Supporting Embryo and Fetus

Maternal System

needed for nourishment, respiration and waste removal

Maternal Adapatations

• increases in

  • respiratory rate and tidal volume

  • blood volume

  • nutrient and vitamin uptake

  • glomerular filtration rate

  • uterus and mammary glands increase size

Yolk Sac

• endoderm lined membrane that surround blastocoel

• degenerates very early in embryonic development in humans

Amnion

• Grows over forming embryo

• Amniotic cavity becomes filled with amniotic fluid

• Amniotic fluid supports and protects the fetus against mechanical shock and provides water and other substances to fetus

Chorion

• derived from the cytotryphoblast and surrounds embryo after 1 month of development

• forms important component of the placenta

Development of Placenta

The process by which the placenta forms from trophoblast layers and maternal tissue, facilitating nutrient and waste exchange between the mother and developing fetus.

Placenta

• Placenta serves as nutrient, respiratory, and excretory organ for fetus

  • fetus receives from placenta

    • oxygen, glucose, growth factors, and other nutrients

  • The placenta serves as a crucial organ that facilitates nutrient, gas exchange, and waste removal for the developing fetus. It provides oxygen, glucose, growth factors, and other nutrients necessary for fetal development.

Decidua Basilis

• maternal part of placenta

Decidua Capsularis

• results of deciduoma response with overgrowth of endometrium

Decidua Parentalis

• endometrium not directly connected to the fetus

  • on a technicality the fetus resides within the uterine wall not in the uterine cavity

Growth of Placenta

• Week 4: 20% of inner uterine wall covered by placenta

• Week 20: 50% of inner uterine wall covered by placenta that weighs 200 g, while fetus weighs 500 grams

• Week 40: placenta weight is 700 g and has 285 L of blood that pass through it each day

Umbilical Cord

The flexible structure that connects the fetus to the placenta, facilitating the transfer of nutrients, oxygen, and waste between them.

Fetal Circulatory System

The system through which blood flows in the fetus, including structures like the umbilical cord, ductus arteriosus, and foramen ovale, allowing oxygenated blood to bypass the non-functioning lungs.

Foramen ovale

A small opening in the septum between the right and left atria of the heart that allows blood to flow directly from the right atrium to the left atrium, bypassing the lungs in a fetus.

Ductus Arteriosis

A vessel connecting the pulmonary artery to the aorta in a fetus, allowing blood to bypass the non-functioning lungs.

Dizygotic Fraternal twins

• two genetically distinct zygotes form and two embryos implant in uterus

Implant spatially seprate

two separate placentas chorions and amnions

Implant close together

single placenta fused chorions +2 amnions

Monozygotic Twins

identical twins formed from a single zygote that splits into two embryos, sharing the same genetic material.

Likelihood of Monozygotic Twins

• Delayed transport through the fallopian tube

• Women who have recently been on combination oral contraceptives because they cause decreased tubal motility

• Minor trauma to blastocyst during assisted reproductive techniques

Timing of Monozygotic Twins

• Division in first 72 hours post conception

  • two embryos with two chorions, two amnions, and one or two placentas will develop

• Division between days 4-8 post conception

  • two embryos with one chorion, two amnions, and one placenta will develop

• Division after day 8 post conception

  • two embryos with a single chorion, amnion, and placenta will develop

• Division occurs from days 9-12 post conception

  • mirror monozygotic twins can develop with have mirror opposite features

• Division occurs even later e.g., on day 14 post conception

  • cleavage is incomplete and conjoined twins are formed

Monozygotic Twins divisions (FIRST 72 Hours)

• division in 1st 72 hours post conception

  • before chorion and amnion

  • 2 embryos

  • 2 chorions

  • 2 amnions

  • 1-2 placentas

Monozygotic Twins divisions (4-8 Days Post Conception)

