TOPIC
BASIC HUMAN EMBRYOLOGY – PART I
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CONTENTS 1. Fertilization 2. Cleavage 3. Blastocyst formation 4. Formation of germ layer 5. Formation of extra embryonic mesoderm and coelom 6. Formation of prochondral plate and primitive streak 7. Formation of 3rd germ layer 8. Formation of notochord 9. Formation of neural tube and neural crest cells 10. Sub divisions of intra embryonic mesoderm 11. Yolk sac and folding of embryo 12. Pharyngeal arches www.indiandentalacademy.com
Introduction : The history of man for the nine months preceding his birth would, probably; be for more interesting and contain events of greater moment than all the three score and ten years that follow it. Samuel Taylor Coleridge, Miscullanies, Aesthetic and literary.
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What is embryology ? Embryology is the study of the formation and development of the embryo (or fetus) from the moment of its inception up to the time when it is born as an infant.
Why do we need to study embryology? 1)
This subject tells us how the organs of the body develop.
2) This knowledge helps us to understand many complicated facts of adult anatomy. Embryology helps us understand why some children are born with organs that are abnormal. Appreciation of the factors responsible for mal development assists us in preventing, or treating such abnormalities
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FERTILIZATION Fertilization of ovum occurs in the ampullary region of the uterine tube within 24 hrs of ovulation.
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STAGES IN MATURATION OF THE OVUM
A. Ovum just before ovulation. B. Ovum at the time of ovulation. C. Ovum at the time of fertilization. D. Ovum just after fertilization.
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BEHAVIOUR OF CHROMOSOMES DURING FERTILIZATION Two daughter cells is still surrounded by ZONA PELLUCIDA Each daughter cell is smaller than ovum. Diploid chromo. is restored Determination of sex takes place Cleavage starts www.indiandentalacademy.com
TEST TUBE BABIES Produced by technique called ‘IN VITRO FERTILIZATION’ GONADOTROPHINS admin. to women to stimulate growth of follicles. Ovum removed just before ovulation & placed in suitable medium. Spermatozoon added to medium Fertilization and early development takes place in medium. At 8- cell stage put inside uterus. Successful implantation takes place in 20% cases. www.indiandentalacademy.com
CLEAVAGE Two cell stage undergo series of subdivisions called as CLEAVAGE. 16- cell stage – MORULA It look like a mulberry. It form 3 days after fertilization Outer layer – TROPHOBLAST Inner cell mass - EMBRYOBLAST www.indiandentalacademy.com
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FORMATION OF BLASTOCYST Some fluid passes into Morula from uterine cavity. Morula acquires shape of cyst. Cell of trophoblast become flattened. Inner cell mass attach to inner side of the trophoblast on one side. It form on 4th day of fertilization www.indiandentalacademy.com
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FUNCTION OF ZONA PELLUCIDA
Trophoblast has the ability of sticking to the uterine epithellium and it cell has the ability to eat up other cells . Zona pellucida prevent embryo from sticking to the epithelium and therefore it travel down to uterine tube.It prevent implantation to abnormal site ABNORMAL EMBRYO www.indiandentalacademy.com
FORMATION OF GERM LAYER Three layered disc – EMBRYONIC DISC 1. ECTODERM 2. ENDODERM 3. MESODERM All tissues of the body derived from one or more of these layers.
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. Formation of ENDODERM
. Formation of ECTODERM
. Formation of AMNIOTIC CAVITY
. Formation of PRIMARY YOLK SAC
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EXTRA EMBRYONIC MESODERM & EXTRA EMBRYONIC COELOM
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FORMATION OF PROCHONDRAL PLATE
It determine the central axis of embryo and also enable us to determine head and tail ends.
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FORMATION OF PRIMITIVE STREAK Some mesodermal cells spread sideways from primitive streak Between ectoderm & endoderm. These cell form “INTRA EMBRYONIC MESODERM” (3RD GERM LAYER) This process of formation of primitive streak & intra embryonic mesoderm called as GASTRULATION
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Formation of cloacal memberane and spread of intra embryonic mesoderm.
