Fertilization and Implantation Notes

Fertilization and Zygote Formation

Fertilization occurs when the male and female pronuclei fuse, forming a zygote. The zygote is initially surrounded by the zona pellucida, which prevents implantation in the fallopian tube where fertilization takes place (specifically, the ampulla).

Early Cleavage and Morula Stage

After fertilization, the zygote undergoes cleavage:

  • First cleavage: Occurs 20−3020-30 hours post-fertilization, resulting in a two-celled stage.
  • Further divisions: Lead to four-celled, eight-celled, and then sixteen-celled stages.
  • Morula: The stage between 88 to 1616 cells. It is called morula because it resembles a mulberry. The morula remains surrounded by the zona pellucida.

Zygote Transport and Uterine Entry

The zygote stays within the fallopian tube for approximately three days. During this time, it receives nutrition from the secretory cells of the fallopian tube.

  • Fallopian tube cells: The fallopian tube is lined with ciliated columnar epithelium, including ciliated cells, secretory cells, and PEG cells.
    • Ciliated cells: Aid in pushing the zygote towards the uterine cavity through ciliary movement, complementing peristalsis.
    • Secretory cells: Provide essential nutrition to the early zygote.
    • PEG cells: Their function is unknown.
  • Movement Mechanism: Zygote moves due to:
    • Peristalsis in the tube (primary reason).
    • Ciliary movement.

Ectopic Pregnancy and Contraception

Contraceptives generally prevent all types of pregnancies, including ectopic pregnancies. However, failures in hormonal contraception, especially those involving progesterone, can increase the risk of ectopic pregnancy.

  • Progesterone: As a smooth muscle relaxant, it can decrease peristalsis in the fallopian tube, leading to implantation in the fallopian tube (ectopic pregnancy).

Entry into Uterine Cavity

The zygote, now in the morula stage, enters the uterine cavity approximately four days after fertilization, typically at the 16-celled stage.

Important points:

  • Timing: Enters four days post-fertilization.
  • Stage: Enters as a 16-celled morula.

Menstrual Cycle Correlation

Assuming a regular 2828-day menstrual cycle, ovulation occurs around day 14. Fertilization is presumed to occur on the same day as ovulation. Therefore, the zygote enters the uterine cavity on day 18 of the menstrual cycle (14 + 4 = 18).

Key Presumptions:

  1. 2828-day regular cycle.
  2. Ovulation on day 14.
  3. Fertilization occurs on the same day as ovulation.

Blastocyst Formation and Implantation

On day 5, the zona pellucida is shed off in a process called zona hatching. Fluid enters the morula, transforming it into a blastocyst.

Blastocyst Structure:

  • Inner cell mass: Cells clustered on one side of the blastocyst's interior that will form the embryo.
  • Trophoblast: The outer layer of cells lining the blastocyst.

Implantation: The blastocyst attaches to the uterine endometrium. In humans, implantation is interstitial, meaning the blastocyst burrows deep into the endometrium.

  • Timing: Begins six days after fertilization (day 20 of the menstrual cycle), with an implantation window between days 20 and 21.
  • Completion: Ends approximately 10-11 days after fertilization.

Decidua Formation and Implantation Phases

Following implantation, the endometrium is now referred to as the decidua.

Phases of Implantation:

  1. Apposition: Blastocyst makes initial contact with the endometrium. It is facilitated by spinopods (villi-like structures) on the endometrium, which are morphological markers of endometrial receptivity.
  2. Adhesion: Attachment is strengthened by proteins like integrins and selectins.
  3. Invasion: Facilitated by metalloproteinase enzymes.

Nita Book's Layer: A layer that limits the penetration of the blastocyst, preventing it from invading the myometrium (muscle layer of the uterus).

Site and Nature of Implantation

  • Location: Implantation typically occurs in the upper posterior wall of the uterus, near the fundus.
  • Eccentricity: Implantation is eccentric, occurring on one side of the uterus, leading to asymmetrical growth in early pregnancy, known as the Piscasek sign.

Gestational Sac: Forms wherever implantation happens and is also eccentric in location. A true gestational sac should be distinguished from a pseudo gestational sac

Hartmann's Sign: Some females may experience bleeding at the time of implantation

Intradecidual Sign: On ultrasound, a sign is visible early that signifies the blastocyst is going deep inside the endometrium and then implanting.

Decidua Subdivisions

After implantation, the decidua is divided into three parts:

  1. Decidua Basalis: Located below the blastocyst; forms the maternal side of the placenta and site for future placenta formation.
  2. Decidua Capsularis: Overlies the blastocyst, separating it from the uterine cavity.
  3. Decidua Parietalis: The remaining part of the decidua lining the uterine cavity.

Identifying Decidua on Images:

  1. Locate the uterine cavity.
  2. Identify decidua capsularis between the uterine cavity and the blastocyst.
  3. Decidua basalis is directly opposite the decidua capsularis.
  4. The rest is decidua parietalis.

Uterine Cavity Obliteration and Decidua Fusion

As the embryo grows, it gradually fills the uterine cavity. Eventually, the decidua capsularis and decidua parietalis fuse together. This fusion and obliteration of the uterine cavity typically occur around 14 to 16 weeks of pregnancy.

