NEET MDS Lessons
Anatomy
ENDOCRINE
Endocrine glands have no ducts
They secrete into the blood from where the secretion (hormone) reaches a target cell
The following is a list of endocrine glands:
- Hypophysis
- Thyroid
- Parathyroid
- Adrenals
- Islets of Langerhans
- Pineal
- Gonads
Hypophysis: Develops from oral ectoderm and nerve tissue, The oral part forms an upgrowth with an invagination (Rathke's pouch) The nervous part grows from the floor of the diencephalon - staying intact .The oral part separates from the mouth
Ectoderm – adenohypophysis - pars tuberalis
- pars distalis
- pars intermedia .
Diencephalon – neurohypophysis - pars nervosa .
- infundibulum
- median eminence
Rathke's pouch remains as Rathke's cysts
Pars Distalis: Forms 75% of the gland, The cells form cords, with fenestrated capillaries in-between
2 Cell types:
Chromophobes : 50% of the cells, do not stain lie in groups, they are resting chromophils
granules have been used
Chromophils: Stain
They can be subdivided according to their reaction with different stains
Acidophils (40%) :Cells have acidophilic granules in their cytoplasm. The cells are secretory.
They have a well developed EPR and Golgi apparatus.They have secretory granules.
subdivided into:
- Somatotropin cells: secrete somatotropin (growth hormone)
- Mammotropic cells: secrete prolactin
Basophils (10%) : These cells have basophilic granules in their cytoplasm and can be subdivided into:
Thyrotropin cells: secrete thyroid - stimulating hormone (TSH)
Corticotrophin cells: secrete adrenocorticotropic (ACTH)
Gonadotropic cells: secrete two hormones: Follicle stimulating hormone (FSH):
Stimulate follicle development and spermatogenesis
Luteinizing hormone (LH): Stimulate the formation of the corpus luteum and Leydig cells
Pars Tuberalis: Cells lie around the infundibulum . It is continuous with the pars distalis
Cells are cuboidal with no granules. Their function is unknown
Pars Intermedia: Poorly developed in the human. Follicles lined by cuboidal cells and filled with colloid are found Known as Rathke's cysts .There are also a few big basophilic cells
Their function is unknown
Pars Nervosa: Contains: - myelinated axons pituicytes, blood vessels
Axons:
The cell bodies of the axons lie in the supra-optic and paraventricular nuclei of the hypothalamus .From the cell bodies the axons go through the infundibulum forming the hypothalamohypophyseal tract to end in the pars nervosa
The axons have dilated blind endings filled with hormones (Herring bodies) coming from the cell bodies.
Two hormones are secreted:
Oxytoxin: - Cause contraction of the uterus
- Cause contraction of the myoepithelial cells of the milkgland
- The hormone is secreted by the paraventricular nuclei
Vasopressin :- Cause reabsorption of H2O in the kidney (also known as antidiuretic hormone ADH) The hormone is secreted by the supraoptic nuclei. A hypophyseal portal system exists
A primary capillary plexus of fenestrated capillaries form around the median eminence. Inhibitory hormones are secreted into these capillaries
The capillaries rejoin to form the portal veins that traverse the pituitary stalk
The portal veins break up into a secondary capillary plexus which lies close to the cells of the adenohypophysis
This portal system regulates the functions of the anterior pituitary function.
Pineal
Surrounded by pia which sends septae into the gland Cells are mainly pinealocytes and astroglial cells
Pinealocytes:Irregular shaped cells. with processes ending in flattened dilatations
Have a well developed smooth surfaced endoplasmic reticulum, Also a rough EPR not well developed, Lots of microtubules
Astroglial Cells: Elongated nucleus, Cells have long processes, They perform a supporting function
Hormones:
Melatonin - secreted during the night .suppress the onset of puberty
Serotonin - secreted during the day
In humans the pineal form concretions of calcified material called brain sand
Brain sand vary in size and number with age and is visible on X-rays
Mast cells are also found in the pineal and cause the high histamine contend of the gland
THYROID
Has a CT capsule that sends septae into the gland to divide it up into incomplete lobes and lobules. In the lobules are follicles, Follicles vary in size, They are surrounded by surrounded by reticular CT and capillaries
Cells of the Follicle:
Follicular Cells : Single layer of cuboidal cells, lie around the colloid, Follicular cells can become columnar when very active, Nucleus central, EPR has wide cisternae ,Golgi present
- microvilli on the free surface
Parafollicular Cells: Also known as C-cells, Form part of the epithelium or form clusters between the follicles
- They never come into contact with the colloid
- Larger and stain less intensely than the follicular cells, Form 2% of the cells, Secrete calcitonin
Hormones: Thyroxine and thyriodothyronine - stimulate the metabolic rate, Calcitonin - lower the blood calcium
Parathyroid:
Has a CT capsule which send septae into the gland to divide it up into incomplete lobules, The CT contains fat which increase with age - may eventually be 50% of the gland, Glandular cells are arranged in cords
