PHYSIOLOGY 1B Flashcards
(88 cards)
1
Q
The thyroid gland is located in front of and wrapped around the sides of what structure?
A
trachea
2
Q
The thyroid hormones are transported around the body in the blood attached to …?
A
thyroxine-binding globulin
3
Q
The thyroid gland function is regulated by a hierarchical control axis
A
hypothalamus and anterior pituatry gland
4
Q
What is the name of the hormone secreted by the hypothalamus, which contributes to the hypothalamic-pituitary-thyroid axis?
A
thyrotropin-releasing hormone
5
Q
The thyrotropin releasing hormone (TRH) then travels via the hypothalamic-pituitary portal vein to the anterior pituitary where it stimulates the release of what hormone?
A
thyroid stimulating hormone
6
Q
The thyroid gland secretes thyroid hormones in response to stimulation from TSH. The secretion of thyroid hormones is then regulated by negative feedback. Which of the following are correct? (there may be more than one!)
A
Negative feedback onto the hypothalamus
Negative feedback onto the anterior pituitary
7
Q
If for some reason there was an increase in the secretion of thyroid hormone, what would happen to the negative feedback by T3 and T4 on the hypothalamus and anterior pituitary?
A
there would be more
8
Q
As a consequence, what would happen to the secretion of thyroid stimulating hormone (TSH)?
A
there would be less
9
Q
negative feedback
A
The secretion of thyroid hormones is regulated by negative feedback loops onto the hypothalamus and anterior pituitary.
If thyroid hormone levels are too high, then there is more negative feedback. This will suppress the synthesis and secretion of thyrotropin releasing hormone (TRH) from the hypothalamus, suppress synthesis and secretion thyroid stimulating hormone (TSH)from the anterior pituitary, and consequently thyroid hormone secretion will also be suppressed.
Similarly, if there was a decrease in thyroid hormone levels, then the negative feedback would be less. This would allow more TRH secretion from the hypothalamus, more TSH from the anterior pituitary and so thyroid hormone secretion would also increase
10
Q
To synthesize thyroid hormone, iodide must be accumulated by the thyroid follicular cells. This is achieved by which protein?
A
sodium iodide symporter
11
Q
What is the approximate concentration of iodide inside the follicular cells of a normal thyroid gland compared to the circulating concentration of iodide?
A
20 times greater
12
Q
Once inside the follicular cell, the higher concentration of iodide (20-times greater) will drive the movement of iodide across the apical membrane into the follicular lumen. This is facilitated by which TWO of the following membrane proteins?
A
Anoctamin
Pendrin
13
Q
Upon entering the follicular lumen, the iodide takes part in a series of reactions that leads to the synthesis of thyroid hormone. Place these reactions in the correct order for the synthesis of thyroid hormone:
A
- oxidation of iodide to iodine
- iodination of tyrosine residue
- conjugation of adjacent tyrosine residue
14
Q
What is the name of the enzyme that is responsible for the oxidation, iodination and conjugation reactions?
A
thyroperoxidase TPO
15
Q
To secrete thyroid hormones, the thyroglobulin is first endocytosed from the follicular lumen. This process is stimulated by:
A
thyroid-stimulating hormone TSH
16
Q
The endocytotic vesicle then fuses with a lysosome. The contents of the lysosome then enter the endocytotic vesicle and as a result, T3 and T4 are:
A
cleaved from the thyroglobulin
17
Q
Thyroid hormone secretion
A
Secretion
The cleavage of T3 and T4 from the thyroglobulin by proteases is accompanied by degradation of thyroglobulin itself. The vesicle then breaks down, allowing T3 and T4 to be exported to the circulation via transport proteins in the basal membrane.
18
Q
Thyroid stimulating hormone (TSH) binds to receptors on the basal membrane of follicular cells and initiates signalling pathways inside the follicular cell. In this way, TSH has three major effects on follicular cells. Which THREE from the list below are actions of TSH?
TSH increases:
A
Iodide uptake
Synthesis of thyroglobulin
Endocytosis of thyroglobulin
19
Q
Major actions of thyroid stimulating hormone (TSH)
A
TSH increases:
iodide uptake
thyroglobulin synthesis
endocytosis of thyroglobulin for secretion of thyroid hormone
20
Q
Which of the following characteristics are visible in the follicular cells from the thiouracil treated glands?
