Also known as the week of two’s:

1. Two embryoblast layers form:

• Endoderm (Hypoblast)
• Ectoderm (Epiblast)

Therefore forming a bilaminar germ disc

2. Two trophoblast layers form:

• Cytotrophoblast
• Syncytiotrophoblast – has lacunae 

3. Two cavities form:

• Amniotic cavity – the cytotrophoblast cells surrounding the amniotic cavity become amnioblast cells
• Primary yolk sac – endodermal cells grow down, line cytotrophoblast forming Heuser’s membrane (exocoelomic membrane)

Heuser’s membrane/exocoelomic membrane – Short lived combination of hypoblast cells and extracellular matrix

4. Two extraembryonic mesodermal layers form:

In the cytotrophoblast, extraembryonic mesoderm forms, in which lacunae appear and fuse to form extraembryonic coelom. Therefore dividing the extraembryonic mesoderm into 2 layers.

• Somatopleuric mesoderm – lines cytotrophoblast and amniotic cavity
• Splanchnopleuric mesoderm – lines primary yolk sac

NB: 

• Secondary yolk sac also forms, when a large part of primary yolk sac separates and disappears leaving the smaller part. It is completely lined with endoderm.

• Primary chorionic villi forms – cytotrophoblast projects into syncytiotrophoblast (more in the next chapter)

Syncytiotrophoblast – Placental barrier between maternal and fetal blood

Ovarian Cycle: Follicular phase – Ovulation – Luteal phase

Uterine Cycle: Menstruation – Proliferation – Secretory

1. Follicle Stimulation Hormone (FSH)

• Produced by anterior pituitary gland
• Stimulates primary follicles to develop

2. Estrogen

• Produced by primary follicles
• Endometrium to proliferate
• Stop production of FSH so that other primary follicle don’t develop
• Stimulate production of LH (Luteinizing Hormone)

3. Luteinizing Hormone (LH)

• Produced by anterior pituitary gland
• Stimulates ovulation
• Rest of Graffian follicle forms corpus luteum – produces progesterone

4. Progesterone

• Maintain thick endometrium (secretory phase of endometrium)

NB:

• If fertilized, corpus luteum will form corpus luteum of pregnancy – produces progesterone and is maintained by gonadotropic hormones produced by the embryo until placenta takes over to produce progesterone
• If not fertilized, corpus luteum will form corpus albicans which will then degenerate
• Reduction in production of progesterone leads to menstruation
• Reduction in production of estrogen leads to FSH production
• Proteolytic enzymes prevent coagulation of blood during menstruation

1. Germinal period – 2 weeks; formation of 3 germ layers
2. Embryonic period – 3rd week to 3rd month; differentiation of the 3 germ layers into organs and systems
3. Fetal period – growth of various organs and systems

1st week after fertilization

Zygote →Cleavage division → Morula → Blastocyst → Hatching (degeneration of zona pellucida) → Late blastocyst

Inner cell mass forms embryoblast

Outer cell mass forms trophoblast; which has 2 poles:

• Embryonic pole – faces site of implantation
• Abembryonic pole – opposite pole

Process of Implantation:

1. Trophoblast erodes endometrium
2. Penetration defect in endometrium
3. Blastocyst embedded in endometrium of uterus
4. Penetration defect closed by fibrin clot

Clinicals:

1. Ectopic Pregnancy – implantation can occur in the abdomen, ovarian and fallopian tube
2. Blighted ovum – dead embryoblast
3. Downs syndrome – Trisomy of chromosome 21
4. Turners syndrome – monosomy of x chromosome
5. Triple x syndrome

• Fusion of sperm nucleus with egg nucleus to form a diploid zygote cell
• Occurs in upper parts of fallopian tube ie. Ampulla

Process of Fertilization

1. Ejaculation in vagina

2. Capacitation – removal of glycoproteins to make sperm active and identify ovum

3. 1st contact – Acrosome reaction

• Contact of head of sperm with zona granulosa cells
• Acrosome reaction – Acrosome burst by exocytosis
• Releases hydrolytic enzymes

4. Break down of first barrier – Corona radiata

• Enzymes digest the sticky substances that holds the follicle cells together
• Cause cells to break away from zona pellucida

5. Break down of 2nd barrier – Zona pellucida

• Another acrosomal enzyme then digests the zona pellucida to create path for entry of the sperm cell

6. Contact with third barrier – Oocyte membrane

Formation of fertilization membrane:

• Head of 1st sperm makes contact with oocyte membrane
• Oocyte membrane become permeable to calcium ions
• Calcium ions rush into oocyte by diffusion
• Calcium ions stimulate rapid exocytosis of cortical granules, which release chemical substances
• Cortical reaction – release of cortical granules
• A new membrane forms, called fertilization membrane
• Fertilization membrane prevents polyspermy (entry of many sperm cells)

Q) How does fertilization membrane prevent polyspermy?

Acrosomal enzymes released by other sperms cannot digest the changed zona pellucida/fertilization membrane

7. Release of sperm nucleus into the secondary oocyte

8.  This stimulates 2ry oocyte to complete 2nd meiotic division and form a true ovum

9. Fusion of male and female pronucleus (enlarged nucleus) to form a zygote

Embryology: study of developmental process from a single cell to a baby in months

Embryogenesis: formation and development of an embryo

Ontogeny: Process of development of an organism

Conceptus: Product of conception

Cleavage: series of mitotic cell divisions of the zygote resulting in blastomeres

Morula: a mass of 12 to 32 blastomeres

After a while, the morula develops a cavity and we call it a;

Blastocyst: structure formed after a cavity forms within the morula.

Blastocele: fluid that enters the cavity of blastocyst

Embryoblast: inner cell mass of blastocyst

Trophoblast: outer cell mass of blastocyst

Endometrium: inner epithelial lining of the uterus

Implantation: attachment and embedding of the blastocyst to the endometrium

Decidua: endometrium during pregnancy

Bilaminar disc: 2 layered embryo, occurs during second the 2nd week

Gastrula: trilaminar disc embryo (3 layered embryo)

ie. ectoderm, mesoderm, endoderm

Gastrulation: process of formation of the trilaminar disc, begins on 15th day

Neurulation: process of formation of neural tube, the primordium of the central nervous system

Primordium: beginning or first discernible indication of an organ or structure

Organogenesis: formation of body organs

Morphogenesis: process of shape development

Fetus: unborn offspring

Abortion: expulsion of an embryo or fetus before its viability (capable of living outside the uterus)

Teratology: study of birth defects

Teratogen: an agent that causes congenital (born with) defects

Anomaly: structural abnormality

Malformation: abnormal developmental process