Embryology Flashcards
1
Q
What are the three embryological periods and time frames
A
Pre-embryonic: conception - week 2
Embryonic: week 2 - 8
Fetal: week 9 - birth
2
Q
Process of oogenesis
A
Primordial germ cells arise from wall of yolk sac during week 2
Migrate to occupy gonadal ridges week 6
Undergo rapid mitotic division
Differentiate into oogonia and then primary oocytes and rapidly multiply in the embryonic ovary until the 5th month (7 mil in number), then undergo atresia
At birth, the primary oocytes enter prophase 1 (1st meiotic division) and remain in diplotene phase until puberty
At puberty, the follicular cells become stratified, forming layer of granulosa cells
Granulosa cells secrete glycoprotein layer and CT cells condense to form theca follicle, which differentiates into the inner vascular and secretory layer (theca internal), and the outer fibrous layer (theca externalities)
3
Q
What causes meiotic arrest
A
Oocyte Maturation Inhibitor (OMI) produced by the follicular cells of the primary follicle
4
Q
Process of spermatogenesis
A
Spermatogonia form in the fetal period and remain dormant in the seminiferous tubules until puberty
5
Q
5 steps of fertilization
A
- Sperm activation and penetration of corona radiata
- Attachment to zona and penetration of zona pellucida
- Fusion of oocyte and sperm cell membranes
- Completion of meiosis in oocyte and formation of pronuclei
- Formation of zygote
6
Q
Process of sperm activation
A
In order to become active, sperm must undergo capacitation
- Removal of glycoprotein and cholesterol of the ayrosomal membrane by secretions from the cervix and uterine tube. Must happen before sperm reaches distal tube.
7
Q
How does the sperm penetrate the corona radiata
A
Viable sperm come in contact with the C.R and surround it. They then undergo an acrosomal reaction and release enzyme hyaluronidase which is needed to penetrate the CR
8
Q
What enzymes are required to penetrate the zona pellucida
A
Esterases, neuraminidase, acrosin
9
Q
What is the zona reaction
A
Once a sperm binds the zona, it initiates the zona reaction, whereby the zona changes its physical properties to prevent attachment of other sperm. Done through the action of lysosomal enzymes into a space between the zona and oocyte cell membrane
10
Q
When does the oocyte complete meiosis
A
Once the sperm has entered, the oocyte completes its 2nd meiotic division to produce a mature oocyte and a second polar body.
11
Q
Explain the process of cleavage
A
Series of rapid mitotic cell divisions by which the single cell zygote becomes the 16 cell stage embryo, in 3/7
12
Q
What is the blastocyst
A
The embryological stage after morula formation, where blastomere cells arrange to form an outer blastocyst containing a fluid-filled blastocoele in the centre. Formed by day 4-5
13
Q
Describe the maturation of the blastocyst in the second week of embryological development
A
Embryo partly implants in the endometrium, which initiates decidualisation
Trophoblast differentiates into the inner layer, the cytotrophoblast, and the outer layer, the syncytiotrophoblast. The ST is more extensive and invasive and invades into the endometrium.
The blastocyst differentiates into the epiblast and the hypoblast (bilaminar disc). Cavity develops in the epiblast –> amniotic cavity, and in the hypoblast –> primary yolk sac
By D12, lacunae appear in the ST which communicate with maternal sinusoids. Extra-embryonic coelom develops between the exocoelomic membrane and the CT. Completely surrounds embryo except for connecting stalk.
By day 13, the CT has formed primary chorionic villi.
By the end of the second week, ST is producing HCG to maintain the pregnancy
14
Q
What is the function of the primary yolk sac
A
To provide nutrition via diffusion until the placenta is formed and functional
15
Q
What is gastrulation
A
The process whereby the bilaminar embryonic disc becomes trilaminar
16
Q
Describe the process of gastrulation
A
Primitive streak and node appear on the epiblast/ ectoderm during the 3rd week.
Ectodermal cells migrate toward the streak, then detach from it spreading out laterally beneath it. This forms the third germ layer, the intra-embryonic mesoderm.
17
Q
At what points are there no mesoderm?
