Cytology · MU Sofia

Periods in embryonic development. Critical periods.

Walks through the four periods of human embryonic development: the pre-embryonic period up to implantation, the embryonic period of weeks 3 to 8, the fetal period from week 9 to birth, and the postnatal period. Each is covered by its time frame and key events, with the sensitivity of each stage to teratogens described for the critical windows. It closes with general characteristics of critical periods, including timing and irreversibility of damage.

Inside: 7 sections · 5 labelled pictures · 13 flashcards · 6 min

Topic 97 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 815 words (6 min of reading), with 13 flashcards.

In this note

  1. Introduction
  2. Periods in Embryonic Development
  3. Pre-Embryonic Period (Weeks 1–2)
  4. Embryonic Period (Weeks 3–8)
  5. Fetal Period (Weeks 9–Birth)
  6. Postnatal Period (After Birth)
  7. General Characteristics of Critical Periods:

Periods in Embryonic Development

Embryonic development can be divided into several stages, each characterized by distinct processes and events crucial for the formation of a fully developed organism. These periods are marked by critical milestones in morphogenesis (the shaping of the organism) and organogenesis (the formation of organs). The critical periods are phases during which the embryo is particularly sensitive to environmental influences, such as teratogens, which can lead to birth defects if exposure occurs during these times.

Pre-Embryonic Period (Weeks 1–2)

  • Time Frame: Fertilization to implantation (the first two weeks post-fertilization).
  • Key Events:
  • Fertilization occurs, and the zygote begins the process of cleavage (cell division).
  • The morula forms, followed by the blastocyst, which is the structure that implants into the endometrium of the uterus around days 6–7 post-fertilization.
  • Trophoblast differentiation: The outer cells of the blastocyst (trophoblast) form the syncytiotrophoblast, which invades the uterine lining, and the cytotrophoblast, which gives rise to the placenta.
  • Inner cell mass (ICM) forms the embryo, while the amnion and yolk sac begin to develop.

Critical Periods: The pre-embryonic period is the stage when the zygote is most vulnerable to chromosomal abnormalities or genetic mutations. Major structural defects are less common here because the embryo is still in a stage of cell division and differentiation, but disruptions in early development can lead to miscarriage.

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