FROM FERTILISATION TO FULL TERM BABY

Fertilisation is the process of fusion of two mature, haploid (23 chromosomes each) gamete cells, one from each parent, i.e., an Ovum and a spermatozoa. It marks the completion of the second meiotic division. 

Stages of fertilisation –

1. Approximation of gametes
2. Contact and fusion of gametes
3. Results of fertilisation 

Fertilisation results in formation of undifferentiated, diploid (46 chromosomes) mononucleated single cell known as “zygote”.

The zygote undergoes a number of mitotic divisions, resulting in formation of “3-celled” [ known as Embryo ] stage, “4-celled” stage, and so on. The process of subdivision of cells is known as cleavage. The stages of subdivision are as follow-

1. Stage of compaction – starts from 8 celled stage ( third division)
2. Morula – 16 celled stage ( fourth division)
3. Blastocyst – 32-64 cell stage (fifth and sixth division)

The process of cleavage lasts for 6 days, which occurs in fallopian tubes, from where it is transferred to the uterine cavity on around 4th day via ciliary beats of uterine epithelium and contraction of uterine tube musculature.

Age	Stage	Special Feature
0 days	Fertilisation
1 day	One celled stage	Zygote
2 days	Four celled stage	Embryo
2-3 days	Eight celled stage	Exhibit polarity
3-4 days	16 celled stage	Morula
4-5 days	32- 64 celled stage	Blastocyst formation
6th day	Hatching of blastocyst
7th day	Implantation in uterine cavity

Layers of Embryo-

At the very early stage, embryo proper has three germ layers-

1. Endoderm (inner)
2. Mesoderm (middle)
3. Ectoderm (outer)

All tissues of the body are derived from these germ layers. Further differentiation in these germ layers occurs as-

1. Amniotic cavity formation
2. Formation of primary yolk sac
3. Formation of extraembryonic mesoderm
4. Formation of extraembryonic coelom
5. Connecting stalk formation
6. Formation of chorion and amnion
7. Formation of secondary yolk sac
8. Circular embryonic disc
9. Formation of prochondal plate
10. Formation of primitive streak
11. Formation of intraembryonic mesoderm
12. Extensions of intraembryonic mesoderm

Age	Stage
8th day	Bilaminar disc formation
14 days	Prochondral plate and primitive streak is seen
16 days	Disc is now three layered
	Allantoic diverticulum starts appearing

Further development of Embryonic disc –

During the 2nd and 3rd week of development, the embryonic disc changes in shape from circular to oval and then finally pear shaped.

The cells of epiblast at caudal end of bilaminar embryonic disc are pluripotent, and forms following structures-

1. Definitive endoderm
2. Intraembryonic mesoderm
3. Definitive ectoderm
4. Notochord
5. Primordial germ cells

Formation of notochord

Notochord is a midline structure, which develops between primitive streak and prochrondal plate. The formation of notochord occurs through following stages-

1. Formation of primitive knot
2. Appearance of depression in centre of primitive knot, known as blastopore
3. Multiplication of cells of primitive knot and formation of notochordal process
4. The cavity of the blastopore extends and covers the whole notochordal process and results in formation of a notochordal canal.
5. Later this canal wall becomes flattened and a notochordal plate is formed.
6. Proliferation of the cells of the notochordal canal results in formation of solid rod of cells, known as definitive notochord.

NOTE- as the embryo enlarges, the notochord also enlarges and lies in mid line and later the space is occupied by the vertebral column. However, notochord do not participate in vertebral column formation, but part of it is present in each intervertebral discs as nucleus pulposus.

Formation and fate of Neural Tube-

Neural tube is formed from an ectoderm lying over the notochord, and extends from the prochondral plate to the primitive knot. Soon it is divisible into cranial and caudal parts and gives rise to the brain and spinal cord.

FOLDING OF EMBRYO

The number of changes in shape of embryo and inner structures occurs due to rapid growth of cells in the central part of the embryonic disc and growth of somites.