A
- At the prochordal plate, which becomes the buccopharyngeal membrane, which breaks down by week 4 to allow communication between the gut tube and the amniotic cavity.
- At the cloacal plate, which becomes the cloacal membrane
18
Q
How does the notochord form and what does it become
A
The notochord is a cylindrical structure that develops from the ectoderm. Cells of the primitive node migrate cranially toward the buccopharyngeal membrane, forming the notochord plate. This folds inwards, forming the notochord. The notochord underlies the future neural tube and forms both the long axis of the embryo, and becomes the nuclei pulps of the intervertebral discs of the vertebral column
19
Q
What is neurulation
A
The process of the formation of the brain and spinal cord
20
Q
Describe the process of neurulation
A
Ectoderm gives rise to neuroectoderm –> most major components of the CNS
D19: notochord induces overlying ectoderm which thickens to form neural plate. Neural plate begins to invaginate, forming neural folds. The plate continues to deepen forming the neural groove, which fuses to form the neural tube.
21
Q
What are the neuropores and when do they close
A
They are the cranial and caudal ends of the neural tube that remain open after the neural tube fuses.
They close at day 25 and 27 respectively
22
Q
Where are the neural crest cells formed and where do they migrate to
A
At the edges of the neural tube, where the neuroectoderm is continuous with the surface ectoderm, the neural crest cells are formed. Before the neural groove fuses to form the NT, the crest cells detach and form aggregates alongside the neural tube
23
Q
Three divisions of the mesoderm and their derivatives
A
Paraxial mesoderm - Skeletal muscle trunk and limbs -Skeleton (except the skull) - Dermis and connective tissue Intermediate mesoderm - Urogenital system Lateral plate mesoderm - Serous membranes -SM and CT - Viscera
24
Q
Divisions and contributions of the ectoderm
A
Surface ectoderm - Epidermis - Tooth enamel - Lens - Inner ear Neural tube - CNS - Retina - Post pituitary Neural crest - Cranial and sensory ganglia - Adrenal medulla -facial bones - melanocytes
25
Describe the process of mesoderm development
Mesoderm cells proliferate on either side of the notochord
by D17, it is thickest closest to the midline --> paraxial mesoderm.
by D19, clefts appear in the lateral plate, which is continuous with the extra-embryonic mesoderm covering the yolk sac and amniotic cavity. The mesoderm around the A.S. is the parietal or somatic layer, and that around the yolk sac is the visceral or splanchnic. Extra-embryonic mesoderm has diverging limbs on either side which open into the extra-embryonic coelom. Clefts within the lateral plate merge to form the intra-embryonic coelom, which is the forerunner for the serous cavities.
26
Development of the paraxial mesoderm
Undergoes differentiation in paired blocks of tissue (somites) in craniocaudal direction.
First pair of somites appears on D20, then appear at a rate of 3/day until 42-44 pairs.
By beginning of the 4th week, the somites beefing to differentiate.
27
Development of the somites
Medially placed mesenchymal cells of the somites migrate toward the notochord to form sclerotomes, which later form bone and cartilage. (week 4)
Ventrolateral cells of the somites become myotomes, and the remaining become dermatomes
Myotomes spilt into dorsal epimeres and ventral hypomeres. Dorsal epimeres become the epaxial muscles (erector spinae). Ventral hypomeres form hypaxial muscles (muscles of the body wall).
Remaining venterolateral parts of somites near the future limb buds migrate to differentiate into the limb musculature.
Dermatomes form dermis of the skin
As dermatomes and myotomes migrate, they take the innervation from the adjacent neural tube with them.
28
Endoderm development and derivatives
Endodermal gut tube formation relies on lateral and longitudinal folding of the embryo
Endoderm gives rise to the lining of the GIT and rest tract, as well as the parenchymal cells of the liver and pancreas, thyroid and parathyroid and the lining of the urinary bladder.
29
Describe folding of the embryo
2 directions: longitudinal and lateral
Longitudinal occurs from D21-24 . It is due to the rapid enlargement of the cranial end of the neural tube to form the brain. Results in the head and tail section being brought together. Causes the endoderm to form a tube like structure, which initially has a large communication with the yolk sac but narrows as the longitudinal folding increases.
