LEC 11: Embryology I - 08.25.14 Flashcards
When does human development begin
Human development begins at fertilization (sperm fertilizes egg)
totipotent stem cells
ability of a single cell to divide and produce all of the differentiated cells in an organism (e.g. zygotes)
gametogenesis
process of formation and development of gametes (oocytes or sperm)
ploidy of gametes
sperm and oocyte are highly specialized sex cells; are haploid (half the necessary chromosomes)
oogenesis
formation of mature ovum during gametogenesis
spermatogenesis
formation of mature sperm during gametogenesis
clinical significance of meiosis during gametogenesis
- germ cells only (to produce haploid gametes)
- reductive division
- diploid germ cells –> haploid gametes (sperms and oocytes)
- contant chromosome number
- variable number of copies
- genetic variability
- random assortment
- recombination
overview of spermatogenesis
- process by which spermatogonia are transformed into mature sperm
- spermatogonia dormant in seminiferous tubules of testes until puberty
- transformed into 1* spermatocytes
- transformed into 2* spermatocytes
- 4 spermatids undergo spermiogenesis
Result: 4 mature sperm (spermatozoa)
1st meiotic division of spermatogenesis
- primary spermatocyte (46 XY) undergoes 1st meiosis
- (2) secondary spermatocytes (23 X, 23 Y)
2nd meiotic division of spermatogenesis
- secondary spermatocytes (23 X, 23 Y) undergo 2nd meiotic division
- (4) spermatids (23 X, 23 X, 23 Y, 23 Y)
spermiogenesis
- 4 spermatids from 2nd meiotic division undergo spermiogenesis
- result is (4) mature sperm
- spermiogenesis is last phase of spermatogenesis
- rounded spermatid transformed into elongated sperm
Where does spermatogenesis occur, and where are mature sperm stored
- spermatogenesis occurs sequentially in the testes
- sperm then transfored from seminiferous tubules to epididymis
- stored
- become functionally mature during puberty
spermatozoa
mature sperm cell
How long does spermatogenesis take
approximately 2 months
Sertoli cells
line the seminiferous tubules and support/nuture germ cells; may be involved in regulating spermatogenesis
When do sperm become functionally mature
sperm are passively transported from seminiferous tubules (testes) to the epididymis, where they are stored and become functionally mature during puberty
location and description of epididymis
elongated coiled duct along the posterior border of the testes; continguous with the ductus deferens which transport sperm to the urethra
characteristics of spermiogenesis
- loss of cytoplasm
- development of the tail
- formation of the acrosome
- acrosome, derived from Golgi region of spermatid, contains enzymes released at beginning of fertilization
- assist sperm in penetrating corona radiata and zona pellucida
Structure of mature sperm
- head
- haploid nucleus
- partially covered by enzyme-filled acrosome
- tail (motility to site of fertilization)
- middle piece = full of mitochondria (ATP)
- principal piece
- endpiece
NB: Y chromosome is essential for normal spermatogenesis; microdeletions = defective spermatogenesis and infertility
oogenesis
process by which oogonia transformed into mature oocytes; begins before birth and is completed after puberty; ends with menopause
NB: timing differs for spermatogenesis and oogenesis
sequence of events in oogenesis
oogonium –> primary oocyte –> secondary oocyte / Polar Body I –> fertilized ovum / Polar Body II
oogonia
- primordial female sex cells
- fetal life proliferate via mitosis
- enlarge to form primary oocytes before birth
folliculus
Latin, “small bag, shell, pod”
What is the difference between a follicle and an oocyte
- oocyte develops within a follicle in the ovary
- follicle is a thin-walled structure containing fluid, with the egg attached to the wall
- Usually, only one follicle develops per month
primary oocytes (primordial follicles vs. primary follicles)
primary oocytes are surrounded by flattened follicular cells
- before or at birth, primordial follicles
- after birth, primary follicles
- NO PRIMARY OOCYTES form after birth
- primary oocyte is in suspended prophase from birth until puberty
- dormant in ovarian follicles until puberty (years)
primary oocytes (primary follicles)
- during puberty, primary oocyte enlarges, follicular cells become columnar
- follicle becomes surrounded by zona pellucida
- NO PRIMARY OOCYTES form after birth
- primary oocyte is in suspended prophase from birth until puberty
- dormant in ovarian follicles until puberty (years)
secondary follicle / secondary oocyte
- Leading up to puberty, follicle matures, increases in size
- oocyte completes 1st meiotic division before ovulation
- this gives rise to secondary oocyte / polar body I
ovulation and formation of fertilized ovum
- at ovulation, nucleus of secondary oocyte begins 2nd meiotic division
- arrests in metaphase
- if sperm penetrates, 2nd meiotic division is completed
- fertilized egg and 2nd polar body
gametogenesis in men vs. women
MEN
- begins at puberty
- one germ cell results in 4 mature spermatazoa (23 X or 23 Y)
WOMEN
- begins during embryogenesis
- arrests until puberty
- one germ cell results in 1 ovum and 2 polar bodies (23 X)
zona pellucida
primary oocyte surrounded by a covering of amorphous acellular glycoprotein material
corona radiata
layer of follicular cells around oocyte
Uterus
thick walled, pear shaped muscular organ (body, cervix)
endometrium
