Gametogenesis, Fertilization, Cleavage and Gastrulation (Germ layers) Flashcards
Spermatogenesis (metamorphosis)
Spermatogonium (2N), 1
——– S phase (mitosis/ differentiation) ——-
Primary spermatocyte (2N, # of chromos decrease, diploid, 16), 1
——-Meiosis 1———
Secondary spermatocytes (1N, haploid, no DNA duplication, 32), 2
——- Meiosis 2———–
Spermatids (1N, haploid, don’t divide, 64), 4
———- Metamophosis ———
Spermatozoa (1N, flagellum), 4
When is mitosis completed for females?
In fetal state, before birth
At birth all eggs are primary oocytes
Oogenesis
Oogonium
——– S phase (mitosis/ differentiation) ——-
Primary Oocyte (diplotene, 2N)
——-Meiosis 1———
Secondary Oocyte and First polar body (1N)
——- Meiosis 2———–
Ootid and second polar bodies (1N)
———- differentiation ———
Ovum
What will the primary oocytes undergo?
Atresia
360 will ovulate out of the 100,000 after puberty
When is meiosis 1 arrested in females?
During primary oocyte at diplotene stage
When is meiosis 1 completed in females?
Just before ovulation (all mammals) or after ovulation in the ampulla without sperm (mare and dogs)
When is Meiosis II completed?
After sperm penetration at secondary oocyte and polar body
What are the ovulated eggs?
Female dogs and mares: primary oocyte
All other species: secondary oocyte
Steps prior to fertilization in females
- Capacitation of sperm
- Binding of sperm with zona pellucida
- Acrosome reaction
- Penetration of zona pellucida
- Sperm-egg cell membrane fusion
- Cortical reaction
- Completion of Meiosis II (2nd polar body forms)
- Pronuclei formation
- Union of male and female pronuclei
Capacitation
Removal of certain sperm membrane proteins
Acrosome reaction
Rupture of outer acrosomal membrane
Hydrolytic enzymes released
Motility increases resulting in zona penetration
Cortical reaction
Release of glycoprotein granules from cytoplasm of egg
Hardens the zona pellucida and alters oocyte membrane to prevent polyspermy
Pronuclei formation
Sperm nucleus is heterochromatic (must condense)
Ovum chromos must coil
Union of pronuclei
Fertilization
Results in restoration of 2N chromo number
Forms zygote
What are the results of fertilization?
Completion of Meiosis II
Restoration of diploid chromo number
Determination of genetic sex
Initiation of cleavage
Cleavage
Division of zygote into many cells without an increase in mass cytoplasm
Blastomeres
Cells that lie within the confines of zona pellucida
Morula
A clumped group of 8-16 cells formed by cleavage
Latin name for “small mulberry”
Most embryos reach the ______ at the _________ stage.
- Uterus
- Morula
When is morula stage (uterus) reached for most species?
4-5 days
What happens to blastomeres after stage 8?
They lose their universal potential
Each blastomere can no longer become an embryo
What is the second stage of the blastomere?
Blastula
Blastocoel
Cavity formed when fluid-filled spaces coalesce among blastomeres
Trophoblast
Formed when blastomeres surround the blastocoel
Future amnion and chorion
Inner cell mass (embyro proper)
When some blastomeres form a mass at one end of the blastcoel
How long does the blastula stage last?
5-9 days after ovulation
What are the stages of the blastomere?
- Blastomere
- Morula
- Blastula
- Hatching (degeneration of zona pellucida)
- Gastrulation (germ layers developed)
Hatching
Degeneration of the zona pellucida
Degenerates one day after completion of blastula
Which species are exceptions to hatching?
Horses
Pigs and ruminants
What happens to the zona pelucida in horses?
It’s replaced by glycoprotein capsule (neo-zona)
Remains until 21 days of gestation
What happens after hatching in pigs and ruminants?
Trophoblast undergoes a rapid elongation
Up to 1 meter long in pigs and 35 cm in ruminant
For pigs, when is elongation completed?
9-16 days of gestation
For ruminants, when is elongation completed?
12-14 days of gestation
Formation of Primitive Streak (P.S.)
Thickening of caudal end of epiblast cells due to proliferation
Formation of 2 ridges and a central groove along the diameter of a streak
What is the significance of PS?
First step in the formation of mesoderm
Establishes cranial and caudal ends of the embryo
Primitive node (Hensen’s node)
Formed by the cranial most end of the primitive streak
What is the notochord remnant in adults?
Nucleus pulposus
How is the mesoderm split?
Paraxial (closer to notochord)
Intermediate
Lateral (develops coelom and divided into somatic and splanchnic)
Differentiation of somite
Paraxial/ mesoderm differentiates on each side of notochord into somite
Somite formation begins cranially and extends caudally
What is each vertebral somite represented by?
One pair of somite, each pair constant for each species
Number of pairs can be related to age of embryo
What are the parts of a somite?
- Sclerotome: vetebrae (axial skeleton)
- Myotome: skeletal musculature of body except head region
- Dermatome: dermis underlying CT of skin
What does ectoderm form?
Epidermis of the skin
Epithelium of the oral and nasal cavities
Nervous system and sense organs
What does mesoderm form?
Muscle and CT
Including bone, components of circulatory, urinary and genital systems
What does endoderm form?
Mucosal epithelium and glands of respiratory and digestive systems
Aplasia/ hyplasia
Dwarfism
Cranio-facial defects
Abnormal neural crest migration
Amelia
Absense of extremities
Meromelia
Partial absence of extremity
Micromelia
Short extremity
Polydactyly
Extra digits
Syndactyly
Fused digits
