the secretory activity of the anterior lobe through the releasing hormones secreted by the hypothalamic nuclear areas. Inferior hypophyseal arteries give rise to a number of end arteries. Venous blood from the anterior lobe is collected by the hypophyseal veins that join the cavernous sinus.
Essay # 8. Thymus:
Thymus is located in the thoracic region in the anterior and posterior mediastina behind the sternum. It acts both as a lymphoid gland and endocrine gland. It lies between the posterior part of the thyroid gland and the anterior end of the pericardium. Thymus is made up of two elongated flask shaped lobes fused together.
Each lobe is subdivided into a number of lobules. The right lobe is bigger than the left. Each lobule in turn consists of a number of follicles measuring about 1 mm in diameter. Thymus gland is largest and well developed during embryonic life and childhood but degenerate gradually and undergoes involution. At the age of 12 -15 years it weighs 38gm but only 3 – 6 gm in old age.
Thymus gland is endodermal in origin. It develops from the third and fourth branchial pouches as primordial lymphoid organs on right and left sides. The two outgrowths fuse together to form a single mass of tissue. This median mass of tissue is invaded by the mesodermal tissue, which surrounds it to develop into the lymphoid gland. The original endodermal part gives rise-to Hassall’s corpuscles.
Sections of the thymus gland reveal two lobes, each in turn divided into a number of lobules. A connective tissue capsule encloses the thymus gland. Each lobule can be differentiated into outer cortex and inner medulla. Cortex is made up of dense, darkly staining cells while the medulla is loose and stains lightly. The central mass of medullary tissue gives rise to projections to the periphery and these are surrounded by cortical tissue (Fig. 18).
The capsule forming the outermost tissue is made of white connective tissue fibers containing a number of macrophages, plasma cells, mast cells, granular leucocytes and adipose cells. The connective tissue envelope penetrates deep into the underlying thymus tissue dividing into lobes and lobules. The trabeculae and septa are formed by the connective tissue capsule extensions. Numerous blood capillaries, lymph vessels and nerves penetrate the capsule and septa.
The cortex forming the outer part of the thymus gland consists of closely packed masses of cells resembling the lymphocytes. These cells can be distinguished into three types depending on their size. Large lymphocytes measure more than 11 µ m in diameter while medium sized lymphocytes vary in diameter from 7 to 11 µ m.
Small lymphocytes or thymocytes have diameter less than µ m. Large and medium sized lymphocytes divide 3-4 times a day and increase the cell numbers. Large lymphocytes can be transformed into medium sized lymphocytes and later into small lymphocytes. In between these lymphocytes are scattered a number of elongated reticular cells arising from the epithelial cells.
The medulla forms the central mass of the thymus gland. It is broad and branched forming the lobes and lobules of the gland. Medulla is filled with reticular cells, few lymphocytes, and variable number of mast cells, plasma cells, eosinophils and melanocytes lying near blood vessels.
The characteristic feature of the thymus medulla is the presence of Hassal’s corpuscles, which are aggregations of the flattened reticular cells. They represent degenerating and hypertrophied reticular cells. Cells in the periphery are in the process of degeneration while the central cells are the degenerated cells.
The pineal body is a flattened body, pyramidal in shape and greyish in color. It measures about 6-8 mm in length and 3-5 mm in width. A small, short and hollow stalk connects it to the roof of the third ventricle. The gland is covered by pia matter and connective tissue separates the gland into a number of septa. A number of blood capillaries travel through the septa into the tissue (Fig. 19).
Pineal gland is highly vascularized. The wall of the blood vessels is rich in alkaline phosphatase and monoamine oxidase. The pineal body is innervated by the autonomic nerve fibers originating from the superior cervical sympathetic ganglia. These fibers travel through two large nerve tracts.
Pineal body is made up of two types of cells:
(i) Chief cells, and
(ii) Interstitial cells.
Both the cells are derived from nerve cells but after their full differentiation, they have no resemblance with the nerve cells.
(i) Chief Cells:
Chief cells contain irregular shaped nuclei and moderate amount of basophilic cytoplasm. These cells possess long processes with club-like endings terminating near the endothelia of the capillaries (Fig. 20).
The cytoplasm contains numerous microtubules extending to the thread like processes. Gogi complex and endoplasmic reticulum are well developed. In addition to these structures, the cytoplasm is also loaded with lipid globules, lipochrome, lysosomes and a number of enzymes.
(ii) Interstitial Cells:
Interstitial cells or supporting cells are distributed between the groups of pinealocytes and spaces surrounding the blood capillaries. The nucleus is dense and cytoplasm contains few mitochondria, dense endoplasmic reticulum and few ribosomes (Fig. 21). Glycogen granules may also be present.
Two testes are enclosed in a sac like structure, the scrotum. Each testis is a flat, oval structure covered by a tough, compact, fibrous capsule, the tunica albuginea (Fig. 22). The gland is divided into a number of conical lobules by trabeculae, which descend from the tunica albuginea.
The lobules contain convoluted seminiferous tubules, which measure about 500 mm in length. A number of seminiferous tubules unite to form a straight tubule, which in turn unite repeatedly forming a rete testis.
These tubules join together to form vasa efferentia. The vasa efferentia combine to form the duct of epididymis. The epididymis is a highly convoluted tubule measuring about 5-6 meters in length. It remains highly coiled together with connective tissue at the back of the testis.