Glandular Cells:
Chief Cells: Small cells so their nuclei lie close together, Rich in glycogen, Biggest omponent
Secrete parathyroid hormone - essential for life
Oxyphil Cells:Develop at puberty, Bigger than the chief cells, Nuclei are smaller, Acidophilic
Hormones:
Parathyroid hormone - regulate calcium and phosphate ions in the blood
ADRENAL
- Thick CT capsule that do not send septae into the gland
Cortex:
Has 3 layers
Zona glomerulosa: 15% of the cortex, Directly under the capsule, Cells are columnar or pyramidal, Arranged in small groups or clusters, Wide fenestrated capillaries surround the clusters, Cells have an extensive smooth EPR
Zona Fasciculata: 78% of the cortex, Cells are arranged in cords ,1 to 2 cells wide perpendicular to the surface, Sinusoids lie between the cords, Cells are polyhedral with a central nucleus which is bigger than that of the zona glomerulosa, Lots of lipid in the cytoplasm cause the cells to stain lightly, Cells have a well developed smooth and rough EPR
The mitochondria in the cells are round with tubular or vesicular cristae
Zona Reticularis: 7% of the cortex, Cells form a network of cords with wide capillaries in-between The mitochondria in the cells are more ofte6n elongated than that in the zona fasciculate Degenerating cells with pyknotic nuclei are found. Cells contain numerous large lipofuscin granules. Cells of the cortex do not store their secretions but form and secrete on demand.
Hormones:
3 Groups:
Glucocorticoids (e.g. cortisol) - have an affection on carbohydrate metabolism
Mineralocorticoid (e.g. aldosterone) - control water and electrolyte balans
Androgens (e.g. dehyroepiandrosterone) - not very important
Medulla:
- Cells are big and oval and lie in groups and cords around bloodvessels
- Oxidising agents stain the granules in these cells brown - cells are therefore called chromaffin cells
- Granules contain adrenaline or non-adrernalin
- A few parasympathetic ganglion cells are also present
Hormones:
- Adrenaline - increase oxygen uptake
- increase blood pressure
- Noradrenaline - maintain blood pressure
Blood Supply:
- Blood vessel enter from the capsule to form the wide capillaries
- They flow into venules that form a central vein
- Between the endothelium of the capillaries and the glandular cells there is a subendothelial
- space.
- The glandular cells have microvilli protruding into this space.
ISLES OF LANGERHANS
Endocrine part of pancreas. The isles are round clusters in the exocrine tissue
- 100 - 200 µm
Islands consists of slightly stained polygonal or rounded cells, The cells are separated by fenestrated capillaries
- Autonomic nerve fibres innervate the blood vessels and the island cells
- 4 different cell types have been described
A cells : 20% of the cells, Bigger than B cells, Lie at the periphery, Have secretory granules ,Contain glucagon
B cells : 80%, Lie in the centre of the island, The cells are small with granules which are crystals, Granules are formed by insulin
D cells : Not numerous, Membrane bound granules, Store somatostatin (inhibit somatotropin)
F cells : Have membrane bound granules, Store pancreatic polypeptide, The hormone inhibits pancreatic exocrine secretion
LYMPHOID SYSTEM
Consists of cells, tissues and organs
Protects the body against damage by foreign substances
Immuno competent cells in the lymphoid system distinguish between the bodies own molecules and foreign molecules.
The response is immunity.
lymphoid tissues have a: - reticular framework (collagen III) consisting of: reticular cells , (indistinguishable from fibroblasts) , lymphocytes, macrophages, antigen presenting cells, plasma cells
Each organ has special features:
Capsulated – spleen, lymph nodes, thymus
Unencapsulated – tonsils, Peyers patches. lymphoid nodules in: - alimentary canal
- Nodules in: respiratory tract, urinary tract, reproductive tracts
2 Types of immunity:
- Cellular: Macrophages - destroy foreign cells
- Humeral – immunoglobulins and antibodies (glycoproteins) interact with foreign substances
- cellular and humeral immune system require accessory cells like: macrophages, antigen presenting cells
Thymus
Lymphocytes develop from mesenchym. The lymphocytes then invade an epithelial premordium .The epithelial cells are pushed apart by lymphocytes. Epithelial cells remain connected through desmosomes to form the epithelial reticular cells. Septae from the capsule divide the thymus up into incomplete lobules (0,5-2 mm ). Each lobule has a cortex which is packed with lymphocytes. In the middle of the lobule is the lighter staining medulla. The cortex and medulla are continuous. Hassall's corpuscles, consisting of flat epithelial cells, lie in the medulla .The corpuscles increase in size and number through life
Thymus cells:
- Cortex and medulla have the same cells – only their proportions differ
- The predominant cell is the T lymphocytes and precursors
- There are also epithelial reticular cells with large oval nuclei. The cells are joined by desmosomes.