• division occurs 4-8 days after conception

  • after chorion and inner mass formation

  • 2 embryos

  • 1 chorion

  • 2 amnions

  • 1 placenta

Monozygotic Twins divisions (8 Days Post Conception)

• Division occurs after day 8 post conception

  • After chorion and amnion have already formed

  • 2 embryos

  • 1 chorion

  • 1 amnion

  • 1 placenta

Monozygotic Twins divisions (9-12 days Post Conception)

• Division occurs from 9-12 days post conception

  • After chorion and amnion have already formed

  • 2 embryos

  • 1 chorion

  • 1 amnion

  • 1 placenta

  • Mirror monozygotic twins may develop which have opposite features

Monozygotic Twins divisions (14 days Post Conception)

• Division occurs from 14 days post conception

  • After the chorion, amnion, and embryonic disc have already formed and cleavage is incomplete

  • 2 embryos are conjoined 1 chorion

  • 1 amnion

  • 1 placenta

  • Conjoined twins can have features of mirror monozygotic twins

Embryonic Development

• During week 3, the epiblast splits into 3 germ layers: ectoderm, mesoderm, endoderm

• By beginning of third week post conception, the embryo develops into a flattened embryonic disc consisting of three germ layers: ectoderm, mesoderm, and endoderm.

• Week 3-8 is embryonic period during which time all major internal and external structures take shape.

• Very sensitive to disturbances (teratogens, mutagens) that could result in death or congenital malformations.

Embryonic Period Week 3

• three layered embryo (ectoderm, mesoderm, and endoderm) develop from epiblast layer

Primitive Streak

raise groove on dorsal surface of epiblast

Gastrulation

the process during which the three germ layers (ectoderm, mesoderm, and endoderm) are formed from the epiblast, leading to the establishment of the body plan.

Embryonic Period Days 14-15

• germ layers begin to form when a raised groove called the primitive streak appears on the dorsal surface of the epiblast

• the epiblast cells migrate inward through the primitive streak and displace the cells of the underlying hypoblast layer to become the endoderm

Embryonic Period Days 16

• the ingressing epiblast cells form a new layer called mesoderm which is positioned between the endoderm and epiblast; the epiblast layers that remain on the embryo’s dorsal surface make up the ectoderm

  • All three of the primary germ layers are derived from epiblast cells

Derived from the Epiblast

• Endoderm

  • forms from migrating cells that replace the hypoblast

• Mesoderm

  • formed between ectoderm and endoderm from epiblast

• Ectoderm

  • form from epiblast cells that stay on dorsal surface

Neurulation

is the process during embryonic development in which the neural tube forms from the ectoderm, ultimately developing into the central nervous system.

Embryonic Development Week 4

• Somite growth results on lateral folding

• The folding at the head and tail is caused by expansion of the brain and the lengthening of the spinal cord

• Primitive gut emerges: site of future digestive tube and respiratory structures

Embryonic Development Week 5

• Rapid growth of brain

• Arm buds flatten and hands become paddle shaped

• One cm in length by end of week 5

Embryonic Development Week 6

• Eyes become pigmented

• Exterior ears began to form

• Head/brain continues to grow

• Leg bud becomes paddle shaped

• Hand rays indicate positions of digits

• Distinct tail is still present

Embryonic Development Week 7

• Toe rays form indicating position of digits

• Rapid development of gut tube causes intestines to protrude into the umbilical cord to produce an umbilical herniation

• Tail is diminished

Embryonic Development Week 8

• Major organs are formed; the fetal position is assumed and the tail disappears completely. Facial features become more recognizable, and sex differentiation begins.

Embryonic Development Week 9-Birth

• organs systems are established in embryonic period continue to develop and differentiate

• period of rapid growth

Placenta Endocrine Support Strategies

• tendency for feto-placental unit to take over endocrine control from mother

RH (Rhesus) Factor

a protein found on the surface of red blood cells that can trigger an immune response in Rh-negative individuals if exposed to Rh-positive blood.