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In this article we will discuss about the structure of different organs of Mammalia.
1. Skin (Vertical Section) Mammalia:
(Fig. 41.1)
The skin consists of two layers; an outer epidermis and a deeper dermis. A subcutaneous layer of loose connective tissue is present beneath the dermis.
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1. Epidermis, a stratified, keratinized epithelium.
It has two layers:
A. Stratum corneum, the superficial keratinized layer of flat cells.
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B. Stratum germinativum, the deep layers of columnar cells.
a. Stratum germinativum bears inward projections, the dermal papillae.
b. Melanin present in the deeper cells of stratum germinativum.
2. Dermis consists of connective tissue.
A. Outer part dense and bears papillae projecting into the epidermis.
B. Deeper part consists of loose connective tissue.
Sweat glands, sebaceous glands and hair follicles present in the dermis.
a. Sweat gland—a coiled tube, the duct runs straight outward and passing between two dermal papillae and epidermis opens on the surface as a sweat pore.
b. Sebaceous gland—small, sac-like, alveolar, simple or branched and opens in the hair follicle.
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c. Hair consists of a shaft projecting above the skin and a root within the skin.
i. The root expands into a hair bulb.
ii. Hair papilla, the vascular portion of the dermis on which the hair bulb is moulded.
iii. Hair follicle encloses the root of the hair.
2. Smooth (Involuntary) Muscle:
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(Fig. 41.2)
1. Arrangement of muscle cells (fibres) may be in groups, form a network or even scattered singly.
2. Muscle cells long and spindle-shaped.
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3. Length of the cell extremely variable.
4. Round or angular in cross-section.
5. Nucleus ovoid and central.
6. Each fibre is bounded by a delicate plasma lemma, the sarcolemma.
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7. Sarcoplasm contains fine longitudinal myofilaments.
8. Absence of cross striations.
3. Skeletal (Voluntary) Muscle:
(Fig. 41.3)
1. Muscle cells united together in bundles, the fasciculi.
2. A dense connective tissue, epimysium, surrounds the muscle.
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3. A muscle cell (fibre) consists of sarcoplasm, a peripherally placed nucleus and en-sheathed by a thin sarcolemma.
4. Myofibrils embedded in the sarcoplasm run parallel to others throughout the length of the fibre.
5. In a cross section of a fibre, the cut ends look like dots.
6. The myofibrils are striated in a regular manner and exhibit alternate light and dark segments.
7. The whole muscle shows cross striations as the position of the bands in the fibrils of a muscle fibre, corresponds with those in adjacent fibres.
4. Cardiac Muscle:
(Fig. 41.4)
1. Consists of many parallel muscle cells (fibres) placed closely.
2. A muscle cell consists of abundant sarcoplasm, one or two round, centrally placed nuclei and en-sheathed by a thin sarcolemma.
3. The fibres branched and anastomosing.
4. Myofibrils embedded in the sarcoplasm run parallel to others throughout the length of the fibre.
5. The fibrils are most closely packed round the periphery of the fibre.
6. In cross section of a fibre, the periphery of the cut ends look like very closely placed dots.
7. Intercalated discs present in the fibre.
8. The myofibrils are striated in a regular manner and exhibit alternate light and dark bands, which are not very definite.
5. Oesophagus Mammalia (Transverse section):
(Fig. 41.5)
1. External fibrous coat:
A narrow layer of loose, connective tissue.
2. Muscular coat (muscularis externa):
Consists of an outer longitudinal and an inner circular layer of smooth fibres containing many bundles of oblique fibres.
3. Submucosa:
Consists of loose connective tissue containing large blood vessels and lymphatic’s.
4. Mucous membrane (mucosa):
It has three layers:
a. Muscularis mucosae. A thin layer of smooth muscles with fibres arranged longitudinally.
b. Lamina propria (stroma). Consists of loose connective tissue, which projects into the indented epithelial layer as long papillae.
c. Non-keratinized stratified epithelium, indented on its deep surface.
6. Cardiac Stomach Mammalia (Transverse and longitudinal section):
(Fig. 41.6A, B)
1. Serous coat (serosa):
A thin layer of connective tissue.
2. Muscular coat:
Consists of ill defined layers, an outer longitudinal, a middle circular and an inner oblique layer.
