Gametogenesis

Formation of gametes

Weeks 4 and 5 post-conception, primordial germ cells (PGC) migrate into yolk sac, later migrate to future gonad when urinary/repro systems develop

Meiosis I

homologous chromosomes separate (ex: X and Y sep)

Meiosis II

Sister chromatids separate

(ex: 2 copies of Y separate)

disjunction

separation of chromosomes

nondisjunction is failure of chromosomes to separate properly, can occur in meiosis I or II

Spermatogenesis

1) PGC arrive in primitive testis wk4, dormant until puberty

2) puberty: type A (replicate to form A and B) and B spermatogonia form

3)Type B-->meiosis I--> primary spermatocytes (2N)

4) complete meiosis I-->2 secondary spermatocytes (N)

5) meiosis II--> 4 spermatids (N)

Spermiogenesis

spermatids-->sperm

Capacitation

sperm are not capable of fertilization until capacitation, which occurs in female reproductive tract

Length (days) of sperm cell cycle

64

Oogenesis

1)Primary oocytes formed in 5th month of fetal life, dormant in prophase of mei I until puberty

2) puberty-->ovarian cycle-->meiosis I complete-->secondary oocyte

3) secondary oocyte-->mei II, arrested in metaphase until fertilization or menstruation

4) fertilization: mei II completed--> mature oocyte

Ovulation

one oocyte(arrested at 2nd meiotic division) discharged by ovarian follicle, surrounded by zona pellucida and corona radiata

"caught" by fimbriae, travels down ampulla (site of fertilization)

Fertilization

sperm penetrates corona radiata, zona pellucida and cell membrane

oocyte finishes 2nd meiotic div-->female pronucleus

zona impenetrable to other sperm

head of sperm=male pronucleus

site of fertilization

ampulla of uterine tube

morula

16 cell stage

enters uterus on 3rd or 4th day after fertilization

blastocyst

after morula enters uterine cavity and blastocyst cavity begins to develop

inner cell mass

ICM of blastocyst develops into embryo

outer cell mass

OCM of blastocyst develops into trophoblast

when does implantation occur?

Week 2

Endometrium

inner lining of uterus

myometrium

muscular uterine layer

perimetrium

1 cell layer thick, outer layer of uterus

site of implantation

anterior or posterior of superior wall

ectopic pregnancy

blastocyst implants outside of uterus: uterine tube, abdominal cavity, peritoneal covering of intestinal tract

embryo usually dies in 2nd month

sx abdominal pain and hemorrhaging

trophoblast

cytotrophoblast: distinct cell boundaries, mitosis-->syncytiotrophoblast and primary chorionic villi

syncytiotrophoblast: blurred cell boundaries-->grows into endometrium, connects with maternal blood supply, produces human chorionic gonadotropin (hCG)

pregnancy testing

hCG detected in blood by day 8 and urine by day 10

embryoblast --> 2 layers

epiblast: extraembryonic somatic mesoderm (outer layer), splanchnopleuric.  will eventally cover embryo and from connecting stalk to suspend embryo (NOT umbilical cord)

hypoblast

bilaminar disc

epiblast and hypoblast (both originate from embryoblast)

week 2

+trophoblast forms 2 layers, cyto and syncytiotrophoblast

+embryoblast forms 2 layers, hypo and epiblast

+extraembryonic mesoderm forms 2 layers, somatopleure, and splanchnopleur

+2 cavities form, amniotic and yolk sac

epiblast and hypoblast cell types

epi--columnar ectodermal

hypo--cuboidal ectodermal

gastrulation

development into 3 layered disc (ecto, meso, endoderm)

1) primitive streak (primitive groove and prim node), loc on cephalic end and surrounded by prim pit

2) invagination of epiblast cells under prim streak, form endoderm

3) cells between epiblast and endoderm-->mesoderm

4) remaining cells=ectoderm

(hypoblast is replaced by endoderm, degenerates)

mvt of primitive streak

posterior-->anterior

notochord

mesoderm-derived, first indication of axial skeleton

elongation cranial-->caudal

inductive role in organ formation

adults: persists in intervertebral discs (nucleus pulposus)

embryonic folding

develops cephalocaudally

orientation: cranial-sacral axis, right-left axis, dorsal-ventral axis

somites form (meso)

neural plate (ecto)

lateral folding

extends laterally, then ventrally, closing off body cavities

Neurulation

3rd week of development

notochord induces ectoderm to thicken and form neural plate (neuroectoderm)

groove forms, closes to form spinal cord

neural crest cells form at endges of neuroectoderm and migrate throughout body

Neural crest derivatives

connective tissue/bones of face and skull, cranial nerve ganglia, "C" cells of thyroid, conotruncal septum in heart, odontoblasts, dermis of face/neck, spinal (dorsal root) ganglia, sympathetic chain and preaortic ganglia, parasympathetic ganglia of GI tract, Schwann cells, Glial cells, Meninges (forebrain), melanocytes, smooth muscle cells to blood vessels of face, adrenal medulla

embryonic period

3 to 8 wks

3 germ layers give rise to tissues and organ sytems

Ectoderm

nervous system, sensory epithelium of ear/nose/eyes, skin, hair, nails, pituitary, mammary, sweat glands, enamel of teeth

Mesoderm

connective tissues, tendons, cartilage, bone, muscle, some urogenital structures, blood cells, adrenal cortex

Endoderm

GI tract, epithelial lining of resp tract, parenchyma of thyroid, parathyroids, liver, pancreas, stoma of tonsils and thymus, epithelial lining of urinary bladder and urethra, epithelial lining of tympanic cavity and auditory tube

Paraxial mesoderm

closest to midline

organized into somites (cephalocaudal development)

Intermediate mesoderm

urogenital structures

Lateral plate mesoderm

parietal and visceral layers line intraembryonic cavity and surround organs

Placenta

cytotrophoblast-derived

fetal portion: derived from chorionic plate

maternal portion: derived from decidua basalis

placental barrier

blood from uterine spinal arteries enters intervillous spaces and bathes fetal villous tree with oxygenated blood

barrier=syncytial membrane and endothelial cells from fetal capillaries

umbilical cord

2 umb arteries, 1 umb vein, and Wharton's jelly

hormones produced by placenta

hCG, progesterone (stimulated by hCG)--maintains endometrium, placental lactogen--induces lipolysis which elevates free fatty acid levels, assisting in fetal growth

3 cavities

1) uterine

2) chorionic

3) amniotic

3 layers of decidua

d. basalis (deep to blastocyst), d. capsularis, d. parietalis

all 3 eventually fuse

yolk sac

nutrient transfer to embryo before placental circulation established, participates in hematopoiesis, contributes to digestive system formation, location of PGC before migration

Amnion

source of amniotic fluid unknown

fetus excretes 500 mL of urine daily, fluid ingested and eliminated by placenta

full term, 1000mL of amniotic fluid

oligohydramnios

too little amniotic fluid, assoc. w renal agenesis and urethral obstruction

polyhydramnios

too much amniotic fluid, assoc with anencephaly, intestinal obstruction or atresia (absence of opening, often anus)