Human Embryonic and Fetal Development

Human Embryonic and Fetal Development

•  The image displays various stages of human embryonic and fetal development.
•  The stages show the fertilized egg, 2-cell stage, 4-cell stage, 8-cell stage, 16-cell stage, blastocyst and subsequent fetal development stages at 4 weeks, 10 weeks, 16 weeks, and 20 weeks.
•  The images show the progression in size and development during the early stages.

Early Development: Cleavage Lecture

•  Four major developmental events occur in human embryogenesis:
  • Cell division (cleavage)
  • Cell differentiation
  • Morphogenetic events
  • Growth
•  Cleavage is the initial mitotic divisions converting the single-celled zygote to a multicellular embryo.
•  Blastomeres are the cells produced during cleavage.
•  Mitotic divisions during cleavage maintain the 2N complement (number of chromosomes) without growth phases, leading to smaller cells over time.

The Timing of Early Development

•  Development from Day 0-35 after fertilization
•  Days 0-7: fertilization, cleavage, and blastocyst formation
•  Days 8-14: implantation
•  Days 15-21: gastrulation, formation of somites, and vascular systems
•  Days 22-28: neurulation, neural crest migration, and development of various organ systems
•  Days 28-35: further development of systems including nervous system, limbs, and genitalia

Cleavage: Functions & Events

•  Cleavage involves initial divisions transforming the zygote into a multicellular embryo.
• During cleavage the cells are called "Blastomeres"
•  These divisions, mitotic divisions, maintain the chromosome number (2N) without intervening growth phases.
• The result is smaller cells over time.
• Complete separation of cells in holoblastic cleavage.

Mitotic vs. Normal Cell Cycle

•  During cleavage, the cells progress directly from S stage (DNA synthesis) to M stage (mitosis) without the interphase growth phases (G1 and G2).
•  This results in cleavages continuing to decrease in size approximating the size of somatic cells.

The Purposes of Cleavage

•  Cleavage converts the zygote from a single cell to many cells.
•  It generates many cells that interact and can be moved around the embryo.
•  It maintains the diploid chromosome complement across all the cells, ensuring genetic fidelity via synchronous divisions that are not identical in timing.
•  Embryos with differing numbers of cells at various points in time are a result.
•  No growth during cleavage; the total embryo will remain about 100 microns in diameter.

The Stages of Early Cleavage

•  Cleavage begins approximately 24 hours after fertilization.
•  The images show the 2-cell stage, 4-cell stage, and 8-cell stage, etc.
• The process is not synchronous (all cells do not divide exactly at the same time).

Mammalian Compaction

•  Compaction is a process occurring at the 8-cell stage.
•  Cells adhere more tightly together, becoming more compact.
•  E-cadherin, an adhesion protein, appears at the time of compaction, causing tighter cell adhesion.

Formation of the Blastocyst

•  The formation of the blastocyst from the morula involves accumulation of fluid forming the blastocoele.
•  The fluid-filled cavity within the blastocyst is called the blastocoel.
•  The blastocyst consists of an outer layer of cells called the trophoblast and an inner cell mass.

Hatching of the Blastocyst

•  Hatching takes place just prior to implantation.
•  The blastocyst secretes proteases, which digest components (e.g., zona pellucida), allowing the blastocyst to free itself.
•  Hatching is crucial for the blastocyst to implant in the uterine wall.

Embryo Implantation, Process & Their Stages

•  The process of implantation involves several stages, from initial hatching through apposition, adhesion, and invasion.
•  Specific time frames within the implantation window are critical for successful implantation, with a limited window of receptivity.

Implantation (Process)

•  Implantation is a crucial stage converting a blastocyst into a functional embryo.
•  It involves multiple phases: hatching, pre-contact, apposition, adhesion, and invasion. This allows the embryo to attach and invade the uterine wall (endometrium).

Factors Related to the Embryo

•  The embryo must reach the blastocyst stage to implant.
•  The embryo consists of an internal cell mass and a trophoectoderm.
• The trophoectoderm forms the placenta and other attachments.
•  Implantation requires the blastocyst to detach from the zona pellucida, grow to full size and achieve maximum expansion before implantating.

Factors Related to the Endometrium

•  The endometrium, the innermost lining of the uterus, must be receptive to allow for successful implantation.
•  Its thickness (between 7-10 mm) and specific trilaminar aspect indicate receptivity.
•  The endometrium's receptivity is influenced by hormonal changes during the menstrual cycle.

When Does Implantation Occur?

•  Implantation will only occur when the endometrium has become receptive.
•  This receptive period, the window of implantation, lasts for about 4 days in most women, typically between days 19-21 of the menstrual cycle.

Phases of Implantation

•  Implantation is a process that consists of specific steps. These are adhesion, apposition, and invasion. The diagram details the stages beginning with the hatching blastocyst.
•  Adhesion molecules bind the blastocyst to the endometrial lining.
•  During apposition, a close relationship between the embryo and the uterine lining is established.
•  Invasion occurs when the trophoblast cells of the embryo penetrate into the uterine lining.

Symptoms of Implantation

• Implantation-related symptoms are not always observable, although certain signs including light bleeding, breast swelling, abdominal pain, or a heightened urge to urinate may occur.

Placenta Development

•  The placenta forms from the invasion and merging of the syncitiotrophoblast and endometrial cells.
•  It's a disc-shaped organ consisting of two plates (chorionic plate and basal plate) to facilitate the exchange of nutrients and waste products.
•  The placenta goes through stages from initial formation to maturation.

Trophoblast Action

•  The trophoblast cells secrete proteolytic enzymes to digest the uterine lining and allows the blastocyst to penetrate and attach to the uterine wall.
•  The secreted enzymes help the embryo penetrate and adhere to the uterine lining, providing nutrients.

Receptivity of Uterus

• Receptivity involves several crucial changes: -Plasma membrane transformation (pinopodes) -Decidualization (change in endometrial thickness) and production of decidual cells.

Process of Implantation of the Blastocyst in the Uterus

•  The blastocyst is transported toward the uterus from the fallopian tube and will implant in the uterine wall after approximately 3-6 days.
•  It relies on the nutritive secretions of the uterine endometrium for support during this time.
•  The trophoblast cells are crucial to implantation, enabling the blastocyst to penetrate and attach to the uterine lining.

Remodelling of Circulation

• The placenta's functionality depends on remodelling maternal blood vessels that allows low pressure and low velocity flow, which occurs by roughly 10-12 weeks.
• Extravillous trophoblasts (EVTs) remodel maternal spiral arteries.

Placental Maturation

• The placenta matures through the development of septa dividing into cotyledons.
• Receiving blood from approximately 80-100 spiral arteries in these structures.

Full-Term Placenta

• By the end of pregnancy, the placenta reaches a size and thickness, and a certain weight.
• The maternal side is covered by a layer of decidua basalis.
• The fetal side is covered by chorionic plate.

Umbilical Cord

•  The umbilical cord connects the fetus to the placenta and consists of 3 blood vessels carrying fetal blood.
• Two arteries carry deoxygenated blood from the fetus to the placenta; a vein carries oxygenated blood from the placenta to the fetus.

Amniotic Fluid

•  Amniotic fluid cushions and protects the fetus, and acts as a transport medium for fetal urine and other substances/metabolites.
•  It's formed from maternal plasma diffusing through the placenta and fetal urine production.

This quiz explores the stages of human embryonic and fetal development, from the fertilized egg to later stages at 4, 10, 16, and 20 weeks. Key concepts include cleavage, cell differentiation, and morphogenetic events that shape the early embryo. Test your understanding of these crucial developmental milestones.