Embryology

Embryology

1. Stem cells

   • unipotent
   • totipotent
     • known as omnipotent cells
     • form embryonic and extra embryonic cell types
   • pluripotent
     • embryonic stem cells
       • from the inner cell mass of blastocysts
     • gives rise to every cell type in the fully formed human body but not the placenta and umbilical cord
     • induced pluripotent stem cells
       • stem cells that have been engineered in a lab
         • converts tissue-specific stem cells into cells that behave like embryonic stem cells
         • stem cells are produced using viruses
   • multipotent
     • tissue-specific/adult/somatic stem cells
       • can generate different cell types for a specific tissue or organ in which they live
       • haematopoietic stem cells
       • mesenchymal stem cells
       • neural stem cells
       • skin and epithelial stem cells

2. Main stages of life before birth

   • weeks 9-38
     • foetal
   • weeks 2-8
     • embryonic
   • week 1
     • pre-implantation
   • critical developmental landmarks
     • day 0 - zygote formation by fertilisation
     • day 1 - cleavage begins (the zygote starts to divide)
       • cleavage is mitotic divisions of the fertilised oocyte
         • individual cells become smaller in size after each division but the overall size remains the same
           • allows for passage through the smallest parts of the uterine tube (isthmus)
             • 4 days after constant mitotic divisions, cells maximise contact with each other
               • a ball of 16-20 cells will be present with peripheral cells flattened against the zone pellucida
               • a cluster of cells is formed which is held together by tight junctions
                 • at this point the cluster enters the uterus and is called a morula
                   • the morula marks the end of cleavage and means mulberry
                   • a blastocyst then forms between days 5 and 7
                     • it has two identifiable different cell types and a fluid filled cavity called the blastocoel
                     • when the blastocyst forms the first signs of cellular differentiation are present
                       • the cells in the blastula communicate via gap and tight junctions
                       • inner mass cells
                       • trophoectoderm
                         • trophoblast
                       • fluid filled blastocyst cavity
                         • blastocoel
                       • as the embryo enters the uterine cavity, fluid enters via the zona pellucida into the spaces of the inner mass cells
                         • eventually there will be no space and the blastocyst needs to find a way to burst out of the zona pellucida
                           • blastocyst will start to run out of nutrients to implantation is needed
                           • the blastocyst "hatches" from the zona to facilitate implamtation
                             • implantation takes place between days 7-12
                             • in order to implant there is an interaction between the implanting embryo and the endometrium (the mucous membrane that lines the inside of the uterus)
                               • if the embryo implants anywhere apart from the uterine cavity, the pregnancy is called an ectopic pregnancy
                               • adplantation begins with the initial adhesion to the uterine epithelium where the blastocyst slows in motility and aligns its inner mass cells closest to the endometrium and then it stops
                               • trophoblast (trophectoderm) cells implant first
                                 • trophoblast extensions penetrate between the uterine epithelial cells and eventually the embryo is completely embedded in the uterine all as uterine epithelial cells grow over the implantation site
                                   • implantation is complete by day 9
                                   • the trophoblast then differentiates into cytotrophoblasts and syncytiotrophooblasts
                                     • the inner mass cells form the epiblast and hypoblast
                                       • the embryo is now a bilaminar embryo
     • week 1 - blastocyst stage and implantation
     • week 2 - bilaminar disc
       • amnion is continuous with the epiblast and provides protection for the developing embryo
         • the yolk sac is continuous with the hypoblast
           • it is important for nutrient transfer in weeks 2-3 but is disappears at around week 20
           • the chorion is the trophoblast and the extra-embryonic mesoderm that forms the foetal components of the placenta
             • disappears as the amnion expands
             • the syncitiotrophoblast secretes enzymes that breaks down the ECM and allows the passage of the blastocyst into the endometrial wall
               • these cells also generate spaces filled with maternal blood called lacunae
                 • human chorionic gonadotrophin is a placental hormone secreted by syncitiotrophoblasts
                   • it has a variety of different functions e.g. cytotrophoblast differentiation, angiogenesis of uterine vasculature, promotion of growth and differentiation of fetal organs and supports the endometrial lining which helps to maintain pregnancy
                   • hCG levels peak between weeks 8-10
     • week 3 - gastrulation
       • the process of cell division and migration resulting in the formation of the three germ layers
         • the bilaminar epiblast forms the trilaminar embryo
           • the embryo epiblast decides which of its ends will be the future head and future tail end of the embryo
           • the primitive streak starts to form
             • it is an area where cells start to proliferate and bunch up
               • cells move towards the primitive streak to cause invagination
                 • once cells have been invaginated, some displace the hypoblast creating the endoderm and others lie between the epiblast and the newly created endoderm to form the mesoderm
                   • the remaining cells in the epiblast then form the ectoderm
                     • at the cranial end of the embryo, the primitive streak expands to form the primitive node
                       • from the fourth week of development, the embryo undergoes a rapid development in size and shape
                         • the trilaminar disc undergoes the process of embryonic folding
                           • folding of the ectoderm in the medial plane results in the development of the cranial and caudal folds
                             • folding in the horizontal/coronal plane results in the development of the two lateral body folds
                               • folding in the endoderm causes the formation of the primitive gut tube
                                 • the primitive gut tube differentiated into the foregut, midgut and hindgut
                                   • the foregut is temporarily closed by the oropharyngeal memrbane which, at the end of the 4th week of development, ruptures to form the mouth
                                   • the midgut remains connected to the yolk sac until the 5th week of development due to embryonic folding narrowing the connection to the yolk sac to a stalk called the vitelline duct
                                   • the hindgut is temporarily closed by the cloacal membrane which, during the 7th week of development, ruptures to form the urogenital and anal openings
     • week 3-8 - organogenesis
     • week 4 - heart begins to beat and limb buds form
       • the heart forms from the mesoderm arounds days 18-19, it begins to develop at the head of the embryo in the cardiogenic area, following chemical signals from the endoderm, the cariogenic area begins to form two strands called the cariogenic cords -> endocardial tubes
     • week 8 - foetus begins to resemble a baby
     • week 10 - male or female expression of genitalia

