Weeks 4-8 Flashcards
What is the 4th-8th week period called
Organogenetic period
Mesoderm vs mesenchyme
Mesoderm: derived from the mesodermal germ layer
Mesenchyme: any loose connective tissue; may be derived from the mesoderm, neural crest, etc
Mesoderm cells to blood vessels signalling pathway
- FGF2 binds to FGFR creating mesoderm cells
- VEGF binds to VEGF-R2 and creates hemangioblasts around mesoderm cells.
- VEGF binds to VEGF-R1 to create tube formation.
Hemangioblasts can become…..
- Angioblasts (outside cells) which create:
- endothelial tube - hematopoetic stem cells (HSC) which create:
- myloid -RBC,WBC, macrophages
- Lymphoid-B cells abd T cells
Mesoderm cells can differentiate into these types
Hermangioblasts, angioblasts and HSC
Vasculogenesis
Blood vessel formstion begins in the extraembryonic mesoderm of the yolk sac, connecting stalk and chorion and eithin the embryo a few days later.
- mesoderm cells differentiate to angioblasts
- aggregate to form blood islands which become endothelial lined channels
- new vessles sprout (angiogenesis)
- adjacent mesoderm becomes muscle
- inital blood crlls form from endothelial cells
Embryonic period
Important period of development as the beginning of all organ systems appear during this time
- each organ system has a particular critical period
- by the end of the embryonic period, the embryo appears human
Folding of the embryo
- during the 4th week, the shape of the emrbyo changes
- flat trilainar embryo becomes cylindrical in shape
- folding results from the rapid growth of the neural tube and amniotic cavity but not yolk sac
- folding occurs in the longitudinal and transverse planes
Folding in the longitudinal plane
- redults in the head and tail ends of the embryo swinging ventrally
- the septum transversum, primitive heart and orophsryngeal membrane turn onto ventral surface
- somatopleure becomes face and chest wall
- part of yolk sac is incorporated as the foregut
- the tail fold occurs after head fold
- during the tail fold, part of the yolk sac is incorporated as the hindgut
- the connecting stalk moves to the ventral surface
Folding in the transverse plane
- the sides of the embryonic disc roll ventrally forming a cylindrial embryo
- part of the yolk sac is incorporated as the midgut
- the splanchnopleure forms the wall of the gut
- the somatopleure forms the body wall
Major events in order so far….. (Up to week 4)
-cleavage, implantation,gastrulation, neurluation, folding of the embryo
Neural tube defects
(Due to lack of closure of different reguons of thr neural tube)
-If anterior neuropore doesnt close…
-if posterior neuropore doesnt close…
Anencephaly (missing parts of brain, skull)
Slina Bifida (Incomplete closing of the spine)
Waht can prevent NTD’s during pregnancy?
Folic Acid
- all women of childbearing age should take folic acid
- 400 ug/day
- 4000 ug/day when trying to concieve if history of NTD’s in family
- prevents 70% of NTD’s
- must be started 3 months prior to conception so all women of childbearing age should have 400ug/day
Control of development
During embryonic development, certain embryonic tissues act as inductors and influence the development of adjacent tissues
- some signal passes from the inducing tissue to the induced tissue such as
- a diffusible molecule
- the extracellular matrix
- physical contact
Examples of control of development
- notochord and neural tube: notochord secretes FGF and shh to form neural plate which then forms neual groove and then neural tube.
- lens formation: Les vesicles induced to form by optic cup location
The placenta is a _____ organ
Fetomaternal
- fetal component is the villus chorion
- maternal component is the decidua basalis of the endometrium
Maternal and fetal blood flow very close together bht normally ______
Do not mix
Blood flows from the fetus to the placenta via ______ (rest of path)
2 umbilical arteries—->capillaries of the chorionic villi—->exchange occurs——>veins—->unbilical vein—-> fetus
Maternal blood goes to the olacenta via _______ endometrial arteries though _______
80-100
Intervillous spaces——> endometrial veins
Exchange occus across the placental membrane in these ways
Diffusion
Facilitated diffusion
Active transport
Pinocytosis
Hemolytic disease of the newborn (Erythroblastosis fetalis)
- usually results from Rh incompatibility between an Rh- mol and an Rh+ fetus.
- Rh- person does not have Rh antigen on RBC nor anti-Rh antiobdies in plasms
- if Rh- person is exposed to Rh+ blood, they will produce anti-Rh antibodies
- anti-Rh antibodies corss the placenta and lyse fetus’s Rh+ RBC
Decidua refers to
The gravid endometirum, the functional layer of the endometrium in a pregnant women that separates from the remainder of the uterus after childbirth (second birth)
3 main regions of the decidua
Decidua basalis-part of the decidua deep to the conceptus that forms the maternal part of the placenta
Decidua capsularis-the superficial part of the decidua overlying the conceptus
Decidus parietalis- all the remaining parts of the decidua
Twins and fetal membranes
Twins that originate from 2 zygotes are dizygotic (DZ) twins or fraternal twins
Twins that originate from ine zygote are Monozygotic (MZ) twins or identical twins
Amniotic cavity expands and occupies all ____
Chorionic space and fuses with the chorionic cavity including the membrane to create a amniochorionic cavity
How many umbilical arteries and veins are there?
