Lecture 13: Early Human Development

3 Main achievements of Embryogenesis

Patterning - Cells acquire identity in space & time

Axis Definition - Anterior/Posterior, Dorsal/Ventral axis established

Germ Layer Formation - 3 germ layers formed (Gastrulation)

3 Steps of fertilisation & their F

1. Path of sperm cell - Travels through corona radiata & zone pellucid to penetrate oocyte

2. Completion of Meiosis 2 - Oocyte completes meiosis, forms a polar body

3. Formation of pronuclei - Male & female genetic material combine

Cleavage & Blastocyst Formation Overview

Cleavage of zygote forms a morula by day, blastocyst forms by day 5 which consists of an inner cell mass (embryoblast) & outer Trophoblast. Around day 4/5 blastocyst cavity forms

Inner cell mass (Embryoblast) F

It gives rise to the embryo itself

Trophoblast F

Gives rise to the extra embryonic tissue

Signs of Gastrulation taking place

First is formation of primitive streak where ectoderm cells displace endoderm cells to create the mesoderm (Primitive streak moves oofrom connecting stalk to future anterior side of the embryo

3 Primary germ layers & they’re F

Ectoderm - Gives rise to external structure e.g epidermis, nails, glands, develops all nervous tissue

Mesoderm - Gives rise to CT, muscle, cartilage, skeleton

Endoderm - Gives rise to gut lining, liver, glands, respiratory epithelium

Trophoblast development Overview

Consists of syncyotiotrophoblast & cytotrophoblast which establishes choironic villi/fetal placenta for nutrient exchange with maternal endometrium

Formation of Bilaminar Embryonic Disc

Contains hypoblast & epiblast which is crucial for further layer formation

Embryonic Folding Overview

Tail & head fold towards each other

Connecting stalk & yolk sack will form future umbilical cord

Sides of embryo will fold down to form a tube

Stem Cells def & F

They’re defined by their ability to generate differentiated daughter cells & self-renew, functions are growth, repair of tissue, they’re not limited in their replication potential (unlike differentiated cells)

Totipotent Stem Cell F

Highest in hierarchy, can give rise to all embryonic tissue

Pluripotent Stem cell F

Second in hierarchy, derived from blastocyst can differentiate other almost all cell types in body except extra embryonic tissue

Multipoint Stem cell F

Third in hierarchy, limited to differentiating into all cell types in a specific tissue/organ

Unipotent Stem cell F

Can only produce one cell type e.g skin stem cell for renewal