3. Ooctye

   • a cell in an ovary which may undergo meiotic cell division to form an ovum

4. Germ layers

   • ectoderm
     • epidermis of skin
     • epithelial lining of mouth and anus
     • cornea and lens of eye
     • nervous system
     • sensory receptors in skin
     • adrenal medulla
       • inner part of the adrenal gland
     • tooth enamel
     • epithelial of pineal and pituitary glands
   • mesoderm
     • notochord
       • very important signalling centre
     • skeletal and muscular system
     • muscular layer of stomach and intestine
     • excretory system
     • circulatory and lymphatic system
     • reproductive system
       • except germ cells
     • dermis of skin
     • lining of body cavity
     • adrenal cortex
       • outer part of the adrenal gland
   • endoderm
     • epithelial lining of digestive tract
     • epithelial lining of respiratory system
     • lining of the urethra, urinary bladder and reproductive system
     • liver
     • pancreas
     • thymus
     • thyroid and parathyroid glands

5. After gastrulation

   • once the mesoderm has formed, it becomes divided into distinct regions
     • the mesoderm differentiates between days 17 and 21 and by day 21 the three mesoderm layers should have formed
     • not all muscles come from the mesoderm as the smooth muscle in the pupil, the mammary glands and sweat glands come from the ectoderm
     • the mesoderm thickens up at day 19 which causes it to push up on the ectoderm (which also thickens up) - a lot of muscle needs to be produced which is why the mesoderm thickens up
     • paraxial mesoderm
       • closest to the primitive streak
         • lies adjacent to the notochord and neural tube
       • formed from cells moving bilaterally and cranially from the primitive streak
       • forms somites
         • somites form alongside the developing neural tube in a craniocaudal sequence from day 20
           • somites always form in pairs, around 3 pairs a day, until the end of week 5
             • counting pairs can be used to estimate a baby's age
             • somitogenesis involves a mesenchymal to epithelial transition where building blocks become more epithelial in structure
               • notochord is the driving force as well as the lateral plate mesoderm
               • clock and wavefront activities regulate the formation of somites
                 • FGF, Wnt, Notch and targets of these families control and regulate somitogenesis
                   • genes/gene products tells the cells to switch between a permissive and non-permissive state in a constantly timed fashion - like a clock
                     • a wave of factors then sweeps along the length of the embryo and interacts with cells that are permissive (at the right time in the right area)
                       • example: the clock gene expression, notch, helped along by ant followed by a wave of FGF8 has an overlapping gradient that controls the formation of somites
                       • by the end of week 5, 42-44 pairs of somites are present
                         • 4 occipital pairs
                           • these pairs all form the axial skeleton
                         • 8 cervical pairs
                         • 12 thoracic pairs
                         • 5 sacral pairs
                         • 5-7 coccygeal pairs
         • somites are a block of paraxial mesoderm that gives rise to muscles
           • unsegmented mesoderm is pre-somitic mesoderm
             • pre-somitic mesoderm will be patterned to from somites
               • once somites have formed they start to differentiate
                 • transform into spheres - epithelialisation
                 • cells in the ventral and medial area transition (epithelial-mesenchymal) to become the sclerotome
                   • this will eventually become the vertebrae and the ribs
                   • in the bottom of the neural tube and the notochord, sonic hedgehog and noggin are produced and they induce sclerotome formation
                   • tendons are derived from the sclerotome under control of the transcription factor SCLERAXIS
                 • cells in the dorsal half form the dermomyotome
                   • sclerotome is more loosely packed than the dermomyotome
                   • splits again
                     • dermatome
                       • directed to become the dermis by neutrophil-3 from the dorsal neural tube
                     • myotome
                       • myotome cells are committed muscle cell precursors