1. Flat pear shape to cylindrical shaped embryo.
2. Head and tail end starts building over the amniotic cavity, however they still lie close to each other. These bulging gives rise to head fold, tail fold and lateral folds.
3. With the formation of head and tail fold, part of the yolk sac becomes enclosed within the embryo, giving rise to formation of primitive gut. The primitive gut is divisible as foregut, hindgut and midgut, based on their relation with head, caudal or intervening part.
4. Gradually, communication with the yolk sac became narrower, smaller and now known as the definitive yolk sac. It is connected to the gut cia channel, known as vitellointestinal duct. Later, it forms an umbilical opening and connects it to the placenta.
5. Umbilical opening- comprises of- vitellointestinal duct, mesoderm of connecting stalk ( later converts to Wharton’s jelly ), blood vessels [connecting embryo and placenta], a part of embryonic coelom.

Age	Stage
15 days	Appearance of primitive streak and definitive yolk sac
17 days	Notochordal process appears, heart tube is seen
19 days	Connecting stalk starts distinguishing, intraembryonic mesoderm formed
21 days	Neural groove is seen, head fold starts forming
3rd week	Umbilical vessels developed
	Blood and vessel forming cells appear
	Heart tube and pericardium formed
	Formation of external genitalia begins
	Neural tube begins to form

FURTHER DEVELOPMENT OF FOETUS OCCURS AS FOLLOWS-

4th week

Days	Structure formed
4th week	Tongue start forming
	Subdivisions of Heart tube visibleHeart begins to beatAortic arch begins to establish in the cranial and caudal end.Veins starts forming.
	Mesonephric tubules starts forming
	Urorectal septum starts forming
	Appearance of optic sulcus and Otic placode
22	Optic vesicle comes in contact with surface ectoderm
	Appearance of 1st and 2nd pharyngeal arch
23	Neural tube closes
	Neural folds begin to fuse.Primordia of sensory ganglia formed.(spinal and cranial)
26	Forelimb bud appears
28	Hindlimb bud appears
	Frontonasal, maxillary & mandibular process identifiable
	Lens and nasal placode appear

5th week

Days	Structure formed
5th week	Four pharyngeal arches are seen .Thyroid and thymus start forming.
	Limbs become paddle shaped
	Nasal pits established
	Hypobranchial prominence become prominent
	Stomach rotates and dilates.Intestinal loops begin to form.Cecal bud identifiable.
	Spiral septum formed.Aortic arches formed.Lymphatic sac formedThe cardinal, umbilical and vitelline veins formed.Conduction system of heart formed.
	Metanephrons formed
	Formation of brain vesicle.Sympathetic ganglia formed.Cerebral hemisphere begins forming.
	Adrenal glands begins to develop.
	Eye primordium surrounded by loose mesenchyme.
	Tubotympanic recess develops.

6th week

Days	Structure formed
6th week	Formations of future digits (fingers) seen.Cartilaginous model of bone starts forming.
	Tubercle for formation of ear pinna starts forming.
	Palate process forms on each side from maxillary process.
	Dental lamina of both upper and lower jaw established.
	Stomach completes its rotation.Intestinal loops well formed.Urorectal septum starts dividing into cloacaAppendix becomes clearly visible.
	Lungs enter in pseudoglandular stage (2nd to 4th month of IUL)
	Coronary circulation estabilising.AV valves and papillary muscles forming.
	Mesonephron well formed.Cloacal membrane divides into urogenital and anal membrane.
	Choroid fissure formed.Lens vesicle seen.
	Auricles start forming.Cochlea and semicircular canal starts forming.

7th week

Days	Structure formed
7th week	Mammary line established.
	Thyroid gland reaches its definite position.
	Rotation of limbs occurs.
	Eyelids are established.Maxillary process fuses with the medial nasal process.
	Salivary glands starts developing.
	Separation in cloaca completed.Intestinal loop herniates out of abdominal cavity.
	Heart septa completely formed.
	Urogenital sinus established.
	Solid eye lens is formed.
	Mesenchymal condensation for 3 ear ossicles appears.