A
pseudopodia on apical surface;The follicular cells show signs of a ‘fuzzy’ apical surface, indicating the presence of pseudopodia.
evidence of endocytosis of colloid; There is evidence of vesicles in the cytoplasm that indicates there is endocytosis.
evidence of protein synthesis;The chromatin in the nucleus is dispersed, which indicates transcription and so protein synthesis is occurring in the cells.
21
Q
. Follicular cells from normal thyroid glands show
less
of these features compared to thiouracil treated glands
A
less
22
Q
Follicular cells from thyroxine treated thyroid glands show
less
of these features compared to thiouracil treated glands.
A
less
23
Q
chemical messenger
A
chemical messenger can be classified according to their function or chemical structure
24
Q
chemical messengers can be classified into 3 categories
A
1 paracrines
2neurotransmitters
3 hormones.
each type of messenger transmits a specific signal by binding to receptors on a target cell.
25
paracrines
are chemicals that communicate with neighbouring cells.
the target cells must be close enough that once the paracrine is secreted into the extracellular fluid, it can reach the target cell by simple diffusion. Paracrines generally include growth factor, clotting factor, and cytokines. Growth factor are proteins that stimulate proliferation and differentiation cells. For example, nerve growth factor is necessary for embryonic development of the nervou system and for maintenanace or regeneration of neurons; vacular endothelial growth factor stimulates proliferation of blood vessels.
clotting factors are proteins that stimulate formation of a blood clot.
cytokines are peptides; usually released from immune cells, their function is to coordinate the body's defense against infections. although most cytokines function as paracrines, others travel in the bloodstream to distant target cells and function more like hormones.
26
histamine
is an exmaple of a paracrine chemical messenger that is important in allergic reactions and inflammation. It is secreted by mast cells scattered throughout the body. it is responsible for the runny nose, red, watery eyes associated with allergic reactions. occurring in response to bacterial infections and various forms of tissue damage, the release of histamine by mast cells is a part of a complex called inflammation. in inflammation, histamines increases blood flow to affected tissues and causes fluid to leak out of the blood vessels and into the tissue.
27
autocrine
are a subclass of paracrine that act on the same cell that secreted them. thus the seceretory cells is also the target cell. often an autocrine regulates its own secretion.
28
neurotransmitters
are chemicals released into interstitial fluid by neurons. these substances are released from a specialised portion of the neuron called the axon terminal, which is very close to the target cell. Because the juncture between the axon terminal and the target cell is called synapse, communications by neurotransmitters is often called synaptic signaling.
the cell that releases the neurotransmitter is called the presynaptic neuron, whereas the target cell is called the postsynaptic cell. upon release from the presynaptic neuron, the neurotransmitter quickly diffuses over the short-distance from the axon terminal and binds to receptors on the postsynaptic cell triggering a response.
communication between a neuron and its target cell is very specific because the neurotransmitter is directed only to cells with which the neuron has an anatomical association; the synapse. an example of neurotransmitter is acetylcholine, which is released by the neurons that triggers contraction of skeletal muscles.
29
hormones
are chemicals released from endocrine glands into the interstitial fluid, where they can diffuse into the blood. the hormone then travels in the blood to its target cells, which can be distant from the site of hormone release. only cells possessing receptors specific for the hormone are able to respond and thus serve as its target cell. an example of hormone is insulin, which is secreted by the pancreas and acts on target cells throughout the body to regulate energy metabolism.
30
neurohormones
are relased by a special class of neurons called neurosecretory cells through a mechanism similar to that of neurotransmitter release. an example of neurohormone is vasopressin, or antidiuretic hormone ADH, which is synthesised by neurosecretory cells originating in an area of the brain called the hypothalamus. Once vasopressin is released from the axon terminals of these neurosecretory cells, which are located in the posterior pituitary gland, it travels in the blood to its target cells. the primary target cells are located in the kidneys where vasopressin affects the volume of urine that is excreted.
31
the most important chemical characteristic
is whether the messenger can dissolve in plasma (primarly water) or cross lipid bilayer in plasma membrane of cells.
32
lipophilic (hydrophobic) molecules
are lipid soluble and therefore readily cross the plasma membrane but they do not dissolve in plasma.
33
lipophobic (hydrophilic)
molecules that are soluble in water and therefore dissolve in plasma but do not cross the plasma membrane.
