(each embryonic cell is called a blastomere)

4 cell pig embryo- totipotent in the pig

8 cell embryo- potipotent in the rabbit, mare, cow, and ewe

products of conception (embryo/fetus + placenta)

1. the early embryo
2. the embryo and extraembryonic membranes (preimplantation phase)
3. fetal and placenta (post-attachment phase)

• nuclei from somatic cells from adult cattle, sheep, goats, horses, swine, cats, and dogs have been transplanted into enucleated oocytes and normal individuals obtain (5% success) - Dolly the sheep!!

• the zygote is one of the largest single cells in th ebody and is contained by the zona pellucida (fixed volume)
• until hatching from the zona pellucida, the size of each blastomere after a mitotic division are halved to form first a morula (solid ball of blastomeres) and then a blastocyst (inner cell mass and trophectoderm cells and the blastocoel)

a morula is a solid ball of blastomeres contained within the zona pellucida

1. the outer blastomeres form tight junctions and the inner cells form gap junctions between them
2. the outer cells become the trophectoderm (TE) cells and pump Na+ into the intercellular spaces on the embryo to form the blastocoele while the inner cells form the inner cell mass cells (ICM)
3. When the distinct cavity is seen, the embryo is called a blastocyst
4. The ICM cells will form the body of the embryo, while the TE cells will form the chorion (outer part of the placenta)
5. Continued mitosis, fluid accumulation and enzymatic degradation result in the weakending and rupture of the zona pellucida (hatching)

Timing of Early Embryogenesis

Cow

Ewe

Mare

Sow

Women

Morula                         Blastocyst                       Hatching

4-7 days                      7-12 days                     9-11 days

3-4 days                     4-10 days                       7-8 days

4-5 days                     6-8 days                         7-8 days 

3.5 days                      4-5 days                         6 days

4 days                         5 days                             5-6 day

after hatcdhing, the conceptus undergoes massive growth

• cow embryo- day 13 (3mm) and on day 17 (250mm)
• pig embryo- day 10 (2mm) day 11-12 (200mm), and on day 16 (800-1000 mm)

splanchnic mesoderm- develops into blood vessels

somatic mesoderm- develops into connective tissue

1. Yolk sak
2. Chorion
3. Amnion
4. Allantois

1) Diffuse placenta- have uniform distribution of chorionic villi covering the surface of the placenta (pig and horse)

2)Zonary placentae- have a ban-like sone of choionic villi (dog and cat)

3)Discoid placentae- form a regionalized disc of chorionic villi (primates and humans)

4)Cotyledonary placenta- have numerous descrete button-like structures called cotyledons (ruminants)

• have closely packed chorionic villi that are distributed over the entire surface of the placenta 
• Initial attachment occurs around day 12 and is well established by day 18 to 20 after ovulation

• while classified as diffuse, this placenta has many specialized "microzones" of chorionic villi known as microcotyledons (increased transport capacity) 
• Also forms 5-10 endometrial cups on its surface (chorionic and endometrial orgin which secrete equine chorionic gonadotropin (eCG)- are present between day 35 and 60 of pregnancy and then are destroyed
• Initial attachment of the the conceptus to the endometrium is about day 24 and becomes well established by day 36-38 after ovulation

• a broad zone of villus attachment for nutrient/waste product exchange only forms around the chorion near the middle of the conceptus
• A pigmented region called the paraplacenta is seen at either end of the placenta (contains small blood clots) is thought to transport iron from the mother to the fetus.