EMBRYOLOGY

It is the branch of biology where we study the development of the embryo during gestation or pregnancy period. 
  1. The Father of Embryology is Aristotle. 
  2. Father of modern Embryology – Von Baer.
  3. There are four basic stages of embryonic development
  • Cleavage
  • Morulation
  • Blastulation
  • Gastrulation

CLEAVAGE (Cellulation or Segmentation)


  1. The term 'Cleavage' was given by "Von Baer".
  2. Fertilized egg undergoes repeated cell divisions which occur rapidly producing a multicellular structure without changing its size.
  3. Due to the process of cleavage, a single-celled zygote, through successive mitotic cell divisions changes into a complex multicellular structure.

     

CLEAVAGE

CLEAVAGE

With each cleavage division:
  • Number of blastomeres increases
  • Size of blastomeres decreases
  • Embryo size – remain same
  • KI (Karyoplasmic index) value increase 

DiFFERENCE B/W CLEAVAGE & MITOSIS

CLASSIFICATION OF CLEAVAGE

A. Based on the fate of blastomeres 
  • Determinate cleavage:- The fate of blastomeres is fixed, determined. Blastomeres form a particular portion of the embryo. Eg. Nematoda, annelida, mollusca .
  • Indeterminate cleavage:- The fate of blastomeres is not definite. These are totipotent in the early stage of the embryo. Eg, humans.
Twins formation


CLASSIFICATION OF CLEAVAGE

B.On the basis of the amount of Yolk:

  • Balfour's law rate of cleavage is inversely proportional to the amount of yolk present in the egg.
  • The yolk present in the egg disturbs the rate of cleavage.
  •  The rate of cleavage is slow in that part of the egg, in which the amount of yolk is more, and the rate of cleavage is faster in the portion of the egg in which yolk is in lesser amount.

Based on the amount of Yolk 

A.Complete or holoblastic:-

  • When cleavage furrow passes through the egg completely.
  • Whole egg divides

B.Incomplete or Meroblastic:-

  • Cleavage does not occur in the part of the egg where the yolk is present.
  • Cleavage occurs only in the cytoplasmic part
  • Found in megalecithal eggs  

GENERAL STAGES OF EMBRYONIC DEVELOPMENT 

Zygote:

  • Single-celled stage 
  • Covered by zona pellucida
  • Present in the fallopian tube 

Morula:

  • The embryo with 8 to 16 blastomeres (celled) is called the morula 
  • Covered by zona pellucida  
  • Solid ball like / mulberry like  
  • Forms in the isthmus of the fallopian tube 

 Blastula/blastocyst

  • Blastula of eutherian and metatherian mammals is called a blastocyst
  • More than 16 celled stage
  • Present in uterine cavity
  • Covered by Zona pellucida but just after implantation it disappears
  • Inner cell mass contains certain cells called stem cells which have the potency to give rise to all tissues and organs 
  • Inner cell mass gets differentiated as the embryo HUMAN REPRODUCTION 
  • The trophoblast layer then gets attached to the endometrium
  • The trophoblast secretes HCG hormone
  • The presence of HGC hormone in the maternal blood or urine is an indicator of pregnancy and is the hormone detected by the home pregnancy test
  • Trophoblast cells secretes lytic enzymes to dissolve cells of endometrium for implantation so help in attachment to uterine epithelium  


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