Embryology LCSC06 BIOSCIENCES FOR SPEECH AND LANGUAGE THERAPY

Embryology LCSC06 BIOSCIENCES FOR SPEECH AND LANGUAGE THERAPY
Pedro Amarante Andrade, PhD

INTRODUCTION TO EMBRYOLOGY
Basic concepts and principles How the embryo develops from fertilisation to implantation – brief overview (for your information) Influences on the growth and development of the embryo; critical period concept Important stages in development: gastrulation, neurulation, development of the nervous system The neural crest cells – and their fate Embryonic development of the head and neck structures

DIRECTED STUDY ELEMENTS
Basic genetics To understand how the pharyngeal arches develop and contribute to the development of the structures of the face, neck and palate Relate this development to the structures of the head and neck you are currently studying

EVERYBODIE’s JOURNEY

WHY STUDY EMBRYOLOGY?

PREGNANCY: MEASURED IN TRIMESTERS

REAL STAGES Prenatal development: Germinal stage: days 1 to 14
3 stages of unequal length Germinal stage: days 1 to 14 (i.e. from conception to implantation) Embryonic stage: begins at implantation approximately 2 weeks after conception and continues through weeks 3 to 8 (the period oforganogenesis )

REAL STAGES The Foetal Stage: from 9th week to the birth
(40 weeks from last period, or 38 weeks from fertilisation) Organs grow and continue differentiation Increase in weight Foetal stage: weeks 9 to 38; increase in weight, refinements of the organ systems especially the lungs and brain.

CHALLENGES IN STUDYING EMBRYOLOGY
Complexity Personal experience Attitudes and beliefs Terminology

PRE-EMBRYONIC PERIOD Germinal Stage
The first 14 days Cells can still repair themselves If damaged  either repairs or dies (spontaneous miscarriage)

CRITICAL PERIOD Cell division and the critical period:
Cells divide (proliferate), migrate into position, then differentiate. The first two of these stages (proliferation and migration) are particularly sensitive to either genetic or environmental insult. Cell division times can be as little as 4 hours. Cell proliferation occurs in preparation for cell division. Proliferative cell stages are highly sensitive to insult from genetic and environmental factors. The embryo itself therefore passes through a stage where it is most sensitive to these insults. If insult occurs early – embryo dies; if later, one or more organs may develop abnormally, resulting in one or more birth defects. As SLTs we are interested in particular in the development of the CNS and PNS, the ear, and the head and neck structures. Go to the hyperlink…identify the extent of the critical period for the development of the palate, the ear, the nervous system

WHAT CAN AFFECT PRENATAL DEVELOPMENT?

