Embryology & Gametogenesis Flashcards
Cleavage
Initial cell division and blastomeres
Unicellular zygote divides by mitosis to become multicellular Embryo
Each daughter cell is called a blastomere
No increase in size so termed as cleavage
The morula
Blastomeres continue to divide into a compact ball of 16-31 cells within the zone pellucid
Looks like a mulberry
The blastocyst
As the morula grows a cavity (blastocoele) forms within it
Inner cell mass (embryo blast) surrounded by outer cells (trophoblast)
Blastocyst: inner cells
Embryoblast forms entire embryo
Forms bilaminar disc of Epiblast (dorsal) and Hypoblast (ventral)
Establishes dorsoventral axis
Blastocyst: Outer cells
Trophoblast will expand to form the extra embryonic membranes (placentation)
Stem cells
Specialized cells capable of self renewal; they may be-
Totipotent
From zygote- can form every cell in embryo and trophoblastic placenta
Pluripotent
from inner cell mass- can form all embryonic cells (not trophoblastic placenta)
Multipotent
Can form a limited number of subtypes within a broader category (epithelial cells, WBCs etc)
Unipotential
can form a specific cell type
Migration, differentiation
a complex interaction controlled by gene interactions and cell signaling
The zygote contains
1 cell
Gastrulation
Inner cell mass –> Bilaminar disc –> trilaminar disc
Cells from epiblast migrate leaving a primitive streak (craniocaudal axis)
Trilamminar disk
Renamed as 3 germ layers:
Ectoderm
Mesoderm
Endoderm
These layers are formed by migration of epiblast cells
Ectoderm derivatives
Epidermal structures (skin), lining of oral, nasal cavity and anus. Nervous system (neuroectoderm - specialized ectoderm)
Mesoderm derivatives
Connective tissues, muscle tissue, mesothelium, cardiovascular system, urogenital tract
Endoderm derivatives
Epithelial lining and glands of digestive and respiratory system
Neural tube
Mesoderm forms notochord under primitive streak
Notochord induces ectoderm to form neural tissue (neuroectoderm)
Neural tube and neural cost cells formed; early somites formed
Cranial part of the neural tube–> brain
Caudal part of neural tube –> spinal cord
Embryonic folding
Cranial-caudal and lateral body folds create the head, tail, primitive anus and mouth
The yolk sac is lined by
endoderm
Mesoderm differentiates further
Axial: notochord
Paraxial: somites
Intermediate: urogenital system
Lateral plate: somatic and splanchnic mesoderm
Somites
Dermatome –> dermis
Myotome –> muscles
Sclerotome –> vertebrae and ribs
Somatic and splanchnic mesoderm separate with formation of coelom (peritoneum) in between
Limb buds- all other cartilage and bones of the body and limbs are formed from lateral plate mesoderm
Skull is formed of a combination of lateral plate mesoderm, paraxial (somite) mesoderm and neural crest cells
The smooth muscle of the GIT arise from
Splanchnic mesoderm
Pluripotent stem cells arise from the
inner cell mass
Summary of embryology
Sperm +ovum =pronucleum –> zygote –> morula –> blastocyst —> trophoblast and inner cell mass
Trophoblast –> placenta
Inner cell mass –> epiblast –> ectoderm, mesoder, endoderm –> foetus or talk sac
Hypoblast –> yolk sac
Teratogens
Any age or factor that can cause congenital (present at birth) anomalies in an embryo or fetus
Genetic factors
Radiation
Chemical agents (nicotine, drugs, alcohol)
Infectious agents (bovine viral diarrhea virus)
Hormones (anti-paramesonephric duct hormone in freemartins)
Teratogenic influences on development
At predifferentiation stageL embryo dies
During organogenesis stage: structural defects (major morphologic defects)
During fetal growth stage: affects functional maturation (and minor structural defects)
At which of the following stages does action by a teratogen result in the most severe consequences
Predifferentiation
Which are part of the brain
Cerebrum
Cerebellum
Brainstem
Pineal gland
Gametogenesis
Production of haploid (1N) germ cells at the right place at the right times
Gametes have 1/2 the chromosome number
Spermatozoa or ova
Fertilization
Fusion of 2 haploid gametes creates a diploid (2N) zygote with full complement of chromosomes
Embryological origins of germ cells
Primordial germ cells from yolk sac and allantois migrate to genital ridge via dorsal mesentery –> become biopotential gonad
Male XY
Seminiferous cords (no lumen)
-spermatogonia
-supporting sertoli cells
Interstitial cells of leydig
Female XX
Primordial germ cells (oogonia) form peripherally arranged primordial follicles in a stroma
Primordial follicles all formed in the developing embryo before birth
Formation of duct systems
Male: mesonephric duct