2 arteries (oxygenated blood) 1 vein
When are each chorionic villi types present?
Primary- end of second week (begin to branch)
Secondary-cover the surface of chorionic sac
Tertiary-blood vessels visbile in them
Digestive tract includes
Accessory digestive organs include
Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus
Liver, gall bladder and pancreas
Primitive gut is a ____consisting of the
Straight tube
Foregut
Midgut
Hindgut
Throughout the digestive tract…
Endoderm gives rise to
Splanchnic mesoderm gives rise to
Epithelium and glands
Connective tissue and muscle of the wall
Umbilical arteries bring?
Deoxygenated blood
The midgut remains attached to the yolk saf via the
Vitelline Duct or omphaloenteric duct
Derivities of the foregut
Pharynx to half of duodenum
Esophagus-develops causal to pharynx
The esophagus is separated from the trachea by the
Tracheoesdophageal septum
Esophageal artresia
1/3000 births
Results from improper division of the trachea and espohagus (usally occurs with a tracheoesophageal fistula (opening) or failure of the espohagus to recanalize in 8th week
How are gut regions established?
From pharynx to colin there is an increasing gradient of retinoic acid! SOX2-specifies esophagus and stomach PDX1-specifies Doudenum CDXC-Small intestine CDXA-Large intestine
Morphogens
Dorsal mesentery suspends…
Ventral mesentery forms from the____ and develops into the
The gut tube from the posterior body wall
Septum transversum
Lesser omentum
3 blood vesseld in developing gut
Celiac arterial trunk - supplies foregut
Superior mesenteric artery -supplies midgut
Inferior mesenteric artery- supplies hindgut
Mesentery
Connective tissue holding in place structures
Dorsal mesogastrium
Holds stomach to dorsal body wall
Which region of the stomach grows faster?
Dorsal region
Liver grows rapidlt and tskes up most of the peritonial space, and splits the _________ into 2 spaces______
Ventral mesentery
- Lesser omentum-between liver and stomach
- Faciform ligament-between liver and ventral wall
Lesser omentum has two ligaments____
Caudal part-Heptoduoderal ligament
Crabial part-Hepatogastric ligament
Foregut endoderm proliferates to form____
Buds for lungs, liver, gall bladder and pancreas
Where does the stomach develop from
The distal portion of the foregut
- in 4th week a dilation occurs
- enlarges and lies ventrally
- dorsal border grows faster than ventral
- dorsal border becomes the greater curvature of the stomach
- ventral border becomes the lesser curvature of the stomach
Congrnital pyloric stenosis
1/150 Males
1/750 Females
- abdnormal thickening of the pyloric sphincter
- restricts movement of materials from stomach into the small intestine
- stomach becomes distended and projectile vomitting occurs
Stenosis=partial blockage
Stomach rotation
- Stomach rotates 90° clockwise around a longitudinal axis
- ventral border moves to the right and doesal border to the left
- before rotation, the cranial and causal ends of the stomach were in midline.
- after rotation, the cranial end is on the left and the caudal end is on the right and the long adis is almost horizontal
What happens to the mesentery during stomach rotation?
Messntsry also rotates 90°, mesentery has spaces that fuse and form a cavity
Now it is called the greater omentum (apron that hangs over) instead of dorsal mesogastrium
Omental Bursa
- isolated clefts develop in the mesenchyme forming he thick dorsal mesogastrium.
- the clefts fuse and form a single cavity=omental bursa or lesser perioneal sac
- as stomach enlarges, the onental bursa expands and acquires an inferior reccess of the omental bursa between the lsyer of the elongated dorsal mesogastrium and is called greater omentum.