WHAT CAN AFFECT PRENATAL DEVELOPMENT?
Chromosomal and genetic factors Teratogens: maternal disease, drugs Mutagens: radiation Other maternal influences on development: Diet, age, chronic illness, environmental hazards and maternal emotions Teratogens include: infectious agents/maternal disease (rubella, cytomegalovirus, varicella, herpes simplex, toxoplasma, syphilis, etc.); physical agents (ionizing agents, hyperthermia); maternal health factors (diabetes, maternal PKU); environmental chemicals such as organic mercury compounds (dentists, dental technicians, semi-conductor manufacturing workers or eating a lot of fish), polychlorinated biphenyl or PCB (also found in fish), herbicides and industrial solvents; lead, arsenic, cadmium drugs (prescription, over- the-counter, ‘recreational’). Teratogens are non-genetic factors that interfere with normal embryonic and fetal differentiation and morphogenesis. They are not mutagens. Mutagens are natural or manmade agents that alter the structure or sequence of DNA; they act randomly on all DNA and do not produce one specific genotype; they are usually also carcinogenic.. Children who have been exposed to teratogens in utero will not pass their defect on to their children. Because the effects of teratogens are seen at birth and are therefore a congenital defect, they are often thought to be genetic and can mimic genetic disorders. Maternal disease: Rubella: rubella virus can cross the placental barrier - deafness, cataracts and ear defects, even death. Vaccine itself may be teratogenic. HIV: can cross the placenta or baby becomes infected at birth. More likely when mother has AIDs rather than HIV positive. Other STDs – transmitted often during birth. Exception = syphilis and Cytomegalovirus (herpes group) – deafness, CNS damage and learning difficulties can arise. Drugs: any drug (prescription, non-prescription, herbal) can be teratogenic. The drug Thalidomide is probably the best known in this group - Smoking correlated with low birth weight and ADHD. Marijuana: tremors, sleep problems. Heroin and methadone: miscarriage, premature labour, early death % of babies will be born with addiction also. Withdrawal symptoms (high pitched cry, irritability, uncontrollable tremors, vomiting, convulsions, sleep problems. Effects can persist if mother continues to use drugs (effects of environment here). Cocaine: interaction between effects of drug and environment. Effects on social and cognitive development have been demonstrated. Alcohol: effects can occur on ovum during development in the ovary and in its journey down the fallopian tube; zygote can be affected. Foetal Alcohol Syndrome: smaller baby with smaller brains. Heart defects and hearing loss, distinctive facies. Mild learning and/or behaviour difficulties. Problems can persist into adolescence and adulthood. A fuller list of features: · In infancy, low birth weight, irritability, feeding difficulties, sleep disturbances, alcohol withdrawal, strong startle reflex · Growth deficiencies · Small head · Vision problems · Dental abnormalities · Hearing problems · Craniofacial abnormalities - narrow eye slits, flat midface, low nasal ridge, loss of groove between nose and upper lip, small jaw · Mental retardation, developmental delays · Learning disabilities · Neurological dysfunction - attention deficit, memory deficit, hyperactivity, difficulty with abstract concepts, poor problem solving skills and judgment, difficulty learning from consequences · Epilepsy · Behavioral problems - mood swings, defensive and stubborn, lack of self-discipline, genuine innocence, detached attitude · Organ and body dysfunction - muscle problems, bone and joint problems, genital defects, heart defects, kidney defects Maternal influences on development: Diet and the role of folic acid Spina Bifida (AKA meningocoele) is a rare birth disorder, affecting approximately 1 in live births, although fortunately its incidence appears to be declining with each decade. It is the most serious birth disorder affecting the nervous system compatible with life. Spina bifida occurs when a portion of the fetal spinal cord, during the third and fourth weeks of pregnancy, fails to properly close. As a result, the child is born with a part of the spinal cord exposed on the back. There appear to be both genetic and environmental influences that contribute to spina bifida. For example, once a couple has one child with spina bifida, the risk of having a second child increases to 2-3%; if the couple has two affected children, their risk of having a third child increases to about 10%. Recently, attention has been focused on nutrition, and in particular the role of folic acid (a B vitamin what foods are rich in folic acid?) on reducing the incidence of spina bifida. Studies show that a woman who takes folate supplementation before and during the early stages of pregnancy has a lower chance of having a child with spina bifida. Any woman who is contemplating getting pregnant should begin taking supplemental folate. Diet: malnutrition during pregnancy (esp final 3 months) has increased risk of delivering a low-birth weight infant with learning difficulties in early childhood; can also be a factor in the development of mental illness as an adult. The nervous system is particularly susceptible to being disrupted – smaller brains, fewer and smaller bran cells. Maternal age: risks of pregnancy increase with increasing age of the mother – exacerbated by poor ante-natal care and by poor health habits eg smoking and drinking. No more likely to have birth defects with the exception of trisomy 21 (Down’s syndrome). At the other end of the age spectrum, children of very young mothers are more likely to exhibit leaning and behaviour problems in school (controlling for socio-economic factors). Chronic illness: emotional/physical illnesses can affect the developing baby. Heart disease, lupus, diabetes, endocrine disturbances and epilepsy can all affect development in utero.

CHROMOSOMAL & GENETIC FACTORS
90% of abnormal embryos are spontaneously aborted Only 1% of live new-borns have a genetic abnormality

CHROMOSOMAL ABNORMALITIES
23 pairs of chromosomes 50+ different chromosomal abnormalities Too many chromosomes Too few chromosomes Vast majority of chromosomal abnormalities are lethal (spontaneous abortion) Discuss Turner’s syndrome XO and Klinefelters XXY syndrome briefly

CHROMOSOMAL ABNORMALITIES
Down’s Syndrome Three copies of chromosome 21 1 /600 birth Learning disability (IQ averages 50) May have congenital eye, ear, heart defect Distinctive physical features: protruding tongue, short limbs, slightly flattened nose…

ANGELMAN’S SYNDROME AND PRADER-WILLI SYNDROME
Genetic defect on chromosome 15 Angelman’s syndrome (maternal case) Prader-Willi syndrome (paternal case)

KLINEFELTER’S SYNDROME
Sex chromosomes: XXY (boys) 1/500 births Poor coordination May have mild learning difficulties

TURNER’S SYNDROME Only affect girls Occur randomly 23rd X missing
Turner syndrome is a chromosomal condition that exclusively affects girls and women. It occurs when one of the two X chromosomes normally found in females is missing or incomplete. The syndrome is named after Dr. Henry Turner, who was among the first to describe its features in the 1930's. A blood test, called a karyotype, analyzes the chromosomal composition of the individual. This is the most commonly used blood test to diagnose Turner syndrome. The most common characteristics of Turner syndrome include short stature and lack of ovarian development. A number of other physical features, such as webbed neck, arms that turn out slightly at the elbow, and a low hairline in the back of the head are sometimes seen in Turner syndrome patients. Individuals with Turner syndrome are also prone to cardiovascular problems, kidney and thyroid problems, skeletal disorders such as scoliosis (curvature of the spine) or dislocated hips, and hearing and ear disturbances.

