Embryonic Development and Hermaphrodites

Vestigial Structures

• Vestigial structures are organs or structures that once served a purpose in our evolutionary history but no longer serve the same or any purpose.

• Examples:

  • Yolk sac
  • Human tail: Suggests evolution from monkey/gorilla-like ancestors who used tails for swinging from trees.
  • Wisdom teeth: Jaws have shrunk due to dietary changes, making wisdom teeth less necessary. Some infants are now born without them.
  • Wisdom teeth often cause problems (infections, crowding) requiring surgical removal.

Yolk Sac

• Similar to the allantois, it becomes part of the umbilical cord.
• The chorion becomes part of the placenta.
• The amniotic membrane and amniotic fluid encase and protect the embryo.

Third Week of Development

• Pregnancy tests can detect pregnancy hormones.
• Morning sickness may begin.
• A woman would be about one week late for her period.
• Around day 18, the heart starts beating but does not pump blood, and its activity can't be detected on an ultrasound.

Fertilization to Neurulation

• The process includes fertilization, cleavage, blastulation, gastrulation, morphogenesis, and neurulation, leading to the formation of germ layers.
• By the end of the third week, the baby's heart is already beating.

Fourth Week of Development

• A woman would be about two weeks late for her period.
• The embryo is about 0.5 cm long.
• Blood starts to form and fill the blood vessels.
• Morphogenesis occurs with the lungs and kidneys starting to take shape.
• Tail is visible, which will become part of the spinal cord and neural tube.
• Limb buds begin to form into arms and legs.
• The head and nose start to take shape.

Fifth Week of Development

• The heart begins pumping blood. This is a crucial sequence:
  • Week 3: Heart beats.
  • Week 4: Blood forms.
  • Week 5: Heart pumps blood.
• The circulatory system enables the movement of oxygen.
• The embryo is about 1.3 cm in size.
• Blood and urine tests are recommended.
• Prenatal appointments are scheduled, including a sexual history to assess the risk of STIs.
• Brain cells begin to differentiate into different areas of the brain.
• The eyes are open, but there are no eyelids or irises yet.

Sixth Week of Development

• Limbs lengthen and can slightly flex.
• Gonads start to produce hormones but sex is still not determined.

Amniotic Fluid and Sac

• The amniotic sac is like a membranous water balloon filled with amniotic fluid.
• When the "water balloon" pops, the amniotic fluid is released, and the sac encases the fetus at delivery and must be removed.
• The umbilical cord and placenta begin to form. The chorion layer becomes the infant side of the placenta, while the other part comes from the endometrium lining of the mother.
• Umbilical and placental formation starts in week three and completes around week ten, with most of the development occurring between weeks three and eight.

Seventh and Eighth Week of Pregnancy

• The embryo is about the size of a paper clip by the end of the eighth week.
• A mother can have her first ultrasound and detect the embryo's heartbeat.
  • Week 3: Heartbeats.
  • Week 4: Blood formed.
  • Week 5: Blood moved by the heart.
  • Week 8: Heartbeat detectable via ultrasound.
• Nostrils form and fill with mucus.
• Eyes develop with thin gelatinous eyelids.
• Cartilage starts to be replaced by bone. Cartilage remains in the nose, ears, and around the esophagus for flexibility.
• External genitalia starts to form but is not differentiated.
• The nervous system starts coordinating body activities.
• Internal organs are formed and start to coordinate together.
• The embryo can flex toes and knees, but the mother cannot feel it.
• Around week eight, a first ultrasound can show the presence of multiples, though sometimes they can be hidden.

Pseudohermaphrodites

• At this stage, the embryo has an overlay of both female and male systems.

• Hormones determine the differentiation into either female or male.

• True hermaphrodites: Organisms with complete male and female sex organs.

• Pseudohermaphrodites: Not true hermaphrodites.

• In female embryos exposed to high levels of male sex hormones:

  • Internal female reproductive organs develop.
  • External male genitalia develop.

• This is a pseudohermaphrodite because they do not have both complete sets of internal organs.

• Medical decisions can be made in infancy or later about altering the genitalia.

• In minor cases, the clitoris may appear large, resembling a penis, and the vulva may resemble a scrotal sac.

Embryonic Development

• As an embryo, mammary tissue and nipples form in both males and females.
• Differentiation into secondary sex characteristics occurs later as a fetus when hormones take effect.
• By the end of the eighth week, most structures are present even if not fully refined. This is building the groundwork for future developement and then refining it afterward.

Sex Hormones and Development

• The conditions and their apparent sex based on chromosomal perspective as well as the outcome from those conditions are important to know.

• Nondisjunction: Errors in chromosome separation during meiosis, leading to variations in the total number of chromosomes.

• Down syndrome and Edward's syndrome are examples of autosomal conditions.

• Presence of a Y chromosome determines male sex, regardless of the number of X chromosomes.

• Females (variations in X chromosomes):

  • One X chromosome: Often leads to sterility due to defective or absent ovaries, lack of follicle and hormone development, and underdeveloped secondary sex characteristics.
  • Additional X chromosomes: Associated with a decline in mental cognitive function.

• Males (variations in X chromosomes):

  • Klinefelter's syndrome (males with extra X chromosomes): Usually sterile and may have impotence and other medical problems. The more X's it has, the more the cognitive function decreases.

• Adding more X chromosomes, whether in females or males, tends to result in a decline in mental cognitive function, though the exact mechanism is unclear. The use of the term "mental retardation" is outdated; "mental cognitive effect" is more appropriate.