Lateral folding is due to the enlarging somites.
30
Three components of the urinary system development, time frames
Pronephros - appears at week 4 - rudimentary and never functional
Mesonephros - appears at week 4 - functions from week 6 - 10, then disappears
Metanephros - appears from week 4/5 - functions from week 12
31
Key points in gonadal formation
Primordial germ cells develop in the endodermal yolk sac - W4
Germ cells migrate to mesodermal endothelium - W5
Gonads remain indifferent - W6
Leydig cells produce testosterone - W8
Secondary cortical/sex cords form - W10-12
Primordial follicles form - W16
32
What is the origin of the urogenital system
Intermediate mesoderm
33
What is the urogenital ridge and where does it form
It is a longitudinal elevation of the intermediate mesoderm that forms on either side of the aorta
34
What are the two parts of the urogenital ridge
Nephrogenic cord, which develops into the urinary system
| Gonadal ridge, which develops in to the genital system
35
What is the pronephros
A rudimentary and non-functional excretory organ that appears in the cervical region at week 4.
All elements disappear 1/52 after they appear
36
What is the mesonephros
Intermediate excretory organ that forms at week4, functions from week 6-10, then disappears by end week 10.
Originates between the upper thoracic and upper lumbar segments
Forms excretory tubules that lengthen and acquire a capillary tuft. The tubules adjacent to the tuft differentiate into the Bowman's capsule, and those laterally open into the wolffian duct.
The tubules then degenerate, but the wolffian duct persists and participates in the formation fo the genital system in males.
37
What is the ureteric bud and what does it become
The ureteric bud is an outgrowth of the mesonephric duct, near to its entrance on the cloaca
The stalk of the bud forms the ureter and the expanded part forms the renal calyx.
38
What is the metanephron
The embryonic kidney which forms the permanent kidney
Appears in week5, functions from w12.
Excretory units develop from metanephric mesoderm, with collecting ducts forming from the ureteric bud.
Each collecting tubule is covered at the distal end by the metanephric tissue cap
Mesoderm adjacent to the collecting tubule forms metanephric vesicles which become renal tubules, and acquire a tuft of capillaries that forms the glomerulus.
39
What controls kidney formation
Proteins RET, GDNF and GFRA1
RET: Rearranged during transfection protooncogene
GDNF: giant cell line-derived neutrophilic factor
GFRA1: GDNF familia receptor alpha 1
40
Glomerular integrity relies on signalling between which 3 major cell lineages
Podocytes, endothelial cells and mesangial cells.
41
How does the kidneys position change
Initially develops in the pelvic region, then ascends cranially due to the differential growth between the lumbar and sacral regions.
As the kidney ascends, it rotates 90 degrees.
It derives its blood supply from the aorta in its final position and the lower vessels disappear
42
Development of the bladder
At week4, cloaca is decided into urogenital sinus anteriorly and anal canal posteriorly by the urogenital septum (mesodermal layer, whose tip forms the perineal body)
Cranial part of the urogenital sinus becomes the bladder, which is continuous with the allantois, the urachus forms and later obliterates to form the medial umbilical ligament
As the cloaca differentiates, the caudal portions of the mesonephric ducts are absorbed into the bladder wall.
43
What are the 3 parts of the developing urogenital sinus
Cranial end becomes the bladder
Narrower middle pelvic part forms the whole urethra in females, and the prostatic and membranous urethra in males
The caudal or phallic part forms the genital organs
44
Process of undifferentiated gonadal development
```
In week 4, there forms a thickened area of mesothelium on the medial side of the mesonephros
Initially appears as a pair of longitudinal ridges (the genital or gonadal ridges) and remain indifferent until week 7
Gonadal cords (finger-like epithelial cords) grow into the underlying mesenchyme dividing the ridge into external cortex and internal medulla
Primordial germ cells from the yolk sac migrate along the dorsal mesentery and invade the gonadal ridges in week 6
```
45
What is SRY and where is it located
Sex determining region on the Y chromosome, on the short arm, Yp11.