GENETIC DISORDERS Through dominant or recessive genes (E.g. Cystic fibrosis) Phenylketonuria (PKU) Lack of enzyme to digest food containing amino acid phenylalanine (e.g.milk) Phenylpyruvic acid accumulates in the body and attacks the developing nervous system Hyperactivity; learning difficulties Test routinely given at birth Treatment: diet

MUTAGENS Caused by physical or chemical agents (e.g. ionising radiation) Act on the DNA in cells, especially during cell division Alters the genes Mutation can be passed on, if it occurs in the gametes

TERATOGENS Any disease, drug or other environmental agent that can harm the developing embryo or foetus The effect is worse on a body part when that structure is forming or growing rapidly Critical period: Period when a body part is most sensitive to teratogenic agents

MATERNAL DISEASE Rubella Most dangerous in first trimester
Blindness, deafness, cardiac abnormalities, mental retardation Syphilis Most harmful in middle or later stages Miscarriage, serious eye, ear, bone, brain damage

TOXOPLASMOSIS ¼ adults have this mild disease – similar to common cold
Parasite (cat) Powerful teratogen Serious eye or brain damage, induce spontaneous abortion

DRUGS Thalidomide In 60s Drug against sickness given to pregnant women in first trimester Violent teratogen Badly deformed eyes, ears, nose, even missing limbs

DRUGS Smoking Low birth weight (less than 2.5 kg)
Nicotine constricts blood vessels  Reduces blood flow to placenta

DRUGS Foetal alcohol syndrome (FAS): Microcephaly
Malformation of heart, limbs, joints, face Smaller Lower IQ - More likely to have learning difficulties (IQ < 85) Even low drinking, social drinking

DRUGS Mercury Poisoning Minamate disease W Eugene Smith, 1972

EMBRYOLOGICAL DEVELOPMENT
Formation of embryonic structures is more advanced in cranial than caudal regions of the embryo Cephalocaudal pattern – from head downwards Proximodistal pattern - from centre body outwards to the extremities Cells organise themselves into layers ‘talk’ to one another through chemical signals, induce neighbouring cells into making proteins And to assume particular shapes eg cylindrical, cuboidal, flattened With a tendency to migrate, ‘flow’ over surfaces, or adhere Genes in cells switch on and off according to the signals received from their neighbours Products of gene activity influence the properties and the behaviour of cells

ITERATIVE PROCESSES IN EMBRYOLOGICAL DEVELOPMENT
Cell division (proliferation) Cell adhesion Separation of cell sheets to form cavities Cell migration Cell differentiation Cell induction Formation of embryonic structures is more advanced in cranial than caudal regions of the embryo Cephalocaudal pattern – from head downwards Proximodistal pattern - from centre body outwards to the extremities Cells organise themselves into layers ‘talk’ to one another through chemical signals, induce neighbouring cells into making proteins And to assume particular shapes eg cylindrical, cuboidal, flattened With a tendency to migrate, ‘flow’ over surfaces, or adhere Genes in cells switch on and off according to the signals received from their neighbours Products of gene activity influence the properties and the behaviour of cells

TIMING IS EVERYTHING Cell proliferation : embryo/organ system vulnerable to genetic or environmental factors Cell migration: cells move into position; can be affected by matrix through which they travel Cell differentiation: cells assume their ultimate form or phenotype – less vulnerable to insult Over 50% fertilised ova are aborted Over 50% of these have chromosomal abnormalities 4-5% of live born infants suffer from a serious structural defect 90% of babies are born healthy!!!! Bee and Boyd (2004). The developing child. Schaffer, D.R. (1996). Developmental psychology.