3. Submucosa:
A layer of loose connective tissue containing fat cells, lymphocytes, mast cells and lymphatic’s.
4. Mucous membrane (mucosa):
It has three layers:
a. Muscularis mucosae:
Consists of smooth muscles arranged as an inner circular and an outer longitudinal layer.
b. Lamina propria (stroma):
Consists of loose connective tissue, which project into the indented mucosae.
i. Fundic glands:
These occupy almost whole of the lamina propria. The gland cells are mainly three types-chief cells, oxyntic cells and mucoid cells.
c. Epithelium:
The lining epithelium is of tall, columnar cells, most of which are mucocytes.
7. Small Intestine (Ileum) Mammalia (Transverse section):
(Fig. 41.7A-C)
1. Serous coat:
A layer of mesothelial cells with underlying sub-serous layer of elastic and collagen tissue.
2. Muscular coat:
Consists of well differentiated external longitudinal and internal circular layers.
3. Submucosa:
A layer of loose connective tissue containing large blood vessels, nerve plexus, adipose tissue and lymphatic plexus.
4. Mucous membrane (mucosa):
It has three layers:
a. Muscularis mucosae:
Consists of an outer longitudinal and an inner circular layer of smooth muscles.
b. Lamina propria (stroma):
A layer of loose connective tissue packed with tubular intestinal glands. A group of pyramid-shaped cells, the paneth cells are present at the base of each intestinal gland (Fig. 41.7B).
Muscularis mucosae and stroma together are thrown up into minute projections, the villi.
c. Epithelium:
Covers the villi and consists of a single layer of columnar epithelium containing many goblet cells.
The muscle layers (stomach and ileum):
The muscle layers of the stomach (Figs. 41 .6A, B) and ileum (Figs. 41.7A, C) look different in transverse and longitudinal sections under the microscope.
Transverse section:
1. Circular muscle layers:
The muscle fibres are cut lengthwise and appear as long and short threads running side by side.
2. Longitudinal muscle layers:
The muscle fibres are cut across and appear as bundles of closely packed, small round cells.
Longitudinal section:
1. Circular muscle layers:
The muscle fibres are cut across and appears as bundles of closely packed, small round cells.
2. Longitudinal muscle layers:
The muscle fibres are cut lengthwise and appear as long and short threads running side by side.
8. Liver (Section) Mammalia:
(Fig. 41.8)
1. Consists of a large number of lobules compactly held together.
2. Lobules are polyhedral, with 5 to 7 sides and separated from one another by Glisson’s capsule made of connective tissue.
3. An intra-lobular vein present in the middle of each lobule.
4. Presence of a portal canal consists of an interlobular vein, branches of hepatic artery, the interlobular bile ducts and lymphatic vessels.
5. Liver cells (hepatocytes) are polyhedral with coarsely granular cytoplasm or ’empty’.
6. Nucleus one or two, large, round and vesicular.
7. Liver cells are arranged in the form of branching and anastomosing plates, termed muralium.
8. The plates separate adjacent lacunae formed by blood containing sinusoids. Kupffer cells or stellate cells present on the walls of the sinusoids.
9. The hepatic parenchyma consists of a continuous mass of cells traversed by a complex system of lacunae.
9. Pancreas Mammalia (Section):
(Fig. 41.9)
1. Consists of a number of secretory alveoli, the exocrine component. Endocrine components or islet of Langerhans, are scattered amongst the exocrine tissues.
2. Exocrine component:
a. Most of the alveoli are tubular and surrounded by a lamina.
b. Cells lining the lumen of the alveolus are somewhat conical with a granular cytoplasm and a spherical nucleus.
c. In a resting alveolus, the lumen is small and the cells are packed with zymogen granules except for a narrow outer zone.
d. In an active alveolus, the lumen is distended and the granules restricted only to the innermost part of the cell.
3. Endocrine component:
a. Solid groups of lightly staining cells, marked off from the alveolar portion by a thin reticular membrane.
b. Blood supply extremely rich.
c. The cells contain granules and are of four types,
i. coarsely granular α cells, less in number.
ii. β cells containing small granules, much more numerous.
iii. δ and F cells contain small granules and are less in number.