                         • Muscle development is regulated by genes and myogenic factors. MYOD and MYF5 are both transcription factors that activate muscle specific genes. They enable the differnetiation of myogenic precursor cells in the dermomyotome into myoblasts. The transcription factors can convert non-muscle cells (fibroblasts, adipocytes, chondrocytes, retinal pigment cells etc) to cells expressing all the muscle proteins i.e. muscle cells.
                         • MYOD1 can make any cell into a myoblast. Fibroblasts and adipocytes can be reprogrammed to become myoblasts if they are transfected with MYOD1 mRNA.
                         • MYF5 is required for myoblast formation. Inactivated MYF5 results in delayed development in the intercostal and paraspinal regions.
                         • Loss of function mutations in MYF5 and MYOD1 results in a complete lack of skeletal muscle formation.
                         • In the neural tube, Wnt proteins, activating, and BMP proteins, inhibitory, combine to activate MYOD in the dermomyotome, creating a group of muscle cell precursors which express MYF5 WNT proteins also direct lip of somite to muscle precursor cells expressing MYF5
                         • In the lateral plate mesoderm, Wnt and BMP activate MYOD and MYF5 and these go on to activate specific genes
                         • undergo cells division under the influence of growth factors like myogenin (a very important growth factor that mediated the differentiation)
                           • when growth factors are depleted, the cell stops dividing and secretes fibronectin onto the ECM
                             • binds to fibronectin via an integral-crucial step
                               • cells align into a chain and fuse
                                 • cell membrane disappears and they form multinucleated myotubes (primary myotubes)
                                   • myoblasts fuse to form long multinucleated fibres called myotubes
                                   • each myotome from both abaxial and primaxial domains receive innervation from spinal nerves of the same segment
                                     • epaxial - true back muscles, innervated by the dorsal primary rami hypaxial - limb and body wall, innervated by the ventral primary rami epaxial and hypoxia are the regions of the muscles once they have formed (the domains that give rise to structures)
                         • MYOD - creates a group of muscle precursor cells which express MYF5
                           • MYOD/MYF5 - myogenic regulatory factors, they activate specific genes and can covert non-muscle cells to muscle cells
                       • dorsomedial muscle cells
                       • ventrolateral muscle cells
                 • the lateral somitic frontier is the border between the dermis from the dermatome and the dermis from the lateral plate mesoderm
                   • the frontier separated the two mesodermal domains
                     • primaxial domain
                       • some examples of muscles that originate from the different domains: primaxial - intercostals, rhomboids, latissimus dorsi and levator scapulae abaxial - pec major and minor, rectus abdomens and transverses abdomens
                       • region around the neural tube
                       • somite derived mesoderm
                       • signals from the neural tube and notochord
                     • abaxial domain
                       • parietal layer of the lateral plate mesoderm
                       • some migratory somatic cells
                       • signals from the lateral plate mesoderm
     • intermediate mesoderm
       • lies between the paraxial mesoderm and lateral plate mesoderm
       • forms the genitourinary system
     • lateral plate mesoderm
       • splits further into two layers with a cavity (intraembryonic coelom) between the upper and lower layers
         • upper somatic or parietal layer
         • lower splanchnic or visceral layer
           • smooth muscle (gut and derivatives) and cardiac muscle comes from the visceral layer MYOD is NOT involed in early cardiac development NKX2.5 is an important gene for specification of cardiac muscles. The homolog in drosophila is called Tinman.
           • Serum response factor is responsible for smooth muscle cell differentiation. SRF is unregulated by kinase phosphorylation pathwsays. Myocardin and mycardin related transcription factors enhance SRF activity.
     • extraembryonic mesoderm
       • not involved with muscle formation
       • most lateral to the primitive streak
   • the cloacal and oropharyngeal membranes breakdown to form the mouth and the anus so these are the only regions of the body that do not have any mesoderm