34
the chemical classifications of chemical messengers are
1 amino acids
2 peptides/proteins
3 steroids
4 eicosanoids
5 amines
35
amino acid messengers ( neurotransmitter)
glutamate, aspartate, glycine, and gamma-aminobutyric acid (GABA) are amino acids that are classified as chemical messengers becuase they function as neurotransmitters in the brain and spinal cord. Glutamate, aspartate, and glycine are among the 20 alpha amino acids, that are used in protein synthesis. Whereas GABA belongs to gamma amino acids. amino acids are lipophobic.
36
amine messenger ( neurotransmitter, paracrine, and hormones)
are chemical messengers derived from amino acids. the maines include a compound called catecholamines, which contain a catechol group and are derived from the amino acid tyrosine.
catecholamines include dopamine, norepinephrine, and epinephrine. dopamine and norepinephrine function primarily as neurotransmitters, whereas epinephrine functions primarily as a hormone. except for the thyroid hormones, all of the amines are hydrophilic.
37
peptide/protein messengers
most chemical messengers are polypeptides. the term peptide generally refers to chains containing fewer than 50 amino acids, whereas proteins are longer chains of amino acids. polypeptides are hydrophilic.
38
steroid messengers (hormones)
a class of compounds derived from cholesterol. all of the bodys steroid messengers function as hormones. they are derived from hydrophobic cholesterol, they are also hydrophobic
39
Eicosanoid messengers (paracrine)
they are lipids and readily cross the plasma membrane, hence they are hydrophobic.
40
synthesis of amino acids
amino acids can be obtained from the diet, and the four amino acids that function as neurotransmitters must be synthesised within the neuron that will secret them.
Glutamate and aspartate are synthesised from glucose through a three step series of reactions. First glucose is catabolised to pyruvic acid by glycolysis; pyruvic acid is in turn converted to acetyl CoA, which then enters the kerbs cycle; and finally the amine groups are added to certain kerbs cycle intermediates to form glutamate or aspartate. glycine is synthesised from a glycolysis intermediate, 3-phosphoglycerate, in a series of 4 reactions. GABA is synthesised from glutamate in a single reaction catalysed by the enzyme glutamic acid decarboxylase. following their synthesis in the cytosol, amino acid neurotransmitters are transported into vesicles, where they are stored until they are released by exocytosis.
41
synthesis and release of amines
all amines are derived from amino acids, and all except thyroid hormones are synthesised in the secretory cell by a series of enzyme catalysed reactions.
catecholamines is derived from amino acid tyrosine. dopamine acts as the precursor for noreinephrine, which in turn serves as the precursor for epinephrine. because dopamine is a precursor for the other catecholamines, all catecholamine-secreting cells possess the 2 enzymes that catalyse its synthesis: tyrosine B-hydroxylase and dopa decarboxylase. both of these enzymes are found in the cytosol. after dopamine is synthesised, it is transported into vesicles, where it is stored for later release by exocytosis or used as the substrate for synthesis of norepinephrine. the enzyme that converts dopamine to norepinphrine is dopamine G-hydroxylase, which is found inside the vesicle. if norepinephrine is the final product, it is stored in the vesicle until released by exocytosis. for cells that secret epinephrine the norepinephrine is transported out of the vesicle into the cytosol, where the enzyme phenylethanolamine N-methyl transferase PNMT is located. PNMT converts norepinephrine to epinphrine and the epinephrine is then transported back into a vesicle and stored until exocytosis.
42
serotonin synthesis and release
or 5-HT is synthesised from the amino acid tryptophan, one of the essential amino acids. serotonergic neurons have tryptophan carriers that actively transport tryptophan into the cell. tryptophan is converted to 5-HT throgh a pair of enzyme catalysed reactions: 1 tryptophan-5-hydroxylase converts tryptophan to 5-hydroxytryptophan, and 2 aromatic L-amino acid decarboxylase converts 5-hydroxytryptophan to 5-HT. like the catecholamines, 5-HT is packaged into vesicles and stored until its release by exocytosis.
43
histamine
produced from histidine in the cytosol by a one step reaction that is catalysed by the enzyme histidine decarboxylase. histamine is then packaged into vesicles and stored until its release by exocytosis.
