What have you been gone through?

Hello everyone.

I have written about cancer, covid19, lungs. Though, it is not much and there is a long road ahead, but I thought that I am missing something. Can you guess?

Here’s a hint. That happens before anything, for each and every organism and not just humans. Did you guess it? Let see.

And that is antenatal development. Or should I say prenatal development.

You guessed it right. This time I will be discussing about:

  • Fertilization
  • Embryological development
  • Germ layers
    • Germ layer formation
    • Germ layer derivatives

Shall we start?


This is the primary event needed for yours, mine or any human’s life.

It is the process where female egg/ovum gets fertilised by male sperm. Also called syngamy.

These sex cells are also called gametes. They both are haploid i.e. cell that contains single set of chromosomes. As you know, our cells contain 2 set of chromosomes, means they are paired. There are 46 chromosomes in 2 pairs, hence making 23 pairs of chromosomes. This is called diploidy(2n). So, all of our cells are diploid(2n)? Well, no.

In gonads (testes or ovary i.e. primary reproductive organs), germ cells(Q1) are diploid. These are the cells that form gametes.

These germ cells undergo mitosis(A1) to increase their number and meiosis(A2) to differentiate into gametes or sex cells i.e. egg or sperm.

These gametes from each parent during reproduction come into contact and fuse to form zygote (2n).

Q1. What makes germ cell different from somatic cells?

A1. Almost all cells in our body are somatic cells. Or in other words, any cell other than germ cell is a somatic cell. Germ cells are only found in our gonads i.e. ovaries of female and testes of male.

Q2. Why gametes are haploid?

A2. As you know that each human is made from zygote which is a diploid fertilised egg. If the gametes are not haploid or are diploid, to say, then zygote will be…. Tetraploid(4n) i.e. zygote will have 4 sets of chromosomes. This condition is actually a thing but a very rare one indeed. Triploidy is more commonly seen and occurs primarily due to fusion of 2 sperms with egg that often leads to miscarriage or stillbirth. By the way, it generates another question, why it is rare to see more than 1 sperm to fertilise egg? But let’s leave this question for some other time.

A1. Mitosis: It is also called as equational division. Why? Because chromosome numbers in daughter cells is equal to that of parent cell. Simply saying, cells before and after division contains 23 pairs of chromosomes. Eg, somatic cells and germ cells. And in germ cell, spermatogonium (undifferentiated male germ cell) multiplies to increase their number. That’s why a male is never empty(well, kind of a weird statement!). Numbers are also increased in female eggs too but it all happens before female fetus birth. This topic is vast on itself, so definitely will cover some other time.

A2. Meiosis– This cell division is seen in germ cells where, for eg, some spermatogonium divides and forms a cell that has the chromosomes number reduced to half i.e. 23 chromosomes now. You can think, what else we call this division. This is also called as reductional division.


After fertilisation or fusion in fallopian tube, fertilised egg(zygote) travels toward uterus with the help of ciliary cells and peristaltic movement of the tube. This process is called implantation that takes around 10 days (variable with ovulation and sexual practice) after conception.

During implantation:

  • Endometrium (innermost wall of uterus) adapts and changes to attach with blastocyst (zygote keeps on dividing till reaching uterus, see A3).
  • During this period, hCG (human Chorionic Gonadotropin) hormone secretion begins.
  • hCG is crucial for pregnancy maintenance as it stimulates corpus luteum (A4) to produce progesterone for about 1st trimester i.e. 12 weeks, which inhibits further ovulation i.e. release of ovum/egg.
  • After fertilisation (I will consider this as reference point), within around 2 weeks, bilaminar disc is formed. This disc (or bilaminar blastocyst) has epiblast and hypoblast.
  • Within 3 weeks, trimlaminar disc is produced by epiblast which forms ectoderm, mesoderm and endoderm. (see Germ layers to learn their formation)
  • After 3 weeks, neural tube is formed by neuroectoderm.
    • The period from 3 to 8 weeks is embryonic period.
    • It is the period of organogenesis i.e. generation of organs.
    • This makes it susceptible to teratogens (substances causing developmental abnormalities to fetus but also can lead to miscarriage).
  • Week 4, the first organ develops. Is the name beating in your mind?
    • Yes, it is heart. You can hear the 1st heartbeat at this time.
    • Another embryonic tissue starts proliferating that forms our upper and lower limbs called limb buds.
    • Organs like brain, lungs etc are developing simultaneously.
  • Week 6, head is developing, chin is taking shape.
    • A medical practitioner could perform a test where they put a transducer in pregnant lady’s vagina. The device emits sound waves to learn about the structures beyond vagina. This is called Transvaginal ultrasound(Q3).
  • After week 8, upper lips are forming.
    • Fetal movements can be noticed.
  • Week 10, fetus can be differentiated as the genitals are formed.