10. Artery (Transverse) Section:
(Fig. 41.10)
1. Circular, hollow, the thick wall consists of three distinct layers.
2. Tunica intima or internal layer:
a. Lined by an endothelium of squamous cells.
b. Sub-endothelial elements oriented longitudinal to the vessel.
c. Separated from tunica media by a membrane, internal elastic lamina.
3. Tunica media or middle layer:
a. The thickest layer and consists chiefly of elastic tissue (elastic artery) or smooth muscles (muscular artery).
b. Elements are arranged circular to the vessel.
c. External elastic lamina separates it from the outer coat.
4. Tunica adventitia or outermost layer:
a. Usually thinner and gradually merges with the tissue surrounding the artery.
b. Consists chiefly of elastic fibres.
c. Elements are usually directed longitudinally.
11. Vein (Transverse) Section:
(Fig. 41.11)
1. Circular, hollow, the thin wall consists of three layers, less clearly differentiated.
2. Tunica intima or internal layer:
a. Lined by poorly differentiated squamous endothelial cells.
b. Consists chiefly of elastic fibres, usually directed longitudinally.
c. The layer merges with the tunica media.
3. Tunica media or middle layer:
a. Relatively thin and consists of a little elastic tissue, smooth muscle and considerable collagen fibres.
b. Elements are arranged circumferentially but sometimes longitudinally in the innermost part.
c. It is continuous with the outer layer.
4. Tunica adventitia or outermost layer:
a. Well developed and thickest of all the layers.
b. Consists of smooth muscles, collagen and elastic fibres.
c. Elements are usually directed longitudinally.
12. Lung (Transverse section):
(Fig. 41.12)
1. Presence of a network of empty spaces (alveoli) enclosed by thin walls, portions of the walls of bronchi, bronchioles and blood vessels.
2. The walls between the alveoli are extremely thin.
3. It consists of a stroma containing numerous anastomosing capillaries and a network of fine reticular and elastic fibres.
4. Some macrophages and fibrocytes are present in the wall.
5. Alveolar pores present in the interalveolar septa.
6. Alveoli are lined with a complete epithelium of squamosal cells.
13. Spleen (Transverse section) Mammalia:
(Fig. 41.13)
1. The capsule enclosing the spleen is covered by a single layer of mesothelial cells and consists of a dense connective tissue containing many elastic fibres and a few smooth muscle fibres.
2. Scattered throughout the spleen are lymphoid follicles (Fig. 41.13B), separated by splenic pulp; the white and red pulp.
3. The white pulp is fusiform or cylindrical in shape.
a. It consists of reticular fibres, reticular cells and lymphocytes.
b. Large type lymphocytes in dividing stages are often present in a germinal centre.
4. The red pulp is a reticular meshwork, honeycombed by sinuses.
a. Non-granular leucocytes are numerous.
b. Splenic cells (macrophages) present.
c. Granular leucocytes and erythrocytes found in the meshes of stroma.
14. Kidney (Sagittal section) Mammalia:
(Fig. 41.14)
1. Enclosed in a capsule of connective tissue.
2. Kidney consists of an outer granular cortex and a radially striated inner medulla.
3. Medulla consists of a number of conical masses, the renal pyramids.
4. The bases of the pyramids are adjacent to the cortex and apices, the renal papillae, point towards the renal pelvis.
5. The pelvis is extended and forms the beginning of the ureter;.
6. Closely packed uriniferous tubules are present both in cortex and medulla.
7. The amount of interstitial connective tissue is small-in the cortex and more in the medulla.
8. The functional unit of kidney is the nephron.
9. A nephron consists of a renal capsule with glomerulus, proximal convoluted tubule, loop of Henle divided into descending and ascending limbs.
10. Leaving medulla, the tubule enters cortex as distal convoluted tubule, near renal capsule.
11. The tubule is connected with a collecting tubule through a short junctional tubule.
12. The collecting tubule receives other junctional tubules, and several of these form a papillary duct.
13. The papillary duct opens at the apex of the pelvis.
15. Thyroid Gland (Transverse Section) Mammalia:
(Fig. 41.15)
1. The thyroid gland is enclosed in a two layered capsule, the outer and inner. Septa of fibro-elastic tissue extend inwards from the inner capsule and divide the gland into lobules.
