Questions and Answers
What type of twins develops independently with their own extraembryonic membranes?
What term is used for abnormal twins that survive by sharing the blood supply of the normal twin?
Which type of twins are usually formed from incomplete division of the embryo during the primitive streak stage?
Which classification of twins refers to those joined at the abdomen and often has partially fused intestines?
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What is the common term for twins formed from a single zygote that separates at different stages?
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What is the term used for a fertilized egg that is in the early stages of development before it reaches the uterus?
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Which structure is formed from the outer shell of the blastocyst that nourishes the embryo?
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During which process do blastomeres divide without growing in volume after fertilization?
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What term describes twins that consist of one normal individual and an extra body part that is attached?
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What is the state of cells within the morula in terms of their developmental potential?
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Study Notes
Causes of Multiple Births
- Multiple births can occur from fertilization of separately ovulated eggs.
- Complete or partial separation of cleavage/blastocyst blastomeres leads to multiple embryos.
- Duplication during gastrulation can also result in multiple embryos.
Classification of Twins
- Free Twins: Unattached and can be dizygotic or monozygotic.
- Cojoined Twins: Twins that are fused together, usually monozygotic.
- Symmetrical Twins: Identical twins that can be free or cojoined.
- Asymmetrical Twins: One twin is normal, while the other is rudimentary or malformed.
Types of Free Twins
- Free, Symmetrical Twins: Develop from a single zygote separated at different growth stages.
- Zygote's cleavage may lead to two embryos with distinct extra-embryonic membranes (EEM).
- May occur through bisection or rupture of zona pellucida, resulting in shared embryonic disc.
- Free, Asymmetrical Twins: One twin can be a normal embryo while the other may be an amorphous globosus or acardiac twin.
Characteristics of Cojoined Twins
- Cojoined Twins: Also known as Siamese twins, they arise from incomplete embryo division.
- Classified based on attachment sites, including thoracopagus, abdomonopagus, pygopagus, and cephalopagus.
- Prefixes indicate body part duplication like dicephalus (two heads) or diprosopus (two faces).
Fertilization to Gastrulation Process
- Human prenatal development consists of three stages: pre-embryonic, embryonic, and fetal.
- Fertilization typically occurs in the ampulla of the fallopian tube, forming the conceptus.
- The conceptus undergoes cleavage, resulting in blastomeres through mitotic cell division.
- After approximately three days, the morula forms and contains around 16 totipotent cells.
Blastocyst Formation
- The morula develops into a blastocyst, consisting of an inner cell mass (embryoblast) and an outer trophoblast.
- The trophoblast's role is to nourish the embryo and later forms the chorion (fetal placenta).
- Trophoblast secretes an enzyme to dissolve the zona pellucida, facilitating implantation.
- Upon implantation, trophoblasts develop into syncytiotrophoblasts, extending into the endometrium for nutrition.
Hormonal Role and Embryonic Layers
- Syncytiotrophoblasts secrete human chorionic gonadotropin (HCG) to sustain the corpus luteum and progesterone production.
- Progesterone is vital for pregnancy maintenance and supports nutrient-rich decidual cell growth.
- By the second week, the embryoblast forms a bilaminar disc (hypoblast and epiblast), crucial for embryo development.
- Gastrulation transforms the bilaminar disc into three definitive germ layers (ectoderm, mesoderm, endoderm) which differentiate into various organs and tissues.
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Description
This quiz explores the phenomena of twinning and embryonic duplication, including the various causes of multiple births such as the fertilization of separately ovulated eggs and the processes during gastrulation. It also classifies different types of twins, highlighting the differences between dizygotic and monozygotic twins. Test your understanding of these fascinating topics in embryology!