46
Which hormones are present in male development
Mullerian inhibiting substance - inhibits the paramesonephric ducts
Testosterone- stimulates the mesonephric ducts
Dihydrotestosterone - stimulates the formation of the external genitalia
47
Which hormones are present in female development
Estrogen, to stimulate the paramesonephric ducts and to stimulate formation of external genitalia
48
Development of the testes
Under the influence of the SRY gene, primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis or medullary cords
Layer of dense CT appears and separates cords from surface epithelium, which later becomes the tunica albuginea
Enlarging testis becomes separated from mesoderm and develops own mesentery (mesochorium)
Cord of cells become seminiferous cords, which becomes the seminiferous tubules, both tubuli seminiferi recti and the rete testis
Sertoli cells develop from surface epithelium, and Leydig cells from the mesenchymal cells of the gonadal ridge
Seminiferous tubules canalise at puberty and enter the ductuli efferentes
49
Development of the ovary
Sex cords dissociate in absence of SRY to form irregular cell clusters
Germ cells in medullar part of every are replaced by vascular storm but surface episode multiplies to cord cords of cells.
Germ cells are incorporated into the cords during multiplication
At the 4th month the cords split into isolated cell clusters around the germ cells
Later the germ cells become oogonia and the epithelial cells surrounding become the follicular cells.
50
Development of the male genital ducts
Influence of testosterone causes the mesonephric ducts to differentiate into the epididymis, ductus deferens sand the ejaculatory duct
As the mesonephros regresses the tubules divide into epigenital and paragenital tubules. Epigenital tubules join the rete testis and become the efferent ductules
Paragenital tubulues do not join, rather form the paradidiymis. The mesonephric duct below the paradidiymis elongates and convolutes to form the epididymis.
The seminal vesicle grows as an outbidding from the tail of the epididymis and the region after the seminal vesicle becomes the ajaculatory duct.
The paramesonephric duct degenerates to for the appendix testis
51
What two ducts comprise the genital ducts
Wolffian - mesonephric
| Mullerian - paramesonephric
52
Development of the female genital ducts
Paramesonephric ducts arise as longitudinal invaginations of mesoderm on the lateral surface of the urogenital ridges
Cranially the duct opens into the abdominal cavity; caudally it merges with the paramesonephric duct of the opposite side to form the uterovaginal primordium
The caudal tip then projects into the posterior wall of the urogenital sinus to form a small swelling, the Mullerian tubercle, which gives rise to the sinovaginal bulb.
As the ducts fuse in the midline, they take a sheet of peritoneum, which forms the broad ligament
The fused paramesonephric duct become the corpus and cervix of the uterus, and the surrounding layer of mesenchyme forms the myometrium
53
Indifferent stage of external genitalia development
In week 3, the mesenchyme around the cloacal membrane becomes slightly elevated, forming the cloacal fold
The folds cranial to the cloacal membrane unite to form the genital tubercle. Caudally, thy divide into the urethral folds anteriorly and anal folds posteriorly
As the urorectal septum fuses with the cloacal membrane, it decides the cloacal membrane into the dorsal anal membrane and the ventral urogenital membrane
On either side of the urethral folds, another pair of swellings becomes apparent , the genital swellings.
54
Development of the male external genitalia
Under the influence of androgens, the genital tubercle enlarges (=phallus)
As the phallus enlarges, it pulls the urethral folds forward so they form the lateral walls of the urethral groove
Epithelial lining of the groove originates in the endoderm and forms the urethrl plate, which extends from the phallic portion of the urogenital sinus
Urethral folds clod over the urethral plate to form the penile urethra
Ectodermal cells penetrating inward and joining the urethra at the tip of the glans form the external urethral meatus
The corpora cavernosa and corpus spongiosum develop from the phallic mesenchyme
The genital swellings fuse to form the scrotum
55
Development of the female external genitalia
As the paramesonpehric ducts reach the urogenital septum, 2 solid invaginations grow out of the pelvic part of the urogenital sinus - the sinovaginal bulbs, which proliferate and form the solid vaginal plate.