It continues as the vas deferens. The vas deferens joins the duct of the seminal vesicles, which are sac-like structures. The seminal vesicles open into the ejaculatory duct that finally opens into the urethra.
The outermost covering of the testis is the tunica albuginea formed by collagenous connective tissue and elastic tissue. The outer surface of this layer is covered by a single layer of squamous epithelium. Inner surface of the tunica albuginea forms the tunica vasculosa containing numerous blood vessels.
The mediastinum is a thickening of the tunica through which ducts, blood vessels and nerves enter the testis. Other components of the testis are the seminiferous tubules and Interstitial cells (Fig. 23).
There are one to three seminiferous tubules in each testis thrown in the form of arches. Each tubule is lined by four to eight layers of cells, each representing a particular stage in the development of spermatozoa. These layers from outside inwards are spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and spermatozoa.
The spermatogonia divide to give rise to primary spermatocytes, which in turn divide to form the secondary spermatocytes. The secondary spermatocytes give rise to spermatids, which undergo transformation into the spermatozoa. The spermatids and spermatozoa are the products of meiotic division and therefore contain only haploid number of chromosomes.
Sertoli cells are elongated cells lying in between the spermatogonia. These give support to the spermatogonia and the spermatozoa bury their heads in them. The Sertoli cells are characterized by the presence of elongated mitochondria and lipid droplets. Besides providing nourishment to the spermatozoa, the Sertoli cells also secrete oestrogens. Interstitial cells or Leydig cells are polyhedral in shape (Fig. 24).
They are located inside the stroma and in between the seminiferous tubules. They increase in number during puberty but gradually decrease in number during old age. These are large cells with eccentric nuclei, two or three nucleoli and scanty cytoplasm. Smooth endoplasmic reticulum is well developed.
Large number of mitochondria, Golgi apparatus, lysosomes, pigment granules, lipid granules and protein crystals occur in the cytoplasm. Leydig cells contribute to 10% of the mass of testis and secrete testosterone.
Ovary is present in the developing fetus even before birth. In an embryo of 30 weeks old, about seven million oocytes enclosed in a layer of granulosa cells are present. During further development, the number of follicles decreases to two million before birth, and to about 300, 000 in the adult. During this stage ovarian cortex is formed by the atretic follicles.
During puberty, cyclic secretion of gonadotrophic hormones leads to ovulation and later the formation of the corpus luteum. The functional units of the ovary are the ovarian follicles each consisting of an ovum surrounded by two layers.
The inner layer is non-vascular and is made of granulosa cells, which secrete progesterone. This layer is surrounded by the theca interna. The ovum is released in the middle of the menstrual cycle and remains viable for only about six hours.
The ovaries are oval or bean shaped bodies measuring about 5 × 3 × 2 cm. They are located one on either side, near the mouth of the fallopian tube. The ovary is suspended by a fold of peritoneum called the mesovarium. Blood vessels and nerves richly supply ovaries.
The structure of the ovary shows much variation during different periods of life cycle, such as childhood, puberty, pregnancy and menopause.
A normal ovary consists of six layers:
(i) Germinal epithelium,
(ii) Tunica albuginia,
(iv) Graffian follicles,
(v) Corpus luteum, and
(vi) Interstitial cells (Fig. 25).
i. Germinal Epithelium:
Germinal epithelium is mesodermal in origin, being derived from the coelomic epithelium. This forms the outermost covering of the ovary and is made of a single layer of cuboidal epithelial cells. It is continuous with the peritoneum. The germinal epithelium gives rise to the primitive Graffian follicles.
ii. Tunica Albuginia:
This forms a thin layer beneath the germinal epithelium. It is formed of eosinophilic connective tissue fibers having low cellularity.
This tissue lends support to the ovarian tissue. It is continuous with the tunica albuginia and is made of connective tissue fibers, which form a network. It contains spindle shaped cells with involuntary muscle fibers. A number of blood vessels, lymph vessels and nerve fibers are present in this layer.
iv. Graffian Follicles:
The ovarian tissue is divided into a number of lobules or follicles, the Graffian follicles. These constitute groups of cells in various stages of development. They are located mostly in the periphery of the ovary. Primordial follicles are the immature follicles. Ovum lies in the center of the follicle surrounded by a single layer of follicular cells.
v. Corpus Luteum:
The primordial follicles undergo various stages of maturation. The fully grown, mature Graffian follicle ruptures to release the ovum. This process is known as ovulation. After ovulation, the remnants of the ruptured follicle develop into the corpus luteum. Corpus luteum secretes progesterone.
vi. Interstitial Cells:
Interstitial cells are derived from the stroma cells or the unruptured ovarian follicular cells. They constitute groups of polyhedral cells. The cytoplasm of these cells contains lipid granules representing the oestrogens.
During childhood, the ovary consists of primordial follicles. These follicles do not multiply or mature. Ovulation and corpus luteum formation do not occur. At puberty, the ovary consists of a large number of mature follicles, both in ruptured and unruptured condition.
From the ruptured follicles small corpora lutea develop. The corpus luteum reaches biggest size during pregnancy. The ovaries atrophy during menopause, their size reduces and the follicles disappear. Interstitial cells also undergo degeneration due to which, very little oestrogen is synthesized.