Development Of The Maternal-Fetal Unit

Knowledge of the growth and development of the maternal-fetal unit provides a basis for the care of the mother and fetus at risk for disease or pregnancy complications. This knowledge can provide a basis for early detection of maternal or fetal problems: therefore more serious complications can be prevented.

Embryo

During the luteal phase of the menstrual cycle, cervical mucus becomes receptive to spermatozoa. Ejaculation of sperm into the vagina is aided by mucoid receptivity, which allows rapid migration of spermatozoa through the cervix, into the uterine cavity, and into the fallopian tube.

Week One, Fertilization and Cleavage

The oocyte becomes fertilized in the fallopian tube and immediately transforms into a zygote. The zygote finally bursts the protective shell of the zona pellucida in order to make contact with the uterine

wall.

Oocyte

An unfertilized human oocyte.

Fertilization

Fertilization

Fertilization means fusion of one spermatozoon with an ovum to form a single cell, the zygote.

Fertilization begins when the sperm fuses with the egg to form a diploid zygote. This usually occurs in the upper reaches of the fallopian tube.

The sperm and ovum meet in the distal portion of the fallopian tube. Fertilization occurs when the head of motile sperm approaches to zona pellucida and start the process of penetration. This is helped by enzymes present at the acrosomal cap of sperm head. Once this happens, the sperm head is engulfed by the ovum and the zona pellucida becomes impermeable to any further sperm.

Cleavage

Cleavage is the process of early mitotic cell divisions, which progressively reduce cell size during the passage of the embryo down the fallopian tube towards the uterus.

During cleavage, the total embryonic mass remains relatively constant.

So the values of fertilization are:

1. Sex determination.

2. Initiation of cleavage.

Cleavage

Cleavage means rapid succession of mitotic division of the zygote with subsequent production of large number of small cells called blastomers

It occurs about 40 - 60 hours after fertilization.

The zygote undergoes first cleavage division into two blastomer stage, four blastomer stages….etc. until a large number of new cells have collected on the inner side of the zona pellucida are known as morula.

When the embryo has about 16 cells, its individual cells begin to adhere to one another, and it coalesces into a morula (Latin for mulberry) shape.

Blastocyst Formation

A cavity forms in the morula when it enters the uterus. This cavitation is an important transition from homogeneous cells to differentiated cell function. This new structure is called a blastocyst The blastocyst consists of an outer layer, the trophoblast, and an inner cluster of cells, the inner cell mass.

Continued expansion of the blastocyst cavity eventually ruptures the protective zona pellucida (shell) surrounding the morula. Before the morula makes contact with the uterine wall, the zona pellucida will be shed.

Implantation begins

Implantation is the process in which the blastocyst attaches to and penetrates into the uterine wall. Upon contact with the uterine lining (the endometrium), newly defined trophoblast cells begin an invasive activity that gives the embryo access to the deeper layers of the uterine wall.

The implanting trophoblast cells differentiate into two new cell types syncytiotrophoblasts and cytotrophoblasts.

Syncytiotrophoblast cells grow without cell division throughout implantation, becoming large, multinucleate cells, or syncytia (meaning fused cells). Cytotrophoblasts, remain individually distinct, mononucleated cells that invade deeper into the uterine wall than the syncytiotrophoblasts.