44
peptides/proteins synthesis and release
cytosolic mRNA serves as the template that codes for the amino acid sequence in the peptide or protein. translation of this mRNA begins on ribosomes that are found in a free state in cytosol.
1 translation starts, the ribosome attaches itself to the rough endoplasmic reticulum, where the rest of the translation occurs. the polypeptide is formed inside the lumen of the rough endoplasmic reticulum, first as a prepropeptide.
2 in the lumen of the endoplasmic reticulum, proteolytic enzymes cleave off some amino acids from the prepropeptide yielding the propeptide,
3 in the smooth endoplasmic reticulum, the propeptide is packaged into transport vesicles
4the vesicles transport the propeptide to the golgi apparatus
5 in the golgi apparatus, the propeptide is packaged into a secretory vesicle for storage until its release is triggered. more amino acids are cleaved off by proteolytic enzymes in the golgi apparatus or in the secretory vesicle to give the final product, an active messenger.
45
steroids (hormones) synthesis and release
steroid messengers are synthesised from cholesterol in a series of reactions catalysed by enzymes located in the smooth endoplasmic reticulum or mitochondria. during this process, the cholesterol molecule is modified, but its basic ring structure remains intact, as does its hydrophobic character. because they are lipophilic, they cannot be stored prior to release and instead they diffuse out of the cell into the interstitial fluid as soon as they are synthesized.
46
eicosanoids (paracrine) synthesis and release
similar to steroids, they are synthesised and released on demand, because they are lipophilic.
the first step in eicosanoid synthesis involves an enzyme called phospholipase A2, which is activated in reponse to chemical signals of various kinds (paracrines, hormones, neurotransmitters, and even foreign chemicals). when active this enzyme catalyses teh release of arachidonic acid from memebrane phospholipids. once arachidonic acid is released from the memebrane, the final product depends on the complement of enzymes present in the particular cell.
to become an eicosanoid, a molecule of arachidonic acid first binds with one of two enzymes: either cyclooxygenase or lipoxygenase. cyclooxygenase is teh first enzyme in a series of reactions, called the cyclooxygenase pathway, that leads to the syntheiss of prostacyclins, prostaglandins, or thromboxanes. prostacyclins and thromboxanes are important in blood clotting; prostaglandins play roles in several systems, including inducing pain and the inflammatory repsonse. lipoxygenase is the first enzyme that participates ina series of reactions, called the lipoxygenase pathway, that leads to the synthesis of leukotrienes, which also contribute to inflammatory response.
because of the eicosanoids role in paina nd inflammation many anti inflammatory drugs such as aspirin, act by targeting enzymes involved in eicosanoid synthesis. by inhibiting the activity of the enzyme cyclooxygenase, aspirin decreases not only inflammation but also blood clotting. it is this latter effect of aspirin that has led physicians to prescribe low doses of aspirin to pateints believed to be at risk for a heart attack or stroke; these conditions can be cuased by blood clots in the coronary and cerebral arteries.
47
transport of messengers
simple diffusion ( neurotransmitters and paracrines). typically these messengers are quickly degraded in the interstitial fluid and become inactive, minimising the spread of their signaling. in contrast hormones are transported in the blood either in dossolved form or bound to a carrier proteins. to be transported in dissolved form, the messenger must be a hydrophilic messenger. peptides and amines (except thyroid hormones ) are transported in this manner. because steroids and thyroid hormones are lipophilic and do not dissolve well in the blood these hormones are largely transported while bound to carrier proteins. catecholaminesfunction as a hormone and is hydrophilic( lipophobic) and are transported in dissolved form.
48
signal transduction mechanisms for responses mediated by intracellular receptors
receptors for lipophilic messengers are usually located in the cytosol or nucleus of target cells and are readily accessible because these messengers easily permeate the plasma membrane. The binding of the messenger to the receptor alters the synthesis of a specific protein by the mechanism depicted, which shows the action of a lipophilic hormone;
1if a receptor is located in the nucleus, the hormone diffuses into the nucleus and binds to it, forming a hormone-receptor complex.
2 if a receptor is located in the cytosol, the hormone binds to it there, forming a hormone receptor complex that then enters the nuclues.
3 inside the nucleus, the hormone receptor complex functions asa transcription factor by binding to a region of dna called the hormone response element (HRE), which is located at the beginning of a specific gene.
