Development of a baby Flashcards
Week 1
- Fertilisation - zygote
- Cleavage - 2cell, 4 cell, 8 cell, 16 cell (morula)
Blastocyst
It’s the early stage of an embryo - 32 cells onwards
- Contains BLASTOCYST CAVITY ( BLASTOCOEL). Some of the cells move towards the outer edge to form the outer cell mass and some move group together to form the inner cell mass.
Trophoblast (the outer cell mass)
Trophoblast contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta.
Embryoblast (inner cell mass)
Responsible for the formation of the embryo itself.
Second Week
During the second week the trophoblast and embryoblast divide into increasingly specialised cells.
Trophoblast (outer cell mass):
-syncytiotrophoblast (multinucleated)
-cytotrophoblast (mononucleated)
Embryoblast (inner cell mass):
- epiblast
-hypoblast
(bilaminar disc)
- The amniotic cavity forms within epiblast.
Decidua Basalis
The blastocyst implants on the surface of the endometrial cell (into the endometrium of the uterus). The part where it implants is called the decidua basalis
- During this process the syncytiotrophoblast becomes continuous with uterus - such that the maternal blood vessels ( known as sinusoids) invade the spaces within the syncytiotrophoblast ( known as lacunae)
- At this point uteroplacental circulation has begun and further embryonic development can occur
Gastrulation
In the 3rd week of embryonic development, the cells of the bilaminar disk (epiblast and hypoblast) undergo a highly specialised process called gastrulation.
Gastrulation involves the migration, invagination and differentiation of the epiblast. It is largely controlled and orchestrated by the primitive streak.
The epiblast cells migrate through the primitive groove and towards the hypoblast cells (invagination), they then replace these hypoblast cells. This layer is now called the Endoderm
More epiblast cells migrate and move down and laterally (ventrally) through the primitive groove and form the mesoderm layer.
We now call the top layer of epiblast cells the Ectoderm
Epiblast – Ectoderm
New layer – Mesoderm
Hypoblast – Endoderm ( they are at the bottom the –end)
Notochord
In the third week of development, the notochord appears in the mesoderm
Notochord releases growth factors that:
1. stimulate mesoderm differentiation
2. neurulation.
Nervous System Development
- Begins in the 3rd week of development
- Derived from the ectoderm layer
- Thickening of the neural plate causes a depression to form in the middle of the neural plate = neural groove
Surface Ectoderm = Epidermis
Neural crest cells = peripheral nervous structures
Neural tube = CNS
The formation of neural tube is known as neurulation
Neural Crest Cell Derivatives
-melanocytes
-craniofacial cartilage
-bone
- smooth muscle
-peripheral and enteric neurons
- glia -Glial cells are a type of cell that provides physical and chemical support to neurons and maintain their environment
BRAIN and SPINAL CHORD
The top of the neural tube becomes the cranial/anterior neuropore = BRAIN
Neuropore closes on day 25
The bottom of the neural tube becomes the caudal/posterior neuropore = SPINAL CHORD
Neuropore closes on day 28
The hole within the neurotube is the neural canal. It will fill with Cerebrus spinal fluid.
Brain and Cerebellum
In the fifth week of development, swellings appear at the cranial end of the neural tube. Three primitive vesicles appear first, and subsequently these develop into five secondary vesicles.
PRIMARY VESICLES:
Prosencephalon
Mesencephalon
Rhombencephalon
SECONDARY VESCILES:
Telencephalon,
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Spinal chord
Cells on the dorsal side form the ALAR plate, which subsequently becomes the dorsal horn (posterior) = the location of the sensory somatic and visceral input
Cells at the ventral end form the BASAL plate, which then becomes the ventral horn (anterior) = location of the motor function output.
Cervical nerves – C1- C8
Thoracic nerves – T1- T12
Lumbar nerves – L1-L5
Sacral nerves – S1- S5
Cardiovascular System Development
During lateral folding the two heart tubes fuse into one forming a single primitive heart tube.
Cephocaudal folding ( head to tail) further positions the heart and causes it to bulge into the future pericardial cavity.
Primitive heart tube:
Aortic roots (Arterial poles)
Truncus arteriosus
Bulbus cordis
Ventricle
Atrium
Sinus venosus (Venous poles)
Blood flow in foetus
The heart tube continues to elongate, and begins looping at around day 23 of development.