A3. Blastocyst: As the zygote keeps on dividing by mitotic division leading to smaller and smaller entities called blastomeres. Ultimately a distinctive structure is formed from morula (containing around 12 to 16 blastomeres) that is containing cell masses in distinguishing manner. One cell mass is called inner cell mass that forms embryo and other is called outer cell mass that forms placenta. As the name says, it also contains a cyst that is filled with fluid. This structure is formed after around 5 days after fertilisation. Now it is ready to implant into endometrium layer of uterus.

A4. Corpus luteum: A temporary endocrinal structure that forms after rupturing of ovarian follicle within ovary during ovulation. It secretes progesterone that thickens uterus for implantation of zygote. If fertilisation occurs, it grows large in size and known as corpus gravidarum that remains active for 10 weeks until placenta is formed. If no fertilisation occurs, it degenerates after 10 days leading to new menstrual cycle.

Q3. Why transvaginal ultrasound is needed when we can perform abdominal ultrasound?

A3. Actually, we could not perform abdominal ultrasound, atleast it won’t be beneficial. Reason is that, during 6 week period, the fetus has the size of a rice grain. Which you can think, will not generate enough data to study. You find it interesting to know that fetus grows at rate of 1 mm/day after 6 weeks.

Germ layers

As I said, trilaminar disc consists ectoderm, mesoderm and endoderm which later form the tissues and organs.

It happens as:

  • Epiblast and hypoblast are arranged as layers siding amniotic cavity and primary yolk sac respectively.
  • At one point, the layers are tightly attached to each other making prechordal plate that later forms mouth opening. At other i.e. caudal point some epiblast cells invaginate forming a linear band called primitive streak.
  • Cells from epiblast migrates towards primitive streak forming new layers that leads to formation of trilaminar disc formation.
  • Some cells proliferate towards cephalic end (head) from priminitive node (widened part of priminitive streak at near prechordal plate/cephalic end) forming notochord.
  • Now these cells layer are if:
    • Above (towards amniotic cavity) notochord called ectoderm.
    • Side of notochord called mesoderm.
    • Below (towards primary yolk sac) notochord called endoderm.
  • Notochord(Q4) induces ectoderm to form neuroectoderm (neural plate) from which neural tube develops.
  • Hypoblast mainly forms yolk sac and extraembryonic mesoderm.
  • You will find it interesting to know that, endoderm is 1st to develop and mesoderm is last.
  • These layers develop into various tissues.

Ectoderm: This outer layer further differentiates into:

  • Surface ectoderm-
    • Epidermis, outer layer of skin
    • Adenohypophysis (anterior pituitary), grows from Rathke pouch (A5). Vs neurohypophysis (posterios pituitary).
    • Eye lens
    • Ear’s sensory organ like utricle and saccule.
    • Olfactory epithelium inside the nasal cavity.
    • Epithelial lining of oral cavity like inner surface of lips, mouth, cheeks, gum, palate.
    • Glands like parotid, sweat and mammary.
    • Lower 2/3 of anal canal i.e. below pectinate line/dentate line (a irregular circular line formed by anal valve that divides anal canal).
  • Neural tube- Mainly forms CNS.
    • Neurohypophysis, posterior lobe of pituitary gland.
    • Oligodendrocytes, myelin sheath producing cells of CNS.
    • Astrocytes, phagocytic cells (along with microglia or macrophages) inside CNS.
    • Ependymal cells, simple columnar cells that lines choroid plexus which forms CSF.
    • Pineal gland, a pea-shaped deeply centred gland inside brain that produces melatonin (hormone that regulates circadian rhythm and reproductive maturation). Widely called ‘third eye’.
    • Neurons of CNS (brain and spinal cord).
    • Spinal cord
    • Retina, innermost layer inside eye that senses light and converts it into electrical impulses.
  • Neural crest-
    • Arachnoid and pia mater of brain, collectively called leptomeninges.
    • Cranial, dorsal root and autonomic ganglia(A6) of PNS.
    • Schwann cells, myelin sheath producing cells of PNS.
    • Melanocytes, cells found in skin that produce melanin (dark skin protecting pigment).
    • Odontoblasts, columnar cells of the pulp that produce dentin(part of tooth between pulp and enamel).
    • Adrenal medulla, inner part of adrenal gland that responds in flight or fight response by producing catecholamines i.e. epinephrine/adrenaline and norepinephrine/noradrenaline.
    • Some bones in skull.
    • Aorticopulmonary septum’s spiral part, separates aorta and pulmonary arteries in heart.
    • Endocardial cushions in heart that forms wall.


  • Endocardial cushions
  • Mesenchyme, that forms
    • Muscle
    • Connective tissue (CT) like tendon, ligaments and specialised CT like bone, cartilage and blood.
  • Serous membranes like peritoneum of abdomen, pericardium of heart, and pleura of lungs.
  • Heart, blood vessels, lymphatics.
  • Spleen, a graveyard of senile RBC’s and 2o lymphoid organ where mature lymphocytes (T and B cells) perform their duties.
  • Kidney
  • Wall and smooth muscle of digestive tract
  • Adrenal cortex, outer part of gland that produces hormones like aldosterone to maintain BP, cortisol that increase availability of glucose for flight or fight response.
  • Dermis, inner layer of skin containing sweat glands and hair follicles.
  • Gonads
  • Vagina (also develops from endoderm)


  • Gut tube epithelium upto anal canal above pecitante line
  • Lower vagina and urethra
  • Lungs
  • Liver and gall bladder
  • Pancreas
  • Eustachian tube, tube like structure connecting pharynx and middle ear that equalises pressure of outside and intratympanic side.
  • Thymus, primary lymphoid organ where T-cells after forming in bone marrow, mature.
  • Thyroid gland, contains majorly follicular cells that form T4 (thyroxine) and T3 (triiodothyronine) and parafollicular or C cells forming thyrocalcitonin hormone that checks PTH function.
  • Parathyroid gland, 4 glands situated on thyroid gland that produces PTH which increases serum calcium and decreases serum phosphorus.

A5. Rathke pouch: It is the ectodermal derived structure which outpouches at the roof of developing mouth. Also called hypophyseal diverticulum.

A6. Ganglia: A collection of neural cell bodies in the various parts of PNS where it function as relay point i.e. where one nerve enters and other exits.

This is so intriguing that I can’t express how a tiny cluster of cells turns into a living creature that influences other organism like ‘it’. And not just itself, but the whole world. Among such living creatures is us, humans, which are now trying to achieve the touch of extra-celestial objects. Wow!

Well, before I become more philosophical, let me wrap up everything with a question.

Q4. Is notochord found in adult humans?

A4. Notochord, a cartilaginous structure derived from mesoderm, is a characteristic feature of all members of chordate (a higher category i.e. phylum of taxonomic classification) that signals the development.

In adult humans, it is found in nucleus pulposus (a jelly-like soft central portion) of intervertebral disc which can be compressed to absorb shock while movement.

This topic was very interesting to discuss. Ask me anything in the comment section.


Published by signaturedoctor

I am a doctor-to-be pursuing my medical studies. I want to share my knowledge to fellow medical students and to other interested people.

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