4 binding of the complex to the HRE activates or deactivates the gene, which affects transcription if mRNA and ulimately increases or decreases synthesis of the protein coded by the gene.
5 the mrna moves into teh cytosol
6 the mRNA is translated by ribosomes to yield proteins.
because changes in protein synthesis can take hours or even days, the effect of lipophilic messengers are generally slow to develop. in addition, because these newly synthesised proteins often remain in the target cells long after the messenger is gone, the effects can persist a long time.
49
signal transduction mechanism for responses mediated by membrane-bound receptors.
lipophobic messengers cannot be permeate the plasma membrane to any significant degree. thus their receptors are located on the plasma membrane with the binding site facing the extracellular fluid. the receptors for these messengers are classified into three general categories: channel-linked receptors, ezyme-linked receptors and G-protein receptors.
ion channels that open or close in response to the binding of a chemical to a receptor or to the channel are called ligand-gated channels. channel linked receptors are a type of ligand gated channels in which the ligand is a messenger that binds to a receptor. these channels are classified into two categories: 1 fast channels in which the receptor and channel are the same protein
2 slow channels in which the receptor and channel are seperate proteins but are coupled together by a third type of protein called g protein.
50
Fast Ligand gated calcium chnanels
binding of a messenger to the receptor/ channel opens calcium channels, enabling calcium ions to enter the cell. calcium entry will change the electrical properties of the cell, but can also trigger a variety of responses such as secretion of some product by exocytosis, muscle contraction, or a change in a protein's activity. in the last instance, calcium acts as a second messenger, binding to the protein calmodulin to form a calcium-calmodulim complex. this complex activates a protein kinase, which phosphorylates a protein that produces a response in the cell.
51
second messenger
an intracellular messenger produced by the binding of an extracellular messenger ( first messenger) to a receptor.
52
signal transduction mechanism ( receptors)
a decrease in the number of receptors is called down regulation, an increase is called up regulation.
53
hydrophilic messengers
bind to 3 types of cell surface, 1 channel linked receptors, which affect the opening and closing of fast ligand-gated channels, 2 enzyme linked receptors which catalyse reactions inside cells; and 3 G protein linked receptors which activates specific memebrane proteins called G-proteins
54
activated G proteins
can actiavte a variety of intracellular proteins including enzymes or channels. many of these enzymes catalyse the formation of a second messengers inside the cells, including cAMP, cGMP, DAG and IP3, and calcium ions.
55
neural connection between the hypothalamus and the posterior pituitary gland
the posterior lobe of the pituitary gland contains neural tissue consisting of the axon terminals of neurons originating in the hypothalamus. these neural endings in the posterior pituitary gland secret 2 peptide hormone: ADH (vasopressin) and oxytocin.
ADH is synthesised primarly in the paraventricular nucleus and oxytocin is synthesised primarily in the supraoptic nucleus
following synthesis, the peptides are packaged into secretory vesicles, which are transported to the axon terminals in the posterior pituitary. the hormones are released by exocytosis when these neurons recieve a signal usually from other neurons; this reaction is called a neuroendocrine reflex.
56
the anterior lobe of the pituitary gland and the cells of the hypothalamus that control it secrete primarily tropic hormones.
tropic hormones can be stimulating hormone which increases the secretion of another hormone or an inhibiting hormone which decreases the secretion of another hormone.
57
hypothalamic-pituitary portal system
neurosecretory cells of the hypothalamus secret tropic hormones into the hypothalamic-pituitary portal system. the tropic hormones travel to the anterior pituitary, where they effect release of anterior pituitary hormones into the blood.
58
tropic hormones of the hypothalamus and anterior pituitary
1. prolactin-releasing hormone PRH(neurosecretory cells in the hypothalamus) and prolactin-inhibiting hormone PIH, stimulates/inhibit the anterior pituitary to release prolactin, which stimulates mammary gland development and milk secretion in females.
2thyrotropin releasing hormone TRH stimulates the release of thyroid stimulating hormone TSH( anterior pituitary tropic hormone) from the anterior pituitary. TSH then stimualtes the secretion of thyroid hormones by the thyroid gland. thyroid hormones regulate metabolism.