- membrane overhangs the developing small intestine
Alimentary system
Digestive tract frim mouth to anus
When does the prinordial gut form
Fourth week
Espohageal stenosis
Incomplete canalization of the espohagus during the 8th week
-no apoptosis
Development of duodenum begins when and where
Early in the 4th week, and develops from the caudal or distal part of the foregut
-grows rapidly forming a c-shaped loop that projects ventrally
During the 5th and 6th weeks, the lumen of the duodenum becomes progressively _____-
Smaller and is temporaily obliterated because of the proliferation of its epithelial cells
Normal vacuolation occurs as the epithelial cells degenerate, as a result, the dupdenum normally becomes recanalized by the end of the embruonic period
Duodenal stenosis
Partial occlusion of the duodenal lumen
- usually results from incomplete recanalization of the duodenum resutlint from defective vacuolization
- bile duct is connected to duodenum therefore the stomachs contents usually contain bile are often vomited
similar symptoms to pyloric stenosis
Can also be determined by ultrasound
Duodenal artresia
Fully blocked duodenum
Recanalization fails to occur
Occurs almost always at the junction between the bile and pancreatic ducts
Amnitoic fluid builds up as it can not pass through
Liver and gull baldder development begins when
4th week as an endodermal outgrowth from the foregut
- liver bud or hepatic diverticulum
- bud projects into splanchnic mesoderm and divides into cranial and caudal parts
What layer gives rise to the heptaocytes
Endoderm
Hapato=liver
Cytes=cells
Cell types like hemopoietuc tissue, fibrous tissue and kupffer cells are derived from?
Mesoderm
The liver grows so rapidly that by week 8 the liver makes up what percent of the fetal body weight?
10%
When does hematopoiesis begin in the liver? And when does bile formation occur?
Week 6 -hematopoietic cells migrating from the dorsal aorta to the liver
Week 12
Liver primordium is the ____ part
Gall bladder and cystic duct are the ____ part of the diverticulum
Larger, causal
Smaller, caudal
Liver bud outgrowth is induced by….?
- secreted factors from cardiac mesoderm (FGF’s) and septum transversum (BMP’s)
- initially, all of foregut can form liver, but is inhibited everywhere but at the site of liver bud outgrowth by factors secreted by surrounding tissue (noncardiac mesoderm, ectoderm, and notochord)
- fibroblast growth factors (FGF), secreted by cardiac mesoderm, inhibit the inhibitors and stimulate bud growth.
- BMP’s enhance the action of FGF’s
- later hepatocyte-specific genes regulate liver cell differentiation.
The pancreas develops where and from what?
Endodermal cells form dorsal and ventral pancreatic buds at caudal end of foregut. Week 4
As duodenum rotates, the vental bud is carried dorsally to lie posterior to the dorsal bud
By week 5 which pancreatic bud is bigger
Dorsal
During week 6 the pancreas is rotated 90° causing
2 buds to become closer and fuse into one pancreas and 1 drainage site
Derivatives of the midgut
Include the small intestine (part of duodenum, jenunum, and ileum), proximal protions of the large intestine (caecum and appendix, ascending colon and 2/3 of the transverse colon)
As the midgut elongates, it forms a U-shaped loop that projects through the ______ into the _____
Umbilical cord
Extraembryonic coelom
Midgut undergoes _______ in the 6th week because there isnt enough room into the abdominal cavity (large liver and 2 sets of kidneys)
Physiological herniation
Midgut enters yolk sac
Midgut loop has a _____ and _____ limb
Cranial- grows rapidly and gives rise to loops of the small intestine
Caudal-gives rise to parts of the large intestine
During herniation, loop rotates 90°________ and causes…
Counterclockwise around the axis of the superior mesenteric artery
-cranial limb to the right and caudal limb to the left
Midgut returns to the abdomen (hernia reduced) in the ____ week
10th
-enlargement of tthe abdominal cavity, and the relative decrease in the size of the liver and kidneyes are contributing factors in intestinal return
The midgut goes through how many rotations?
3-90° counterclockwise rotations
After the rotations of the midgut are complete, the intestines become fixed in position by____
Fusion of the mesenteries with the abdominal wall
Congenital omphocele
- midgut herbia is not reduced and intestines remain in proximal part of umbilical cord (1/5000)
- can be born woth intestines put of body if ruptured (gastroschisis)
Ileal diverticulum
Connection between midgut and yolk sac remains to birth (2-4%)
Derivatives of the hindgut
Distal portion of transverse colon, descending colon, sigmoid colon, rectum and superior anal canal
Cloaca
- Is the expanded terminal part of the hindgut
- is an endoderm-lined chamber that is in contact with the surface ectoderm at the clocal membrane
- membrane is composed of endoderm of the cloaca and ectoderm of the proctodeum or anal pit
Partitioning of the Cloaca
- by 7th week, urorectal septum (mesenchyme) divides cloaca into vental urogenital sinus and dorsal GI part.
- anal membrane ruptures at the end of the 8th week which brings digestive tract into communication with amniotic cavity
The superior two thirds of the adult anal canal are derived from the
Hindgut
By end of week 8, the fetus can now take in amniotic fluid and
Dispose of it
The proctodeum and hingut connect where
Pectinate line
Proctodeum=ectoderm
Hibgut=endoderm
Communiction between germ layers since there isnt room for mesoderm
Annular pancreas
Ring like annular part of the pancreas consists of a thin, flat band of the duodenum which may cause obstruction to the dupdenum
