1
Q
How does a morula form?
A
From rapid mitotic divisions of the ovum
2
Q
What is a blastocyst
A
Fluid filled hollow sphere, which consists of an inner cell mass at one pole, surrounded by a thin wall of torphoblast cells.
3
Q
What forms the fetal portion of the placcenta
A
Trophoblast cells attach he blastocytes to the uterine wall to form the fetal portion of the placena.
4
Q
WHat do trophoblasts differentiate into?
A
Trophoblasts differntiates into syncyiotrophoblast and cytotrophoblast. Lacunae appear in synctiotrophoblast ans merge to communicate with maternal blood sinusoids. By this the embryo derives nutrition from the maternal ciruclation. The cytotrophoblast froms villi, which are finger like protrsions into the lacunae
5
Q
WHat constitues the fetal membrane?
A
The amniotic and chorionic cavities come to surround the ermbryo and their walls constitue the fetal membrane.
6
Q
how does the endometrium respond to implantation?
A
The endometrium responds to implantation by mounting the decidual reaction. The decidual absalis forms the maternal component of the planta.
7
Q
What is gastrulation
A
Inner cell mass differentiates into 2 of 3 primary germ layers: epiblast (later the ectoderm) and the hypoblast (later endoderm). This results in bilaminar embryonci disc.
8
Q
What does the ectoderm further differentiate into
A
- Ectoderm further differentiates and forms the proimitive streak, whic gives rise to the third germ layer, the mesoderm. This the trilaminar embryonic disc fors.
9
Q
What does the mesoderm differentiate into?
A
- Mesoderm differentiates into paraxial, intermediate and lateral late mesodern. The lateral plate mesoderm encloses th intraembryonic coelom.
- paraxial mesoderm - forms segmented somites which give rise to muscles, skeletal structures and dermis.
- Intermediate mesoderm - contirbutes to urogenital system
- Lateral plate mesoderm - forms parietal and visceral layer of serou membranes of body.
10
Q
A
11
Q
What is neurulation
A
- Ectoderm gives rise to neural tube and neural crests of developign brain and spinal cord.
12
Q
WHat does the notochord form?
A
- Notochord forms midline axis of the body: largely disappears before birth, but perissts as the centres of IV discs (nucleu polposus)
13
Q
What does the endoderm give rise to?
A
Endoderm gives rise to linings of GI and respiratory systems and of the urinary bladder and the parenchyma of the liver and pancreas.
14
Q
How does the trilaminar disc become a folder embryo
A
- Trilaminar disc undergoes longitduinal and lateral folding this forming the folded embryo
15
Q
Where does the epidermis and dermis of the skin and its derivatives devlop from?
A
- The epidermis of the skin and its derivatives develop from the ectoderm.
- The dermis is from the lateral plate of mesoderm and dermatomes.
16
Q
WHat does the MSK develop from
A
- MSK is mesenchymal in origin, derived from somites, the lateral plate somatotropic layer and the neural crest
17
Q
What 2 areas do the somites subdivide into
A
Sclerotomes and dermyotomes
18
Q
How do individual vertebrae develop?
A
From sclerotome cells of two adjoining somites
19
Q
WHat are the 2 parts of the developing skull?
A
- Neurocranium
- Viscerocraniu,
- Flat bones of the skull develop in mmebrane but the base of the skull is derived from crtilaginous components
20
Q
When do limb buds develop and from where?
A
Limb buds develop from the lateral body wall between 24-28 days.
21
Q
How do limb bones develop?
A
Limb bones develop from endochondral ossifiction of cartilaginous precursors. The only exception is the clavicle which ossifies in membrane. The ossification of cartilaginous models of limb bones begins between 8th adn 12th week
22
Q
What factors contribute to limb anomalies
A
- Limb anomalies are multifactoral and form as a result of genetic defects, drug actions and mechanical effects
23
Q
What are dorsal and ventral muscles of body alls formed from (what layer myotome)
A
- Dorsla muscles of the body walla nd trunk are formed from the epimere and the ventral muscles from the hypomere divisions to myotomes. The developmental origins of the muscles are reflected in their innervation by the dorsal and ventral rmai of spinal nerves.
- The ventral rami supplying limb muscles ar eorganised into brachial and sacral plexuses.
24
Q
Def of ectoderm
A
One of the 3 germ layers – groups of cells that coalesce early during the early embryonic life.
