Human Development - Week One
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Introduction
Several hours after ovulation, the egg is in a semi-dormant state and usually encounters the sperm in the Fallopian tube.
The fertilized embryo remains in the fallopian tube for 72 hours after ovulation, and an additional 72 hours in the uterus before implantation.
Fertilization
Fertilization is the first stage of embryonic development, with a spermatozoan and a secondary oocyte combining to form a zygote several hours after ovulation.
Fertilization occurs in eight steps:
- The sperm head weaves past the follicular cells and attaches to the zona pellucida, which is comprised of glucoproteins.
- Interaction of the sperm with glycoprotein ZP3 leads to the acrosomal reaction
- Zona pellucida hydrolysis occurs
- Fusion of the sperm-egg plasma membranes and excocytosis of acrosomal contents occurs
- The spermatozoan's presence activates the cortical reaction, whereby Ca2+ influx causes a hardening of the zona pellucida and prevents further sperm penetration
- The oocyte's second meiotic division, which was arrested in metaphase, occurs, resulting in a second polar body
- The male pronucleus forms
- The male and female pronuclei fuse, forming the zygote bearing 46 chromosomes
- Intermingling of the chromosomes before cleavage (mitosis) means that the embryo is genetically unique
Cleavage, Morula, and Blastocyst Formation
Division of the zygote occurs approximately 30 hours after fertilization.
Repeated cleavages results in the formation of a morula (12-32 cells in a sphere) three days after fertilization.
Shortly after the morula enters the uterus, a fluid-filles space begins to grow and transforms the morula into a blastocyst. As fluid accumulates, the blastocyst separates into two parts:
- the trophoblast, a thin outer cell layer that gives rise to the embryonic part of the placenta
- the embryoblast, an inner group of cells that is the beginning of the the embryo
Subsequent disintegration of the zona pellucida (the egg 'hatching') allows the blastocyst to rapidly grow in preparation for implantation.
Implantation
The presence of the blastocyst induces further decidualization of the endometrium during the secretory phase in preparation for implantation.
Before the embryo implants in the endometrium, indirect communication is established through the synthesis and secretion of several hormones, matrix and adhesion molecules, and surface receptors.
Small, finger-like protrusions called pinopodes develop on endometrial cells and persist for only 2-3 days.
Implantation occurs 7-8 days after fertilization, with three steps occurring - apposition, adhesion, and invasion.
About 6-8 days after fertilization, the blastocyst moves alongside the endometrium and adheres to it, inducing the rapid proliferation and gradual differentiation of the trophoblast into two layers:
- the outer syncytiotrophoblast - a multinucleate cell mass that invades the endometrium and its constituent blood vessels
- the inner layer of cytotrophoblast
Fingerlike projections of the syncytiotrophoblast extend through the endometrium epithelium and invade the connective tissue.
The syncytiotrophoblast acts as an endocrine gland. Among other things, it secretes human chorionic gonadotropin (hCG), closely related to lutenizing hormone, into maternal blood to signal the corpus luteum to continue producing progesterone to maintain the endometrium, until the placenta is producing enough on its own.
Implantation at sites other than the uterus leads to ectopic pregnancy, and implantation low in the uterus leads to placenta previa.
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