Cytology · MU Sofia

Blastocyst formation.

Explains blastocyst formation from fertilisation through cleavage and the solid morula to the hollow blastocyst, with its blastocoel, inner cell mass and trophoblast. Also covers hatching from the zona pellucida, chorion development after implantation, and the hormonal support provided by hCG and progesterone. Seven pictures accompany the text.

Inside: 4 sections · 7 labelled pictures · 14 flashcards · 4 min

Topic 83 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 494 words (4 min of reading), with 14 flashcards.

In this note

  1. Introduction
  2. Key Stages in Blastocyst Formation:
  3. Key Features of the Blastocyst:
  4. Hormonal Support:

Introduction

Blastogenesis refers to the early stages of embryonic development after fertilization, which includes the formation of the morula and the blastocyst. This process is crucial as it leads to the development of a structure that will implant into the uterine wall and eventually form the embryo.

Key Stages in Blastocyst Formation:

  1. 1. Fertilization:
  • The process begins when a sperm fertilizes an egg (oocyte) to form a zygote. This typically occurs in the fallopian tube.
  • The fertilized egg undergoes cleavage, a series of rapid mitotic cell divisions, producing smaller and smaller cells called blastomeres.
  1. 2. Morula Formation:
  • As cleavage continues, the zygote divides into a solid ball of cells known as the morula (Latin for "mulberry"), typically around 16-32 cells.
  • At this stage, the cells are tightly packed together, and the structure is still solid with no fluid-filled cavity.
  1. 3. Blastocyst Formation:
  • As the morula moves toward the uterus (around day 3 to 4), it undergoes a transformation into the blastocyst.
  • A fluid-filled cavity begins to form within the morula, called the blastocoel.
  • The blastocyst is now a hollow sphere with an inner cell mass, a layer of outer cells, and a cavity in the center.
  1. 4. Differentiation into Key Structures:
  • Inner Cell Mass (ICM):
  • A group of cells located at one pole of the blastocyst, destined to become the embryo and eventually give rise to all the tissues and organs of the developing fetus.
  • Trophoblast:
  • The outer layer of cells surrounding the blastocoel, which will later develop into the placenta.
  • The trophoblast cells begin to differentiate into two layers:
  • Cytotrophoblast: The inner layer of the trophoblast cells, which will proliferate and contribute to placental growth.
  • Syncytiotrophoblast: The outer layer of the trophoblast cells, which invades the endometrium during implantation and secretes human chorionic gonadotropin (hCG) to support the pregnancy.
  1. 5. Blastocyst Hatching:
  • Before implantation can occur, the blastocyst must "hatch" from the zona pellucida, a protective glycoprotein layer surrounding it.
  • Hatching allows the blastocyst to become free and ready for implantation into the uterine lining (endometrium).
  1. 6. Development of Chorion:
  • After implantation, the trophoblast begins to form the chorion, which will contribute to the formation of the placenta and facilitate the exchange of nutrients, gases, and waste products between the mother and developing fetus.

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