Proliferation of the vaginal plate at the cranial end continues until it reaches the cervix.
The lower parts of the paramesonephric ducts are absorbed into the sinovaginal bulb, forming the vaginal fornices
Later, the vaginal plate acquires a lumen through breakdown of the central cells to form the vagina
In the presence of oestrogen's the genital tubercle elongates slightly to form the clitoris and the urethral folds develop into the labia minor.
Genital swellings enlarge to form the labia major and the urogenital groove becomes the vestibule.
56
Descent of the gonads - male
Urogenital mesentery attaches the testis and mesonephros to the posterior abdominal wall
As the mesonephros degenerates, the attachment serves as mesentery for the gonad. Caudally, it becomes the caudal genital ligament or Gubernaculum. As the foetus grows the testis passes through the inguinal canal and the lower part of the Gubernaculum form the scrotal floor and going the intrabdominal part.
The testis reaches the inguinal canal by w6, migrates through the canal by w28 and reaches the scrotum by w33.
As it descends, the covering peritoneum is called the processes vaginalis, which becomes the tunica vaginalis.
57
Embryological gonadal ligament derivatives in the female
The cranial genital ligament forms the suspensory ligament of the ovary
The caudal genital ligament (Gubernaculum remnant) forms the ligament of the ovary proper and the round ligament.
58
When does the brain and spinal cord development begin
week 3
59
Time frames of neural development: NT, primary brain vesicles, secondary brain vesicles, brain bends, spinal cord spanning vertebral canal
NT develops from week 3, completed by week 4
3 primary brain vesicles and primary bend during week 4
5 secondary brain vesicles and 4 ventricles visible from week 6
Bends start on day 40
spinal cord spans vertebral canal by 8th week
60
Function of brain bending
To fit developing brain into the cranial cavity
61
Resultant conditions if neuropores do not close and time of closure
Cranial NP - anencephaly - day 25
| Caudal NP - spina bifida - day 27
62
Describe the cranial neural tube dilatation and differentation
3 primary dilatations: prosencephalon, mesencephalon and rhombencephalon
Then
prosencephalon further differentiates into telencephalon and diencephalon
mesencephalon remains the same
Rhombencephalon differentiates into metencephalon and myencephalon
63
Primitive brain structures and their derivatives plus ventricular spaces
Telencephalon - cerebral cortices and basal nuclei; lateral ventricle
Diencephalon - thalamus, hypothalamus and epithalamic; 3rd ventricle
Mesencephalon - cerebral peduncle, tactile and segmental parts of the midbrain; aqueduct of Sylvius
Metencephalon - cerebellum and pons
Myencephalon - medulla
4th ventricle housed by both metencephalon and myencephalon
64
Development of the mantle layer and its divisions
Neural tube cavity is lined with neuroepithelial cells which proliferate and form the mantle layer, which becomes the grey matter of the spinal cord.
Layers of the mantle layer are the alar plate and basal plate
65
Structure of the primitive spinal cord
Central neural tube with neuroepithelial lining neural tube cavity
Dorsally, bilateral alar plates form, which give rise to sensory fibres
Ventrally, bilateral basal plates, give rise to motor fibres
In between, sulcus limitans forms
In the thoracic and cervical area, the intermediate plate starts developing which fits rise to the intermediolateral horn, which is sympathetic in nature.
Roof plate and floor plate develop, and the area between the borders of the spinal cord and the grey matter is the marginal zone/layer. These 3 components are white matter structures; no grey matter is found here.
66
Development of motor fibres
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motor cell differentiates into ,multipolar cells, with the axon extending from the basal plate out the anterior horn.
Myelinated fibres (by Scwann cells)
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67
Development of sensory fibres
1st order sensory neurons arise from neural crest cells peripherally; not from the neural tube/ spinal cord.