25
Mesoderm
Germ layer that arises during gastrulation and is present between ectoderm which will turn into skin and ACNS cells
26
Endoderm
Innermost of the 3 germ layers, or masses of cells which appears in early development of an embryo. Will produce gut and lungs
27
Lateral folding
= Lateral edges of the embryonic disc flex sharply ventral. The edges of each germ layer make contact at head and tail regions and zipper toward the umbilicus.
28
Neurulation
= Process in which the neural plate bends up and later fuses to form the hollow tube that will eventually differentiate into the brain and the spinal cord of the CNS.
29
Dermomyotome
= Epithelial cell layer constituting of the dorsal part of the somite lying under the ectoderm.
30
Sclerotome
= Any of the paired block like segments of the mesoderm alongside neural tube.
31
Notochord
= Flexible rod formed of a material similar to cartilage.
32
Somites
= Blocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryo. Also determine migratory paths of neural crest cells and of the axons of spinal nerves.
33
Fate of the germ layers
* Epidermis – epidermis, CNS & PNS, eyes and internal ears, CT structures of head
* Endoderm – epithelial linings, glandular cells
* Mesoderm – everything else, including bones and muscles
34
Fate of the primtiive streak
35
Notochondal process and notochord
* Some mesenchymal cells migrating through primitive node extend cranially to form notochordal process. Notochordal process extends to prechordal plate. Notochordal process acquires a lumen.
* Floor of notochordal plate obliterates – notochordal canal is not continuous with umbilical vesicle.
* Fate of notochord – Defines longitudinal axis, signals development of axial MSC structures and CNS. Remnants (nucleus pulposis of IV disc). Vestigigal remnants can lead to development of chordomas
36
Somites
: Develop from paraxial mesoderm, paired cuboidal blocks that form lateral to developing neurtube. 38 pairs between days 20-30, 42-44 bu end of 5th week. First somties appear in future occipital region. Develop craniocaudally. Give rise to most of facial skeleton and musculature and adjacent dermis of skin.
37
Limb and spine development. 3 regions for limb and how
* Development of the limb: Limb bud -\> limb.
* Forelimb = upper limb
* Hindlimb = lower limb
* Overview = Human embryo (5wks) -\> outgrowth + patterning -\> growth + differentiation -\>proximal
* **3 Regions for the limb:**
* Stylopod – humerus/ femur
* Zeugopod – ulna/radius, tibia/fibula
* Autopod – wrist/fingers, ankle/toes
* Apical ectodermal ridge (AER) – at distal part of limb bud. Produces FGF used in signalling pathways. Has a progress zone which rapidly dividng mesodermal cells to cause elongation of the limbs.
* Zone of polarising activity – determines ulnar, radial, axis. Produces SHH (sonic hedgehog). Cells surrounding ZPA then higher conc SHH. Further away then cells have lower conc SHH. A cells fate can be determine by conc of SHH its exposed to.
* Syndactyly = digit malformation. Apoptosis does not occur so webbed digits.
38
Development of vertebrae
Parts sclerotome move up and down.Fuse together to vertebrae. Fissures allow channels for spinal Neves etc to get to myotome which become muscle’s
39
How are long bones formed
* Long bones are formed through the process of endochondral ossification. This is the formation of bone from a cartilaginous.
* The formation of separate ossification centres ensures an epiphyseal growth plate remains. This plate allows long bone growth.
40
What is Amelia / Meromelia for congenital abnromalities
In some instances, developmental signalling pathways and processes involved in osteogenesis fail. This results in congenital abnormalities such as: Spina Bifidia, polydactyly, syndactyly, amelia, meromelia.
* Amelia – this is total absence of limb or limbs
* Meromelia – Absenc eof a part of the limb.
41
Spina Bifida
condition wherin some of the vertebral laina (usually of the lumbar region) do not develop properly an so do not protect the spinal cord posteriorly. Can manifest in a number of ways:
* Spina bifida occulta is the most common and most mild form.
* Meningomyelocele is the most severe form.
Consequences: Lack of protection of the spinal cord will increase the likelihood and severity of damage. If the spinal cord is exposed to the open air, there I also an increased risk of infection. Symptoms can also include: Seizures, hydrocephalus, loss of sensation, paralysis.
42
Poly/Syndactyly:
* Polydactyly = presence of more than fie digits on the hands or feet
* Syndactyly = is fusion or webbing of the digits of the hands or feet
* Bidactyly = presence of only radial and ulnar or tibial and fibular digits.