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Spread of intra embryonic mesoderm into connecting stalk
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FORMATION OF NOTOCHORD Develop in the region extending from cranial end of primitive streak to the caudal end of the prochondral plate. Primitive Knot Blastopore formation Notochordal process or head process NOTOCHORD CANAL
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NOTOCHORD PLATE
Definitive notochord
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Importance of NOTOCHORD
Present in all animals belong to phylum CHORDATA
Notochord does not give rise to vetebral column but part of it persist in the region of each intervertebral disc as the NUCLEUS PULPOSIS
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FORMATION OF NEURAL TUBE The ectoderm overlying notochord undergoes changes result in Formation of NEURAL TUBE Neural tube give rise to brain and spinal chord
The process of formation of neural tube is called as NEURULATION www.indiandentalacademy.com
Neural crest cells
At the time when the neural plate is being formed, some cells at the junction between the neural plate and rest of ectoderm become specialized(on either side) to form the primordia of neural crest. With the separation of neural tube from the ectoderm, the cells of neural crest appear as groups of cells lying along the dorso lateral sides of the neural tube. These neural crest cells migrate to various parts of the embryo and give rise to several important structures. www.indiandentalacademy.com
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Derivatives of neural crest cells Derivatives Nervous system Neurons, including
Sensory ganglia Sympathetic ganglia (V, VII, IX, X) Parasympathetic ganglia of neck Neurological cells Schwann cells
Skeletal system Branchialarch cartilages Bones including
Maxilla Mandible Palatine Facial complex Cranial vault www.indiandentalacademy.com
Adenohypophysis Lingual glands Thymus Connective tissues
Thyroid and parathyroids
Connective tissue component of
Vascular and dermal smooth muscles Odontoblasts and pulp of the teeth Corneal endothelium and stroma Melanocytes and melaonpores Pigment cells Epidermal pigment cells Secretory cells Carotid body type 1 cells Calcitonin producing cells of ultimobranchial body. www.indiandentalacademy.com
SUB DIVISIONS OF INTRA EMBRYONIC COELOM Intra embryonic mesoderm is continuous with extra embryonic mesoderm at the edge of embryonic disc.
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Paraxial mesoderm
Lateral plate mesoderm
Intermediate mesoderm
Somitomeres www.indiandentalacademy.com
Formation of intra embryonic coelom
Small cavities appear in lateral mesoderm which coalesce to form large cavityIntra embryonic coelom
It divide lateral mesoderm o: Somatopleuric or parietal ra embryonic mesoderm.
Splanchnopuric or visceral ra embryonic mesoderm It give rise to perecardial,www.indiandentalacademy.com plural and peritonial cavities.
Septum transversum Cranial to cardiogenic area both somatopluric and splanchnopluric mesoderm are continuous with each other. This unsplit mesoderm form a structure called SEPTUM TRANSVERSUM
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YOLK SAC AND FOLDING OF EMBRYO
1.Progressive increase in embryonic disc 2. Increase length of embryonic disc bulge the yolk sac upward in the amniotic cavity forming HEAD AND TAIL FOLDS 3. Formation of primitive gut 4. Formation of definitive yolk sac www.indiandentalacademy.com
Stages in development of umbilical cord
Connecting stalk : only connecting Link between embryo and placenta. It provides : nutrition Oxygen Removes waste products
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Contents of umbilical opening
1. 2. 3. 4.
Vitello- intestinal duct Wharton’s jelly protecting blood vessels Blood vessels that passes from embryo to placenta Small part of extra embryonic coelom
Umbilical cord too short or too long May cause problem during delivery of the fetus
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ALLANTONIC DIVERTICULUM Arises from the yolk sac near the caudal end of the embryonic disc. It helps in development of urinary bladder
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Effects of head & tail folds on position of other structures 1. Pericardial cavity comes to lie on the ventral side of embryo,ventral to foregut.
2. Septum transversum now lie caudal to heart. At later stage Diaphragm and liver develop in relation to it. 3. Region of prochondral plate now forms the buccopharengeal memb. Which closes the foregut. www.indiandentalacademy.com
4. Cranial most part of embryo is now the enlarged cranial part of neural tube which later form the brain.
5. Primitive streak disappear at tail end. Remnnant of it may give rise to sacrococcygeal tumours Distal end of hind gut is closed by cloacal memb.