The 1st order neutrons run in at the dorsal horn and either branch off and ascend or cross over
The sensory nerve neurons in the alar plate form 2nd order neurons
68
Development of intermediate horn and sympathetic fibres
Intermediate horn makes preganglionic sympathetic fibres, which are also myelinated by Schwann cells
Exit via the ventral horn into the white ramus and to the sympathetic ganglion
Post ganglionic fibre in the sympathetic ganglion also arise from NCC, and are unmyelinated, therefore run through the grey ramus
69
4 NCC origins
Cranial
Truncal
Vagal and sacral
Cardiac
70
Derivatives of cranial NCCs
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Cranial NCCs enter the pharyngeal pouches/ arches and contribute to:
Thymus
Middle ear bones
Jaw bones
Odontoblasts of the teeth
```
71
Derivatives of the trunk NCC
Contribute to:
Melanocytes
Dorsal root ganglion (1st order sensory cells)
Sympathetic ganglion (grey ramus communicans)
Adrenal medulla
Nerves around the aorta
72
Derivatives of the sacral/ vagal NCCs
Contribute to:
Enteric nervous system ganglia
Parasympathetic ganglions
73
Derivatives of cardiac NCCs
Contribute to:
Melanocytes
Cartilage, CT and neurons of some pharyngeal arches
Musculo-CT of some of the large arteries
Part of septum dividing the pulmonary vessels from aorta
Semilunar valves
74
Development of the parasympathetic system
Preganglionic fibres come from the cranial region, therefore develop from the neural tube
Postganglionic come from the NCCs
75
Role of retinoid acid in the formation of blood vessels
Increased concentration of retinoic acid causes venous formation
Decreased concentration of retinoic acid causes arterial formation
76
Vasculogenesis vs angiogenesis
Vasculogenesis is the development of new blood vessels where none existed prior.
Angiogenesis is the development of new blood vessels by branching off from existing vessels
77
How does the cardiogenic area form
Cardiac progenitor cells lie in the epiblast, immediately lateral to the primitive streak
These cells migrate through the streak and proceed toward the cranium, coming to lie in the splanchnic layer of the lateral plate mesoderm
They are then induced by cardiac myoblasts
Blood islands appear in this mesoderm and undergo vasculogenesis. With time, these islands unite and form a horse-show shaped endothelial-lined tube surrounded by myoblasts, which becomes your cariogenic field or cariogenic area
78
Which factor induces vasculogenesis and where is it secreted from
Vascular-endothelial growth factor, secreted by the endoderm
79
When is the intra-embryonic coelom formed
day 15
80
What lies above the cardiogenic area
The septum transversum, which becomes the diaphragm
81
Results of cranial folding in relation to the CVS
Heart area moves to the thoracic region
Septum transversum moves below the heart
Mouth area moves above the heart
Pericardial sac region moves in front of the heart
Foregut gorms and the heart hangs with the foregut through the mesochordium
82
Which two tissue types form the heart
Splanchnopleuric mesoderm
| Neural crest cells
83
What forms the pericardial cavity
Lateral folding brings the intra-embryonic coelom together, and at the level of the heart, this coelom fuses creating the pericardial cavity.
Cells from the sinus venosis migrate to the intra-embryonic coelom at the cardiac level and form the epicardium and visceral pericardium
84
Results of lateral folding in relation to the CVS
2 heart tubes come together in the middle of the chest cavity, which later fuse
Pericardial sacs come together, which later fuse to become one sac
Heart tubes come connected to the foregut through the mesocardium
85
By what point is lateral and longitudinal folding complete
End of week 4
86
4 sources forming the heart
Cardiac progenitor cells from the ectoderm which migrate to the cardiogenic area form 2 streaks, which form the primitive heart tubes or primary heart forming regions, destined to become the endocardium
Pharyngeal area mesoderm contributes further cells which allow the heart tubes to develop myocardium
Myocardium starts secreting hyaluronic acid and other CT-like substances to form the cardiac jelly (becomes CT of the heart)
Pericardial cavity surrounds the heart and makes the epicardium. Lower part of the heart tube, the sinus venosis, contributes cells that sit outside the myocardium to form the visceral pericardium
87
Describe the cardiac dilatations
```
From cranial to caudal:
Truncus arteriosus
Bulbus cordis
Primitive ventricle
Primitive artium
Sinus venosus
```
88
Describe the folding of the heart
DUe to differential growth of the bulbus cords and ventricle, the heart folds onto itself, known as the cardiac or bulboventricular loop. Occurs at day 28. Also known as dextrolooping.