43
What is the funciton of the urinary system?
* Removal of metabolic waste products like uric acid, urea and creatinine.
* Maintain electrolyte, water and pH balance
* Reg of BP, BV and erythropoiesis and vitamin D production
44
WHta are the 3 overlapping sequential systems in the development of the kidney and summmarise when they appear and what they develop into?
* **Pronephros** - Appears 4th week as cervical region. Segmented divisons intermediate mesoderm form nephrotomes which join to pronephric duct (connects it to cloaca of mebryo). Non funcitonal and regresses completely by end week4.
* **Mesonephros** - dveelops caudally to pronephros. Mesonephric tubules which receive tuft of capilalries form dorsal aorta so filtration blood and drian to mesonephric tuct. Pirmatory excreotry system in embryo and regress by end 2nd month. Additonally meosnephric duct sprouts ureteric bud and induces development of kidney.
* **Metanephros** - forms defintiive kdiney, appears 5th week and funcitonal about 12weeks. metanephric blastema which for metanephric system with two comaprtments:
* collecting system - form ureteric bud to ureter, renal pelvis, major and minor calyces and colelcitng tubules.
* Excretory system - from metanephric blastema. each tubule fcovered by tissue cap which gives rise to excretory tubules and these with glomeruli form nephron.
45
Where does the kidney develop and move to and how to do vessels accomdoate this
* Definitive kidney intiailly developsin pelvic region and then ascends into abdomen.
* In the pelvis, the kidney receives its blood suppy from pelvic branch of abdominal aorta and as it ascends, new arteries from abdominal aorta suppy kidney. Pelvic vessels usually regress but can persist as accessory renal arteries.
46
What is. horse shoe kdiney
Horseshoe kidney = (cake kidney/fused kidney) where 2 developing kidneys fuse into single horse shoe shape structure. Occurs if kidneys becomes too close together during ascent from pelvis to abdomen – become fused and stuck under inferior mesenteric artery. This is still drained by two ureters and is usally asymptomatic but can be prone to obstruction.
47
What are the bladder and urethra derived from
* Cloaca - hindgut structure this divides in 4-7th week bu uro rectal septum:
* Urogenital sinus (anterior) -\> upper part forms blaffr, pelvic part forms urethra and some repr tract in females + prostatic + membranous urethra in males then phallic/caudal part forms part female repr and spongy urethra uin males
* Anal canal (posterior)
48
How does the male bladder form?
* As the kidneys ascend into abdomen the ureteric openings move cranially
* The mesonephric ducts (wolffian fucta) move caudally and closer together, entering prostatic urethra to become ejaculatory ducts
49
How does the femal ebladder form
* As the kidneys ascend into the abdomen, the ureteric opneings move cranially.
* The mesonephric ducts degernate due to lack of testicular androgens
50
How fdoes the male urethra form
* Pre-prostatic, prostatic and membraous is formed form eplvic part of urogenital sinus
* The psongy urethra is formed form phallic part od urogenital sinus
51
How femal urethra formed
Urethra is forme from pelvic part of urogenital sinus.
52
Summarise how the kidney is formed
* metanephric mesoderm = excretory units / nephrons
* ureteric bud = collecting system (from the mesonephric duct)
* nephrons are formed until birth ~ 1 million/kidney
* urine production begins in early gestation, shortly after differention of glomeruli (by week 10)
* kidneys have lobultaed appearance at birth which disappears during infancy as the nephrons grow in size but not in number!
* kidneys initially develop in the pelvic region, but ascend to abdominal region as the body becomes more linear & elingates in the lumbar & sacral regions
53
Describe the indifferent stag in gonad develop
* Impossibel to differentiate male and female gonad.
* Bgein as genital ridges (deirved from itnermediate mesoderm + overlying epithelium)
* In 4th week, germ cells begin to migrate from endodern lining of yolk sac to genital ridges via dorsal mesentery of hindgut. Reach genital ridges in sixth week.
* Sitmultaneously- epithelium of genital ridges proliferates + penetrates intermediate mesoderm to form primitive sex cords. Combo of germ cells and primitie sex cords forms indifferent gona (from whcih devleopment into testis or ovaries can occur)
54
Testes - how these develop from gonad
* In male mebryo, the XY sex chromsoomes are present.