6. Congenital disorders

   • definitions
     • congenital - a disease or physical abnormality present from birth
       • often associated with craniofacial, cardiac and/or genitourinary defects
     • anomaly - something that deviates from the normal or expected
     • teratogen - an agent or factor which causes malformation of an embryo
   • limbs
     • Developmental dysplasia of the hip is when the acetabulum and head of the femur are poorly developed in utero. Dislocation commonly occurs after birth. There can be complications like avascular necrosis of the femoral head. This is treated with a Pavlik harness or surgery if it is detected late.
     • failure of formation
       • amelia or meromelia
         • Amelia is the absence of an entire limb and acheria/apodia is the absence of hands or feet
         • Phocomelia is the absence or shortening of proximal limb segments
         • Hemimelia is the absence of preaxial or postaxial parts of a limb
         • Meromelia is the general term for absence of any number of digits
     • failure of differentiation
       • sirenomelia or synostoses
     • duplication
     • overgrowth
       • hemihypertrophy
     • undergrowth
       • micromelia
     • constriction band syndromes
     • generalise anomalies and syndromes
       • achondroplasia or Marfan's syndrome
         • achondroplasia affects 1 in 25000 births and it is diagnosed with a prenatal ultrasound
         • Vasoritide (BMNIII) is a CNP analogue that reduces the effects of overactive FGFR3 Other treatments include limb lengthening surgery and gene therapy
         • Phase 1 trials to the drug establish safety, tolerance and optimum dosing schedule
         • Marfan's syndrome is an autosomal dominant disorder of connective tissue. It is a mutation of the FBNI gene. The syndrome presents as tall stature, long, thin digits and limbs, hyper extensible joints, arched palate, eye problems and mech, heart & lung problems.
   • hands and feet
     • failure of formation
       • ectrodactyly
         • the absence of any number of digits, genetic causes, mainly mutations
     • failure of differentiation
       • syndactyly
         • interdigital webbing, occurs in Poland syndorme, Apert's syndrome
     • duplication
       • polydactyly or mirror hand
         • excessive number of digits, genetic causes, mainly mutations
         • mutations of Hox genes, sonic hedgehog or wnt, genetic recessive trait
     • overgrowth
       • macrodactyly
     • undergrowth
       • bachydactyly
         • shortened digits, occurs in Poland syndrome, downs syndrome and is autosomal dominant
     • constriction band syndromes
       • the pathogenesis of constriction is not fully understood, it could be due to adhesions between the amnion and foetal structures or tears in the amnion leaving tissue that can wrap around parts of the foetus
     • generalised anomalies and syndromes
       • cleft hand and foot, club foot, syndactyly
         • split hand or foot is the absence of central components of a hand or foot
         • macrodactyly is when digits are enlarged and is often seen in tandem with syndactyly
   • risk factors
     • intrinsic
       • chromosomal abnormalities
       • inherited
       • sporadic mutations
     • extrinsic
       • teratogens (thalidomide)
         • an agent or factor which causes the malformation of an embryo
         • the most sensitive period for teratogen-induced limb defects is weeks 4-5 of development
         • other teratogens like warfarin, alcohol, cocaine, valproic acid and phenytoin can cause similar defects
       • nutrient deficiency (folate)
       • infections
       • failed abortion
       • removal of IUD
   • screening is important for prenatal diagnosis including foetal ultrasounds and amniocentesis
   • management
     • failure in formation
       • prosthesis, microvascular transfer of phalanges
     • failure of differentiation
       • separation
     • duplication
       • excision
     • overgrowth
       • epiphysiodesis, total physical resection, debunking, partial amountation
     • undergrowth
       • politicisation, web space deepening, metacarpal transposition, phalangeal transfer, bone graft
     • constriction band syndrome
       • prothesis, excision of groove and functionless tissue