8th week

Days	Structure formed
8th week	Melanoblast appears.
	Elbow and knee established.Fingers and toes are free.Primary centre of ossification starts appearing in many bones.
	Eye shift from lateral to frontal position.Bucconasal membrane ruptures
	Enamel organs are formed.
	Intestinal loop rotates counterclockwise.
	Cerebellum starts forming.
	Rathke’s pouch loses its connection with oral cavity.
	Cochlea and semicircular canal assume their definitive external form.

9th to 12th week (till end of 3 months)

Days	Structure formed
9th week	Anal membrane breaks down.
10th week	Palatal process and nasal septum fuse with each other.
	Enamel organs become cup shaped.
	Corpus callosum formed.
	Scala vestibuli and scala tympani appear.
11th week	Epidermal ridges appear.Collagen and elastic fibres appear.
12th week	Cells of neural crest begin to migrate to the skin.Melanoblast invades epidermis.
	Primary centre of ossification is seen in all long bones.
	Head and tail foldings are completed.Herniated coils of the intestine return back to the abdominal cavity.
	Urethral folds fuse with each other.Prostate begins to develop.
	Definite kidney becomes functional.
	Cerebellar cortex and purkinje cells are formed.
	Urine foration begins
	Male and female external genitalia visible through USG.

Till end of 6 months

Days	Structure formed
4th month	Surface ectoderm becomes multi-layered.
	Dermal papilla formedNails appear.
	Lungs completed their pseudoglandular stage.
	Dentate nucleus seen.Myelination of nerve fibre begins.
5th month	Development of eccrine sweat glands, rudiments of lactiferous ducts.
	Lungs enter the canalicular stage.
	Vagina gets canalised.
	Length increases considerably.
	Foetal movement felt by mother.
	Hair on eye and head seen
6th month	Enamel and dentate formed considerably.Formation of the tongue is almost complete.
	Rapid eye movement begins
	Lungs secrete surfactant. (sign of lung maturity)

Till end of 9 months

Days	Structure formed
7th month	Lungs complete the canalicular stage and enter the saccular stage.
	External acoustic meatus canalised.
	CNS able to control respiration, if born preterm
8th month	Testes reaches scrotum.
	Body weight increases rapidly.
Just before birth	Cementum formed
	Sulci and gyri appear over the cerebral hemisphere.

After birth

Days	Structure formed
Soon after birth	Periodontal ligaments formed before teeth eruption.
	Lungs enter alveolar stage ( remain in this stage till 8 years of life)
	Changes in foetal blood vessels-Contraction of thick muscle wall of umbilical arteries.Lumen of umbilical veins and ductus venosus is occluded.Ductus arteriosus occluded.Pulmonary vessels increase in size.
Over a period of time	Occluded vessels become fibrous tissue and form ligament.
	Closure of frontal and occipital fontanelle.

{REFERENCE- Inderbir Singh’s Human Embryology}

Suggested book for further read on the topic with Homoeopathic Therapeutics – 

1.  A Concise Textbook Of Obstetrics And Neonatology With Homoeopathic Therapeutics by Dr Trupti M. Deorukhkar, Find this book here- https://www.bjainbooks.com/product-detail/a-concise-textbook-of-obstetrics-and-neonatology-with-homoeopathic-therapeutics 
2. Gynaecology & Obstetric Therapeutics, by Shrikant Kulkarni, Find this book here- https://www.bjainbooks.com/product-detail/gynaecology-obstetric-therapeutics 
3. Homoeopathic Medicines For Pregnancy & Childbirth, by Richard Moskowitz, Find this book here- https://www.bjainbooks.com/product-detail/homoeopathic-medicines-for-pregnancy-childbirth