* The Y chromosome contaisn SRY gene, which stimulates developmentof primtiive sex cords to form testis (medullary) cords.
* The tunica albuginea, a fibrous connective tissue layer forms are cords
* A portion of the testis cords breaks off to form future rete testis. Remaining cord has germ cells and sertoli cells
* In puberty these cords acquire lumen and become seminferous tubules (where sperm will be formed)
* Located between testis cords are Leydig cells deived from itnermediate mesoderm. In 8th week, the begin production of testosterone, which dirves diferentiate of internala dn external genitalia.
55
Ovaries development form gonads
* In female emrbyo, XX chromosomes ar erpesent. As there is no Y chromosome, there is no SRY gene to influence development. Without it the primtiive sex cords degenerate and do not form the testi cords.
* Instead the peithelium of the gonad continues to proliferate, producing cortical cords.
* In 3rd months, these cords break up into clisters, surroundign each oogonuum (germ cell) with layer of epithelial follicular cells, forming primordial follic.e
56
Descriebt he internal genitalia stage of internal gentialia
* **Indifferent stage** = in firts weeks of urogenital development, all embryos have 2 pairs ducts, both ending at cloaca. These are the Mesonephric (Wolffin) ducts and Paramesonephric (Mullerian) ducts
57
Descirbe how the male internal genitlia develops
* In presence of testosterone (produced by leydig cells), the mesonpehric ducts develop to form primary mal egenital ducts. They give rise to efferent ductules, epididymis, vas deferns and seminal vesicles.
* Meanwhile, parameosnephric ducts degenerate in presence of sni-Mullerian hormone- produced by sertoli cells in testes. Its development remnan is appendic testis; small portion of tissue located on uppe rpole of each testice, which has no physiological function
58
Female internal genitalia development
* No leydig cells so mesonephric ducts degenerate, leaving behind only vestigial remnant (Gartners duct)
* No Mullerian hormone so paramesonephric ducts develop. These intiially have 3 parts: Cranial (-\> fallopian tubes), Horizontal (-\> becomes fallopian tubes), Caudal (-\> fuses to form uterus, cervix and upper 1/3 vagina) . Lower 2/3 vagina formed by sinovaginal ducts.
59
What is a bicornuate uterus
Relatively common structural defect. It occurs when there is incomplete fusion of paramesonephric ducts. This results in two district uterine horns, both opening into single vagina. As it is asymptomatic, the condition is often only picked up on US scan during pregnancy. This malformation is considered high risk in pregnancy as associated risk of miscarriage and premature delivery
60
indifferent stafe of external genitalia
o The development of the external genitalia begins in the third week. Mesenchymal cells from the primitive streak migrate to the cloacal membrane to form a pair of cloacal folds.
o Cranially, these folds fuse to form the genital tubercle. Caudally, they divide into the urethral folds (anterior) and anal folds (posterior).
o Genital swellings develop either side of the urethral folds
61
Male external genitalia development
· The development of the indifferent genitalia into the male genitalia is driven by the presence of androgens from the testes, namely dihydrotestosterone (DHT).
· There is rapid elongation of the genital tubercle, which becomes the phallus. The urethral folds are pulled to form the urethral groove – this extends along the caudal aspect of the phallus. The folds close over by the 4th month, forming the penile urethra.
The genital swellings become the scrotal swellings, moving caudally to eventually form the scrotum
62
Female xternal gentialia development
o Oestrogens in the female embryo are responsible for external genital development. The genital tubercleonly elongates slightly to form the clitoris.
o The urethral folds and genital swellings do not fuse, but instead form the labia minora and labia majorarespectively.
o The urogenital groove therefore remains open, forming the vestibule into which the urethra and vagina open.
63
Hypospadias - what are these
Condition where one or more abnormal openings of urethra along inferior side of penis. This is a result of inomplete closure of urethral folds during developmen. Surgery is usually performed to correct defect.
64
How do the gonads decend,
* Arise in upper lumbar region tethered to scrotum or labia by gubernaculum.
* Testes - mroe cadual as fetus grows and pass through inguinal canal and reach scrotum. Retain original blood uspply with testicular arteries branchign from lumbar aorta. Scrotal lgimaent is audlt remnant of gubernaculum
* Ovaries - Intiially migrate caudally and dont go as far, they reach true pelvis as final psoition. Gubernaculum become sovarian ligament and orudn ligament of uterus.
Anatomy - y4
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