Chapter 14. Week 1 of Embryonic Development: Ovulation to Implantation - Review of Medical Embryology Book

Introduction: the unfertilized ovum reaches the ampulla of the uterine tube and is fertilized, in the distal third of the tube by 12-24 hours, to form a zygote. During its passage through the uterine tube, until the end of its morula stage, the egg undergoes almost no change in volume and is about 150 mm. It remains surrounded by the zona pellucida which it loses upon entering the uterine cavity. The zygote has made its way under the influence of peristaltic movements of the uterine tube and of ciliary movements of the tube epithelium. During its passage, the egg maintains itself on its own reserves, which are reduced since it is an alecithal* egg, and on tubular secretions. Survival of the egg and its transport down the tube, as well as implantation of the blastocyst, depends on hormonal secretions of the ovary and anterior pituitary gland

Cleavage is the rapid mitotic cell division that the zygote, a single cell, undergoes as it passes down the uterine tube. It may occur without fertilization as a part of parthenogenesis (naturally occurring or artificially induced)

DIVISION OF THE ZYGOTE into 2 daughter cells, the blastomeres, takes place by 30 hours. Further divisions follow rapidly upon one another, forming progressively smaller and smaller blastomeres: 4 are seen in 40-50 hours, 8 by 60 hours, and 12-16 by day 3 or 4

The 12-16 blastomere stage, arrived at by cleavage of the fertilized ovum, is a solid ball resembling a mulberry and is called a morula (morula stage). As it forms, the morula enters the uterine cavity from the tube

ABOUT DAY 4, fluid enters the morula from the uterine cavity and occupies the intercellular spaces. The fluid-filled spaces fuse to form a single, large cavity, the blastocele, and the morula is now called a blastocyst (blastocyst stage)

An outer cell layer, the trophoblast, which gives rise to the placenta
A group of centrally located cells, the inner cell mass or embryoblast, which gives rise to the embryo proper

The free blastocyst is seen in the uterine cavity on day 4 or The zona pellucida disappears rapidly

In the human, the trophoblast cells over the embryonic pole of the blastocyst penetrate the epithelial cells of the uterine mucosa at about day 6 or 7 or about 20 days after the beginning of the last menstruation

Penetration and erosion of the epithelial cells of the mucosa result from proteolytic enzymes produced by the trophoblast
The uterine mucosa also promotes proteolytic action of the blastocyst so implantation is a mutual action of the endometrium and trophoblast

An inner cytotrophoblast or cellular trophoblast
An outer syncytiotrophoblast or syncytial trophoblast consisting of a multinucleated protoplasmic mass in which intercellular boundaries are absent

The fingerlike processes of the syncytial trophoblast grow into the endometrial epithelium and invade the endometrial stroma
By the end of week 1, the blastocyst is superficially implanted in the compact layer
At the time of implantation, the uterine mucosa is at day 21 of the menstrual cycle and is richly vascularized, edematous, and secreting mucus and glycogen - all favoring implantation of the blastocyst
As the blastocyst is implanting, early differentiation of the inner cells mass occurs

The embryonic endoderm (entoderm), a flattened layer of cells, appears on the surface of the inner cell mass facing the blastocyst cavity, at about day It is the first of 3 germ layers of the embryo that forms during the first 3 weeks