Special topics Lecture 1

Embryonic development

Embryogenesis

It achieves patterning, identification of major axis, three germ layers are defined, rudiments of the major organs

Events in week 1

Fertilisation, cleavage of the zygote, formation of the morula and blastocyst, implantation

Events in week 2 - 4

Trophoblast development and embryonic disc, gastrulation, structures derived from three primary layers

Major axis

Anterior (head), posterior (butt), dorsal (back), ventral (stomach)

Patterning

Cells acquire identity in space (spatial) and time (temporal)

Fertilisation

Sperm enters an oocyte and a diploid zygote is created and meiosis II is completed in the fallopian tube

Path of sperm cell

Corona radiata to zona pellucida to plasma membrane of secondary oocyte to cytoplasm of secondary oocyte

Corona radiata

Cells that surround the egg that come from the ovaries

Zona pellucida

Fibrous mat

Plasma membrane of secondary oocyte

Fuse and delivery of genetic material

Cytoplasm of secondary oocyte

Gives rise to the first polar body

Polyploidy

The condition of having more than two complete sets of chromosomes, which can occur in some organisms due to errors in cell division, such as two sperm entering one egg cell

Cleavage and formation of the blastocysts

Cleavage is rapid cell division that produces blastomeres that are still in the zona pellucida, day 4 produces a morula and day 5 produces a blastocyst which is hollow, contains an inner cell mass that will form the embryo, and does not have the zona pellucida around it. This occurs in the fallopian tube

Implantation

The process by which a blastocyst attaches to the uterine wall, beginning the development of an embryo, typically occurring around 6 to 10 days after fertilization. During this process, the blastocyst burrows into the endometrium, establishing a connection for nutrient exchange.

Ectopic pregnancy

A condition where a fertilized egg implants outside the uterus, commonly in a fallopian tube, leading to potential complications. This is prevented by the zona pellucida which keeps the cell from being sticky

Twins

They occur due to two eggs being released at the same time

Identical twins

Result from the splitting of a single fertilized egg into two embryos, sharing the same genetic material.

Development of the trophoblast

Synctiotrophoblast and cytotrophoblast

Synctiotrophoblast

The outer layer of the trophoblast that invades the uterine lining and facilitates nutrient exchange between the mother and developing embryo.

Cytotrophoblast

The inner layer of the trophoblast that provides support to the outer synctiotrophoblast and contributes to the embryonic portion of the placenta.

Chorionic villi

Finger-like projections that extend from the trophoblast into the uterine lining, playing a crucial role in nutrient and gas exchange between the mother and fetus.

Fetal placenta

A structure formed by the union of maternal tissue and trophoblastic cells, which supports fetal development by facilitating nutrient and gas exchange between the mother and fetus.

Development of the bilaminar embryonic disc

Hypoblast cells and epiblast cells

Hypoblast cells

Gives rise to the endoderm layer

Epiblast cells

Gives rise to the ectoderm layer

Gastrulation

The process during which the bilaminar disc transforms into a trilaminar structure via formation of the primitive streak, forming the three primary germ layers: ectoderm, mesoderm, and endoderm.

Embryonic folding

Refers to the process during which the flat embryonic disc transforms into a three-dimensional structure, leading to the formation of body axes and organ rudiments.

Invagination

A process during embryonic development where an area of cells folds inward to form a pocket or pouch, contributing to the formation of germ layers.

Stem cells

A cell that divides to form one daughter that goes on to differentiate, and one daughter that retains its stem-cell properties, playing a crucial role in replacing differentiated cells in growth, renewal, and repair. Differentiated cells have limited divisions

Stem cell hierarchy

Totipotent, pluripotent, mutlipotent, nullipotent (unipotent)

Totipotent stem cells

Capable of giving rise to all cell types of body and extra-embryonic tissues

Pluripotent stem cells

Capable of giving rise to all cell types of body

Multipotent stem cells

Capable of giving rise to all cell types of a particular tissue or ogran

Nullipotent

Not capable of giving rise to other cell types, only one cell type

Keratinocytes stem cell

Only gives rise to keratinocytes cells in the skin and are located in the basal layer of the epidermis.

Epidermolysis bulls (EB)

A group of genetic skin disorders that cause fragile skin, leading to blistering and tears from minor injuries.