2. The glandular tissue (parenchyma) consists of groups of irregularly spherical follicles.
3. The follicles are supported by connective tissue carrying blood vessels, lymphatic’s and nerves.
4. The follicles are lined by cuboid cells with spherical nucleus and supported by a thin basement membrane.
5. Microvilli present on the apex of the cells.
6. Cytoplasm contains granules and droplets.
7. Golgi apparatus well developed and the apical cytoplasm contains numerous membrane bound dense granules.
16. Adrenal Gland (Transverse Section) Mammalia:
(Fig. 41.16)
1. The gland is enclosed in a thick capsule of fibrous connective tissue containing a few muscle fibres.
2. Gland consists of a firm, outer cortex and a soft, central medulla.
3. Cortex consists of three layers, somewhat vaguely differentiated and merge into one another.
4. The outer zona glomerulosa is a narrow zone.
a. The cells are small, columnar and closely packed in rounded groups.
b. The free surface of each cell adjoins a capillary.
c. The nuclei round, stain deeply and the cytoplasm is basophil.
5. The middle zona faciculata is thickest.
a. Cells are large, polyhedral with vesicular nucleus and rich in lipid.
6. The inner zona reticularis is thicker than zona glomerulosa.
a. The cords of cells are arranged as an anastomosing network round the sinusoids.
b. Cells are large, polyhedral, with vesicular nucleus and less lipid.
7. The medulla consists of polyhedral cells arranged in groups and in contact with venous sinuses.
a. Cells are supported by a meshwork of fine fibres and contain small granules.
17. Testis (Transverse Section) Mammalia:
(Fig. 41.17)
1. Presence of a fibrous capsule, tunica albuginea.
2. A number of seminiferous tubules and interstitial tissue between them constitute the testis.
3. The wall of the tubules consists of an outer connective tissue coat, a basal lamina and a lining stratified epithelium, the germinal epithelium.
4. Two kinds of cells, Sertoli and germinal are present in the epithelium.
a. Sertoli cells are slender, pyramidal, with ovoid and vesicular nucleus and placed at intervals on the basal lamina.
b. Germinal cells are in various stages of development, 5 to 6 layers thick and lie between the Sertoli cells.
5. Proceeding to the lumen of the tubule from the basal lamina, the cells in different layers are—spermatogonia, primary spermatocytes, secondary spermatocytes and spermatids several layers thick.
6. In some places, spermatids developing to spermatozoa are present.
7. Mature spermatozoa are free in the lumen of the tubule.
18. Ovary (Transverse Section) Mammalia:
(Fig. 41.18)
1. The surface is covered with a layer of cubical cells.
2. The capsule, tunica albuginea is present inside the epithelium.
3. Ovary constituted by stroma, Graafian follicles and groups of interstitial cells, scattered throughout the stroma.
4. Stroma consists of elongated, fusiform, connective tissue cells and elastic network.
a. Interstitial cells are large, polyhedral and contain lipid droplets.
5. Graafian follicle consists of:
a. An ovum, eccentric in position.
b. Discus proligerus, several layers of cells, continuous with the outer pole of the follicle.
c. Membrana granulosa, peripheral layers of cells.
d. Theca interna, inner vascular layer.
e. Theca externa, outer fibrous layer of Graafian follicle.
19. Long Bone (Transverse) Section:
(Fig. 41.19)
1. The bone is en-sheathed by a vascular connective tissue, the periosteum.
2. A thick, compact wall surrounds a large, central cavity containing bone marrow.
3. The wall consists of circumferentially arranged bone lamellae and Haversian system or osteones, arranged in the long axis of the bone.
4. In a Haversian system 20 to 40 lamellae lie concentrically around a central space, the Haversian canal.
5. Osteones are united to each other by oblique communications.
6. The spaces between the individual systems are filled with interstitial lamellae arranged irregularly.
7. At the periphery of each osteone, a narrow fibre free band, the cement line is present.
8. Circumferential lamellae enclosing all the osteones are present beneath the periosteum and also lining the marrow cavity.
9. Bone cells lie in the lacunae between or within the lamellae and in the Haversian canals.
