Embryology Notes Up to Gametogenesis




Embryology: It deals with the prenatal stages of development from the fertilization of the ovum to the birth of a new individual.

  The zygote by series of changes – cell division, growth, differentiation, cell migration, delamination, and folding.


Ontogeny: Complete the life cycle of an organism. It consists of prenatal development and postnatal growth and maturity then aging, sanity and death.

Phylogeny: Encompasses the evolution or ancestral history of a group of organisms.

Ascending order of vertebrates -fish?? amphibians?? reptiles?? birds?? mammals.

The basic process of development: Two process-

  1. Growth
  2. Differentiation


  1. Growth: means an increase in bulk. Which may take place by one of the three ways


  • Multiplicative: by cell division cause an increase in cell number. It occurs by mitotic cell division, obstructed in most of the tissue and organs in prenatal life.
  • Auxetic: by increasing cell size. E.g oocyte.
  • Accretionary: by increasing accumulation of intracellular substance .e.g bones, cartilages (connective tissue).
  1. Differentiation: it is a complicated process in which a group of cells assume special characteristics and are assigned with specific functions.


  • Totipotent: after the formation of zygote each of the cell-derived from first cleavage division possess a totipotent character. And each cell produces a separate embryo. Totipotent nature may be retained up to the 8 cells of cleavage division.
  • Plastic phase
  • Chemo differentiation
  • Histo differentiation
  • Organogenesis
  • Functional differentiation

Cell cycle: The period of time between the birth of a cell as a result of the division of its parent cell and it’s own division to produce two daughter cells is known as cell cycle. Or

The alteration between mitosis and interphase is called the cell cycle.

Events; 1.karyokinesis-nuclear division

               2.cytokinesis-cytoplasmic division

Q.where mitosis occurs?

Ans. Mitosis occurs in all somatic cells and immature germ cells .e.g. spermatogonia, oogonia.

  1. Where meiosis occur?

Ans. Meiosis occurs in primary spermatocyte, primary oocyte, sec. Spermatocyte, sec. Oocyte.


Prophase :

  1. Chromosome begins to shorten, thicken and recognizable.
  2. Each chromosome split longitudinally into two chromatids except at the centromere.
  3. Outside the nucleus, centriole begins to separate moving to opposite poles of the cell.
  4. Central spindle formed by continuous elongation of microtubules between the centrioles and astral rays form.
  5. Nuclear membrane and nucleolus disappear.



  1. Chromosome move to the equator of the spindle.
  2. Chromosomes attached by their centromeres to spindle microtubules.


  1. Centromere split longitudinally.
  2. Two chromatids separate, to form two new chromosomes and move towards each pole


  1. The daughter chromosomes collected at each end of their spindle and involved by the nuclear membrane and the nucleolus reappear.

        Before mitosis ?? chromosome no.:46

           After mitosis ?? chromosome no.:46

Non-disjunction :(VVI)

After the splitting of chromosome one or more chromosomes failed to migrate properly. Due to the abnormal function of the spindle apparatus. This phenomenon is known as non- disjunction.

As a result of trisomy, monosomy occurs.


Meiosis l

  • Prophase l
  1. Laptotene
    1. Chromosomes begin to shorten, thicken and recognizable and attached to the nuclear membrane.
    2. Chromosomes appear as beads.
  2. Zygotene
    1. The pairing of chromosome .i.e synapsis occur
    2. Chromosomes have come together side by side in homologous pairs.
  3. Pachytene
    1. Pairing is complete and tetrad formation occurs.
    2. Crossing over also occurs.
  4. Diplotene
    1. Chiasmata form where crossing over has occurred.
  5. Diakinesis
    1. Nucleoli and nuclear membrane disappear.
  • Metaphase l

Chromosome pairs attached to the spindle and are arranged in an equatorial plane.

  • Anaphase l

Paired chromosome separate and move towards the pole.

  • Telophase l

(Like mitosis)

The purpose of the result of meiosis:

  1. Genetic variability enhanced through crossing over.
  2. Haploid no. Of chromosomes form. So that at fertilization the diploid number is restored.


Go phase: In this phase further cell division does not occur and cells do not enter the S phase

Permanent cell: Nerve cell and skeletal muscle cells.

      Before meiosis: DNA-4n

     After meiosis l : DNA-2n

     After meiosis ll: DNA-1n




 Spermatogenesis: The process of formation of sperm is called spermatogenesis.


  1. Spermetoctosis
  2. Meiosis
  3. Spermiogenesis

Spermatocytosis: It is the process by which spermatogonia are transformed into primary spermatocyte.

Meiosis: It is the process by which primary spermatocyte transformed into spermatids.

Spermiogenesis: The series of changes resulting in the transformation of spermatid into spermatozoa is called spermiogenesis.

  The changes include:

  1. Condensation of the nucleus.
  2. Formation of the acrosome.
  3. Formation of the neck, middle piece, and tail.
  4. Shedding off extra cytoplasm. (That is called residual body)

Oogenesis: It is the process by which ovum is formed from oogonia.

process :

Primordial germ cell (develop oogonia and spermatogonia) arises from the endoderm of the yolk sac and reach to the gonads (testes, ovary), where it differentiates into oogonia. Oogonia undergo mitotic divisions and by the end of the third-month cluster of oogonia are surrounded by a layer of flat epithelial cells known as follicular cells, originate from surface epithelium. Then oogonia enter into mitotic cell division but arrested in prophase of meiosis l and form primary oocyte. Most of the oogonia and primary oocyte become degenerate. All surviving primary oocytes surrounding by layers of flat epithelial cells. A primary oocyte with its surrounding flat epithelial cells known as the primordial follicle. Primary oocyte remains in prophase In the diplotene stage and does not finish their meiotic division before puberty is reached.

The total no. Of primary oocyte at puberty is estimated as 4,00,000 and fewer than 500 will be ovulated. At puberty 15-20 follicle begins to mature.

Thereafter primary follicle develops where primary oocyte surrounded by follicular cells changed to form flat to cuboidal epithelial. And proliferate to form stratified epithelium of granulosa cells. Surrounding stroma (connective tissue) cells proliferate to form theca follicle. Finally mature or graffian follicle develops.

Graffian follicle:

A graffian follicle consists from outside to inside :

  1. Theca externa
  2. Theca Interna
  3. Stratum granulosum
  4. Zona pellucida
  5. Perivitelline space with first polar body
  6. Oolema
  7. Ooplasm
  8. Germinal vesicle