Corticotropin-releasing hormone CRH, stimulates the release of adrenocorticotropic hormone ACTH by the anterior pituitary. ACTH then stimulates the secretion of glucocorticoids such as cortisol from the adrenal cortex, the outer layer of the adrenal gland. cortisol is the main hormone that regulates metabolism when the body is stressed.
3growth hormone releasing hormone GHRH stimualtes teh secretion of growth hormone GH by teh naterior pituitary. GH regulates growth and energy metabolism but also functions as a tropic hormone by stimulating the secretion of insulin-like growth factors by the liver.
5 growth hormone inhibiting hormone GHIH inhibits the secretion of growth hormone by the naterior pituitary, thereby decreasing IGF release from the liver.
6 gonadotropin releasing hormone GnRH stimulates the release of the gonadotropins follicle stimulating hormone FSH and luteinzing hormone LH by the anterior putuitary. LH stimulates ovulation in females and it stimulates the secretion of sex hormones (estrogens and progesterone in females and androgens in males) by the gonads. FSH promotes the development of egg cells in females and sperm cells in males. it stimulates the secretion of estrogens in females and inhibin in both sexes.
59
Testis
is the site of spermatogenesis.
60
Leydig Cells
produce and secrete testosterone
61
Epididymis
is the site for maturation and storage of sperm.
62
Sertoli Cells
nurture sperm and control their development
63
Rete Testis
is the structure into which the seminiferous tubules drain
64
Human Spermatozoa
contain 23 chromosomes; during spermatogenesis diploid germ cells (spermatogonia) undergo meiosis to generate haploid sperm cells.
contain a vesicle that hold enzymes needed for fertilisation.; The acrosome is a large vesicle found within the head of the sperm that contains enzymes and other proteins that enable the sperm to penetrate and fuse with the egg during fertilisation.
65
the blood-testis barrier
protects sperm from immune cells; suring spermatogenesis haploid sperm cells are produced from the pool of diploid spermatogonia. the haploid spermatids and spermatozoa are found on the lumenal side of the blood-testis barrier and thus are protected from being attacked by the male's immune system.
maintains a differential fluid composition of the seminiferous tubule lumen that facilitates the development of sperm; the blood-testis barrier limits the diffusion of materials between Sertoli Cells. this isolates the lumenal fluid from the fluid on the basal side of the blood-testis barrier, and allows the Sertoli cells to control the composition of the lumenal fluid so that it specifically meets the requirements of the developing sperm.
66
in the first step of spermatogenesis, spermatogonia differentiate into cells called primary spermatocytes
in the first step of spermatogenesis, the spermatogonium's chromosomes are replicated and it differentiates into a primary spermatocyte which has 46 duplicated chromosomes. the primary spermatocyte undergoes meiosis I to give rise to 2 secondary spermatocytes, which then undergo meiosis 2 to become spermatids. the spermatids then differentiate into spermatozoa with the characteristic head, midpiece, and tail.
67
what is the primary function of the fallopian tubes?
to transport the egg from the ovaries to the uterus
68
what is the endometrium?
the endometrium is the inner layer of the uterus that proliferates, and then is shed during menstruation
69
what is the cervix?
the cervix is the lower, narrower portion of the uterus, that leads directly to the vagina.
70
ovaries
Oval-shaped - 3 to 5 cm in length.
* Female gonads: contain all of the ovum.
* Ova grow under the influence of anterior pituitary hormones (FSH, LH).
Each ovary is composed of dense connective tissue that is well supplied with blood vessels and enclosed within allayer of fibrous connective tissue.
71
fallopian tubes
* Extend from top of uterus to ovaries.
* Transport egg from ovary to uterus.
* Fimbriated ends that collect and sweep the released
ovum into uterine tubes.
* Site where fertilisation usually occurs.
72
uterus
* Consists of 2 layers:
* Myometrium: smooth muscle forming the uterine wall.
* Endometrium: epithelial layer that proliferates and is shed
during menstruation.
* Uterus houses and nourishes the developing fetus during pregnancy.
73
vagina
Canal of about 8-10 cm in length, leading from cervix to outside the body.
* Thin wall; a middle layer of smooth muscle; inner layer of epithelial cells
that change in response to hormone levels.
74
what is a follicle? what is the earliest form of follicle within the ovary?
follicles are spherical structures within the ovary that each contain a single developing oocyte. the primordial follicle is the earliest form of follicle- it consists of the primary oocyte, surrounded by a single layer of follicular cells.