APOPTOSIS

EMBRYONIC DEVELOPMENT
Weeks 1 and 2: Fertilisation Implantation Formation of the placenta and the early embryo

FERTILISATION TO IMPLANTATION

2 cells; Genes ‘switch on’
DAY 1 DAY 2 2 cells; Genes ‘switch on’ DAY 3 Cleavage divisions – divisions take place without net cell growth, resulting in the cell size halving at each division – dividing up the cytoplasm into manageable amounts Embryo begins to function independently at the 4 stage cells – ie it’s genes ‘switch on’. 8 to 16 cell stage (morula)

DEVELOPMENT OF THE BLASTOCYST
Differences in adhesive qualities of the cells help them to ‘recognise’ each other

THREE SOURCES OF EMBRYONIC
STEM CELLS Adult stem cells are any stem cells taken from mature tissue. Because of the stage of development of these cells, they have limited potential compared to the stem cells derived from embryos and foetuses Embryonic Stem (ES) Cells are found in the early stages of development, before implantation. They are derived form the inner cell mass of the blastocyst and can potentially develop into all types of tissue. The third type of cells are Embryonic Germ Cells (EG cells). These cells originate from the reproductive cells of foetal cadaver tissue in the gonad ridge. EG cells are further along in development, and therefore cannot be derived from embryos, but instead must be isolated from foetal tissue. This fact has prevented extensive research to date and raises many ethical issues concerning abortion, the destruction of human tissue, and other issues dealing with new and unprecedented scientific research. There are three classes of stem cells: totipotent, multipotent, and pluripotent. A fertilized egg is considered totipotent, meaning that its potential is total; it gives rise to all the different types of cells in the body. Stem cells that can give rise to a small number of different cell types are generally called multipotent. Pluripotent stem cells can give rise to any type of cell in the body except those needed to develop a fetus.

DAY 14 By day 14, chorionic cavity greatly enlarged relative to amniotic cavity and the yolk sac

"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986)

GASTRULATION Gastrulation:
A PROCESS THAT LASTS 2 WEEKS Gastrulation: At this point, I suggest you do a Google search of gastrulation images, as the best ones are unfortunately subject to copyright On day15. Custard cream phase. Faint groove appears along the midline of the germ disc – first sign of any structure within the embryo itself – this is called the primitive streak and defines the central axis, where the backbone will form; the caudal and rostral, ventral and dorsal , and left and right of the body. Gastrulation is the process involving cell movements that transforms the bilaminar embryonic disc into a trilaminar structure comprised of the 3 primary germ layers. All 3 layers derive from the original epiblast layer via a process of migration and differentiation Say goodbye to epiblast and hypoblast – 3 layers now take the names ecto, meso and endoderm

THE FATE OF THE 3 GERM LAYERS
Ectoderm: CNS, PNS, epidermis, hair, nails, sensory epithelium (nose, ear, eye) Mesoderm: part of skull, muscles, vertebrae, urogenital system, serous membranes, body wall, limbs Endoderm: gut tube and its derivatives; glands, lungs, liver, gall bladder, pancreas

BIRTH DEFECTS ORIGINATING DURING GASTRULATION
Situs inversus Teratoma (formed from epiblast cells – contain hair, skin, bone, liver etc cells) Caudal dysgenesis Situs inversus: reversal of the organs ie liver on the left, spleen on the right. Teratoma – continued migration of epiblast cells beyond the time when the caudal-most regions have formed Caudal dysgenesis: epiblast cells stop their migration too soon – lower limbs might be fused: sirenomelia –more common in insulin dependent mothers

WEEKS 3 TO 4 Formation of the neural and gut tubes
Embryo transformed from a trilaminar disc into something more recognisable!

NEURULATION Formation of neural plate
Elevation and curling of lateral edges ‘zippering’ and formation of the neural tube neurulation video

NEURAL CREST

FINAL DESTINATION NEURAL CREST CELLS
Connective tissue and bones of the face and skull C cells of thyroid gland Septum of the heart Odontoblasts Dermis in the face and neck Dorsal root ganglia Sympathetic chain and pre-aortic ganglia

Parasympathetic ganglia of the GI tract Adrenal medulla Schwann cells Glial cells Arachnoid and pia mater melanocytes

WEEK 3: 0.5 MM a-bifida.jpg

FOLDING OF THE EMBRYO Dorsal surface – formation of the neural tube
‘zippering’ effect cervical first, then caudally Ventral surface – formation of gut tube and body cavities Body stalk – eventual umbilical cord

FOLDING OF THE EMBRYO Body axis formation: dorsal and ventral, rostral and caudal Formation of the notochord (beginning of the backbone) Day 17 Neural folds or ridges form (beginnings of the nervous system) Formation of somites (future muscles either side of the backbone) Neural crest cells form

FOLDING OF THE EMBRYO

FORMATION OF THE UMBILICAL RING
Proliferation and differentiation of mesoderm Causes ventral folding along sides of embryonic axis; amnion surrounds embryo Formation of gut tube Brain grows Head and tail folding ‘purse strings’ effect Still connected to yolk sac via a duct – yolk sac degenerates week 12 Connecting stalk – allantois projects into this- develops blood vessels and becomes umbilical cord folding of the embryo in two planes