- A few mesenchymal reticular cells are also present.
- There are many macrophages.
Cortex:
- Only capillaries (no other vessels)
- small lymphocytes predominate
- here they do not form nodules
- epithelial cells surround groups of lymphocytes and blood vessels
- around the capillary is a space
- forms blood thymus barrier
- Layers of the blood thymus barrier:
- capillary wall endothelium
basal lamina
little CT with macrophages
- epithelial reticular cells - basal lamina
- cytoplasm of epithelial reticular cells
Medulla:
- Stains light because of many epithelial reticular cells
- 5% of thymic lymphocytes found in medulla
- mature lymphocytes - smaller than that of cortex
- leave through venules to populate organs such as the spleen and lymph nodes
- In the medulla the covering of capillaries by epithelial reticular cells is incomplete - no barrier
- Hassall's corpuscles
- 30 - 150µm .
- consists of layers of epithelial reticular cells
- the central part of the corpuscle may only be cell remnants
- unknown function
Lymph nodes
- Encapsulated
- found throughout the body
- form filters in the lymph tracts
- lymph penetrate through afferent lymph vessels on the convex surface
- exit through efferent lymph vessels of the hilum
- capsule send trabeculae into the node to divide it up into incomplete compartments
- reticular tissue provide the super structure
- under the capsule is a cortex – the cortex is absent at the hilum
- At the centre of the node and at the hilum is a medulla
- The cortex has a subcapsular sinus and peritrabecular sinuses
The sinuses:-
- Incompletely lined by reticular cells
- Have numerous macrophages
- fibres cross the sinuses
- they slow the flow of lymph down -
- so that the macrophages can get a chance to perform their function.
Primary and secondary lymphoid nodules
- Some lymphocytes in the cortex form spherical aggregations 0,2-1 mm Ø called primary nodules (or follicles)
- They contain mainly B lymphocytes but some T- lymphocytes are also present
- A germinal centre may develop in the middle of the nodule when an antigen is present. The nodule then becomes a secondary nodule, which is:
- light staining in the centre because:
- many B lymphocytes increase in size to become plasmablasts
- plasmablasts undergo mitosis to become plasmacytes
- plasmacytes migrate to the follicular periphery and then to the medullary cords where they mature
into plasma cells that secrete antibodies into the efferent lymph.
- lymphocytes that don’t differentiate into plasma cells remain small lymphocytes and are called memory
cells – which migrate to different parts of the body
- memory cells are capable of mounting a rapid humoral response on subsequent contact with the same antigen.
- In the nodules there are also follicular dendritic cells which are:
- non phagocytic
- with cytoplasmic extensions
- trap antigens on their surface
- present it to B and T lymphocytes which then respond
Paracortical Zone
- Between adjacent nodules and between the nodules and the medulla are loosely arranged lymphocytes which form the paracortical area or deep cortical area.
- The main cell type in this area is the T lymphocyte.
- They enter the lymph node with the blood and migrate into the paracortical zone.
- T lymphocytes are stimulated when presented with an antigen by the follicular dendritic cells.
- They transform into large lymphobasts which undergo mitosis to produce activated T lymphocytes.
- These activated T lymphocytes must go to the area of antigen stimulation to perform its function.
- When this happens the paracortex expand greatly.
- Later they join the efferent lymph to leave the lymph node.
- These lymphocytes disappear when the thymus is removed - especially if done at birth
The medulla
- Consists of medulla with branching cords separated by medullary sinusses.
- Througout the medulla are trabeculae.
- The cords contain numerous B lymphocytes and plasma cells.
- A few macrophages and T lymphocytes may also be present.
- Receive and circulate lymph from the cortical sinuses.
- Medullary sinuses communicate with efferent lymph vessels.
Spleen
- Largest lymphatic organ
- Many phagocytic cells
- Filters blood
- Form activated lymphocytes which go into the blood
- Form antibodies
General structures:
- Dense CT capsule with a few smooth muscle fibres encapsulate the spleen
- The capsule is thickened at the hilum.
- Trabeculae from the hilum carry blood vessels and nerves in and out of the spleen.
- The capsule divide the spleen into incomplete compartments.
- The spleen has no lymph vessels because it is a blood filter and not a lymph filter like the lymph nodes.
Splenic pulp
- The lymph nodules are called the white pulp
- The white pulp lies in dark red tissue called red pulp
- Red pulp is composed of splenic cords (Billroth cords) which lie between sinusoids
- Reticular tissue forms the superstructure for the spleen and contains:
- reticular cells
- macrophages
Blood circulation
- The splenic artery divide as it enters the hilum
- The arteries in the trabeculae are called trabecular arteries
- The trabecular arteries give of braches into the white pulp (central arteries).
