Talk:2011 Lab 2 - Week 2

From Embryology


Hatching leaves the blastocyst now free of the zona pellucida and should have occured approximately at the end of the uterine tube or in the body of the uterus. It is now floating in the uterine glands rich mucus secretion and able to directly access this nutrition for continued growth.

The blastocyst initially weakly adheres to the endometrial wall rolling across its surface. Increased adhesion may lead to attachment, adplantation, on the inner cell mass side of the blastocyst. This will be the site where implantation will begin and the placenta will develop. In humans, receptivity for implantation occurs about 6 days after the post-ovulatory progesterone surge and lasts about 2 to 4 days.

Trophoblast cells at the site of adplantation proliferate and form an additional layer the syncitiotrophoblast layer. This layer of cells rapidly divide, secrete enzymes that degrade the endometrial extracellular matrix and secrete human Chorionic Gonadotropin (hCG).

Implantation Dynamics

The uterine epithelium (white cells) are invaded by the trophoblast cells (green, syncitiotrophoblasts) with the inner cell mass now having 2 layers: an epiblast (blue) and hypoblast (yellow). The blastoceol is covered in cytotrophoblast cells (green).

Later in the movie the amniotic cavity forms adjacent to the epiblast layer(blue) and spaces in the syncitiotrophoblast layer are filled with maternal blood, lacunae.

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This animation shows the process of implantation, occurring during week 2 of development in humans.

The beginning of the animation shows: the uterus lining (endometrium epithelium), the hatched blastocyst with a flat outer layer of trophoblast cells (green), the inner cell mass which has formed into the bilaminar embryo (epiblast and hypoblast) and the large fluid-filled space (blastocoel).

green cells - trophoblast layer of the conceptus

blue cells - epiblast layer of the bilaminar embryo

yellow cells - hypoblast layer of the bilaminar embryo

white cells - uterine endometrium epithelium

red - maternal blood vessel

Identify the embryoblast and trophoblast layers of the conceptus.

Carnegie Stage 4 represents the beginning of implantation. The blastocyst initially attached to the uterine endometrium (adplantation), syncitiotrophoblasts then secrete enzymes that digest extracellular matrix, allowing the blastocyst to sink into the uterine wall, eventually being completely enclosed within the uterine wall. Note the majority of growth occurs in the trophoblastic shell. The inner cell mass divides initially into 2 layers; epiblast and hypoblast (bilaminar embryo). Hypoblast cells migrate around the original blastoceol cavity forming the primary yolk sac. A second cavity (amniotic) forms between the inner cell mass and the cytotrophoblast shell; this cavity is lined by epiblast cells.

Gray0032.gif Day 8 to 9
  • am. - amniotic cavity
  • b.c. - blood clot, at the site of initial implantation
  • b.s. - body-stalk, or connective stalk later forming the placental cord region with placental blood vessels
  • ect. - embryonic ectoderm that will contribute to embryonic and placental membrane development
  • ent. - entoderm (endoderm), this was the historic term for what we today call endoderm that will contribute to embryo development
  • mes. - mesoderm, consisting of both embryonic mesoderm (in the trilaminar embryonic disc) and extraembryonic mesoderm (outside the trilaminar embryonic disc)
  • m.v. - maternal vessels, spiral arteries that have been opened at their ends
  • tr. - trophoblast, relative to the embryonic disc the outer syncitiotrophoblast and inner cytotrophoblast layers that will contribute to placental development
  • u.e. - uterine epithelium, the epithelial layer that lines the unerus
  • u.g. - uterine glands, the glands that secrete nutrients to support the initial growth both before and after implantation
  • y.s. - yolk-sac, the endoderm lined and extraembryonic mesoderm covered cavity that will contribute to the gastrointestinal tract, blood and blood vessels

Corpus Luteum

Ovary corpus luteum.jpg

Corpus luteum.jpg

An endocrine signal (hCG human Chorionic Gonadotropin) secreted from the implanting conceptus syncitiotrophoblast cells maintains the ovarian corpus luteum, which in turn provides hormonal support to the uterine functional lining, preventing menstruation. The corpus luteum is formed during the luteal phase (secretory phase) of the menstrual cycle by proliferation of both follicular granulosa cells (granulosa lutein cells) and thecal cells (theca lutein cells), which produce progesterone and oestrogens.

Following ovulation

  1. If implantation does not occur (non-pregnant), the remnant of the ovulating follicle will degenerate forming a corpus albicans.
  2. If implantation occurs (pregnancy), the remnant of the ovulating follicle will be maintained forming a corpus luteum.

