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
Intrinsic muscles
all innervated by recurrent laryngeal nerve except cricothyroid: external laryngeal nerve
adductors of vocal folds: bring folds together at midline
Transverse and oblique arytenoids: pull arytenoids together
Lateral cricoartenoids: spin and slide arytenoids up
only one abductor of vocal folds
Posterior cricoarytenoids—down and up
adjustors of shape and tension of vocal folds
Cricothyroid muscle
o superficial to lateral cricoarytenoid
o tenses vocal folds by tilting thyroid cartilage forward and sliding forward
Thyroartenoid and vocalis muscles
o vocalis: sometimes treated as medial most fibers of thyroartenoid muscle
o different fiber directions
lateral: adduct
medial: change shape of folds
control voice by bring bringing together different parts of folds
o as move from epithelium to vocalis muscle, fold becomes stiffer
o near connections, vocal folds are stiffer
o vocal fold: complex, multilayered vibrator
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 Sublingual Glands
- These are the smallest of the three paired salivary glands and the most deeply situated.
- They are almond-shaped and lie in the floor of the mouth between the mandible and the genioglossus muscle.
- The paired glands unite to form a horseshoe-shaped glandular mass around the lingual frenulum.
- Numerous small ducts (10 to 12) open into the floor of the mouth.
- Sometimes one of the ducts opens into the submandibular duct.
- The nerves the accompany the submandibular and sublingual glands are derived from the lingual and chorda tympani nerves and from the sympathetic nerves.
- The parasympathetic secretomotor fibres are from the submandibular ganglion.
Mylohyoid Muscle
- Origin: Mylohyoid line of the mandible.
- Insertion: Median raphe and body of the hyoid bone.
- Nerve Supply: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
- Arterial Supply: Sublingual branch of the lingual artery and submental branch of the facial artery.
- Action: Elevates the hyoid bone, base of the tongue, and floor of the mouth; depresses the mandible.
The Auditory Ossicles
The Malleus
- Its superior part, the head, lies in the epitympanic recess.
- The head articulates with the incus.
- The neck, lies against the flaccid part of the tympanic membrane.
- The chorda tympani nerve crosses the medial surface of the neck of the malleus.
- The handle of the malleus (L. hammer) is embedded in the tympanic membrane and moves with it.
- The tendon of the tensor tympani muscle inserts into the handle.
The Incus
- Its large body lies in the epitympanic recess where it articulates with the head of the malleus.
- The long process of the incus (L. an anvil) articulates with the stapes.
- The short process is connected by a ligament to the posterior wall of the tympanic cavity.
The Stapes
- The base (footplate) of the stapes (L. a stirrup), the smallest ossicle, fits into the fenestra vestibuli or oval window on the medial wall of the tympanic cavity.
Functions of the Auditory Ossicles
- The auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane.
The Palate
- The palate forms the arched roof of the mouth and the floor of the nasal cavities.
- The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.