Unit 4

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• Gestation


  • Measured from the first day of the last normal period (LMP)
  • In humans, it is approximately 280 days
• Calculating due date


  LMP-3 months + 7 days
• Fertilization


  Usually 14-21 days after 1st day of last period
• Stages of human development

  • Zygote from fertilization
  • Embryo from end of week 1 to week 8
  • Fetus from week 9 till birth
• Blastogenesis


  1. Cells start orientating into inner cell mass (embryoblast) and outer ring of cells (trophoblast) forming blastocyst

  2. Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast

  3. Embryoblast differentiates into hypoblast and epiblast layer
• Gastrulation


  1. Epiblast differentiates into 3 germ layers: ectoderm, endoderm, mesoderm

  2. Hypoblast forms extra-embryonic structures
• Trophoblast cells


  • Cytotrophoblast (cellular trophoblast)
  • Syncytiotrophoblast (acellular outer layer)
• Embryoblast/Inner cell mass


  • Hypoblast - forms extra embryonic endoderm, yolk sac
  • Epiblast - forms embryo and 3 embryonic membranes
• Amniotic ectoderm becomes fetal placenta
• Embryonic endoderm and mesoderm form bilaminar embryonic disc
• Gestation


  • Measured from the first day of the last normal period (LMP)
  • In humans it's approximately 280 days
• Calculating due date


  LMP-3 months + 7 days
• Fertilization


  Usually 14-21 days after 1st day of last period
• Stages of human development

  • Zygote from fertilization
  • Embryo from end of week 1 to week 8
  • Fetus from week 9 till birth
• Blastogenesis


  1. Cells start orientating into inner cell mass (embryoblast) and outer trophoblast cells

  2. Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast

  3. Embryoblast differentiates into hypoblast and epiblast
• Gastrulation


  1. Epiblast differentiates into 3 germ layers: ectoderm, endoderm, mesoderm

  2. Primitive streak forms and cells invaginate to form definitive endoderm and mesoderm
• Structures formed from trophoblast and embryoblast

  • Trophoblast forms extra-embryonic structures and placenta
  • Embryoblast forms bilaminar embryonic disc, embryo and 3 embryonic membranes
• There is no study material provided in the latest message.
• Gastrulation


  The process whereby the three primary germ layers of the trilaminar embryonic disc are formed
• Neurulation


  1. The first cells to invaginate through the primitive groove form the definitive endoderm

  2. The remaining cells of the epiblast are called the ectoderm

  3. Cells that remain in the space between the ectoderm and definitive endoderm form a layer called the mesoderm
• Neurulation


  The process whereby the neural plate forms into a neural tube, marking the beginning of the formation of the central nervous system
• Neurulation


  1. The neural plate forms at the cranial end of the embryo and grows in a cranial to caudal direction

  2. The lateral edges of the neural plate become elevated and move together to form the neural folds

  3. The neural folds fuse together, and the neural plate transforms into the neural tube

  4. Fusion of the neural tube usually begins in the middle of the embryo, extending in both cranial and caudal directions

  5. During the closure of the neural tube, cells on the crest of the neural folds detach, forming the neural crest
• Once the neural tube has completely fused, the process of neurulation is complete
• Neurulation occurs during the third week of embryonic development
• Embryonic folding


  1. A neural plate of thickened cells forms in a cranial to caudal direction

  2. The lateral edges of the neural plate elevate and fuse to form the neural tube

  3. Neural crest cells contribute to the formation of the peripheral nervous system
• From the fourth week of development, the embryo undergoes a rapid development in size and shape
• Embryonic folding


  1. The trilaminar disc undergoes a process called embryonic folding to create a basic three dimensional human body plan

  2. Embryonic folding occurs in two planes: the horizontal plane and the median plane

  3. Folding in the horizontal plane results in the development of two lateral body folds

  4. Folding in the median plane results in the development of the cranial and caudal folds
• The cylinder consists of three layers derived from the trilaminar embryonic disc; these are the endoderm, the ectoderm and the mesoderm
• Endoderm


  The innermost layer, mainly responsible for the formation of the gastrointestinal tract
• Embryonic folding


  1. The endoderm moves towards the midline and fuses, incorporating the dorsal part of the yolk sac to create the primitive gut tube

  2. The primitive gut tube differentiates into three main parts, the foregut, midgut and hindgut

  3. The foregut is temporarily closed by the oropharyngeal membrane, which at the end of the fourth week of development ruptures to form the mouth

  4. The midgut remains connected to the yolk sac until the fifth week of development, when the connection narrows into a stalk known as the vitelline duct

  5. The hindgut is temporarily closed by the cloacal membrane, which during the seventh week of development ruptures to form the urogenital and anal openings
• As a result of embryonic folding, the major body plan is established, and the three germ layers continue to differentiate, giving rise to their own specific tissues and organ systems
• Mostly covered in lectures by Dr. Wessels
• Concentrate on the following: 280 days/40 weeks
• MEIOSIS


  1. Spermatogenesis

  2. 4 x haploid(n) spermatocytes

  3. Oogenesis

  4. 1 haploid(n) oocyte
• Somatic cells


  Have diploid (2n) 46 chromosomes
• Gametes


  Have haploid(n) 23 chromosomes
• Types of chromosomes


  • 44 autosomes
  • 2 sex chromosomes
• Males determine the sex of the offspring
• Sex chromosomes carry many genes for traits unrelated to gender