Week 4 embryo

ONTOGENY REPLICATES PHYLOGENY
At week 5, embryo is 8mm long, and resembles the embryonic newt, chick, or sheep, so similar is the basic plan of the body. Not until the 8th week does the embryo take on the clear and distinct human form

ONTOGENY REPLICATES PHYLOGENY
Recapitulation theory or The "biogenetic law" (Ernst Haekel, 1899) states that "ontogeny recapitulates phylogeny." This means that the developing embryo (ontogeny) of each vertebrate species retraces (recapitulates) its evolutionary history (phylogeny). Specifically, each embryo in the course of its development, is said to pass through a progression of abbreviated stages that resemble the main evolutionary stages of its presumed ancestors. Thus, in the case of the human embryo, the recapitulation scenario goes something like this: 1) The fertilized egg starts as a single cell (just like our first living evolutionary "ancestor"). 2) As the fertilized egg repeatedly divides it develops into an embryo with a segmented arrangement (the "worm" stage). 3) These segments develop into vertebrae, muscles and something that sort of looks like gills (the "fish" stage). 4) Limb buds develop with paddle-like hands and feet, and there appears to be a "tail" (the "amphibian" stage). 5) By about the eighth week of development, most organs are nearly complete, the limbs develop fingers and toes, and the "tail" disappears (the human stage). Modern observations Generally, if a structure pre-dates another structure in evolutionary terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonal development and differ in later stages. Examples include: The backbone, the common structure among all vertebrates such as fish, reptiles and mammals, appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, the most sophisticated part of the brain, develops last. If a structure vanished in an evolutionary sequence, then one can often observe a corresponding structure appearing at one stage during embryonic development, only to disappear or become modified in a later stage. Examples include: whales, which have evolved from land mammals, don't have legs, but tiny remnant leg bones lie buried deep in their bodies. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos (like all mammalian embryos) have hair at one stage, but lose most of it later. All land vertebrates, which have evolved from fish, show gill pouches at one stage of their embryonal development. The common ancestor of humans and monkeys had a tail, and human embryos also have a tail at one point; it later recedes to form the coccyx. The swim bladder in fish presumably evolved from a sac connected to the gut, allowing the fish to gulp air. In most modern fish, this connection to the gut has disappeared. In the embryonal development of these fish, the swim bladder originates as an outpocketing of the gut, and the connection to the gut later disappears. Modern theory One can explain connections between phylogeny and ontogeny if one assumes that one species changes into another by a sequence of small modifications to its developmental program (specified by the genome). Modifications that affect early steps of this program will usually require modifications in all later steps and are therefore less likely to succeed. Most of the successful changes will thus affect the latest stages of the program, and the program will retain the earlier steps. Occasionally however, a modification of an earlier step in the program does succeed: for this reason a strict correspondence between ontogeny and phylogeny, as expressed in Ernst Haekel's discredited recapitulation law, fails.

NEURAL CREST CELLS: IN THE HEAD AND NECK REGION……
Form ganglia of the cranial nerves Connective tissue and some of the bones of the skull and face Dermis in the face and neck Odontoblasts Arachnoid and pia mater Glial cells Note that the muscles of the face and neck are derived from paraxial mesoderm.

Body axis formation: dorsal and ventral, rostral and caudal
Formation of the notochord (beginning of the backbone) Day 17 Neural folds or ridges form (beginnings of the nervous system) Formation of somites (future muscles either side of the backbone) Neural crest cells form

WEEK 3: 0.5 MM

FATE OF THE MESODERM Paraxial mesoderm forms paired somites from occiput caudally along the length of the neural tube In the head region, somitomeres form part of skull, muscles, vertebrae, and dermis of the skin.

DEVELOPMENT OF THE SKULL
Neurocranium (protective covering of the brain) derived from paraxial mesoderm Membranous portion (flat bones) Cartilagenous portion = Chondrocranium (base of the skull) Viscerocranium (skeleton of the face – the ‘middle third’ - & including the mandible) derived entirely from neural crest cells

DEVELOPMENT OF THE SKULL
viscerocranium Skull can be considered as 3 interdependent parts: Neurocranium, viscerocranium and chondrocranium. Viscerocranium derives from the ectomesenchyme of the pharyngeal arches. Brain and eyes are the chief functional matrices influencing the growth of the neurocranium. Fontanelles: bones grow outwards from centres of ossification; where they meet and touch, sutures develop – interdigitate in the mature skull, in the newborn they are straight edged at birth – allows for bones to move relative to each other. Where more than 2 bones meet, there are large areas of fibrous tissue between the bones – fontanelles. The anterior fontanelle persists well into the second year of life; others are closed by age 3 months. Hydrocephaly, microcephaly and anencephaly (where the neural tube fails to close in the cranial region) are some of the sequelae to hydrocephalus or lack of development of the forebrain. Other skull abnormalities are the result of genetic defects, which result in the sutures fusing too early in development – craniosynostosis and abnormally shaped skulls result. Skull and jaw bones develop in tandem with tissues and organs they enclose – these together form what is called a functional matrix. A functional matrix is a tissue, organ or space which determines the particular shape of a particular bone by influencing its growth and subsequent maintenance.

