**Phases of Cleavage in Human Reproduction**

Cleavage is the process of cell division that occurs after fertilization in human reproduction. It is a highly regulated and intricate series of events that leads to the formation of the blastocyst. The cleavage process is characterized by rapid cell divisions without significant growth, resulting in the formation of smaller cells called blastomeres. Here is a detailed explanation of the phases of cleavage in human reproduction:

**1. Zygote formation:**
- After fertilization, the sperm and the egg fuse to form a zygote, which is the first cell of the new individual.
- The zygote contains a set of chromosomes from each parent and is diploid.

**2. M-phase:**
- The zygote undergoes the first mitotic division, known as the M-phase or the first cleavage.
- In this phase, the zygote divides into two daughter cells called blastomeres.
- Each blastomere contains the same number of chromosomes as the parent cell.

**3. Blastomere division:**
- The blastomeres continue to divide rapidly through subsequent cleavage divisions.
- These divisions are characterized by the formation of two daughter cells from each blastomere.
- The daughter cells become progressively smaller with each division, as the cytoplasm is divided equally among them.

**4. Morula formation:**
- After several rounds of cleavage divisions, the blastomeres form a solid ball of cells known as the morula.
- The morula consists of 16-32 blastomeres and resembles a mulberry in shape.

**5. Blastocyst formation:**
- Further cleavage divisions result in the formation of a fluid-filled cavity within the morula, leading to the formation of the blastocyst.
- The blastocyst consists of two distinct cell types: the outer layer of cells called the trophoblast and the inner cell mass.

**6. Implantation:**
- The blastocyst undergoes implantation, where it attaches to the endometrium of the uterus and establishes a connection for nutrient exchange.
- Implantation occurs about 6-7 days after fertilization.

In summary, the phases of cleavage in human reproduction involve the formation of the zygote, subsequent divisions of blastomeres, the formation of the morula, and finally the blastocyst. These phases are crucial for the development of the embryo and the establishment of pregnancy.