Week 2

Introduction

Week 1 and 2 development

Key events of human development during the second week (week 2) following fertilization or Clinical week 4 (LMP).

Week 2 is about the implantation process and blastocyst differentiation. Note that all cells produced from the initial fertilization event are defined as the "conceptus" and will include cells with both embryonic and extraembryonic futures. In the conceptus, this is a period of blastocyst "hatching" rapid blastocyst differentiation into extraembryonic and embryonic tissues and proliferation. In placental animals, this is the first physical interaction between the conceptus and the maternal uterine wall with adplantation and the commencement of implantaion.

Week 2 Links: Lecture - Week 1 and 2 | Implantation

Timeline

Day	Stage	Event
8		Implantation
9
10
11
12
13	Stage 6	Chorionic Cavity
14

Week 2 and 3 Movies


Implantation	Mesoderm	Chorionic Cavity	Amniotic Cavity	Week 3

Implantation

The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

implantation commences about day 6 to 7
Adplantation - begins with initial adhesion to the uterine epithelium
- blastocyst then slows in motility, "rolls" on surface, aligns with the inner cell mass closest to the epithelium and stops
Implantation - migration of the blastocyst into the uterine epithelium, process complete by about day 9
coagulation plug - left where the blastocyst has entered the uterine wall day 12

Normal Implantation Sites - in uterine wall superior, posterior, lateral

Uterus

Uterus proliferative phase
Uterine gland proliferative phase
Uterus secretory phase
Uterine gland secretory phase

Endometrium - 3 layers in secretory phase of menstrual cycle: compact, spongy, basal
Myometrium - muscular layer outside endometrium, contracts in parturition
Perimetrium - tunica serosa of the uterus continuous with the peritoneal wall

Endometrial Layers

Compact - implantation occurs in this layer, dense stromal cells, uterine gland necks, capillaries of spiral arteries
Spongy - swollen stromal cells, uterine gland bodies, spiral arteries
Basal - not lost during menstruation or childbirth, own blood supply

Decidual Reaction

occurs initially at site of implantation and includes both cellular and matrix changes
reaction spreads throughout entire uterus, not at cervix
deposition of fibrinoid and glycogen and epithelial plaque formation (at anchoring villi)
presence of decidual cells are indicative of pregnancy

Cervix - at mouth of uterus, secretes mucus (CMP), forms a plug/barrier, mechanical and antibacterial Vascular - increased number of blood vessels

Decidua

The endometrium becomes the decidua and forms 3 distinct anatomical regions (at approx 3 weeks)

Decidua Basalis at implantation site
Decidua Capsularis enclosing the conceptus
Decidua Parietalis the remainder of uterus
- Decidua Capsularis and Parietalis fuse eventually fuse and uterine cavity is lost by 12 weeks

Bilaminar Embryo

Human development about day 8 to 9

The outer trophoblast and inner embryoblast layers now both differentiate to form two distinct cellular layers. The trophoblast layer forms the syncitotrophoblast and cytotrophoblast layers. The embryoblast (inner cell mass) forms the epiblast and hypoblast layers. This early stage of embryo development is referred to as the bilaminar embryo.

Syncitiotrophoblasts

secrete proteolytic enzymes, enzymes break down extracellular matrix around cells
Allow passage of blastocyst into endometrial wall, totally surround the blastocyst
generate spaces that fill with maternal blood- lacunae
secrete Human Chorionic Gonadotropin (hCG), hormone, maintains decidua and Corpus Luteum, basis of pregnancy diagnostic test, present in urine is diagnostic of pregnancy
- Later in development placenta will secrete hCG

MH - more information in lecture 4 and laboratory.

Human Chorionic Gonadotropin

levels peak at 8 to 10 weeks of pregnancy, then decline and are lower for rest of pregnancy

0-1 week: 0-50 mIU/ml  

1-2 weeks: 40-300 mIU/ml

3-4 weeks: 500-6,000 mIU/ml

1-2 months: 5,000-200,000 mIU/ml

2-3 months: 10,000-100,000 mIU/ml

2nd trimester: 3,000-50,000 mIU/ml

3rd trimester: 1,000-50,000 mIU/ml

Non-pregnant females: <5.0 mIU/ml Postmenopausal females: <9.5 mIU/ml

Week 2 Abnormalities

Abnormal Implantation

Ectopic tubal pregnancy

Ectopic Pregnancy | Movie - Ectopic pregnancy ultrasound

Abnormal implantation sites or Ectopic Pregnancy occurs if implantation is in uterine tube or outside the uterus.

sites - external surface of uterus, ovary, bowel, gastrointestinal tract, mesentry, peritoneal wall
If not spontaneous then, embryo has to be removed surgically

Tubal pregnancy - 94% of ectopic pregnancies

if uterine epithelium is damaged (scarring, pelvic inflammatory disease)
if zona pellucida is lost too early, allows premature tubal implantation
embryo may develop through early stages, can erode through the uterine horn and reattach within the peritoneal cavity

Uterus Abnormalities

Endometriosis endometrial tissue located in other regions of the uterus or other tissues. This misplaced tissue develops into growths or lesions which respond to the menstrual cycle hormonal changes in the same way that the tissue of the uterine lining does; each month the tissue builds up, breaks down, and sheds. Endometriosis

Monoygotic Twinning

Monozygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.

Week		Week 1							Week 2
Day	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Cell Number	1	1	2	16	32	128				bilaminar
Event	Ovulation	fertilization	First cell division	Morula	Early blastocyst	Late blastocyst Hatching	Implantation starts			X inactivation

Monoygotic Twin Type		Diamniotic Dichorionic			Diamniotic Monochorionic			Monoamniotic Monochorionic			Conjoined