The SINUS VENOSUS is responsible for the inflow of blood to the primitive heart, and empties into the primitive atrium. It receives venous blood from the right and left sinus horns.
After looping:
Oxygen and nutrients from the mother’s blood are transferred across the placenta to the fetus through the umbilical cord.
This enriched blood flows through the umbilical vein toward the baby’s liver. There it moves through a shunt called the ductus venosus.
This allows some of the blood to go to the liver. But most of this highly oxygenated blood flows to a large vessel called the inferior vena cava and then into the right atrium of the heart.
When oxygenated blood from the mother enters into the right atrium. Most of the blood flows across to the left atrium through a shunt called the foramen ovale.
From the left atrium, blood moves down into the left ventricle. It’s then pumped around the body.
Blood returning to the heart from the fetal body contains carbon dioxide and waste products as it enters the superior vena cava -> right atrium -> right ventricle -> bypasses the lungs and flows through the DUCTUS ATERIOSUS (AIR) into the DESCENDING AORTA, which connects to the umbilical arteries. Blood flows back into the placenta.
There the carbon dioxide and waste products are released into the mother’s circulatory system. Oxygen and nutrients from the mother’s blood are transferred across the placenta. Then the cycle starts again.
At birth, major changes take place. With the first breaths of air, the lungs start to expand, and the ductus arteriosus and the foramen ovale both close over the first minutes and days of life.
Foramen Ovale
- The septum primium and secundum form the foramen ovale
- Septum primum is flexible and will open into the left atrium when blood from the right atrium pushes through the foramen ovale
- Foramen ovale = Fossa ovalis
Embryonic Heart Derivatives
Truncus Arteriosus:
- Aorta
- Pulmonary Trunk
Bulbus Cordis:
- Right Ventricle ( trabeculated part)
- Ventricle outflow tracts
Primitive Ventricle:
- Left Ventricle ( trabeculated)
Primitive Atrium:
- Right and Left Atrium ( trabeculated parts )
Sinus Venous:
- Right Atrium
Development of the RESPIRATORY SYSTEM
- Are formed from the ENDODERM layer - proximal part of the primitive gut tube - respiratory diverticulum
- The diverticulum bifurcates into two buds, which become the left and right primary bronchi. The primary bronchi then proliferate to give rise to secondary and tertiary bronchi.
-Initially, the respiratory diverticulum is continuous with the foregut; but this is not functionally suitable. The formation of a longitudinal ridge known as the tracheoesophageal septum rectifies this to make the two structures compatible with life.
- Actual LUNGS = ENDODERM
- Pleura = SPLANCHNIC MESODERM
TRACHEOESOPHAGEAL FISTULA
-Initially, the respiratory diverticulum is continuous with the foregut; but this is not functionally suitable. The formation of a longitudinal ridge known as the tracheoesophageal septum rectifies this to make the two structures compatible with life.
- These defects mean the baby won’t be able to swallow safely, if at all.
They could also develop life-threatening problems such as choking and pneumonia if not treated quickly, so surgery will usually be carried out within a few days of birth.
Lung Maturation Period
There are 5 stages in the lung maturation period:
-Embryonic stage (3-6 weeks)
-Pseudo glandular stage (5-16weeks)
- Canalicular stage (16-24 weeks)
- Terminal sac (24 weeks until birth)
- Alveolar stage (36 weeks - 8years)
Embryonic stage (3-6 weeks):
- respiratory diverticulum appears on foregut
- 4th week - 6th week: primary -> secondary -> tertiary bronchial buds
Pseudo glandular stage (5-16 weeks):
- extensive morphogenesis - bronchial tree
- splanchnopleuric mesoderm begins to differentiate to give rise to the intrapulmonary arteries
- As the respiratory bronchioles have not developed yet, infants born at this stage will not be able to facilitate gas exchange
Canalicular stage (16-24 weeks):
- Form respiratory bronchioles
- Pneumocytes produce pulmonary surfactant (type 2)
- At this stage, some respiration is possible due to the formation of the gas exchanging portion of the lungs.
However, they often do not survive due to the lack of surface area for gas exchange and limited production of pulmonary surfactant by type II pneumocytes.
Saccular stage (24 weeks until birth):
- the gas-exchange surface area of the lungs significantly expands.
- Saccules form. Thin walled terminal sacs.
-Capillaries invade the thin walls of the sacculi to form the blood air barrier.