DEVELOPMENT OF THE BRAIN VESICLES
Cranial end of neural tube expands Neural tube closure complete in week 4 Brain vesicles form the future brain Anterior vesicle: prosencephalon This subdivides: telencephalon (future cerebral hemispheres) and diencephalon (optic and thalamic tissues and other structures) Telencephalon: This term is technically the anterior-most embryological division of the brain that develops from the prosencephalon. Other term: cerebrum.

Fluid-filled enlargements that develop within the first 4 weeks of embryonic growth
Prosencephalon divides into anterior telecephalon (which becomes the cerebral hemispheres and basal ganglia) and posterior diencephalon (further divides – see next slide); rhombencephalon also subdivides

PRIMARY BRAIN VESICLES
(rhombencephalon) Durng the 5th week, the primary brain vesicles differentiate into 5 parts of the brain: Telencephalon and diencephalon, mesencephalon, metencephalon and myencephalon. Cells in floor of lateral ventricles become the basal ganglia. Diencephalon: masses in its walls develop into thalamus and hypothalamus. The diencephalon also forms the optic stalk (which becomes the optic nerve), part of the pituitary gland Mesencephalon: grows more slowly compared to other parts of the brain, forms mid-brain structures. Posterior brain vesicle is the rhombencephalon which forms the hindbrain (pons and medulla) AND the cerebellum Rhombencephalon shows a series of transverse elevations called rhombomeres shortly after it has formed – see later in the context of the pharyngeal arches and their derivatives. Formation of the vesicles cause the brain to lengthen and bend. During the 6th week of development, commisures are established between the hemispheres of the developing brain, and the optic chiasma forms in the diencephalon.

PRIMARY BRAIN VESICLES
During the last 2 months of gestation, the surface hemispheres grow so rapidly that many gyri (convolutions) form, separated by fissures and sulci. At birth, about 86 billion neurons plus glial cells. Increase in brain size x 5 to adulthood; myelination of oligodendrocyes in the CNS, and Schwann cells in the PNS.

FURTHER DEVELOPMENT OF THE NS
The nervous system continues to develop and changes occur up until the early 20’s The main changes include: Myelination Formation of synapses Synaptic pruning Apoptosis

SYNAPTIC PRUNING Microglia -nerve cells which are usually involved in response to injury in the NS They also pluck off or ‘prune’ some of the synapses between neurons In conjunction with apoptosis of neurons, ensures that only the most-used ie strongest connections, remain Keeps the brain operating efficiently Microglia –involved in the response of the NS to injury; phagocytic; engulf microbes and clear away debris. Occurs during rapid periods of brain growth. At toddlerhood, synapses start to be pruned – those connections and neurons that are most used are spared. Helps the brain to operate more efficiently. The greatest amount of synaptic pruning occurs during the most rapid period of growth for the brain, or during the earliest years of a child's life.

SYNAPTIC PRUNING OCCURS
Prenatally In childhood At puberty “use it or lose it” Learning causes synaptic connections to increase in strength

IMPLICATIONS FOR SLT Aberrant synaptic pruning may be at the root of MND, MS AD – by the time it is identified, people have lost over HALF their synapses Children with ASD have increased cerebral volume, ? ? have not undergone the same extent of synaptic pruning?

MYELINATION Starts late in embryonic development and continues into adolescence/early adulthood Forebrain the last part to complete myelination Q what is the function of the myelin sheath? Postmortem studies suggest that axon diameters and myelin sheaths undergo conspicuous growth during the first two years of life, but may not be fully mature before adolescence or even late adulthood Babinski reflex: a test used to check for incomplete myelination in babies. Softly rub the sole of a baby’s foot from the heel towards the large toe. The baby’s toes will fan out, with the large toe pointing up. In contrast, when you do the same to an older child or an adult, his toes will curve inwards. Myelination of the corticospinal tract months means this positive sign disappears. In UMN damage, it can reappear. ‘positive Babinski sign’ or plantar reflex Regeneration of damaged nerves in the periphery of adults is in some ways similar to what happens in the embryo as its nervous system grows. Development of white matter structure ie myelinated neurons in children correlates with development of motor skills and cognitive function

THE SPINAL CORD Extends from the foramen magnum to the level of the second lumbar vertebra Shorter than the vertebral column because it does not grow as rapidly during embryonic development. Because the spinal cord is shorter, spinal nerves do not always exit the vertebral column at the same level as their origin in the spinal cord.

