General Embryology

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    • Rostral (cranial/superior) in embryology nomenclature refers to the head end.
    • Caudal (inferior) in embryology nomenclature refers to the tail end.
    • Dorsal (posterior) in embryology nomenclature refers to the back.
    • Ventral (anterior) in embryology nomenclature refers to the front.
    • Proximal in embryology nomenclature refers to structures near the body centre.
    • Distal in embryology nomenclature refers to structures further from the body centre, for example, the hand is distal to the elbow.
    • Medial in embryology nomenclature refers to the centre or middle.
    • Human Development Timeline: Zygote is formed at 30 hours, Morula is formed between 30 hours and Day 4, Blastula is formed from Day 4 to 7, Gastrula is formed from Day 7 to Week 3, Embryo is formed from Weeks 3 to 8, and Foetus is formed from Weeks 8 to Birth.
    • Fertilisation is the fusion of sperm with ovum to produce a zygote, eventually leading to the development of an embryo.
    • Gametes are sex/germ cells: Sperm and Ovum.
    • Ovum is the female germ mature egg cell, the largest single biological cell, haploid with a single set of chromosomes.
    • Sperm is a haploid cell with a single set of chromosomes.
    • Fertilisation occurs 12 - 24 hours after ovulation in the fallopian tube, with the penetration of the oocyte using acrosomal enzymes in the head, sperm dissolves and penetrates the zona pellucida (secondary oocyte’s).
    • The head of sperm fuses with the oocyte’s plasma membrane, sperm head then disconnects from the flagellum tail.
    • The ovum nucleus and sperm head nucleus are pronuclei which fuse, intermixing their chromatin to form a diploid nucleus which is a zygote.
    • By the end of week 2, the bilaminar embryonic disc is connected to trophoblast by a connecting stalk, forming the future umbilical cord.
    • Invagination of epiblast cells from the primitive streak displace hypoblast and form endoderm (GIT tract/lung epithelial lining).
    • The initial event in embryonic development is Primitive streak formation, a morphological groove formed by migrating epiblast cells, which will form two germ cell layers (endoderm and mesoderm) and the embryo body axis (head-tail/left-right orientation).
    • Invagination of epiblast cells through Primitive streak and displacement of hypoblast form endoderm.
    • Thickening of endometrial layers is due to extra-embryonic mesoderm, which fills the space between trophoblast, amnion, and chorion, forming an extra-embryonic coelom.
    • Neurulation is the next process in embryonic development.
    • The embryonic disc now has endoderm, mesoderm and ectoderm, and this flat disc begins to fold ventrally at the edges, to form a hollow cylinder (tube) of the three germ layers.
    • The two parts derived from trophoblast are the extraembryonic mesoderm and coelom, which surrounds the embryo and later the foetus, becomes part of the placenta.
    • Cells remaining between epiblast and endoderm are mesoderm (connective tissue/mesenchyme).
    • The chorion acts as an exchange site between mother and embryo, for nutrients and waste.
    • Cells remaining in epiblast are ectoderm (skin epidermis & nervous system).
    • The three germ layers are the ectoderm (external layer), mesoderm (middle layer), and endoderm (internal layer).
    • The Primitive node is a region at the cranial end of the primitive streak.
    • Gastrulation is the major event in mammalian embryonic development where the bilaminar embryonic disc is reorganised into a trilaminar structure, containing three germ layers, which will form the embryo's entire body organs and tissues.
    • Cleavage is where the zygote undergoes rapid mitotic divisions.
    • Cytotrophoblast, the inner trophoblast layer, eventually forms primary chronic villi and is considered to be stem cells for syncytiotrophoblast.
    • The implantation process involves the blastocyst being free in the uterine cavity for 2 days before attaching to the endometrium.
    • Syncytiotrophoblast, the outer trophoblast layer, invades the maternal endometrium, facilitating implantation of the blastocyst.
    • At Day 7, the blastocyst attaches more firmly and the endometrium becomes more vascularised.
    • The blastocyst then secretes enzymes and burrows into the endometrium.
    • The inner cell mass forms the epiblast, which forms three germ layers (primitive ectoderm), and the hypoblast, which becomes the primitive endoderm.
    • Bilaminar embryonic disc development occurs after implantation and prior to embryonic folding during week 2.
    • Trophoblast and inner cell mass layers differentiate to form two distinct cellular layers each.
    • Implantation occurs between Days 6 - 7.
    • Hypoblast becomes primitive endoderm and is a transient epithelium during week 2, lying beneath the epiblast layer.
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