7. Limb development

   • limbs develop from small buds of undifferentiated mesodermal cells which are covered by ectoderm
     • limb morphogenesis takes place between weeks 4 and 8
       • the lower limbs lag slightly behind the upper limbs but catch up at the end of the developmental period
         • there are no nerves in early limb buds
         • the first sign of limb appearance is a condensation of mesenchyme
           • mesenchyme is derived from the dorsolateral mesoderm of the somites
             • limb buds consist of a mesenchymal core from the parietal layer of the lateral plate mesoderm
               • limb buds are covered by a layer of cuboidal ectoderm
                 • inductive relationship with the mesoderm forms thickening and the AER
                 • the apical ectodermal ridge is at the very tip of the limb bud and it control differentiation
                   • remains undifferntiated
                     • FGF4 and FGF8
                   • as the limb grows, the cells furthest from the AER begin to differentiate into cartilage and muscle
                     • limb outgrowth is initiated by the secretion of FGF10 by the lateral plate mesoderm
                       • retonoic acid at the proximal end starts the differentiation into proximal components
                         • at week 6, the terminal portion of the buds become flattened into hand plates and footplates
                           • they are separated from the proximal segments by constriction
                             • a second constriction further divides the proximal portion into two segments
                               • development of the limb occurs proximodistally into three components
                                 • stylopod
                                   • humerus and femur
                                 • zeugopod
                                   • radius/ulna and tibia/fibia
                                 • autopod
                                   • carpals, metacarpals, digits, tarsals, metatarsals
                                 • cell death in the AER separated the ridges into 5 for the 5 digits
                                   • the mesenchyme will condense further to form the early cartilaginous digit models
                     • its existence is controlled by the mesoderm
                       • development stops when the mesoderm reaches an appropriate age, not the ectoderm
                   • key signalling centre
                 • the dorsal ectoderm expresses a signalling molecule (radical fringe gene) and the ventral ectoderm expresses a transcription factor (engrailed-1) which represses radical fringe
                   • radical fringe restricts AER to the distal tip, engrailed at the ventral side
   • limb positioning
     • occurs along the craniocaudal axis and is regulated by the Hox genes expressed in overlapping patterns
       • most important hox genes for limb development are hox9-13 an overlapping pattern of hox genes causes limb patterns
       • these factors are designated to the upper and lower limbs
         • t-box transcription factors: UL - TBX-5 LL - TBX-4 and PITX1
         • development of the upper and lower libs are similar but the upper limbs are 1-2 days faster
           • during week 7 limb rotation occurs and the upper and lower limbs rotate in opposite directions
             • the upper limbs rotate 90 degrees laterally and the elbows point backwards
             • the lower limbs rotate 90 degreed medially so the extensor muscles lie on the anterior surface
             • as the external shape is being established, mesenchyme in the buds becomes condensed
               • cell proliferation, increased density, differentiation and then cell death is induced by wnt14
               • synovial joints begin to form when the mesenchymal condensations are first seen mesenchyme between two bones differentiates into dense fibrous tissue and this differentiates into articular cartilage, synovial membrane, menisci and ligaments within the joint capsule
               • the expression of BMPs causes cells to differentiate into chondrocytes
                 • cartilage is laid down where bones will eventually be
                   • primary ossification centres present is all long bones by week 12
                     • appositional growth
                       • growth in width
                     • interstitial growth
                       • bone is not capable of interstitial growth -> endochondral ossification

Images and files in the map