lining of uterus; during menstrual cycle at peak of development, lining is 4-5cm thick
NB: basal layer of endometrium has own blood supply, not sloughed off during menstruation; functional layers (compact, spongy) disintegrate and are shed during menses
GRH
Gonadotropin Releasing Hormone
- released by neurosecretory cells in hypothalamus
- carried into anterior pituitary
- stimulates release from anterior pituitary of FSH and LH
FSH
Follicle Stimulating Hormone
- stimulate development of ovarian follicles
- stimulate production of estrogen by follicular cells
- promotes growth of several primordial follicles into 5-12 primary follicles
- ONLY 1 FOLLICLE USUALLY DEVELOPS into mature follicle and ruptures, expelling its oocyte
LH
Luteinizing Hormone
- trigger of ovulation
- release of secondary oocyte
- stimulates follicular cells and corpus luteum to produce progesterone, inducing growth of ovarian follicules and endometrium
Ovulation
- early in cycle, ~50 follicles recuited, only one is dominant follicle
- primary follicle = zona pellucida
- secondary follicle = antrum, proliferation of follicular cells
- tertiary (mature) follicle = presses on surface of ovary
corpus luteum
- walls of ovarian follicle collapse to form folds
- develop into grandular structure
- secretes progesterone and some estrogen
What triggers ovulation
- LH surge (12-24 afterwards)
- ovary expells secondary oocyte surrounded by zona pellucida and layers of follicular cells (corona radiata)
- resumption of 1st meiotic division
Female menstrual cycle
- menstrual phase
- proliferative phase
- growth of ovarian follicles
- controlled by estrogen
- endometrium thickens
- luteal phase
- formation, functioning, growth of corpus luteum
- progesterone stimulates epithelium –> glycogen-rich material
What happens if no egg fertilization
- Corpus luteum degenerates
- estrogen and progesterone levels fall
- secretory endometrium becomes ischemic (lose oxygen)
- menstruation occurs
What happens if fertilization occurs
- clevage of zygote and formation of blastocyst
- blastocyst impoants in endometrium about day 5-6
- hCG (human chorionic gonadotropin), produced by syncytiotrophoblasts, keeps CL producing estrogen and progesterone
- luteal phase continues and menstruation does not occur
syncytiotrophoblasts
- produces by hCG
- keeps corpus luteum producing estrogen and progesterone
How are oocytes transported
- during ovulation, fimbriated end of tubule applied to ovary
- sweeping action of fimbriae and fluid currents sweep secondary oocytes into infundibulum of uterine tube
- oocyte passes into ampulla of tube (peristalsis of tube)
ovary –> fallopian tube (ampulla)
How are sperm transported
- sperm transported from epididymis to urethra through peristalsis of ductus
When is meiosis arrested (oogenesis)
Meiosis is arrested at two points:
- 1* oocyte arrests before birth in prophase; this arrest continues until puberty
- division resumes shortly before ovulation for formation of 2* oocytes + 1st polar body
- 2* oocyte arrests in metaphase during ovulation
- if fertilization occurs: meiosis will resume
- no fertilization: oocyte remains arrested
syncytiotrophoblast (Greek roots)
- syn *= “together”
- cytio *= “of cells”
- tropho *= “nutrition”
- blast *= “bud”
Where does fertilization usually occur
In the ampulla (ampullary region) of fallopian tube
endometriosis
endometrial tissue that lines uterus is also appear and flourish outside the uterine cavity, most commonly on membrane that lines abdominal cavity (peritoneum)
capacitation
phase in which ejaculated sperm are transformed so that they can fertilize an egg
- freshly ejaculated sperm CANNOT fertilize an egg
- glycoprotein coat and seminal proteins removed/digested from acrosome
- membrane cholesterol/phospholipid ratio altered
- increased motility
- changes allow acrosome reaction to occur
acrosome reaction
reaction that occurs in sperm during fertilization
- when capacitated (changed) sperm comes into contact with corona radiata surrounding 2* oocyte, undergo complex molecular changes
- point fusions occur between plasma membrane of sperm and external acrosomal membrane
- enzymes **hyaluronidase **and acrosin released from acrosome to facilitate fertilization
When and where does fertilization occur
~12-24 hours after ovulation occurs; typically in ampulla (ampullary region of fallopian tube)
Phases of fertilization
- penetration of corona radiata
- penetration of zona pellucida
- fusion of sperm and egg plasma membranes
- zona reaction
- completion of meiosis II by 2* oocyte (was arrested at metaphase)
- breakdown of female/male pronucleii and aligment of chromosomes
- zygote
zona reaction
once the sperm reaches the zona pellucida, there are changes in properties of the zona pellucida to make it impenetrable
In Vitro Fertilization (IVF)
- hormones given to woman to get multiple mature follicles
- collection of oocytes from follicles during laparoscopy
- place oocytes in petri dish with capacitated sperm; IVF occurs
- cleavage of zygotes in culture medium until 4-to-8-cell stages reached
- transfer of 1-2 cleaving embryos into uterine cavity
ICSI
Intracytoplasmic sperm injection
- injection of 1 sperm into 1 mature oocyte (specific)
- used when IVF fails or when there are too few sperms available
ectopic pregnancy
occurs when embryo implants anywhere outside uterus (most common is fallopian tube)
- can be treated early with medication, but if not caught in time can pose serious complications and even death
