2ND WEEK OF EMBRYONIC DEVELOPMENT

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IMPLANTATION (2ND WEEK)

Implantation of the blastocyst begins at the end of the first week and is complited by end of the second week. The cellular and molecular events related to implantation are complex. It involves a receptive endometrium and hormonal factors 

  1. Estrogen 
  2. progesteron 
  3. prolactin
  4. growth factors
  5. HOX genes

  • The Zona pellucida degenerates (day 5). Its disappearence results from enlargement of the blastocyst and degeneration caused by enzymatic lysis. The lytic enzymes releases from the acrosomes of the sperm that surround and partially penetrate the zona pellucida.

  • The blastocyst adheres to the endometrial epithelium mainly seen at (DAY 6)

  • The trophoblast differentiate into two layers 
  1. syncytiotrophoblast 
  2. cytotrophoblast (Day 7)
  • The syncytiotrophoblast  erodes enometrial tissues and the blastocyst starts to embed in the endometrium (day 8)

  • Blood filled lacunae appear in the syncytiotrophoblast (day 9)

  • The blastocysts sinks beneath the endometrial epithelium and the defect is filled by a closing plug (day 10)

  • lacunar networks form by the fusion of adjacent lacunae (Day 10 and Day 11)

  • The syncytiotrophoblast erodes enometrial blood vessels , allowing maternal blood to seep in and out of lacunar networks , thereby establishing a uteroplacental circulation (day 11 to day 12)




  • The defect in the endometrial epithelium is repaired (day 12 and day 13)

  • Primary chorionic villi develop (day13 to day14)


PLACENTA PREVIA 
Implantation of a blastocyst in the inferior segment of uterus near the internal os results in placenta previa, a placenta that partially or completely covers the os . Placenta pervia may cause bleeding because of premature separation of the placenta during pregnency or at delivery of the fetus.

SECOND WEEK DEVELOPMENT
  • Rapid prolifiration  and differentiation of the trophoblast occurs as the blastocysts completes its  implantation  in the endometrium.

  • The endometrial changes resulting from the adaptation of these tissues in preparation for implantation are known as the decidual reaction.

  • The primary umbilical vesical forms and extraembryonic mesoderm develops . the extraembryonic coelom forms from spaces that develop in extraembryonic mesoderm . The coelom later becomes the chorionic cavity .

  • The primary umbilical vessicle becomes smaller and gradually disappears as secondary umbilical vesicle develops 

  • The amniotic cavity appears as  a space between the cytotrophoblast and embryoblast .

  • The embryoblast differentiates into bilaminar embryonic disc consisting of epiblast , related to the amniotic cavity and hypoblast , adjacent to blastocyst cavity.

  • The prechordial plate develops as a localized thickening of the hypoblast , which indicates the future cranial region of the embryoand the future site for mouth and the prechordal plate is also an important organizer of the head region (ie the formation of head)


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