Fetal Development

Fetal Development Notes

Fertilization and the Preembryonic Stage

  • Embryo Definition:
    • The term 'embryo' has varied meanings across different authorities.
    • Some authorities define the embryo as the fertilized egg or the two-cell stage.
    • Others assert that an individual becomes an embryo when it is 16 days old and consists of three primary germ layers: ectoderm, mesoderm, and endoderm.
  • Preembryonic Stage:
    • This stage covers the first 16 days following fertilization.

Sperm Migration

  • Fertilization Window:
    • The egg must be fertilized within 12 to 24 hours of ovulation to survive.
    • Sperm must encounter the egg somewhere in the distal one-third of the uterine tube.
  • Challenges Faced by Sperm:
    • A vast majority of sperm do not reach the egg due to various factors:
    • Destroyed by vaginal acid or draining from the vagina.
    • Fail to penetrate the mucus of the cervical canal.
    • Destroyed by leukocytes in the uterus.
    • Half of the sperm travel up the wrong uterine tube.
    • Of the 300 million sperm ejaculated, approximately 200 spermatozoa successfully reach the vicinity of the egg.

Fertilization

  • Sperm Viability:
    • Sperm can remain viable for up to 6 - 7 days after ejaculation.
    • Fertilization is optimal if sperm are deposited a few days prior to ovulation or within 14 hours after.
  • Enzymatic Action:
    • When a sperm encounters an egg, it releases enzymes necessary for penetrating the egg, allowing it to pass through granulosa cells and then the zona pellucida.
  • Mechanism of Fertilization:
    • Fertilization combines the haploid (n) set of sperm chromosomes with the haploid set of egg chromosomes, producing a diploid (2n) set.
  • Polyspermy Prevention Mechanisms:
    • Fast Block:
    • The binding of a sperm to the egg opens sodium (Na+) channels in the egg membrane.
    • Slow Block:
    • Involves secretory vesicles and cortical granules located just below the membrane.

Meiosis II and Formation of Zygote

  • Secondary Oocyte:
    • The secondary oocyte begins meiosis II prior to ovulation but completes it only upon fertilization.
    • It forms the second polar body, discarding one chromatid from each chromosome.
  • Pronuclei Formation:
    • Both sperm and egg nuclei swell and become pronuclei.
  • Zygote Formation:
    • Each pronucleus ruptures, and the chromosomes from the two gametes mix into a single diploid set.
    • The fertilized egg is now called the zygote, which is prepared for its first mitotic division.

Major Stages of Prenatal Development

  • Pregnancy Timeline:
    • Pregnancy is divided into three-month intervals known as trimesters.
  • First Trimester (Weeks 1-12):
    • More than half of all embryos die from natural causes during this time.
    • The conceptus is notably vulnerable to stress, drugs, and nutritional deficiencies.
  • Second Trimester (Weeks 13-24):
    • Most organs complete their development.
    • The fetus begins to resemble a distinctly human form.
    • Chance of survival increases (with intensive care) if born near the end of this trimester.
  • Third Trimester (Weeks 25-Birth):
    • The fetus experiences rapid growth and achieves sufficient cellular differentiation to survive outside the womb.
    • By 35 weeks and weighing 5.5 lbs, the fetus is considered mature.

The Preembryonic Stage

  • Overview of Preembryonic Stage:
    • The preembryonic stage is the first two weeks of development, culminating in the formation of an embryo, described as involving three major processes:
    • Cleavage
    • Implantation
    • Embryogenesis

Cleavage and Early Development

  • Cleavage Process:
    • Cleavage refers to the mitotic divisions occurring in the first 3 days as the conceptus migrates down the uterine tube.
    • The first cleavage happens within 30 hours after fertilization, resulting in the zygote splitting into two daughter cells known as blastomeres.
    • By the time the conceptus reaches the uterus (around 72 hours post-ovulation), it becomes a morula—a solid ball of approximately 16 cells, resembling a mulberry.
    • The morula remains no larger than the zygote while producing increasingly smaller blastomeres.

Blastocyst Formation

  • Blastocyst Development:
    • The morula lies free in the uterine cavity for 4 to 5 days, further dividing into about 100 cells.
    • The zona pellucida disintegrates, releasing the conceptus, now referred to as a blastocyst.
    • Blastocyst Structure:
    • Outer layer: Trophoblast (destined to form the placenta and assist in nourishing the embryo).
    • Inner cell mass: Embryoblast (destined to develop into the embryo).

Implantation Process

  • Attachment to Uterine Wall:
    • The blastocyst attaches to the uterine wall approximately 6 days after ovulation, usually on the fundus or posterior wall.
    • Implantation refers to the process in which the blastocyst adheres to the endometrium.
  • Embryogenesis:
    • Refers to the arrangement of blastomeres into three primary germ layers within the embryoblast: ectoderm, mesoderm, and endoderm.

Hormonal Support During Early Development

  • Human Chorionic Gonadotropin (HCG):
    • The trophoblast secretes HCG which stimulates the corpus luteum to secrete estrogen and progesterone.
    • Progesterone plays a crucial role in suppressing menstruation.
    • HCG levels rise in the mother’s blood until the end of the second month.
  • Chorion Formation:
    • The trophoblast develops into a membrane called the chorion, which takes over the role of the corpus luteum, making HCG unnecessary.
    • After this point, ovaries become inactive for the remainder of the pregnancy, while both estrogen and progesterone levels rise from the chorion.

Embryonic and Fetal Stages

  • Embryonic Stage:
    • Begins when all three primary germ layers are present, typically on day 16 post-fertilization.
    • The placenta forms over the next 6 weeks, becoming the primary source of nutrition for the embryo.
  • Organogenesis:
    • This is the process by which the germ layers differentiate into organs and organ systems.
    • By 8 weeks, organs are present, although not fully functional, during which the embryo transitions to being termed a fetus.

Embryonic Folding and Organogenesis

  • Embryonic Folding:
    • During the embryonic stage, the flat embryonic disc transforms into a cylindrical form around week 4.
    • The embryo grows quickly, folding around a yolk sac, forming a C-shape with the head and tail nearly touching.
    • The lateral margins of the disc fold around the yolk sac to create the ventral surface of the embryo.
  • Tissue Formation:
    • Ectoderm now covers the entire surface, eventually forming the epidermis of the skin.
    • Mesoderm splits into two layers: one adheres to the ectoderm, and the other to the endoderm, resulting in the formation of the coelom—the body cavity between the two mesoderm layers.

Organ Formation Timeline

  • Presence of Organs:
    • At 8 weeks, all organs are present in the fetus, measuring approximately 3 cm long.
    • Key derivatives from the primary germ layers are as follows:
    • Ectoderm:
      • Epidermis, nervous system, lens and cornea, integumentary glands, internal ear.
    • Mesoderm:
      • Skeleton, muscle, cartilage, blood, dermis, lymphoid tissue, gonads and ducts, kidneys and ureters.
    • Endoderm:
      • Gut and respiratory epithelium and glands, bladder, and urethra.