Results in TA and BC brought forward, PV and PA brought back.
Later looping causes the PA and SV to move cranially.
89
How is the atrioventricular canal partitioned
In week 4, endocardial cushions form on the dorsal and ventral walls of the AV canal
These approach each other and fuse forming the AV septum, which divides the AV canal into right and left canals.
This effectively separates the primordial atrium from ventricle
90
How is the atrium partitioned
Occurs from D27- D37
Thin crescent-shaped membrane (septum primum) grows from the roof of the primordial atrium toward the endocardial cushions, which partially divides the primordial atrium into R and L halves
The septum has a large opening, the foramen/ostium premium, which acts as a shunt allowing blood flow from right to left
As the septum primum fuses to the endocardial cushion, the foramen decreases until it disappears. However, simultaneously, perforations appear in the central part of the septum which fuse to form the foramen/ostium secundum
A thick crescentic muscular fold grows from the ventrocranial wall of R atrium adjacent to the septum primum. As it grows, the top half of the septum disappears. A flap -like calve is formed between the atria; THE FORAMEN OVALE. Which allows unidirectional blood flow
91
Partitioning of the sinus venosus
Occurs between w4-10
Initially, the SV opens into the centre of the dorsal wall of the primordial atrium, and R and L horns are equal in size
Between w4-5, L-->R shunts in the venous system result in R sinus horn progressively increasing.
L horn loses importance as R umbilical vein and L vitclline vein obliterate (w5). and L common cardinal vein also obliterates in W10; therefore only remaining drainage to L horn is oblique vein of the L atrium and coronary sinus.
Enlarged R horn receives all blood from cranial and caudal regions through primitive IVC and SVC.
R horn becomes incorporated into body of RA, forming the smooth walled part (the sinus venarum)
The demarcation between the smooth section, and the rough trabeculated section is seen internally by a vertical ridge (crest terminalis) and externally by a shallow groove (sulcus terminalis)
L atrial wall formed by incorporation of primordial pulm vein; therefore is smooth
92
Partitioning of the ventricles
Muscular septum arises in floor of primordial ventricle near its apec
Interventricular septum contains a crescent-shaped foramen, which permits communication until week 7
Thereafter, the foramen closes, and the pulmonary trunk communicates with the RV and the aorta with the LV
93
Partitioning of the bulbus cordis and truncus arteriosus
In week 5, mesenchymal cells in wall of BC proliferate to form bulbar ridges, which are continuous with truncul ridges forming in TA. These ridges (together- conotruncal ridges) undergo 180deg spiralling --> spiral aorticopulmonary septum once the ridges fuse.
Partitioning divides the BC and TA into 2 distinct channels; the ascending aorta and the pulmonary trunk;
94
Process of formation of the AV valves
After fusion of the AV endocardial cushions, each AV orifice is surrounded by proliferations of mesenchymal tissue
The bloodstream hollows out and thins these proliferations on the ventricular surface --> valves
They remain attached to ventricular walls by muscular cords. Over time, the muscular tissue degenerates, and is replaced by dense CT
Calyces then consist of CT covered by endocardium
Connected to thick trabeculae in the vent wall (papillaey muscles) by the chordae tendinae
95
Formation of the semilunar valves
As the TA is being partitioned, 3 swellings of subendocardial tissue develop around the aortic and pulmonary trunk orifices
These become hollowed out and reshaped into 3 thin-walled cusps --> semilunar valves
96
Formation of the conducting system
Functional pacemaker is assumed by the atrium, and later by the sinus venous
As SV is incorporated into RA, pacemaker tissue comes to lie near the opening of the SVC --> SA node (w5)
AV node and bundle of His derived from cells in the L wall of the SV and AV canal
Fibres from AV bundle pass from atria to ventricles, split into L+R and distribute through the ventricular myocardium
Innervation occurs later
97
2 layers of the endometrial storm involved in placentation
Bottom layer: stratum basalis, which increases in size due to increased glycogen. Becomes termed the decidua basalis.