THE SPINAL CORD https://quizlet.com/24950848/spinal-cord-flash-cards

BY THE END OF THE 4TH WEEK Neural folds have closed
Head region distinguished by presence of 3 brain vesicles Lens and otic placodes for eye and ear development; Primitive oral cavity (stomatodeum) 3 pairs of pharyngeal arches

BY THE END OF THE 4TH WEEK

FORMATION OF THE PHARYNGEAL ARCHES
Neural crest material grows from the rhombencephalic region (rhombomeres) Migrates and forms 6 paired bands These form the pharyngeal arches Each is accompanied by its own artery, nerve and cartilage In the cervical region of the embryo, the back of the primitive mouth cavity is continuous with the upper part of the foregut (will become the pharynx and oesophagus). Mesoderm sparse in this region; neural crest cells grow from the rhombencephalic region and migrates between the foregut tube and the ectoderm in 6 paired bands…these form the pharyngeal arches; each outgrowth is accompanied by its own artery and nerve; these arches eventually meet and fuse in the midline. On the outside, a cleft is formed between each arch, and the corresponding depression on the interior aspect forms a pouch.

Eventually, 5 well developed pairs of pharyngeal arches form in cranial to caudal sequence (labelled I –VI above, V is vestigial). Adult Derivatives of Pharyngeal Arches First (mandibular) pharyngeal arch: Trigeminal nerve (CN V) Muscles of mastication, mylohyoid muscle tensor veli palitini muscle, tensor tympani muscle, anterior belly of the digastric muscle Maxilla, zygomatic bone, temporal bone, palatine bone, vomer, mandible, malleus, incus, sphenomandibular ligament Second (hyoid) Facial (CN VII) Muscles of facial expression, stylohyoid muscle, stapedius muscle posterior belly of digastric muscle Stapes, styloid process, stylohyoid ligament, lesser horn and superior body of the hyoid bone Third Glossopharyngeal (CN IX) Stylopharyngeus muscle Greater horn and inferior body of the hyoid bone Fourth Vagus (CN X) – Superior laryngeal branch Muscles of soft palate (except tensor veli palatini) and muscles of pharynx (except stylopharyngeus), cricothyroid muscle, cricopharyngeus muscle, Thyroid cartilage, cricothyroid cartilage, arytenoid cartilage, laryngeal cartilages Sixth[1] Vagus (CN X) –Recurrent laryngeal branch Intrinsic muscles of the larynx (except cricothyroid), upper (skeletal) muscles of esophagus Laryngeal cartilages Adult Derivatives of Pharyngeal Pouches 1 Lining of auditory tube and tympanic cavity (middle ear cavity) 2 Lining of intratonsillar cleft (tonsillar fossa) 3 Inferior parathyroid glands, thymus 4 Superior parathyroid glands, parafollicular cells of thyroid gland Reference:

Pharyngeal pouch derivatives include the palatine tonsils, the parathyroid glands, the thymus. Pouch derivatives migrate inferiorly, those from pouch 3 travel the furthest (forming the inferior parathyroid gland and the thymus).Thyroid originates and projects ventrally from the floor of the pharynx at a mid-line depression, the foramen caecum – and begins to descend toward the anterior surface of the developing trachea.

First arch (mandibular arch) : forms maxilla, zygomatic bone, part of the temporal bone, malleus and incus, and mandible; digastric, mylohyoid, muscles of mastication, tensor palatine and tensor tympani, covering skin and floor of mouth and tongue(general sensation) (V CN mixed , mandibular division). Skeletal components of each arch are cartilagenous initially; some are converted to bone later . Meckel’s cartilage is transitory and replaced by bone; traces remain in the form of the malleus and the incus; the stapes is formed by part of the second arch. 2nd arch: muscles of facial expression,part of the hyoid bone, the stapes, the styloid process and stylohyoid ligament; VII CN 3rd arch: stylopharyngeus (the only muscle), greater horn an lower portion of the body of the hyoid, post 1/3 of tongue and pharyngeal mucosa (IX CN) 4-6 arch: muscles of larynx, pharynx and palatee cricothyroid, levator palatine, pharyngeal constrictors; laryngeal cartilages. X CN (vagus)