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TIME TABLE 1. 2 days after fertilization – two cell stage 2. 3rd day – Morulla 3. 4th day – Blastocyst 4. 8th day – Bilaminar disc 5. 14th day – Prochondral plate and primitive streak 6. 16th day – Intra embryonic mesoderm(3rd layer) 7. 17th day – Notochordal proces, Heart tube 8. 19th day – Connecting stalk distinguished 9. 21st day – Neural groove,head fold began to form 10. 23rd day – Closure of neural tube www.indiandentalacademy.com
PHARYNGEAL ARCHES When buccopharyngeal memb. breaks, the foregut opens to the exterior thru the stomodeum. the neck is formed by the elongation of the region between stomodium and the pericardium, achieved by a descent of the developing heart.
This elongation takes place due to appearance of a series of mesodermal thickening in the wall Of the cranial most part of the foregut. This is Called PHARYNGEAL or BRANCHIAL ARCHES
Coronal section thru foregut before the appearance Of pharyngeal arches
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At this stage, endodermal wall of the forgut is seperated from surface ectoderm by a layer of mesoderm. Mesoderm arrange in form of six bars that run dorso-ventrally in the side wall of foregut .They fuses with corresponding bar of opp. Side to form pharyngeal arches. In the interval between any two adjoining arches, the endoderm extends outward in the form of pouch – ENDODERMAL or PHARYNGEAL POUCH to meet the ectoderm which dips into this interval as an ECTODERMAL CLEFT 1ST ARCH – MANDIBULAR ARCH 2ND ARCH – HYOID ARCH 3RD , 4TH , 6TH ARCHES – No specific name 5th ARCH – Disappear soon after formation
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Contents of pharyngeal arch
1.
A skeletal element
2.
Striated muscles
3.
An arterial arch
A. B.
Ventral aorta Dorsal aorta
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In human embryo, a double innervation is seen only in 1st pharyngeal arches(mandibular n. – post trematic n. chorda tympani is pretrematic n.)- seen in Anterior 2\3rd of the tongue.
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DERIVATIVES OF SKELETAL ELEMENTS 1ST ARCH – 1.Meckel’s cartilage, 2. Incus,Mallus ,3. Sphenomandibular ligament. 4.Mesenchyme responsible for bone formation of : maxilla, mandible, zygomatic bone, palatine bone & part of temporal bone. 2nd ARCH – 1.Stapes, 2. Styloid process, 3.stylohyoid(from sheath), 4.Smaller(lesser)cornu of hyoid, 5.Superior part of body of hyoid 3rd ARCH –1. Greater cornu of hyoid bone, 2.lower part of body of hyoid bone 4th & 6th ARCH – Cartilage of larynx www.indiandentalacademy.com
Nerves and muscles of the arches
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The mesenchyme giving rise to muscles of pharengeal arches is derived from the PARAXIAL MESODERM cranial to occipital somites.Paraxial Mesoderm shows partial segmentation into seven masses of mesenchyme called SOMITOMERES. Structures derived from these somitomeres are :
1. Somitomere 1 & 2 : muscles supplied by oculomotor nerve. 2. Somitomere 3 : sup. Oblique muscle supplied bytrochlear n. 3. Somitomere 4 :Muscles of 1st pharengeal arch 4. Somitomere 5 : lateral rectus muscle supplied by abducent n. 5. Somitomere 6 : muscles of 2nd pharengeal arch 6. Somitomere 7 : stylopharengeus supplied by glossopharengeal n. Occipital somites 1 & 2 : larengeal muscles supplied by vagus n. Occipital somite 2 to 5 : muscles of tongue supplied by hypoglossal n. www.indiandentalacademy.com
Fate of ectodermal clefts
Cleft of first arch –epithelial lining of external auditory meatus,pinna(or auricle) Cleft of 2nd arch – cervical sinus – branchial cyst located just below the angle of jaw
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Fate of endodermal pouches First pouch 1. Tongue 2. Tubo tympanic recess 3. Auditory tube 4. Middle ear cavity including Tympanic antrum 2nd pouch 1. Lingual tonsils 2. Tubo tympanic recess 3rd pouch 1. Inferior parathyroid glands 2. Thymus 4th pouch Sup. Parathyroid glands www.indiandentalacademy.com
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References 1. Gray’s Anatomy, 38th edition- Kenry Gray 2. Human Embryology, 6th edition- Inderbir Singh 3. Oral Histology,5th edition – A.R. Ten Cate
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