Decidua basalis replicates and proliferates to form apical layer - stratum/ decidua functionalis
98
Process of attachment of the blastocyst to the endometrium
Trophoblastic cells have microvilli which enable attachment to pinopod receptors on the endometrial surface, which allows a loose attachment
Trophoblastic cells also express interns, which bind selections on endometrial surface --> tight attachment
Once tight attachment occurs, the trophoblastic cells then release chemokines which stabilise the connection and allow for endometrial invasion
99
Steps of trophoblast differentiation
Starts once trophoblast binds endometrium
Differentiates into cut-trophoblast and syncytiotrophoblast
CYtotrophoblast surrounds embryo blast and is well defined and proliferative
Outer laters of CT membranes break down, resulting in protoplasmic pool with free-floating nuclei --> syncytiotrophoblast.
100
Process of endometrial invasion/ implantation
ST releases hydrolytic enzymes --> breakdown of surrounding stroll tissue allowing deeper invasion
101
How is the corpus luteum maintained
ST secretes HCG which maintains the corpus luteum, which continues progesterone production and presents endometrial shedding (for 10-12 weeks) until placenta takes over
102
How do the intervillous spaces form
Lacunae form between the finger-like projections of the ST (D9)
BVs of endometrium lie within the lacunae
ST releases hydrolytic enzymes that breakdown BVs allowing leakage of blood into lacunae
103
When and where does the extra-embryonic mesoderm form and what is it called
forms between embryo blast and cytotrophoblast on D12 --> forms the chorion
104
How are primary chorionic villi formed
On D14, the extra-embryonic mesoderm. breaks down and forms septa, which fuse to form the extra-embryonic coelom. The outer layer around the cytotrophoblast is somatopleuric extra-embryonic mesoderm
CT proliferates and moves toward the ST villi, (=1 villi), then breaks through the ST and moves around, outwards and surrounds the intervillous spaces forming the outer trophoblastic shell
105
What is the difference between 1, 2 and 3 villi
1: CT cells
2: somatopleuric extra-embryonic mesoderm starts invading into the core of the primary villi
3: the mesoderm starts undergoing vasculogenesis, forming a capillary and vascular network that runs from the chorionic plate up into the villi --> 3 villi
106
Blood flow from Umbilical artery to cotyledon
umbilical artery runs through the umbilical cord, spreads out along the chorionic plate, forming chorionic arteries, then moves into the villi, forming cotyledon arteries
107
When does the CT component of villi breakdown and why
at week 20, to make a thinner membrane to allow for more efficient exchange
108
How are cotyledons formed
Placental septa form around 4-5th month. Decidual tissue gives rise to septa which form placental fissures. Divides the villi into segments which are known as cotyledons. 15-20 cotyledons form in the placenta.
109
Which two parts make up the placenta
decidua basalis and the chorionic frondosum (the tertiary villi structure which protrude and interact with the decidual lining)
110
What are the other two parts of the decidual membranes and their outcome
Decidua capsularis - lines the chorionic villi
Decidua parietalis - part of the membrane that has no fetal involvement of attachment. As the foetus grows, it obliterates the uterine cavity and the space between the two membranes, which then fuse
111
What is the chorionic laeve
The part of the chorion on the anembryonic pole which has no extensive villi system
112
Metabolic functions of the placenta
Gas exchange
Nutrient delivery
Waste removal
113
Hormonal functions of the placenta
Estrogens and progesterons :
- maintain endometrial thickness
- increase vasculature and secretions to enable fetal nutrition
- create cervical mucus plug
- role in fetal development
Thyroid hormone: development of CNS
Human placental lactogen:
- decrease fetal insulin sensitivity and alters fetal insulin release
- promotes maternal lipolysis releasing fatty acids to fetus
- promotes maternal gluconeogenesis for fetal glucose demands
- causes maternal insulin resistance to allow for higher concentration of circulating glucose
Relaxin : relaxes ligaments, especially pubic symphysis
Corticotropin releasing hormone: NB for lung development and surfactant production