- The artery may not lie in center but is still called a central artery.
- The central arteries give off branches to the white pulp which go through the white pulp to end in the marginal sinuses on the perimeter of the white pulp.
- The central artery continues into the red pulp (called the pulp artery) where it branches into straight arteries called penicilli.
- The penicilli continue as arterial capillaries some of which are sheated by macrophages.
- The blood from the arterial capillaries flow into the red pulp sinuses that lie between the red pulp cords.
- The way the blood gets from the capillaries into the sinuses is uncertain. It can either:
- Flow directly into the sinuses - closed theory
- Or flow through the spaces between the red pulp cord cells and then enter the sinusoid - open theory.
- Presently the open theory is popular.
- From the sinusoids the blood flow into the: - Red pulp veins
- which join the trabecular veins
- to form form the splenic vein
(Trabecular veins form channels without a wall lined by endothelium in the trabeculae.)
White pulp:
- Forms a lymph tissue sheath around the central artery
- The lymphocytes around the central artery is called the periarterial lymphatic sheath (PALS).
- Which contains mainly T lymphocytes
- So the PALS is chracterized by a central artery.
- True nodules may also be present as an extension of the PALS.
- They displace the central artery so that it lies eccentric.
- Nodules normally have a germinal center and consists mainly of B lymphocytes
- Between the red and white pulp there is a marginal zone consisting of:
- Many sinuses and of loose lymphoid tissue.
- There are few lymphocytes
- many macrophages
- lots of blood antigens which
- play a major role in immunologic activity.
Red Pulp:
- In the fresh state this tissue has a red colour because of the many erythrocytes.
- Red pulp consists of splenic sinusses separated by splenic cords (cords of Billroth).
- Between reticular cells are macrophages, lymphocytes, granulocytes and plasma cells.
- Many of the macrophages are in the process of phagocytosing damaged erythrocytes.
- The splenic sinusoids are special sinusoidal vessels in the following ways:
- It has a dilated large irregular lumen
- Spaces between unusually shaped endothelial cells permit exchange between sinusoids and adjacent tissues. (The endothelial cells are very long arranged parallel to the direction of the vessel)
- The basal lamina of the sinusoid is not continuous but form rings.
Tonsils
- Tonsils are incompletely encapsulated lymphoid tissues
- There are - Palatine tonsils
- pharyngeal tonsils
- lingual tonsils
Palatine Tonsil
- Contains dense lymphoid tissue.
- Covered by stratified squamous non-keratinized epithelium
- with an underlying CT capsule
- Crypts that enter the tissue end blind.
Lingual Tonsil
- Lie on the posterior 1/3 of the tongue.
- Crypts link up with underlying glands that flush them.
- Epithelial covering is the same as that of the palatine tonsil.
The Oral Cavity
- The oral cavity (mouth) consists of two parts: the vestibule and the mouth proper.
- The vestibule is the slit-like spaced between the cheeks and the lips and the teeth and gingivae.
- It is the entrance of the digestive tract and is also used for breathing.
- The vestibule communicates with the exterior through the orifice of the mouth.
- The oral cavity is bounded:
- Externally: by the cheeks and lips.
- Roof of oral cavity: formed by the palate.
- Posteriorly: the oral cavity communicates with the oropharynx.
The Auditory Tube
- This is a funnel-shaped tube connecting the nasopharynx to the tympanic cavity.
- Its wide end is towards the nasopharynx, where it opens posterior to the inferior meatus of the nasal cavity.
- The auditory tube is 3.5 to 4 cm long; its posterior 1/3 is bony and the other 2/3 is cartilaginous.
- It bony part lies in a groove on the inferior aspect of the base of the skull, between the petrous part of the temporal bone and the greater wing of the sphenoid bone.
- The function of the auditory tube is to equalise pressure of the middle ear with atmospheric pressure.
The Laryngopharynx
- The laryngeal part of the pharynx lies posterior to the larynx.
- It extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage, where it narrows to become continuous with the oesophagus.
- Posteriorly, the laryngopharynx is related to the bodies of C4 to C6 vertebrae.
- Its posterior and lateral walls are formed by the middle and inferior constrictor muscles, with the palatopharyngeus and stylopharyngeus internally.
- The laryngopharynx communicates with the larynx through the aditus or inlet of the larynx.
- The piriform recess is a small, pear-shaped depression of the laryngopharyngeal cavity on each side of the inlet of the larynx.
- The palate has a rich blood supply from branches of the maxillary artery.
- Long bones (e.g.. femur and humerus)
- Short bones (e.g.. wrist and ankle bones)
- Flat bones (e.g.. ribs)
- Irregular bones (e.g.. vertebrae)