What is cleavage in embryology?

Cleavage is the process of rapid mitotic divisions where the zygote divides into smaller cells called blastomeres. It occurs as the zygote travels down the uterine tube, remaining enclosed by the zona pellucida.

What are the key stages of cleavage?

Cleavage progresses through several stages: 2-cell stage (unequal division with the larger cell dividing first), 3-cell to 8-cell stages (sequential divisions), and 16-cell stage (morula, resembling a mulberry and segregating into inner and outer cell masses).

How does a morula form and what are its components?

A morula forms at the 16-cell stage of cleavage. Its cells segregate into the inner cell mass (embryoblast), which develops into the embryo, and the outer cell mass (trophoblast), which forms extraembryonic tissues.

What is a blastocyst and how does it form from a morula?

A blastocyst forms when the morula enters the uterine cavity and absorbs endometrial fluid, creating a cavity called the blastocele. It consists of the embryoblast, trophoblast, and blastocele, with the zona pellucida disappearing to allow enlargement and preparation for implantation.

What is the function of the zona pellucida during cleavage and blastocyst formation?

The zona pellucida prevents premature implantation, such as in the uterine tube, and ensures that implantation occurs only in the uterine cavity. It disappears during blastocyst formation to allow for enlargement.

What is a hydatidiform mole and how is it related to trophoblast development?

A hydatidiform mole is a trophoblastic disorder where the trophoblast proliferates excessively without a viable embryo. It often results from fertilization of an empty oocyte by a single sperm and can progress to choriocarcinoma in rare cases.

What are the clinical features of a hydatidiform mole?

Clinical features include high HCG levels (>100,000 mIU/ml), preeclampsia in the first trimester, vesicles resembling grape bunches and a snowstorm appearance on ultrasound, and absence of fetal movements or heartbeat.

Cleavage & Blastocyst – Morula to Implantation

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