FIRST ARCH DEFORMITIES
Micrognathia Cleft palate (indirectly) Conductive hearing loss External ear malformations

MIGRATION/DIFFERENTIATION
ABNORMAL NEURAL CREST MIGRATION/DIFFERENTIATION Abnormal neural crest migration/differentiation can result in severe facial deformities; often also cardiac abnormalities as neural crest contribute towards the development of the heart. First arch deformities characterise a number of deformities – effects the growth of the mandible (results in micrognathia and malocclusion) affecting lip seal and labiodental sounds. Conductive deafness can arise due to deformity of the tiny bones of the middle ear (malleus and incus in the case of the first arch). External ear is often malformed. Nerves and muscles rarely affected. Treacher Collins syndrome: Treacher Collins syndrome presents with different severities. That is, sometimes the syndrome is so mild that it is hard to tell if a child even has the syndrome. Other times, it can be quite severe. The following is a list of traits that a child may, or may not have. A narrow forehead. Eyes that tilts downward (called an "antimongoloid" slant). Pulled down lower eyelids (sometimes erroneously called a "coloboma"). Absent eyelashes on the lower eyelids. Thin skin overlying absent cheekbones (orbital clefts with absent zygomas). Absent ears (microtia), or malformed ears. Cleft palate. Small lower jaw Some children may be born without a soft palate (back part of the roof of the mouth) and small, or absent thumbs. These children may have what is called Nager variant. Robin Sequence (Pierre- Robin syndrome): In this genetic sequence, during development the chin does not come away from the baby's chest as completely as it should during the development of the oral cavity and associated structures, and the result is a combination of characteristics including a small, pushed back lower jaw, a large tongue, and often a wide, U-shaped cleft of the hard and soft palate. Sometimes the problem also affects the growth of the trachea and soft tissues of the pharynx, and the baby's airway can be severely decreased in size or development. Because of the small jaw and large tongue, the airway may become obstructed. Some of these children require tracheostomies or other surgical interventions to increase the airway size. •Micrognathia (extreme smallness of the jaw) or retrognathia (facial disharmony in the jaw) •Cleft palate (usually U-shaped, but V-shape is also possible) •Glossoptosis (abnormal back placement of the tongue), often accompanied by airway obstruction. The small mandible causes the tongue to obstruct the airway in relationship to the size of the mouth. There is a wide scale of severity in each of these three features. The series of anomalies of PRS are all initiated by one developmental problem. Hence, the main concerns for newborn babies and infants with PRS are airway obstruction and feeding problems. Treacher-Collins Robin sequence

First AND second pharyngeal arches malformed
GOLDENHAR SYNDROME First AND second pharyngeal arches malformed One side of the face more developed than the other

DEVELOPMENT OF EXTERNAL EAR

DEVELOPMENT OF EXTERNAL EAR
First cleft deepens to form external auditory meatus; the auricle is formed by an outgrowth of the mandibular branch above and the second arch below.

development of the head, neck,
FORMATION OF THE FACE development of the head, neck, face and palate The tectoseptal processes: these grow out from the medial aspect of each maxillary process, below the developing brain; these fuse in the midline and grow downwards to form the secondary nasal septum (NS in diagram above). Tongue development begins very early, even before the pharyngeal arches fuse anteriorly. At 6 week stage, palatal shelves arise from the medial aspect of each maxillary process. Because the developing tongue is so large, tongue occupies whole of the oronasal cavities and deflects the palatal shelves downwards. Go to to view whole sequence. By 8 weeks, the mandibular arch enlarges considerably, allowing tongue to drop to the floor of the mouth; palatal shelves abruptly change their orientation from vertical to horizontal and meet in the midline. Anterior edges meet the posterior edge of the primary palate. By this time, the secondary nasal septum has elongated sufficiently for all 3 processes to converge at the midline of the palate. Palatal fusion begins around 10 weeks, starts in the middle and spreads anteriorly and posteriorly. Bone develops in the palate after fusion, except posteriorly where it is invaded by muscle. Nasal conchae form as the nasal septum and palatine shelves fuse. Bone begins to form in the anterior portion – forms the hard palate; posterior portion lacks bone – soft palate. Incisive foramen indicates the midline junction between primary and secondary palate.

FORMATION OF THE FACE

FORMATION OF THE PALATE

WEEK 8

Embryology LCSC06 BIOSCIENCES FOR SPEECH AND LANGUAGE THERAPY

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Embryology LCSC06 BIOSCIENCES FOR SPEECH AND LANGUAGE THERAPY

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Presentation on theme: "Embryology LCSC06 BIOSCIENCES FOR SPEECH AND LANGUAGE THERAPY"— Presentation transcript:

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