Clinical Significance:

  • Timing: Obliteration occurs at 14-16 weeks.
  • Superfetation: Theoretically, superfetation (fertilization of another ovum during an existing pregnancy) would only be possible until the uterine cavity is obliterated.

Ultrasound Signs in Early Pregnancy

  1. Intradecidual Sign: Initial sign of implantation.
  2. Double Decidual Sac Sign (or Ring Sign): As the blastocyst grows, it indents the uterine cavity, creating a bulge visible on ultrasound. This blastocyst is surrounded by two decidual layers:
    • Inner Layer: Decidua capsularis.
    • Outer Layer: Decidua parietalis.

Blastocyst Cellular Components

A fully formed blastocyst has approximately 58 cells:

  • Inner Cell Mass: 5 cells, form the embryonic disc, which will develop into the entire embryo.
  • Trophoblast: Remaining 53 cells.

Trophoblast Differentiation

On day 8 after fertilization, the trophoblast differentiates into two layers:

Cytotrophoblast (Langhan cell layer): Retains cell boundaries.

Syncytiotrophoblast: Loses cell boundaries, forming a multinucleated syncytium. It is derived from cytotrophoblast and is the hormone factory of the placenta.

Cytotrophoblast Subtypes and Functions

The cytotrophoblast further differentiates into:

  • Villous Cytotrophoblast:
    • Near Decidua Basalis: Forms villi-like structures called chorion frondosum, which becomes the fetal side of the placenta. If chorion frondosum is not given in the options, cytotrophoblast could be the answer, and trophoblast as the last option.
    • Rest of Cytotrophoblast: Remains smooth, forming the chorion laeve, which contributes to the fetal membrane chorion.
  • Extravillous Cytotrophoblast:
    • Endovascular Cytotrophoblast: Replaces the lining of maternal spiral arteries, converting them from high-resistance to low-resistance vessels, which is called trophoblastic invasion. It occurs in two waves, completed by 12 and 16 weeks of pregnancy.
    • Interstitial Cytotrophoblast: Forms the cytotrophoblastic shell.

Inner Cell Mass and Germ Layer Formation

The inner cell mass becomes the embryonic disc, which divides the blastocyst into two cavities:

  • Amniotic Cavity: Above the embryonic disc, lined by ectoderm. The ectoderm gives rise to the amnion.
  • Yolk Sac: Below the embryonic disc, lined by endoderm (also called the endodermal sinus). The second germ layer to appear.

Germ Layer Formation:

  1. Ectoderm: First germ layer to form; gives rise to the amnion.
  2. Endoderm: Second germ layer to form; lines the yolk sac.
  3. Mesoderm: Third and last germ layer to appear (from the primitive streak).

Double Bleb Sign and Embryonic Disc Folding

The appearance of the yolk sac on ultrasound results in the double bleb sign, where the amniotic cavity and yolk sac are visible.

The embryonic disc undergoes folding to form cranial and caudal folds, which are seen as fetal poles on ultrasound. The crown-rump length (CRL) measures the distance between these folds.

Fetal Heartbeat and Ultrasound Structures

Heartbeat initiation: The heart starts beating around 21 days after fertilization.

Ultrasound Structures:

  1. Gestational Sac: Visible on transvaginal sonography (TVS) around four weeks and one to three days.
  2. Yolk Sac: Visible around four weeks and five days.
  3. Crown-Rump Length (CRL) and Fetal Cardiac Activity: Visible on TVS between five to six weeks.

Calculating Pregnancy Age

Pregnancy is calculated from the first day of the last menstrual period (LMP), not from the day of fertilization. For example, if fertilization occurs two weeks after the LMP, the pregnancy is considered four weeks along at the time of the missed period.

Ultrasound Signs and Discriminatory Zone

In intrauterine pregnancies, a gestational sac should be visible on TVS if beta-hCG levels are above 2,000 international units per liter. This value is known as the discriminatory zone or critical value of hCG. On TAS, a gestational sac should be visible if beta-hCG levels are above 6,500 international units per liter.

If hCG levels are less than 2,000, repeat beta-hCG after 48 hours. In a normal pregnancy, the levels would double.

Pseudo vs. True Gestational Sac

Pseudo Gestational Sac (Ectopic Pregnancy):

  • Thickened decidua that appears like a gestational sac.
  • Centrally located.
  • Size remains constant.
  • No double decidual sac sign.
  • No yolk sac; no double bleb sign.

True Gestational Sac (Intrauterine Pregnancy):

  • Eccentric location.
  • Grows by 1-2 mm per day.
  • Double decidual sac sign is present.
  • Yolk sac and double bleb sign are present.

If mean sac diameter is greater than or equal to 10 mm and no yolk sac is seen, it is likely a pseudo gestational sac.

Diagnosis of Blighted Ovum and Missed Abortion

Blighted Ovum (Anembryonic Pregnancy):

  • Mean sac diameter >= 25 mm.
  • Yolk sac present, but no fetal tissue (no fetal pole, crown-rump length, or cardiac activity).

Missed Abortion:

  • Gestational sac, yolk sac, and crown-rump length are present.
  • CRL >= 7 mm, but no cardiac activity is detected.
  • No bleeding symptoms.