75
what are the phases of the ovarian cycle
the ovarian cycle consists of the follicular phase and the luteal phase, the follicular phase begins with the first day of menstrual bleeding and ends with ovulation. the luteal phase begins with ovulation and lasts for the remainder of the cycle.
76
A follicle in the earliest stage of development is called
primordial follicle, a simple structure consisting of a developing ovum oocyte surrounded by a single layer of specialised epithelial cells called follicle cells
As the follicle develops the epithelial cells increase to more than one layer and the cells are then called a granulosa cells. In later stages of development, the granulose cells proliferate and the outermost layer is transformed into another cell type known as theca cells.
77
Ova and their develop pment
A process called oogenesis I ova developed from relatively undifferentiated germ cells called oogonia. The oogonia are fixed in number prior to birth and they are not continuously regenerated . The process of meiosis, which transforms oogonia into fully mature ova , begins in fetal life but is not completed until fertilisation.
Oogenesis begins at approximately 2-3 months of embryonic life, when oogonia (diploid) undergo mitosis to yield 2-4 million clones from which all ova are ultimately derived. These oogonia differentiate into primary oocytes, which begin the first meiotic division. However, during the remainder of fetal development , meiosis of most primary oocytes stops during prophase of meiosis 1 ; at this point, the primary oocytes enter a state known as meiotic arrest and remain Here until just before ovulation. primary oocytes that do not enter meiotic arrest undergo apoptosi, leaving 1-2 million primary oocytes at birth in meiotic arrest with 46 chromosomes that have 2 chromatids . Meiosis stays in arrest until just before ovulation.
78
By the time a female reaches puberty
Only some 400,000 primary oocyte still exist. The other primary oocyte have undergone degeneratin (atresia). Once puberty starts, one primary oocyte per month continues meiosis 1 completing the process prior to ovulation, producing 2 daughter cells possessing 23 chromosomes each (nx2) . One of the daughter cells, called a secondary oocyte recieves most of the cytoplasm during the cell division and continues to develop further. The other daughter all called the first polar body , degenerates.
Only in the event that the secondary oocyte is fertilised would meiosis 2 occur.
79
The menstrual cycle comes in 2 phases
The ovarian & uterine cycle
80
The Ovarian cycle
is divided into 2 phases :1 the follicular and 2 the luteal phase
Each around 14 days long. The follicular phase commences with the start of menstruations and ends with ovulation
In the follicular phase of ovarian cycle; Most of He follicles are in the primordial phase and about 2mm. follicular phase begins at 1 where a small fraction of follicles begin to develop, with each follicle's development proceeding independently. As a follicle begins to develop, it cells proliferate into multiple layers around the oocyte and differentiate into granulose cells.At the 2nd stage of the follicular phase the primordial follicle becomes a primary follicle, this stage of development is called the preantral stage . During the preantral stage , granulosa cell secrete noncellular material that forms a thick membrane between them and the oocyte called zona pellucidaBecause the zona pellucidum separates the oocyte from other tissue , nourishment to oocyte is provided by gap junctions located between the oocyte and granulose cell. Additionally at the preantral stage certain connective tissue differentiate to form theca cells. Some follicles do not develop and undergo atresia.
At the 3rd stage of the follicular stage ,, the follicles that continue developing form a fluid filled cavity called an antrum, which increases in size, at this point the follicle is called secondary follicle and the stage of development is culled the early antral stage. Approximately 10-25 follicles from the preantral and early antral stage one of the follicles are selected to develop after seven days. The ovary containing the dominant follicle is the dominant ovary for that cycle.
Follicular growth are stimulated by FSH and estrogen secreted by follicle. During follicular phase ,FSH levels gradually fall which tends to cause rates of estrogen secretion to decrease.
As the dominant follicle continues to develop into the late antral stage, the antrum grows and displaces some at the cellular tissue around the oocyte. In the 4th stage of the follicular phase several layers of granulosa cells continue to surround the oocyte , forming the corona radiata, while other granuloses cells form the cumulus oophorus , a bridge of cells that attaches the oocyte and corona radiata to the wall of the follicle now called a graafian follicle. Eventually after meiosis 1 occurs and the secondary oocyte attaches from He follicle wall and floats freely in the antral fluid along with the surrounding corona radiata. The antrum continues to expand and becomes 2- 2.5 mm just before ovulation.