Alveolar Stage (36 weeks – 8 years):
-Until the third year of life, enlargement of lungs is a consequence of the increasing number of alveoli; after this point, both the number and size of alveoli increases until the mature lungs form at around 8 years of age.
DUCTUS ARTERIOSUS
- Is how we bypass the lungs
Development of the GASTROINTESTINAL SYSTEM: FOREGUT
- Derived from the ENDODERM layer
FOREGUT:
The foregut derivatives include: oesophagus, stomach, proximal duodenum.
Also: liver, gallbladder, pancreas
Liver – begins as the liver diverticulum which becomes the liver bud.
Off of liver bud ventrally - you get the gall bladder and cystic duct.
You get ventral and dorsal pancreatic buds, eventually both ends fuse including their pancreatic ducts.
Key process: Stomach rotation: rotation and dilation and growth of accessory digestive organs.
Development of the GASTROINTESTINAL SYSTEM: MIDGUT
Midgut derivatives: distal duodenum, intestines (jujenum, ileum), ascending colon, proximal 2/3 of transverse colon.
Key processes that take place: physiological umbilical herniation.
Rapid growth of liver requires more space than in growing abdomen.
Occurs as a result of the bowel (particularly ileum) growing faster than the abdominal cavity during the early gestational period
Intestines temporarily leave the body – into umbilical region – during week 6.
Retract round week 10 - what also contributes to the position of colon.
Omphalocele
Is a birth defect of the abdominal (belly) wall. The infant’s intestines, liver, or other organs stick outside of the belly through the belly button. The organs are covered in a thin, nearly transparent sac that hardly ever is open or broken.
- Treated with surgery to put organs back into belly.
Development of the GASTROINTESTINAL SYSTEM: HINDGUT
Hindgut derivatives: distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, proximal part of anal canal.
URORECTAL SEPTUM: At the end of 6 weeks, the urorectal septum derived from the mesoderm completely separates the urogenital sinus from the anorectum. By the twelfth week, the anal canal, vaginal and urethral openings are established
Vitelline Duct
The vitelline duct (VD) is an embryonic structure providing communication from the yolk sac to the midgut during fetal development
Allantois
- providing for gas diffusion and removal of wastes
Yolk Sac
- Before the placenta forms and can take over, the yolk sac provides nutrition and gas exchange between the mother and the developing embryo
Development of the Urinary System
The renal system starts to form at about week 4 of gestation from a portion of the urogenital ridge called the nephrogenic cord.
The nephrogenic cord gives rise to three overlapping developmental stages:
- the pronephros
- the mesonephros
- the metanephros
Pronephros consists of an early and nonfunctional system, which regresses by week 4. Next is the mesonephros, which functions as a primitive excretory system in the embryo. Most tubules regress by week eight and are replaced by the metanephros.
Metanephros give rise to actual kidneys, which appear at around week five, and become mature enough to secrete urine around week ten.
Metanephros to kidneys
- intermediate mesoderm near the mesonephric duct differentiates into metanephric mesoderm ( aka the metanephric blastema) - secretes growth factors causing the ureteric bud to grow
- Mesonephric duct - sprouts the ureteric bud which grows and will secrete growth factors.
- The ureteric bud and metanephric mesoderm join and this is what makes up the metanephric kidney aka renal system (kidney, nephrons ….)
Ureteric stalk = ureter
The definitive kidney initially develops in the pelvic region before ascending into the abdomen.
Cloaca
The bladder and urethra of the urinary system are ultimately derived from the cloaca
In the 4th-7th weeks of development, the cloaca is divided into two parts by the uro-rectal septum:
- Urogenital Sinus
- Anal canal
Urogenital Sinus is divided into 3 parts:
- upper part = bladder
- middle/pelvic part = URETHRA and some of the reproductive tract in females and prostatic and membranous urethra in males.
- bottom/caudal part = female reproductive tract and spongy urethra in males
How is the urinary bladder drained in foetus
The urinary bladder is initially drained by the allantois. However, this is obliterated during fetal development and becomes a fibrous cord – the urachus.
A remnant of the urachus can be found in adults; the MEDIAN UMBILICAL LIGAMENT, which connects the apex of the bladder to the umbilicus.
Aminotic fluid made up of
In the early weeks of pregnancy, the amniotic fluid is mostly water that comes from your body. After about 20 weeks of pregnancy, your baby’s urine makes up most of the fluid
