Development up to 2nd weekThe website below has fantastic animations and descriptions of embryology:

http://www.embryology.ch/indexen.html

Day 1-2 - Fertilisation occurs at the ampulla of fallopian tube resulting in fusion of male and female pronucleus, followed by alignment of maternal and paternal DNA along the middle of cell, followed by splitting of chromatid by spindles, followed by formation of two cells containing 46 chromosmes which are a mix of both maternal and paternal DNA (this is called 1st Division/cleavage and occurs within the fallopian tube).

Day 2 - These cells divide again forming 4 cell. They divide again forming 8 cell – at this stage the cells in the centre of the embryo become very compact with each other (called compaction). Then at multiple points their membranes fuse forming channels between each other (called gap junctions), allowing communication between the cells. These inner cells (inner cell mass) will form the embryo proper (baby), and the outer cells (outer cell mass) will make the placenta and supporting coverings.

Day 3 – They divide again forming morula (16 cell mass) – there remains an inner cell mass and an outer cell mass. It is at this point that the morula enters the uterine cavity form the fallopian tube.

Day 4 - Next fluid starts entering the morula from outside via the zona pellicuda, pushing the inner cell mass to one side and forming a fluid filled cavity. This structure is now called a blastocyst. At this stage the inner cell mass is called embryoblast and the outer cell mass is called trophoblast (trophic = neutrition) .

Day 5 – Zona pallicuda breaks down allowing the blastocyst to be released and be ready for implantation.

6th day implantation will occur, this corresponds to the 20th day in the female reproductive cycle (as ovulation occurs at day 14) . So at the time of implanataion:

FSH + LH is low – As don’t need more follicles Corpus Leuteum is secreting oestrogen and progesterone Endometrium – due to progesterone is:

o Highly vascularo Highly glandular

Most common site of implantation is the anterior or posterior wall of the body of the uterus.

Endometrium at this stage has three clearly defined layers:

Compact layer – most superficial Spongy layer – soft because it is full of glandular and vascular tissue Basal layer – always there, compact and spongy layers are lost in menstruation and develop

from basal layer throughout cycle. (during menstruation the compact and spongy layers are lost but this layer always remains)

Implantation (day 6-12):

Day 6-9o On the ovum there are molecules called selectins, these bind to selectin receptors

on the endometrium (this is a loose attachment). Once they bind to them the blastocyst induces cell death in these endometrial cells allowing the blastocyst to move into the endometrium. This death of the cells also provides nutrition for the blastocyst. This local change to the endometrium at the site of implantation is called decidual reaction. In this phase Endometrial cells become polyhedral and Accumulate glycogen and lipids to provide nutrients to blastocyst.

o Also on the ovum there are ingtegrins which bind to dysintegrins on the endometrium and provide a strong attachment, thus further aiding embedding of the ovum.

o Thirdly there are fibronectin and laminin which help the ovum to migrate in the endometrium.

Day 10o The ovum is deep in the endometrium and the hole it enter through is closed by a

mucous plug Day 11-12

o New mucousa seals over the hole.

Changes to Trophoblast and Embryo (day 8-12)So what happens in this period (rule of 2’s)

Cytotrophoblast makes 2 layers Embryoblast makes 2 layers 2 cavities form around the embryoblast Extracoelomic mesoderm has 2 layers

Lets go through these changes:

o Day 6 – attached to endometriumo Day 7 – travels deepero Day 8 The trophoblast is formed by clearly defined nucleated cells forming a sphere

around the inner cell mass. The trophoblast replicates (via mitosis) its cells and pushes them on the outside. These cells don’t have cell membranes so theur cytosplasms forms a protoplasm mass around the trophoblast with nuclei scattered throughout. Now the inner layer is called the cytotrophoblast and the outer layer is called the synsitiotrophoblast.

o Day 9 – cavities from in synsitium called lacunae.

o Day 10 – lacunae link with each other and form a network of cavities (network of lacunae). We say that the syncitiotrophoblast has entered the lacuna phase.

o Day 11-12 - By now the capillaries in the endometrium have become larger and so are called synusoids. The syncytium may spread and surround the sinusoids and break down their walls, allowing this blood to enter the lacunar system. This lacunar system will be connected at some point with mothers arterial system and veins system. This is the initial establishment of utero-placental circulation.

Spotting

If at this time the hole which the ovum entered the endometrium trough is not plugged properly by mucousa cells then when the synsitium breaks down the mother sinusoid may drain into the uterine cavity.

This occurs at day 13 gestational date which would normally coincide with day 27 of menstrual cycle so she may think It is her period and not become aware that she is pregnant

o Day 13-14 - The cytotrophoblast develop columns which extend out into the syncitiotrophoblast and are called primary vili.

Development of embryobalst

Cell which are facing the side of the cystic cavity make a cuboidal layer. This layer, as it becomes more evident, is called hypoblast. The cells which are above this layer are columnar cells and they form epiblast. Above this there is a third layer. These cells make fluid filled cavities which combine to form the amniotic cavity. The cells lining this cavity are called amniotic cells (or amnioblasts).

Next

Hypoblasts cells make more cells which migrate around the inside of the cytotrophoblast and line the blastocoele below the hypoblast. This cavity is now called the Primary (primitive) Yolk Sac.

The epiblast and hypoblast are collectively called the bilaminar germ disc.

Next the cells lining the primary yolk sac start secreting connective tissue causing it to be depositied between these cells and cytotrophoblast. This connective tissue also travels up around the amnioblasts. This is called extraembryonic mesoderm.

Next cavities develop in the extraembryonic mesoderm, and eventually these cavities become confluent forming the extraembryonic coloemic cavity. This extraembryonic coloem extends right up around the amniotic cavity also. The inner layer of cells (i.e. between the Extraembryonic coelom and yolk sac) are called Extraembryonic Splanchnopleuric mesoderm (splanchno = visceral = inside layer) and the outer layer between the extraembryonic cavity and cytotrophoblast are called Extraembryonic somatopleuric mesoderm (somato = outside).

Next the hypoblast produces more cells and forms a new yolk sac (secondary yolk sac). A piece of the primary yolk sac brakes away and form the Exocoelomic cyst within the Exocoelomic cavity.

It is worth noting that the extraembryonic cavity becomes known as the chorionic cavity. All of the tissues aoutside this are called the chorionic plate (i..e Extraembryonic somatopleuric mesoderm, cytotrophoblast and surrounding syncitiotrophoblast).

Also notice that the whole of the amniotic cavity, bilaminar dsic and secondary yolk sac are only connected to the surrounding at one point: the connecting stalk. This will eventually from the umbilical cord.

Reminder fo Rule of 2 for 2nd week:

Trophoblast becomes 2 layers:o Syncitiotrophoblasto cytotrophoblast

Embryobalst becomes 2 layerso Hypoblasto epiblast

2 cavities in the endo Amniotic cavity

o Chorionic cavity Extraembryonic mesoderm makes 2 layers:

o Extraembryonic splanchnopleuric mesodermo Extraembryonic somatopleuric mesoderm

So at this point (14 days post gestation) the female will be 28 days into her cycle and be expecting her period. However as we have seen the syncitiotrophoblast is producing B-HCG which stimulates Corpus Leuteum to continue producing progesterone, so endometrium remains perfused and so cannot shed and cause a period.