FETAL DEVELOPMENT

REPRODUCTIVE CYCLE

  • Overview: The reproductive cycle is characterized by regular changes in the secretions of the anterior pituitary gland, ovaries, and endometrial lining of the uterus.

  • Anterior Pituitary Role:

    • The anterior pituitary, in response to the hypothalamus, secretes FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone).

  • Functions of FSH:

    • FSH stimulates the maturation of a follicle in the ovary, which contains a single ovum.

  • Hormonal Changes:

    • The maturing ovum and the corpus luteum (the empty follicle after ovum release) produce increased amounts of estrogen and progesterone.

    • This increase leads to the enlargement of the endometrium.

  • LH Surge:

    • A surge in LH stimulates the final maturation and release of the ovum.

OVARIAN CYCLE

THE MENSTRUAL CYCLE

  • Hormonal phases in the menstrual cycle include:

    • Low estrogen at the end of menses

    • Follicular phase

    • Rise of estrogen levels

    • Ovulation phase triggered by LH surge.

    • Luteal phase where progesterone is secreted by the corpus luteum.

    • The uterine cycle consists of three phases:

    • Proliferative Phase:

      • Influenced by estrogen; the thickness of the endometrium rapidly increases.

    • Secretory Phase:

      • Influenced by progesterone; the lining becomes highly vascular and edematous.

TUBAL TRANSPORT

  • During transport:

    • The zygote undergoes rapid mitotic divisions known as cleavage.

    • Although the zygote size does not change, individual cells become smaller with each division, forming a solid ball called the morula.

  • Development of the Morula:

    • The morula enters the uterus, and cells form a cavity with two distinct layers:

    • Inner layer: Blastocyst, develops into the embryo and embryonic membranes.

    • Outer layer: Trophoblast, develops into the chorion.

IMPLANTATION

  • The zygote typically implants in the upper section of the posterior uterine wall.

  • Cells burrow into the prepared lining of the uterus, known as the endometrium, which is then referred to as the decidua.

DEVELOPMENT

  • After implantation:

    • Cells begin to differentiate and develop specialized functions, forming the chorion, amnion, yolk sac, and primary germ layers.

    • Chorion:

    • A thick membrane with finger-like projections (villi) that envelopes the amnion, embryo, and yolk sac.

    • Amnion:

    • A thin second membrane that envelops and protects the embryo.

    • The chorion and amnion together form the amniotic sac filled with fluid, allowing the embryo to float freely.

    • Yolk Sac:

    • Appears on the ninth day after fertilization and functions during embryonic life to initiate red blood cell production.

    • Eventually, the umbilical cord encompasses it, leading to its degeneration.

    • Germ Layers:

    • The primary germ layers include the ectoderm, mesoderm, and endoderm.

PREGNANCY

  • Description involves observing a 3D animation showcasing the developing fetus.

PLACENTA

  • The placenta is a temporary organ responsible for fetal respiration, nutrition, and waste excretion, also functioning as an endocrine gland.

  • Formation: Occurs when the chorionic villi of the embryo extend into the blood-filled spaces of the decidua.

  • Appearance:

    • Maternal Side: Exhibits a beefy red appearance, referred to as “Dirty Duncan.”

    • Fetal Side: Covered by the amnion, giving it a shiny appearance, referred to as “Shiny Schultz.”

  • Physiological Importance: Plays a crucial role in fetal development through the transfer of nutrients and waste excretion.

MATERNAL SURFACE OF PLACENTA VS. FETAL SURFACE OF PLACENTA

  • Cotyledon: Functional unit within the placenta.

  • Septum of Cotyledon: Divides the cotyledons for better nutrient transfer coordination.

PLACENTAL HORMONES

  • Progesterone:

    • Maintains uterine lining for the implantation of the zygote.

    • Reduces uterine contractions.

    • Prepares breast ducts for lactation.

  • Estrogen:

    • Stimulates uterine growth and increases blood flow to uterine vessels.

    • Stimulates development of breast ducts for lactation preparation.

  • Human Chorionic Gonadotropin (hCG):

    • Signals that conception has occurred and forms the basis of pregnancy tests.

  • Human Placental Lactogen:

    • Causes decreased insulin sensitivity and utilization by the mother, making more glucose available for the fetus.

UMBILICAL CORD

  • Development: The umbilical cord develops alongside the placenta and fetal blood vessels, acting as the “lifeline” between mother and fetus.

  • Structure:

    • Contains 2 arteries that carry blood away from the fetus and 1 vein that returns blood to the fetus.

    • The blood vessels are cushioned and separated by Wharton’s jelly.

  • Normal Length: Approximately 22 inches, usually protrudes from the center of the placenta.

FETAL CIRCULATION

  • Establishment: Circulation of blood through the placenta to the fetus becomes well established after the fourth week of gestation.

  • Physiological Diversions:

    • Because the fetus does not breathe air and the liver does not process waste, certain diversions are necessary:

    • Ductus Venosus:

      • Diverts some blood away from the liver as it returns from the placenta.

    • Foramen Ovale:

      • Allows most blood to pass from the right atrium directly to the left atrium, bypassing the lungs.

    • Ductus Arteriosus:

      • Diverts most blood from the pulmonary artery into the aorta.

MULTIFETAL PREGNANCY

  • Monozygotic Twins (Identical):

    • Develop when the embryonic tissue from a single fertilized ovum splits to form two individuals.

    • Each develops into its own amnion; if they share an amnion, they may be conjoined.

    • Genetically identical, same sex, and exhibit similar appearances.

  • Dizygotic Twins (Fraternal):

    • Develop when two different ova are fertilized simultaneously by two different sperm, resulting in separate zygotes.

    • Each has its own amnion, chorion, and placenta.

    • May be the same or different sexes, often resemble regular siblings in appearance.

COMPARISON OF TWINS

  • Monozygotic Twins:

    • Derived from one egg,

    • Always the same sex,

    • Very similar in appearance.

  • Dizygotic Twins:

    • Derived from two eggs,

    • May be the same or different sex,

    • Different in appearance.