Notes on Human Development – Embryology and Tissues

The human body has a distinct organization that is formed during the embryonic period.
How long the embryonic period lasts is not specified in the transcript; the focus is on the developmental events and their sequence.

A short video covers embryonic and fetal development to illustrate what happens and when (timings are approximate in the video; exact timings are not required for this course).

Human pregnancy lasts ~9 months.
Stages within pregnancy:
- Zygote formation and early development
- Embryonic period (First Trimester): Weeks 1–8 (stages such as fertilization, cleavage, implantation, gastrulation, neurulation)
- Fetal period (Second and Third Trimesters): Weeks 9–40, characterized by growth and maturation of organs and systems.
Major phases and terms:
- Uterus, Menses, Proliferative, Secretory phases (relating to the menstrual cycle)
- Zygote → Embryo → Fetus progression
- Embryonic period includes the stages: fertilization, cleavage, morula, blastocyst, implantation, hatching
- Embryonic development phases named as Embryonic, Fetal; First Trimester (Embryonic), Second Trimester (Fetal), Third Trimester (Fetal)
Key milestones and terms from the slides:
- Zygote, first cell after fertilization
- First cellular divisions (cleavage) leading to morula and then blastocyst
- Implantation of the blastocyst into the endometrium
- Hatching from the zona pellucida
- Formation of the bilaminar embryo (epiblast and hypoblast) and bilaminar structures (amniotic cavity and yolk sac)
- Gastrulation leading to the trilaminar embryo (ectoderm, mesoderm, endoderm)
- Early differentiation of tissues and organs from germ layers

Zygote formation:
- Occurs when sperm fertilizes the oocyte; the resulting cell contains $46$ chromosomes.
- The zona pellucida surrounds the zygote to prevent entry of additional sperm.
Cleavage:
- Holoblastic cleavage: cells divide completely (holoblastic) to increase cell number while the overall size of the embryo remains the same initially.
- The morula forms after successive cleavages and typically contains ~ $32$ cells.
- The zygote and morula remain the same size despite increasing cell number because cells become progressively smaller.
Key sequence:
- Ovulation releases the oocyte; fertilization occurs in the fallopian (uterine) tube; the zygote is formed; cleavage begins; morula forms; progression to blastocyst; implantation into the uterus.
Important detail: the blastocyst has distinct populations:
- Trophoblast: outer superficial layer that supports implantation and helps modulate maternal immune recognition.
- Inner cell mass (embryoblast): gives rise to the embryo proper.
Visual cues (conceptual):
- The zona pellucida initially encloses the embryo; after hatching, the blastocyst interacts with the endometrium.

Blastocoele: fluid-filled cavity formed during blastocyst formation.
Cellular movements:
- Cells migrate to the outer edge to create the blastocoele, enabling diffusion with fewer cellular layers.
Result:
- A blastocyst with a trophoblast (outer layer) and an inner cell mass (embryoblast) surrounding the blastocoele.

Trophoblast:
- Outer superficial cell layer.
- Aids in embryo’s interaction with the maternal immune system and supports implantation.
Inner Cell Mass (Embryoblast):
- Gives rise to the bilaminar embryonic disc (epiblast and hypoblast).
Function and significance:
- The trophoblast’s invasion and remodeling of the endometrium are essential for establishing the placenta and maternal-fetal exchange.

Hatching from the zona pellucida occurs to enable implantation.
The trophoblast interacts with the endometrium (lining of the uterus) to implant the blastocyst.
The side of the blastocyst containing the inner cell mass (embryoblast) embeds into the endometrium first.
Clinical note:
- Implantation failure occurs at a relatively high rate of $$ ilde{