If implantation does not begin until very late in the current menstrual cycle, or not at all, then that cycle will continue with loss of both the functional layer and the conceptus. Many human fertilization events never form an embryo or develop as a pregnancy.

Pregnancy test.gif

Additional Information - Birth Control

There are a number of different chemical and mechanical methods of birth control. The most comon is the "birth control pill" taken daily and made up of two hormones, estrogen and progestogen and these stop a woman's ovaries from releasing an egg each month (ovulation), which means that a pregnancy cannot begin. Recently the drug RU486, which is an abortive rather contraceptive drug, has been the centre of political and medical discussions in Australia.


  • Estrogen - the hormone estrogen in birth control pills act on the pituitary gland (supress FSH and LH) which then blocks ovulation.
  • Progesterone - the hormone progesterone in birth control pills act on the uterus to both alter the lining to prevent implantation and forms a cervical mucus plug that mechanically blocks acceess of sperm. There is also an inhibition of sperm capacitation.
  • Injectable Control - there are commercial (Lunelle, USA) injectable estrogen/progestin contraceptives administered on a monthly basis.
  • Mifepristone (RU486) - is a progesterone receptor antagonist (antiprogesterone) which can prevent between 92-100 % of pregnancies on oral intake of a 10-600 mg dose within 72 h of unprotected intercourse. (alternative commercial name: Mifegyne)

Links: Clinical Methods - Birth Control

Placental Abnormalities

Placental abnormalities

Placenta Accreta - Abnormal adherence, with absence of decidua basalis. The incidence of placenta accreta also significantly increases in women with previous cesarean section compared to those without a prior surgical delivery.

Placenta Increta - occurs when the placenta attaches deep into the uterine wall and penetrates into the uterine muscle, but does not penetrate the uterine serosa. Placenta increta accounts for approximately 15-17% of all cases.

Placenta Percreta - placental villi penetrate myometrium and through to uterine serosa.

Placenta Previa

Placenta Previa
In this placenatal abnormality, the placenta overlies internal os at the cervix of the uterus, essentially covering the birth canal. This condition occurs in approximately 1 in 200 to 250 pregnancies.

In the third trimester and at term, abnormal bleeding can require cesarian delivery and can also lead to abruptio placenta.

Ultrasound screening programs during 1st and early 2nd trimester pregnancies now include placental localization. Diagnosis can also be made by transvaginal ultrasound.

Hydatidiform Mole

Hydatidiform Mole
A placental tumor with no embryo development.

Several forms of hydatidiform mole: partial mole, complete mole and persistent gestational trophoblastic tumor.

Many of these tumours arise from a haploid sperm fertilizing an egg without a female pronucleus (the alternative form, an embryo without sperm contribution, is called parthenogenesis).

The tumour has a "grape-like" placental appearance without enclosed embryo formation. Following a first molar pregnancy, there is approximately a 1% risk of a second molar pregnancy.


  • bilaminar- having 2 layers
  • blastocyst- the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the conceptus to adplant and then implant into the uterine wall.
  • corpus albicans - (Latin, corpus = body, albicans = whitish) The histological structure formed by luteolysis of the corpus luteum in the ovary. If implantation does not occur and the hormone hCG is not released the corpus luteum degenerates and the structure is white, not yellow, because of the absence of steroid hormone synthesis/accumulation.
  • corpus luteum - (Latin, corpus = body, luteum = yellow) The remains of ovarian follicle formed after ovulation that acts as an endocrine organ (produce progesterone and oestrogens) supporting pregnancy and preventing menstruation (loss of the endometrial lining). Formed during the luteal phase (secretory phase) of the menstrual cycle by proliferation of both follicular granulosa cells (granulosa lutein cells) and thecal cells (theca lutein cells), which produce progesterone and oestrogens. If fertilization and pregnancy does not occur, the corpus luteum degenerates to form the corpus albicans. Regnier de Graaf (1641 – 1673) first observed it in the ovary of a cow as a yellow structure, the yellow colour is caused by accumulation of steroidal hormones. (More? Menstrual Cycle | Ovary Development | Week 2 Ovary | Week 1 - Oogenesis)
  • inner cell mass- the clump of cells found inside the blastocyst. These cells will go in to form the embryo, these are the "stem cells" (we here about in the media) that are totipotential, they can form any tissue in the embryo. Mature oocyte-the female germ cell released at ovulation from the ovary.
  • trilaminar embryonic disc- the 3 layered embryo stage.
  • Trophoblasts- (Gr. trophe = nutrition) outer layer of cells on blastocyst that will generate the embryonic part of the placenta.