The Blastocyst

Later, a cavity develops at one end of the morula as fluid collects, and this structure is known as the Blastocyst. By the 4th to 5th day, the blastocyst arrives at the uterus.

The blastocyst structure consists of:

• Trophoblast: The outer layer of cells.

• Inner Cell Mass: Layers of cells collected at one end of the pole, which will form the embryo.

• Body Stock: The cells connecting the inner cell mass to the trophoblast.

Formation of the Decidua

As a result of conception, the Corpus Luteum continues to grow and produces more Progesterone under the influence of Human Gonadotrophin Hormones.

• This promotes the growth and development of the secretory Endometrium into the Decidua.

• The glands in the Endometrium dilate, become more torturous, and fill with secretions.

• The Endometrium becomes soft, spongy, and highly vascularized to allow for embedment.

Embedment (Implantation)

The blastocyst usually rests on the decidua of the fundus of the uterus.

• The trophoblast secretes an enzyme which erodes the decidua and some blood vessels.

• This allows the blastocyst to embed in the decidua with the inner cell mass in the lead.

• After embedment, the decidua is named based on its location:

  • Decidua Basalis: The decidua directly under the blastocyst (where the placenta will form).

  • Decidua Capsularis: The decidua surrounding and covering the blastocyst.

  • Decidua Vera: The undisturbed decidua lining the rest of the uterus.

THE TROPHOBLAST

Trophoblast Structure and Function

The trophoblast cells multiply, forming finger-like projections called villi on the surface of the blastocyst. These villi invade the decidua and maternal blood vessels so that each projection is surrounded by maternal blood.

The trophoblastic cell layers are:

1. Syncytium or Syncytiotrophoblast: The outer layer.

2. Cytotrophoblast: The inner layer.

3. Mesoderm: The layer below (loose connective tissue).

Syncytiotrophoblast

This outer layer is composed of a nucleated layer of protoplasm. It is capable of breaking down tissue (eroding the decidua) and blood vessel walls, making nutrients and oxygen in the maternal blood accessible to the developing embryo.

Cytotrophoblast

This inner layer secretes the Human Gonadotrophic Hormone (H.C.G.), which stimulates the corpus luteum to continue producing Oestrogen and Progesterone. This maintains the secretory phase and prevents shedding (menstruation) from taking place, thus maintaining the pregnancy.

The Mesoderm

The mesoderm consists of loose connective tissue, similar to that of the inner cell mass, and the two are continuous at the point where they join the body stalk.

THE INNER CELL MASS

Three Germ Layers

The Inner Cell Mass forms the fetus itself and develops into the three primary germ layers:

1. Ectoderm: Mainly forms the skin, hair, nails, the lens of the eyes, enamel of the teeth, and the nervous system.

2. Mesoderm: Forms bones and muscles, the heart and blood vessels (including those in the placenta), kidneys, ovaries, testes, and other internal organ muscles.

3. Endoderm: Forms the alimentary tract, liver, pancreas, lungs, and thyroid glands.

• Summary of Blastocyst Derivatives: The Trophoblasts form the chorion and placenta, while the Inner Cell Mass forms the fetus, amnion, and umbilical cord.

The Amniotic Cavity

This cavity lies over the side of the ectoderm. It is filled with amniotic fluid and gradually enlarges, folding around the embryo to enclose it. The amnion membrane forms from its lining and swells out into the chorionic cavity (which eventually dissolves as the amnion and chorion fuse).

The Yolk Sac

This lies on the side of the endoderm. It provides early nourishment before the placenta takes over. Part of the yolk sac contributes to the formation of the primitive gut. The remainder resembles a balloon, floating in front of the embryo until it atrophies and becomes trapped under the amnion on the surface of the placenta.

DEVELOPMENT OF THE PLACENTA

Chorionic Development

Three weeks after fertilization, one portion of the primitive chorion, next to the Decidua Basalis, outgrows the other portion, which later atrophies.

• Chorion Frondosum: The portion that outgrows the other (will form the placenta).

• Chorion Laeve: The portion that atrophies (will form the smooth chorion membrane).

Villi and Placental Structure

Each villus on the chorion frondosum branches into tree-like projections. Each branch has three layers, and the intervillous spaces are filled with maternal blood.

• Anchoring Villi: Some larger branches penetrate deep into the decidua to anchor the placenta.

• Nutritive Villi: The smaller villi which float into the maternal blood and facilitate exchange.

The collection of villi of the chorion frondosum forms the Placenta. The placenta is fully developed at the $12^{th}$ week of pregnancy.