Implantation & Animal Cloning

Process of Implantation

Animal Cloning

How to Clone

Advantages of Using Animal Cloning

Process of Implantation

Figure 1. Stages of the peri-implantation period.

Senger, P. L. (2015). Pathways to pregnancy and parturition (3rd ed.). Current Conceptions Inc.

Figure 2. Early pregnancy events that occur within the ewe after fertilization.

Spencer, T. E., Johnson, G. A., Bazer, F. W., & Burghardt, R. C. (2004). Implantation mechanisms: Insights from the sheep. Reproduction, 128(6), 657–668. https://doi.org/10.1530/rep.1.00398

What is implantation?

The attachment of a fertilized egg (blastocyst) to the endometrium
Implantation is the initial step in the formation of the placenta in a pregnant ewe
Occurs between days 11-16 after mating
A highly synchronized process of complex interactions between the conceptus & the endometrium during a restricted period of time (23)
Endometrial proteins that regulate implantation in the ewe: 1. Glycosylated cell adhesion molecule 1 (GlyCAM-1)
Mucin glycoprotein secreted by the epithelium
Mediates leukocyte-endothelial cell adhesion (regulates implantation)
Expressed mostly at the luminal surface
GlyCAM-1 levels are low between days 11-13, increase at day 15, & are high at days 17-19 after mating (23)

2. Galectin-15

Bind beta-galactosides that cross-link glycoproteins & glycolipid receptors on the cell surface
Located in the endometrial lumen epithelium & superficial ducts of the ovine uterus
Increased secretion between days 10-14 (23)

3. Integrins

Aid in the implantation adhesion cascade
Mediate cell migration, proliferation, & differentiation
Implantation does not change patterns of integrin expression in ewes (23)

4. Osteopontin (OPN)

Integrin-binding ligand
Progesterone induces expression of OPN in the endometrial glands
OPN binding causes trophoblast & lumen epithelium cells to activate integrin receptors
OPN binds to integrin receptors on the conceptus trophoblast & endometrial lumen epithelium (23)

Figure 3. Pre-attachment, apposition, and adhesion demonstrated as the first three steps in implantation of the ewe.

Spencer, T. E., Johnson, G. A., Bazer, F. W., & Burghardt, R. C. (2004). Implantation mechanisms: Insights from the sheep. Reproduction, 128(6), 657–668. https://doi.org/10.1530/rep.1.00398

Steps of implantation in the ewe:

Shedding of the zona pellucida

The zona pellucida is shed between days 8-9 after mating
Loss of the zona pellucida occurs due to rupture & hatching after blastocyst growth or after enzymatic lysis
The zona pellucida functions to prevent the trophoblast from contact with the endometrial lumen epithelium (LE)

Precontact & blastocyst orientation

The blastocyst becomes positioned & immobilized in the uterus after the zona pellucida is shed
On day 11 after mating the spherical blastocyst begins to elongate & resembles a long filament
By day 13 the conceptus, initially in the uterine horn ipsilateral to the corpus luteum, elongates into the contralateral uterine horn
Blastocyst elongation is crucial for regulated production of interferon tau which acts as the signal for maternal recognition of pregnancy in the ewe

Apposition

Apposition of the conceptus can occur only if the trophectoderm becomes closely associated with the endometrial lumen epithelium
Apical microvilli on the uterine epithelium are not lost in sheep as they are in most species
Apposition occurs in the prescence of the inner cell mass (the embryo) & on the uterine glands
The trophoblast develops papillae that pentrate the superficial ducts of the uterine glands between days 15-18
These trophoblastic papillae anchor the peri-attachment conceptus & facilitate interactions between the trophoblast & the endometrial lumen epithelium

Adhesion

The mononuclear trophoblast cells begin to adhere to the endometrial lumen epithelium (LE) on day 16 after mating
Binucleate cells (BNC) differentiate from mononuclear trophoblast cells as a result of consecutive nuclear divisions
Adhesion of the trophectoderm to the endometrial LE continues along the uterine horn until around day 22
The binucleate cells (BNC) fuse with the endometrial LE & form syncytia of trinucleate cells which replaces the endometrial epithelium
The trinucleate cells continue to enlarge due to BNC migration & fusion & form syncytial plaques connected by tight junctions
These syncytial plaques cover the caruncular surface & help form the placentome
BNC continue to migrate & fuse with the uterine epithelial cells throughout the majority of gestation

Animal Cloning

How to Clone

Cell Collection

Collect cells from the animal you want to clone and allow them to multiply until there is enough division

Egg Preparation

Take an egg cell from another sheep and remove the nucleus

Nuclear Transfer

Join the cell without a nucleus and the cell you collected earlier using electricity. This way, the egg cell contains all of the DNA from the other.

Stimulation of the cells

Trigger the cells to start dividing to create an embryo.

Implantation

Implant this new embryo into a surrogate sheep to carry and deliver the baby.

"Dolly" The first cloned animal

The first cloned animal using this technique was Dolly the sheep, cloned in 1996 at the Roslin Institute in Scotland. She was the first mammal cloned from an adult somatic cell, marking a significant breakthrough in genetic research.

Advantages of Using Animal Cloning

Medical Research

Provides a way to produce genetically identical animals for studying diseases, genetics, and drug testing.

Genetic Preservation

Allows preservation of specific desirable traits, like high milk yield or disease resistance, in livestock.

Endangered Species Conservation

Offers the potential to clone endangered or extinct animals, helping with biodiversity conservation.

Agricultural Productivity

Produces consistent, high-quality livestock, which can improve food production efficiency.

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