-forms epididymis, ductus deferens
Female: paramesonephric duct
-formed uterine tubes, uterus, cervix
Anti-mullerian hormone produced by sertoli cells directs male development
The gonads arise from
intermediate mesoderm
The cell cycle
G1 phase
S phase
G2 phase
M phase
Mitosis
DNA duplicates in S phase of interphase
Each chromosome has 2 sister chromatids
Final products are 2 2n
Clones of the original cell
Meiosis in males
2 cells ends in 4 haploid cells at end of meiosis 2
Spermatogenesis
- Spermatocytogenesis: Spermatogonia divide by mitosis several times and produce primary spermatocytes
- Meiosis: primary spermatocytes divide by meiosis into secondary spermatocytes and spermatides
- Spermiogenesis: transformation (metamorphosis) of spermatids into spermatozoa
Meiosis in females
Stops in prophase 1 until puberty- primary oocyte
puberty meiosis continues
Stops on metaphase 2 in oviduct until sperm contact then it will complete meiosis 2
Oogenesis
Production of an ovum
- oogonia- prenatal mitosis produces primary oocytes
- Primary oocytes (diploid) arrested in meiosis I until puberty
- After puberty- orderly maturation of the oocyte and supporting cells within a protective ovarian follicle under hormonal influence
- Ovarian follicle- meiosis I is completed just before ovulation to produce secondary oocyte and 1st polar body except for dogs and mares (these ovulate primary oocytes)
- Meiosis II starts immediately but arrests until
- In oviduct, secondary oocyte (haploid) awaits fertilization
- Meiosis II completed on sperm contact (2nd polar body is produced)
- Result is ovum (haploid) and 2nd polar body (haploid)
Polar bodies are remnant of cytoplasm and nucleus formed during meiosis
Mitosis vs meiosis
Mitosis results in formation of 2 diploid daughter cells
Meiosis results in formation of 4 haploid cells and exchange of genetic material
Gametes are haploid and of different genetic constitution
Common features of male and female gametogenesis
Occur in specialized sexually dimorphic organs (testis/organs)
Multiplication of spermatogonia and oogonia by mitosis
Formation of gametes by meiosis- importance of crossing over and independent assortment for variability= extensive morphological variation (evolution)
Incapacity of surviving for very long if fertilization does not occur
Differences of male and female gametogenesis
Sperm: supply continuously renewed
- one primary spermatocyte produces 4 sperm
- Gametogenesis results in small cells
- Motile cells
- X or Y sex chromosome
Oocyte: all present at birth
- one primary oocyte produces 1 ovum
- Gametogenesis results in a large cell
- immotile cell
- x chromosomes only
Chromosome number halved during
meiosis 1
Ovulation
Oocyte departs the ovary leaving behind a structure known as the corpus hemorrhagicum- a (transient structure) that becomes the corpus luteum of luteal cells
Spermatozoa
Haploid cell (nucleus) for fertilization Motility to deliver the nucleus to the ovum Energy Mechanism of entry into ovum
Maturation of sperm
acrosome covered by extension of pm
Spermatogenesis: Histology
Peripheral on basement membrane (basolateral): spermatogonia -divid and produce more spermatogonia and primary spermatocyte
-sertoli cells- support
Towards Lumen (apical): primary spermatocytes (secondary spermatocytes are short lived) -round and elongated spermatides
Lumen: sperm
Spermatogenesis: metamorphosis
Transformation of the round spermatid to a spermatozoon
- formation of the acrosome from golgi vesicles (contains enzymes)
- condensation of the nuclear chromatin
- outgrowth of a motile tail (flagellum-from centrioles) surrounded by mitochondria
- loss of excess spermatid material (cytoplasm, water, organelles)
Spermatozoon morphology
Head: nucleus, plasma membrane, acrosome (enzymes)
Mid piece: spirally arranged mitochondria. Intense energy production
Tail: Motility. classified’s flagellum/cilia structure. Axoneme- outer ring of 9 doublet microtubules
Fertilization: capacitation
Occurs in uterine environment
Biochemical change in response to uterine secretions
Hyperactivity of the spermatozoon tail
Allows acrosomal reaction to occur
Fertilization: Acrosome reaction
enzymes released ti digest zone pellucida
Fertilization: cortical reaction
Sperm contact with oocyte plasma membrane
Release of proteases from cortical granules in oocyte
Zona pellucida is then a barrier to other seem penetration
Fertilization
Nuclear material of spermatozoon and ovum are called pronuclei
Syngamy
Syngamy
Union of male and female haploid pronuclei. They merge restoring diploid set of chromosomes and enter the first mitotic division= zygote