In the luteal phase starts with ovulation in which the wall of the graafian follicle ruptures (1) hence causing a flow of antral fluid that carries the oocyte to the ovary's surface. Then ( 2) the ruptured follicle is transformed into a gland called the corpus luteum , which secretes estrogens and progesterone . Both ovulation und formation of corpus luteum are triggered by an abrupt rise in LH. After ovulation the oocyte enters the uterine tube . If the oocyte is not fertilised, the corpus luteum reaches its maximum activity within 10 days of formation (3) Then begins to degenerate to scar tissue called the corpus albicans (4) . This degeneration causes a decline in estrogen and progesterone levels that sets the stage for menstruations and the beginning of the next follicular phase. If the oocyte is fertilised and implants in the whereas then hCG, is released from the endometrium then the placenta.
81
Uterine cycle
The . uterine cycle is divided into 3 stage; menstrual, proliferative , secretory
In the menstrual cycle menstruation is triggered by the fall in estrogen and progesterone. At first, blood vessels in the outermost layer of the endometrium begin to constrict , which reduces blood flow to the tissue
The proliferative phase; during this phase which begins when menstrual end, the rhesus renews itself. The endometrial glands enlarge and blood vessels increase in abundance. In the cervical canal glands begin to secrete a thin mucus that bathes the inner surface; should sperm be deposited in the vagina, this mucus will facilitate Heir migration through the uterus . Uterine changes in the proliferative phase are promoted by estrogens .
The secretory phase : the endometrium is transformed in such a way as to make a favourable environment for implantation and subsequent housing and nourishment of the developing embryo. The endometrial glands enlarge further and begin to secrete fluids rich in glycogen , which the embryo uses as an energy source in its early stages of growth. Furthermore, the secretions of the cervical canal by forming a plug that isolates the uterus from microorganisms in the outside environment that could possibly harm a developing embryo. These uterine changes are promoted by progesterone & estrogen during the secretory phase due to the action of the corpus luteum. As the end of the secretory phase approaches, the corpus luteum degenerates, causing plasma estrogen and progesterone levels to fall. If fertilisation occurs the corpus Interim doesn't degenerate and estrogen and progesterone remain elevated.
82
Hormonal changes during the menstrual cycle
Hormonal changes in the early to mid follicular phase: the early follicular phase is marked by short-lived declines in plasma estrogen and progesterone. FSH binds to receptors on granulosa cells and promotes their growth and proliferation, which causes follicles to increase in size . Under the influence of FSH the outer layer of granulosa cells differentiate into theca cells which posses LH,receptors . ,LH stimulates the theca cells to secrete androgens which then travels to granulosa cells and are converted to estrogens.
The drop in FSH is responsible for triggering atresia of nondominant follicles.
83
Hormonal Changes in the late follicular phase
There is high level of estrogen and thus a stimulation of the secretion of the LH.These events constitute a positive feedback. During the mid follicular phase estrogens stimulate granuloses cells to express LH receptors on their surface making them responsive to LH . As a result the rising tide of LH that occurs in the late follicular phase triggers the following changes in the dominant follicle: 1 Granulosa cells begin to secrete paracrine that stimulate the oocyte to complete meiosis 1 . 2 Estrogen secretion by granulosa cells fall , causing plasma estrogen levels to decrease. 3 Granulosa cell begins to secrete progesterone , which causes a small rise in the hormone. 4 Granulosa cells begin to secrete enzymes and paracrine that begin to break down the follicular wall. About 18 hours following the LH surge , the wall ruptures. 5 Granulosa cells und theca cells begin to differentiate into the cells forming the corpus luteum.
84
Hormonal change in the luteal phase
Early in the luteal phase, the corpus luteum is growing . Estrogen level continues to fall because the rate of estrogen secretion by the corpus luteum is not high enough to compensate for the loss of the estrogen secreting dominant follicle. The decline in estrogen terminates the LH surge. Progesterone secreted by corpus luteum cause LH to begin rising.
85
thyroid hormones and steroids are solube in
lipids
86
hydrophobic hormones
easily cross cell membrane and provide slower response
87
hydrophilic hormones
metabolised fast and are short, and stored in vesicles inside the cell
88
cortisol acts via
nuclear receptors
